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<p>6360tp.indd 1 6/20/08 9:36:33 AM</p><p>Exploring an Earthlike World</p><p>TITAN</p><p>Second Edition</p><p>SERIES ON ATMOSPHERIC, OCEANIC AND PLANETARY PHYSICS</p><p>Series Editor: F. W. Taylor (Oxford Univ., UK)</p><p>Vol. 1: TITAN: The Earth-Like Moon</p><p>Athena Coustenis & Fredric W. Taylor</p><p>Vol. 2: Inverse Methods for Atmospheric Sounding: Theory and Practice</p><p>Rodgers Clive D.</p><p>Vol. 3: Non-LTE Radiative Transfer in the Atmosphere</p><p>M. López-Puertas & F. W. Taylor</p><p>Vol. 4: TITAN: Exploring an Earthlike World (2nd Edition)</p><p>Athena Coustenis & Fredric W. Taylor</p><p>CheeHok - TITAN (2nd Edn).pmd 9/19/2008, 6:58 PM2</p><p>World Scientific</p><p>6360tp.indd 2 6/20/08 9:36:35 AM</p><p>Series on Atmospheric, Oceanic and Planetary Physics — Vol.4</p><p>TITAN</p><p>Exploring an Earthlike World</p><p>Second Edition</p><p>Athena Coustenis</p><p>Paris -Meudon Observatory</p><p>Fredric W Taylor</p><p>University of Oxford</p><p>With illustrations by D J Dylor</p><p>NEW JERSEY . LONDON . SINGAPORE . BEIJING . SHANGHAI . HONG KONG . TAIPEI . CHENNAI</p><p>British Library Cataloguing-in-Publication Data</p><p>A catalogue record for this book is available from the British Library.</p><p>For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center,</p><p>Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from</p><p>the publisher.</p><p>ISBN-13 978-981-270-501-3</p><p>ISBN-10 981-270-501-5</p><p>Typeset by Stallion Press</p><p>Email: enquiries@stallionpress.com</p><p>All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or</p><p>mechanical, including photocopying, recording or any information storage and retrieval system now known or to</p><p>be invented, without written permission from the Publisher.</p><p>Copyright © 2008 by World Scientific Publishing Co. Pte. Ltd.</p><p>Published by</p><p>World Scientific Publishing Co. Pte. Ltd.</p><p>5 Toh Tuck Link, Singapore 596224</p><p>USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601</p><p>UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE</p><p>Printed in Singapore.</p><p>Series on Atmospheric, Oceanic and Planetary Physics — Vol. 4</p><p>TITAN (2nd Edition)</p><p>Exploring an Earthlike World</p><p>CheeHok - TITAN (2nd Edn).pmd 9/19/2008, 6:58 PM1</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>Contents</p><p>Prologue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii</p><p>Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii</p><p>1. Introduction 1</p><p>1.1 Early History . . . . . . . . . . . . . . . . . . . . . . . . . . . 1</p><p>1.2 Titan in Mythology . . . . . . . . . . . . . . . . . . . . . . . . 5</p><p>1.3 Space Exploration of the Solar System . . . . . . . . . . . . . . 8</p><p>1.4 The 20th Century, Before Voyager . . . . . . . . . . . . . . . . 13</p><p>2. The Voyager Missions to Titan 16</p><p>2.1 Space Missions to the Saturnian System . . . . . . . . . . . . . 16</p><p>2.2 Voyager Observations of Titan . . . . . . . . . . . . . . . . . . 20</p><p>2.3 Atmospheric Bulk Composition . . . . . . . . . . . . . . . . . 22</p><p>2.4 Vertical Temperature Structure . . . . . . . . . . . . . . . . . . 22</p><p>2.5 Energy Balance and the Temperature Profile</p><p>in the Thermosphere . . . . . . . . . . . . . . . . . . . . . . . 26</p><p>2.6 Atmospheric Composition . . . . . . . . . . . . . . . . . . . . 28</p><p>2.7 Photochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . 31</p><p>2.8 Cloud and Haze Properties . . . . . . . . . . . . . . . . . . . . 34</p><p>2.9 Speculations on the Surface and Landscape of Titan</p><p>from Voyager . . . . . . . . . . . . . . . . . . . . . . . . . . . 35</p><p>2.10 The Aftermath of Voyager . . . . . . . . . . . . . . . . . . . . 36</p><p>3. Observations of Titan from the Earth 38</p><p>3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38</p><p>3.2 Space Observatories . . . . . . . . . . . . . . . . . . . . . . . 38</p><p>3.2.1 Hubble Space Telescope . . . . . . . . . . . . . . . . . 39</p><p>3.2.2 The James Webb Space Telescope . . . . . . . . . . . . 40</p><p>3.2.3 Infrared Space Observatory . . . . . . . . . . . . . . . 41</p><p>3.3 Ground-Based Observatories . . . . . . . . . . . . . . . . . . . 44</p><p>3.3.1 Mauna Kea Observatories . . . . . . . . . . . . . . . . 44</p><p>3.3.2 The European Southern Observatories . . . . . . . . . . 46</p><p>v</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>vi Titan: Exploring an Earthlike World</p><p>3.3.3 The University of Arizona and Steward Observatory</p><p>Telescopes . . . . . . . . . . . . . . . . . . . . . . . . 48</p><p>3.3.4 Radio Astronomy . . . . . . . . . . . . . . . . . . . . . 49</p><p>3.4 Earth-Based Studies of Titan . . . . . . . . . . . . . . . . . . . 52</p><p>3.4.1 Occultations of Titan . . . . . . . . . . . . . . . . . . . 52</p><p>3.4.2 The Radar Search for Oceans, Seas or Lakes . . . . . . 54</p><p>3.4.3 Spectroscopic Measurements of Titan’s Albedo . . . . . 57</p><p>3.4.4 Imaging Titan’s Atmosphere in the Near-Infrared . . . . 64</p><p>3.4.5 Imaging the Surface . . . . . . . . . . . . . . . . . . . 66</p><p>3.5 Ground-Based Observations and Cassini–Huygens . . . . . . . 71</p><p>4. Cassini–Huygens: Orbiting Saturn and Landing on Titan 72</p><p>4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72</p><p>4.2 The Spacecraft and its Systems . . . . . . . . . . . . . . . . . . 73</p><p>4.3 Scientific Objectives . . . . . . . . . . . . . . . . . . . . . . . 77</p><p>4.4 The Long History of the Cassini–Huygens Mission . . . . . . . 79</p><p>4.5 Departure for the Saturnian System . . . . . . . . . . . . . . . 82</p><p>4.6 Journey to Saturn and Orbit Insertion . . . . . . . . . . . . . . 84</p><p>4.7 Huygens Descends onto Titan . . . . . . . . . . . . . . . . . . 85</p><p>4.8 Experiments and Payloads . . . . . . . . . . . . . . . . . . . . 89</p><p>4.8.1 The Scientific Instruments on the Orbiter . . . . . . . . 89</p><p>4.8.2 The Scientific Instruments on the Probe . . . . . . . . . 99</p><p>4.9 Touring the Saturnian System . . . . . . . . . . . . . . . . . . 107</p><p>4.9.1 Observations of Saturn . . . . . . . . . . . . . . . . . 107</p><p>4.9.2 The Icy Satellites, and Saturn’s Rings . . . . . . . . . . 109</p><p>4.9.3 Saturn’s Magnetosphere and Titan . . . . . . . . . . . . 112</p><p>4.10 Being Involved: Scientists and Instrument Providers . . . . . . . 113</p><p>4.11 Reaping the Benefits . . . . . . . . . . . . . . . . . . . . . . . 115</p><p>Colour Plates 117</p><p>5. Titan’s Atmosphere and Climate 129</p><p>5.1 The Climate on Titan . . . . . . . . . . . . . . . . . . . . . . . 129</p><p>5.1.1 Atmospheric Pressure Profile . . . . . . . . . . . . . . 129</p><p>5.1.2 Atmospheric Thermal Structure . . . . . . . . . . . . . 131</p><p>5.1.3 Troposphere . . . . . . . . . . . . . . . . . . . . . . . 133</p><p>5.1.4 Stratosphere . . . . . . . . . . . . . . . . . . . . . . . 135</p><p>5.1.3 Mesosphere . . . . . . . . . . . . . . . . . . . . . . . . 136</p><p>5.1.6 Thermosphere . . . . . . . . . . . . . . . . . . . . . . 136</p><p>5.1.7 Exosphere . . . . . . . . . . . . . . . . . . . . . . . . 136</p><p>5.2 Radiation in Titan’s Atmosphere . . . . . . . . . . . . . . . . . 137</p><p>5.2.1 Solar and Thermal Radiation . . . . . . . . . . . . . . . 137</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>Contents vii</p><p>5.2.2 Energy Balance and Surface Temperature . . . . . . . . 137</p><p>5.2.3 Model Temperature Profile . . . . . . . . . . . . . . . . 138</p><p>5.2.4 Radiative Equilibrium Temperature Profile . . . . . . . 139</p><p>5.3 Remote Atmospheric Temperature Sounding . . . . . . . . . . . 141</p><p>5.4 Titan’s Ionosphere and its Interaction with the</p><p>Magnetosphere of Saturn . . . . . . . . . . . . . . . . . . . . . 144</p><p>5.5 Climate Change on Titan . . . . . . . . . . . . . . . . . . . . . 147</p><p>6. Chemistry and Composition 150</p><p>6.1 Titan’s Chemical Composition . . . . . . . . . . . . . . . . . . 150</p><p>6.2 The Bulk Composition of the Atmosphere . . . . . . . . . . . . 154</p><p>6.3 Ionospheric Chemistry . . . . . . . . . . . . . . . . . . . . . . 155</p><p>6.4 Trace Constituents in the Neutral Atmosphere . . . . . . . . . . 157</p><p>6.4.1 Stratospheric Composition Measurements with Cassini . 160</p><p>6.4.2 Vertical Distributions . . . . . . . . . . . . . . . . . . . 175</p><p>6.4.3 Spatial Variations . . . . . . . . . . . . . . . . . . . . . 177</p><p>6.4.4 Temporal Variations of the Trace Constituents . . . . . . 180</p><p>6.5 Photochemistry . . .</p><p>of the surface. These “smog”</p><p>particles form a layer that enshrouds the entire globe of Titan and stretches from</p><p>the surface to an altitude of about 200 km, with the altitude of unit vertical optical</p><p>depth in visible light at about 100 km. Views of the limb from Voyager showed the</p><p>presence of a detached haze layer at 340–360 km altitude with large, irregular dark</p><p>particles.</p><p>The most obvious global feature seen by Voyager in the haze cover was a differ-</p><p>ence in the brightness of the two hemispheres, about 25% at blue wavelengths, falling</p><p>to a few percent at ultraviolet and at red wavelengths. This so-called north-south</p><p>asymmetry is apparently related to the circulation of the atmosphere pushing haze</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>The Voyager Missions to Titan 21</p><p>Figure 2.4 The first close-range image of Titan from the Voyager spacecraft showed little detail, just</p><p>a hint of the north-south atmospheric asymmetry and a dark polar collar. The surface of Titan is hidden</p><p>under a deep haze layer (NASA/JPL).</p><p>and gases from one hemisphere to the other. The asymmetry has been observed to</p><p>reverse — when the Hubble Space Telescope (HST) first observed Titan in 1990, a</p><p>little over a quarter of a Titan year after the Voyager encounters, the northern hemi-</p><p>sphere was found to be brighter than the south. Whereas Voyager only observed up</p><p>to red wavelengths, HST can image Titan in the near-infrared. At these wavelengths,</p><p>the asymmetry is reversed, and indeed is somewhat stronger than in the visible. This</p><p>is due to the wavelength dependence of the atmospheric brightness (bright at short</p><p>wavelengths due to Rayleigh scattering, dark in the near-infrared due to methane</p><p>absorption) and the haze (which seems to be dark in blue and bright at red and longer</p><p>wavelengths, by analogy with synthetic haze material generated in the laboratory).</p><p>The visible bright hemisphere has less haze than the darker one.</p><p>The limited photometric data we had from 1970 until the Cassini arrival sug-</p><p>gests that the hemispheric contrast varies smoothly, and that limb-darkening is also</p><p>strongly wavelength-dependent. The disk at UV and violet wavelengths is fairly</p><p>flat, while it is near-lambertian (coefficient ∼1.0) at green and red wavelengths and</p><p>shows limb brightening in the near infrared. Voyager also saw a dark ring around</p><p>the north (winter) pole. This feature, part of a polar hood extending from 70◦ to</p><p>90◦ north latitude, is most prominent at blue and violet wavelengths, and it has</p><p>since then been suggested that it may be associated with lack of illumination in the</p><p>polar regions during the winter (since the subsolar latitude goes up to 26.4◦), and/or</p><p>subsidence in global circulation.</p><p>The data from the brief encounter covered only a few hours, as Voyager was trav-</p><p>elling past Titan at a relative speed of over 17.3 kilometres per second, but it imme-</p><p>diately clarified a lot of questions, and of course raised many others. In particular,</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>22 Titan: Exploring an Earthlike World</p><p>a combination of radio occultation, infrared spectroscopic and ultraviolet observa-</p><p>tions from the spacecraft came out in favour of a model like Hunten’s, albeit with a</p><p>lower surface pressure and temperature of about 1.5 bars and 100 K, respectively.</p><p>During the rush that usually follows the arrival of a space mission at a planet,</p><p>especially when it is the first visit to a new world, scientists work on tight time</p><p>schedules to extract new findings as quickly as possible from masses of data, in</p><p>order to inform the public, their sponsors and the media who are eager for news.</p><p>Once the initial stir subsides, longer-term, more thorough analyses are undertaken,</p><p>generally based on improved laboratory measurements and computer models, and</p><p>designed to extract all of the new information. The following sections discuss the</p><p>results obtained fromVoyager and serve as an introduction to the much more detailed</p><p>information about Titan’s atmosphere, surface, and interior that is being provided</p><p>by the Cassini–Huygens mission, as described in the following chapters.</p><p>2.3 Atmospheric Bulk Composition</p><p>The first reasonably complete picture of the basic nature of Titan’s atmosphere turned</p><p>out to be a combination of the two most popular pre-Voyager models. Molecular</p><p>nitrogen, N2, was detected by the ultraviolet spectrometer as the major component</p><p>of the atmosphere, at about 95%, with methane as the next most abundant molecule,</p><p>with abundances determined by the Infrared Interferometer Spectrometer (IRIS) to</p><p>be 0.5–3.4% in the stratosphere and from 4–8% at the surface. Traces of hydro-</p><p>gen and of various moderately complex organic gases, consisting of several hydro-</p><p>carbons, nitriles and CO2 were found, with just one firm detection of a condensate</p><p>(C4N2) at first. Some simple oxygen compounds like CO2, were also observed</p><p>by IRIS.</p><p>2.4 VerticalTemperature Structure</p><p>From the Voyager radio-occultation experiment, the surface temperature of Titan</p><p>was found to be 94 ± 1.5 K, or −179◦C, at a pressure of about 1.5 bar, 50% higher</p><p>than Earth. A more precise value for Titan’s radius was also derived: 2575 ± 2 km.</p><p>These, and the other main properties of Titan as established at the time, are collected</p><p>in Table 2.2.</p><p>In the radio-occultation experiment, the refraction of the radio beam from the</p><p>spacecraft was measured as it flies behind (or emerges from) the planetary disk, as</p><p>seen from Earth, provided density profiles as a function of altitude near the equator.</p><p>These can be converted into temperature versus mean molecular weight vertical</p><p>profiles in the atmosphere. From an analysis of the rate at which Voyager’s radio</p><p>signal was attenuated by the atmosphere as the spacecraft passed behind the satellite,</p><p>and the opposite effect when it re-emerged, two vertical refractivity profiles were</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>The Voyager Missions to Titan 23</p><p>Table 2.2 Physical characteristics of Titan obtained from Voyager 1 data, shown</p><p>in familiar, terrestrial units. The length of Titan’s day and month are the same, as</p><p>they are for Earth’s Moon.</p><p>Mass 1.346 × 1023 kg (0.0226 of Earth)</p><p>Equatorial radius 2,575 km (0.202 of Earth)</p><p>Mean density 1.88 gm cm−3 (1.88 of water)</p><p>Mean distance from Saturn 1,221,850 km (20.32 Saturn radii)</p><p>Mean distance from Sun 1.422 × 109 km (9.546 times Earth’s)</p><p>Sunfall 1.1% of Earth’s</p><p>Orbital period 15.945 Earth days</p><p>Rotational period Same as above</p><p>Titan day (period of rotation) 15.945 Earth days</p><p>Titan month (period around planet) 0.584 Earth months</p><p>Titan year (period around Sun) 29.46 Earth years</p><p>Mean orbital velocity 5.58 km s−1</p><p>Orbital eccentricity 0.0292</p><p>Orbital inclination 0.33◦</p><p>Escape velocity 2.65 km s−1</p><p>Visual geometric albedo 0.21</p><p>Magnitude (V◦) 8.28</p><p>Mean surface temperature 94 K (−179◦C)</p><p>Atmospheric pressure 1496 ± 20 mbar (1.5 of Earth’s)</p><p>obtained, near Titan’s equator. These were converted into vertical density distribu-</p><p>tions, which in turn were used to calculate vertical pressure profiles, and finally to</p><p>obtain temperature profiles as a function of altitude over the range 0 to 200 km. The</p><p>uncertainty in the temperature at 200 km was as much as 10–15 K, because profiles</p><p>determined by radio occultation depend on the assumed atmospheric composition,</p><p>in particular the proportions of nitrogen, methane and argon. The most likely value</p><p>for the molecular weight of the air on Titan was found to be around 28 amu, the</p><p>value that would correspond to nearly pure N2, but could be as high as 29.4 amu</p><p>(assuming the perfect gas law), and still be within the limits of experimental error.</p><p>This left scope for the possible presence of a heavier component than nitrogen in</p><p>Titan’s atmosphere. Based on cosmological abundance data, the presence of several</p><p>percent of argon was suggested. The formal result from Voyager IRIS data was that</p><p>the argon abundance could be up to approximately 7%, but the gas escaped detection</p><p>until the arrival of Cassini, and then was found only in trace amounts. Along with</p><p>uncertainties</p><p>in composition, calibration errors, deviations from the ideal gas law</p><p>due to the low temperature, and so on, meant that the radio occultation data were</p><p>reliable only up to about 100 km. At higher altitudes, the thermal structure became</p><p>more and more sensitive to the initial conditions adopted in the integration of the</p><p>refractivity profiles.</p><p>In the troposphere the temperature was found to fall from the surface value</p><p>to a minimum of about 71 K, at the level known (by analogy with Earth) as the</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>24 Titan: Exploring an Earthlike World</p><p>Figure 2.5 Titan’s atmospheric temperature profile, as inferred from Voyager radio occultation mea-</p><p>surements (Lindal et al., 1983).</p><p>tropopause, located around 40 km in altitude. A profile in which temperature stops</p><p>falling with increasing height and starts to rise again, is common in planetary atmo-</p><p>spheres. It manifests itself in the infrared spectrum by the presence of emission, as</p><p>opposed to absorption, bands of the more strongly absorbing minor components.</p><p>The atmospheric temperature on Titan is everywhere higher than the condensation</p><p>temperature of molecular nitrogen, making it improbable that nitrogen clouds could</p><p>form. Condensation of methane, however, can occur if the methane stratospheric</p><p>mixing ratio exceeds 1.6%, and the surface pressure and temperature conditions</p><p>found by Voyager were consistent with the presence of methane in liquid form. It</p><p>was tempting for the scientists examining the data to imagine that methane rain</p><p>accumulates in a vast ocean on the surface. However, they noted that the vertical</p><p>gradient of temperature, called the ‘lapse rate’, observed near the surface, was close</p><p>to the value expected for a dry nitrogen atmosphere, indicating that methane sat-</p><p>uration probably did not apply, at least at the particular place and time probed by</p><p>the Voyager occultation. The precise CH4 abundance was difficult to determine; the</p><p>best estimate was a height-dependent value that varied from 2.5% up to 6%.</p><p>Another candidate for the main component of lakes or seas on Titan is ethane,</p><p>which is one of the main photochemical products of methane and which is more</p><p>stable as a liquid under the temperature and pressure conditions on the surface.</p><p>A global ethane ocean, 1 km deep, in which methane is dissolved in appreciable</p><p>quantities, as well as traces of nitrogen and other atmospheric condensates, including</p><p>higher hydrocarbons like propane and acetylene, was suggested by Jonathan Lunine</p><p>of the University of Arizona in Tucson and his colleagues in 1983. The Voyager-</p><p>determined lapse rate changes near a height of 3.5 km, an observation which could</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>The Voyager Missions to Titan 25</p><p>be interpreted as marking the boundary between a convective region near the surface</p><p>and a radiative equilibrium zone higher up, although it could also be the sign of the</p><p>bottom of a methane cloud with a clear mixture of nitrogen and methane gas below.</p><p>Two additional data sets were available to constrain the temperature structure on</p><p>Titan. Firstly, the Voyager ultraviolet spectrometer observed an occultation of the</p><p>Sun by Titan, obtaining a value for the air density at 1265 km of 2.7 ± 0.2 × 108</p><p>molecules cm3. This translates to a temperature of 186 ± 20 K at that level, and</p><p>an average temperature of 165 K in the 200 to 1265 km altitude range. The UVS</p><p>experiment also allowed the detection of a methane mixing ratio of 8 ± 3% around</p><p>1125 km, and placed the homopause level at around 925 ± 70 km. Secondly, the</p><p>occultation of the star 28 Sgr by Titan was observed from places as widely dispersed</p><p>as Israel, the Vatican, and Paris on July 3, 1989. This rare event provided information</p><p>in the 250–500 km altitude range, including a temperature value of 183 ± 11 K near</p><p>450 km.</p><p>In principle, a combined analysis of the radio-occultation atmospheric refraction</p><p>data with the infrared spectral radiance data from IRIS allows the temperature pro-</p><p>file to be retrieved from the ground up to about 200 km, along with estimates of the</p><p>abundances of the major components (N2, CH4 and H2) that affect the distribution</p><p>of infrared opacity sources. In reality, the usual approach is to assume a given com-</p><p>position of the atmosphere (about 98% of nitrogen, about 2% of methane and 0.2%</p><p>of hydrogen) and then to infer the temperature profile by finding the temperature</p><p>profile which fits the measured spectrum assuming this composition. IRIS data in</p><p>the methane band at 1304 cm−1 probe the 0.01 to 10 mbar (about 150 to 450 km)</p><p>atmospheric region, and the retrieval can be extended to include the thermal struc-</p><p>ture in the thermosphere, up to 800 km, if the RSS and IRIS data are augmented by</p><p>the ultraviolet spectrometer (UVS) solar occultation measurements.</p><p>Outside the equatorial region, where no radio occultation data were available and</p><p>only the ν4 CH4 band was available to infer temperature, the results were restricted to</p><p>the upper stratosphere and lower mesosphere. Assuming that methane is uniformly</p><p>mixed in the atmosphere, with a methane mole fraction of 1.8%, nominal temperature</p><p>profiles were retrieved for seven different latitudes in the 0–450 km altitude range</p><p>by extrapolating downwards so that the various retrieved stratospheric profiles all</p><p>join smoothly to a single tropospheric profile. Obviously, this is not completely</p><p>satisfactory, since the tropospheric and surface temperatures also vary latitudinally,</p><p>but we had no information about that with Voyager. Also, the thermal structure</p><p>derived through inversion of the radiative transfer equation, using methane as an</p><p>opacity source, did not exactly join the radio-occultation temperature profile in the</p><p>zone where the latter was thought reliable.With that caveat, the retrieved temperature</p><p>profiles revealed a larger temperature decrease from the equator to the north pole</p><p>than from the equator to the south pole, with the south pole 2–3 K, and the north pole</p><p>up to 20 K, colder than the equator. The Voyager encounter took place at the time</p><p>of the northern spring equinox on Titan and the equator-to-north-pole temperature</p><p>gradient, is expected to reverse as the seasons change, with a lag due to the thermal</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>26 Titan: Exploring an Earthlike World</p><p>500</p><p>400</p><p>300</p><p>200</p><p>100</p><p>0</p><p>H</p><p>e</p><p>ig</p><p>h</p><p>t</p><p>(k</p><p>m</p><p>)</p><p>P</p><p>re</p><p>ss</p><p>u</p><p>re</p><p>(</p><p>b</p><p>a</p><p>r)</p><p>10-5</p><p>10-4</p><p>10-3</p><p>10-2</p><p>10-1</p><p>100</p><p>500</p><p>400</p><p>300</p><p>200</p><p>100</p><p>1109070 130 150 170 190</p><p>0</p><p>H</p><p>e</p><p>ig</p><p>h</p><p>t</p><p>(k</p><p>m</p><p>)</p><p>P</p><p>re</p><p>ss</p><p>u</p><p>re</p><p>(</p><p>b</p><p>a</p><p>r)</p><p>Temperature (K)</p><p>10-5</p><p>10-4</p><p>10-3</p><p>10-2</p><p>10-1</p><p>100</p><p>5° N</p><p>30° N</p><p>50° N</p><p>70° N</p><p>7° S</p><p>32° S</p><p>50° S</p><p>Figure 2.6 Temperature profiles retrieved from Voyager data, at different latitudes. The top set is for</p><p>the northern hemisphere, at the latitudes shown; the bottom set for the southern hemisphere (Coustenis</p><p>and Bézard, 1995).</p><p>and dynamical inertia of the atmosphere. The observed cooling in the north polar</p><p>region could also be the result of an enhanced concentration of infrared emitters</p><p>(gases and possibly aerosols), although this would require the hemispheres to have</p><p>long-term compositional differences for some unknown reason.</p><p>2.5 Energy Balance and theTemperature Profile</p><p>in theThermosphere</p><p>With no significant internal energy source of its own, Titan is expected to be in overall</p><p>energy balance with the Sun. In addition, the individual layers of the atmosphere need</p><p>to be in equilibrium with the surrounding layers, the Sun, and the surface. Computer</p><p>calculations of this balance are a well-established way of predicting the temperature</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>The Voyager Missions to Titan 27</p><p>radiation absorbed</p><p>in haze</p><p>200</p><p>400</p><p>600</p><p>800</p><p>1000</p><p>1200</p><p>0</p><p>60 80 100 120 140 160 180 200</p><p>H</p><p>ei</p><p>gh</p><p>t</p><p>(k</p><p>m</p><p>)</p><p>Temperature (K)</p><p>Figure 2.7 Model temperature profile for Titan, including the thermosphere, derived from energy</p><p>balance considerations (Lellouch et al., 1990).</p><p>structure on bodies with atmospheres. In 1983, Robert Samuelson developed an ana-</p><p>lytical radiative equilibrium</p><p>model for Titan that was vertically homogeneous and</p><p>that considered the radiation balance in three broad spectral intervals. His results</p><p>confirmed pre-Voyager models, which gave a temperature inversion on Titan as a</p><p>result of strong absorption of solar UV in the stratosphere, and penetration to near</p><p>the surface of longer wavelength solar visible radiation. To obtain agreement with</p><p>Voyager temperature data, Samuelson needed new sources of opacity in the height</p><p>regions near 20 and 65 km, and he suggested these might be due to possible conden-</p><p>sation clouds of methane and C2H2–C2H6–C3H8, respectively. He also identified the</p><p>wavenumber range from 400 to 600 cm−1 as a thermal infrared “window”, a spectral</p><p>region of relatively high transparency throughout the atmosphere. The existence of</p><p>such a window, at wavelengths where Titan’s surface is emitting strongly, reduces</p><p>the greenhouse effect on Titan and is responsible for cooling the surface by about</p><p>9 K. The stratospheric haze, which absorbs efficiently at short wavelengths, blocks</p><p>much of the incoming solar radiation, but is more transparent in the thermal infrared</p><p>and does not ‘close’ the window.</p><p>In 1984, Friedson and Yung undertook more detailed calculations of the thermal</p><p>balance of individual layers in Titan’s atmosphere to infer the temperature profile</p><p>for the whole 0–1300 km altitude range. They solved simultaneously the equations</p><p>of heat transfer and of hydrostatic equilibrium, using the UV occultation measure-</p><p>ments of density and temperature as boundary conditions, taking into account the</p><p>sources of energy (solar radiation and a contribution from electron precipitation</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>28 Titan: Exploring an Earthlike World</p><p>into the magnetosphere) and the cooling through emission of the minor components</p><p>(dominated by acetylene in the thermosphere). The heat is transported downwards</p><p>by molecular conduction as far as the mesopause level at about 736 km and 110 K.</p><p>Later, in 1990, the problem was revisited by Lellouch and colleagues, following</p><p>the discovery of some errors in the solar heating profile and in the expressions</p><p>of the collision-induced absorption rates in the earlier work. New thermal balance</p><p>profiles were derived and tested for a match with the emission observed in the ν4</p><p>methane band, and for consistency with the Voyager UVS temperature-density point</p><p>at 1265 km. The temperature profile that satisfies these two conditions consists of</p><p>a “warm” stratospheric region (mean temperature about 175 K) up to 500 km, to</p><p>compensate for a cold mesopause (135 K) at 800 km. The associated methane mole</p><p>fraction is 1.7%. The results of the retrievals for the lower atmosphere impose an</p><p>additional constraint at 200 km of T = 173 ± 4 K, and provide at the same time the</p><p>continuity of the thermal profile down to the surface. The model thermal structure</p><p>in Titan’s thermosphere is extremely dependent on the solar heating efficiencies and</p><p>on the relaxation rates of the cooling agents (mainly acetylene). Given the lack of</p><p>information on these parameters, one may, in theory, obtain several solutions. The</p><p>only firm conclusion of this exercise seems to be the existence of a temperature</p><p>minimum at the mesopause, due to the very efficient cooling resulting from radiative</p><p>emission in the ν5 band of C2H2.</p><p>In 1991, Roger Yelle published realistic models of the thermal profiles for the</p><p>upper atmosphere of Titan (0.1 to 10−2 nbar), including non-LTE effects (that is</p><p>departed from local thermodynamical equilibrium conditions), heating/cooling in</p><p>the rotation-vibration bands of the main hydrocarbons and HCN and aerosol heat-</p><p>ing. These remained the most precise and complete knowledge on Titan’s thermal</p><p>structure until 2004 when the Cassini/Huygens mission began to greatly add to our</p><p>knowledge through in situ measurements at the equator and from data acquired by</p><p>the experiments on board the orbiter (see Chapter 4).</p><p>2.6 Atmospheric Composition</p><p>The IRIS infrared spectrometer aboard Voyager 1 studied the emission from Titan’s</p><p>atmosphere with a field-of-view of 0.25◦, which gave relatively high spatial reso-</p><p>lution. The spectra taken by IRIS cover the spectral region from 200 to 1500 cm−1</p><p>with a resolution of 4.3 cm−1. The data confirmed nitrogen as the main constituent,</p><p>allowed the abundance of other minor components to be determined, and identified</p><p>several other gases not previously detected. The presence of the simple hydrocar-</p><p>bons methane (CH4), acetylene (C2H2), ethylene (C2H4) and ethane (C2H6) was</p><p>confirmed, while the signatures of hydrogen cyanide (HCN), an important ‘prebi-</p><p>otic’ molecule (that is one of the building blocks of amino acids), and two other</p><p>nitriles, cyanoacetylene and cyanogen (HC3N and C2N2), were found. Abundances</p><p>were also estimated by comparison with laboratory spectra for some more complex</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>The Voyager Missions to Titan 29</p><p>hydrocarbons: diacetylene (C4H2), methylacetylene (C3H4), propane (C3H8) and</p><p>monodeuterated methane (CH3D). Finally, carbon dioxide (CO2) was found in the</p><p>IRIS spectra at 667 cm−1. The Voyager instrument did not cover the part of the</p><p>spectrum where carbon monoxide (CO) might have been detected. The search for</p><p>this common and widely-occurring gas was continued by other means, and it was</p><p>eventually discovered in 1983 from ground-based observations in the near infrared.</p><p>Its abundance was estimated to be about one part in ten thousand (a mixing ratio of</p><p>10−4), in agreement with the photochemical model predictions at the time.</p><p>Some of the gases which were observed directly or inferred to be in the atmo-</p><p>sphere of Titan, including molecular nitrogen, carbon monoxide and molecular</p><p>hydrogen — also argon, although it was not detected by Voyager — are expected</p><p>to be uniformly mixed throughout the lower atmosphere without undergoing phase</p><p>changes. Since methane may condense in the troposphere, it may not be vertically</p><p>uniform, and its abundance may increase with temperature down to the surface.After</p><p>methane, the next most abundant hydrocarbons are ethane, acetylene and propane.</p><p>Each of these organics and the less abundant ones could condense at some level in</p><p>the lower stratosphere and precipitate out, which would restrict the amount present</p><p>as a gas to smaller amounts below this level.</p><p>Latitudinal variations were observed for the least abundant molecules, while the</p><p>most abundant gases appeared to be approximately constant from equator to pole.</p><p>Cyanoacetylene and cyanogen (HC3N and C2N2) showed an obvious enhancement</p><p>at high northern latitudes, indeed they could not be detected equatorwards of about</p><p>60◦N. Again, this is probably a manifestation of seasonal effects; Titan’s northern</p><p>hemisphere was in early spring, suggesting these species are most abundant in the</p><p>darkness of the polar winter.</p><p>Voyager 2 flew by Titan nine months after Voyager 1 (on August 27, 1981),</p><p>170 times further from Titan’s surface than Voyager 1. Because the Titan year is</p><p>nearly 30 Earth years long, nine months is equivalent to only about a week in terms</p><p>of the change of seasons on Titan, so the Voyager 2 encounter was also not long</p><p>after the spring equinox, when the northern hemisphere was emerging from winter</p><p>into spring. The Voyager 2 IRIS instrument, identical to that on Voyager 1, took</p><p>a total of 115 infrared spectra, most of them between 15◦S and 60◦N latitude and</p><p>at emission angles lower than 50◦. The projected field of view on Titan’s disk was</p><p>of course much larger for Voyager 2’s more distant encounter, and covered more</p><p>than half Titan’s diameter, allowing for only two locally independent locations to</p><p>be used for atmospheric composition and temperature determinations. The results</p><p>nevertheless confirmed the temperature variations in latitude found by Voyager 1,</p><p>and all of the molecules previously found in the Voyager 1 data were detected by</p><p>Voyager 2 IRIS, with the exception of C2N2, HC3N, CH3D, C2H4 and C4N2. The</p><p>absence of HC3N, C4N2 (which had been detected only in</p><p>the solid phase) and C2N2</p><p>signatures in the later encounter was undoubtedly due to the fact that these molecules</p><p>were detected by Voyager 1 only in horizontal viewing measurements near 70◦N,</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>30 Titan: Exploring an Earthlike World</p><p>that is, in the particularly favourable limb geometry conditions. The other species</p><p>have weak bands in the spectrum so that they did not rise above the noise level.</p><p>Ethane, acetylene and propane, which are the most abundant carbon-containing</p><p>compounds after methane, mono-deuterated methane and carbon monoxide in</p><p>Titan’s atmosphere, were found to be lacking any significant compositional varia-</p><p>tions in latitude, in accordance with Voyager 1 results. This tends to confirm that</p><p>these molecules are homogeneously mixed in Titan’s atmosphere from pole to pole.</p><p>In the Voyager 2 analysis, methylacetylene (C3H4) and diacetylene (C4H2) tend to</p><p>increase (by a factor of 2) near the northern region with respect to the equator. This</p><p>tendency, although less marked, confirms the Voyager 1 results, in which the C3H4</p><p>Figure 2.8 An infrared spectrum obtained by Voyager-IRIS viewing the north polar region of Titan,</p><p>showing the emission features of several of the molecules discussed in the text.</p><p>C2H6</p><p>C2H2</p><p>C3H8</p><p>HCN</p><p>CO2</p><p>C3H4</p><p>C4H2</p><p>10-9</p><p>10-8</p><p>10-8</p><p>10-7</p><p>10-7</p><p>10-6</p><p>10-6</p><p>10-5</p><p>10-4</p><p>10-8</p><p>10-9</p><p>10-9</p><p>-60 60 80-40 40-20 200</p><p>Latitude</p><p>M</p><p>ix</p><p>in</p><p>g</p><p>r</p><p>at</p><p>io</p><p>Figure 2.9 Mean molecular abundance of gases in Titan’s stratosphere observed with Voyager 2</p><p>(Letourneur and Coustenis, 1993).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>The Voyager Missions to Titan 31</p><p>abundance factor of increase was between 3 and 4. The C4H2 mixing ratio in the</p><p>Voyager 1 data is a factor of about 20 higher near the north pole.</p><p>HCN showed a steady increase in abundance in the Voyager 1 data from pole-to-</p><p>pole by a factor of about 12 (of which a factor of about 4 from the equator to 50◦N</p><p>latitude). In the Voyager 2 data, the HCN mixing ratio at high latitudes is only about</p><p>two times higher than at the equator. The much larger projected field of view of the</p><p>Voyager 2 observations is probably responsible for the less marked enhancement.</p><p>In summary, Voyager 1 and Voyager 2 data show that the northern polar regions</p><p>were associated with enhanced abundances for the nitriles and some hydrocarbons</p><p>at the time of the Voyager encounters, probably because the north polar region was</p><p>just coming out of the long winter darkness, having accumulated maximum nitrile</p><p>abundances. Carbon dioxide (CO2) was the only gas showing a possible decrease</p><p>at the time, rather than increase, near the north pole.</p><p>Argon was not detected in Titan’s atmosphere prior to Cassini–Huygens, because</p><p>it shows no emission lines in the infrared spectrum and because, as we now know,</p><p>the amount present is very small. A long search took place, driven by cosmogo-</p><p>nic arguments that suggested a substantial presence was likely. The estimates of</p><p>the mean molecular weight of Titan’s atmosphere from the Voyager radio science</p><p>experiment which allowed for an element heavier than nitrogen to be present in a</p><p>substantial amount, originally thought to be as high as 27%. The expected argon</p><p>resonance lines at 1048 and 1067 Å in Titan’s EUV dayglow, due to solar and</p><p>photoelectron excitation processes, did not appear in the Voyager UV spectra, and</p><p>this led to the imposition of an upper limit of 14% by D. Strobel and colleagues</p><p>in 1992. The solar occultation data from the same instrument, and a spectrum of</p><p>the north polar region dayglow, combined to reduce the upper limit on the argon</p><p>mixing ratio at the tropopause to less than 10%. Thermal considerations using</p><p>Voyager radio-occultation measurements next led R. Courtin and colleagues to fur-</p><p>ther lower the upper limit on the argon mole fraction, to 6%. These authors also</p><p>used the IRIS intensity ratio derived in 1981 to check their analysis and found</p><p>that only 5% of argon produced the best fit. Samuelson and his co-workers reanal-</p><p>ysed the Voyager infrared and radio-occultation data and found an upper limit of</p><p>7% for argon. A search for the spectrum of H2–Ar Van der Waals (loosely bound)</p><p>molecules at 2.1 µm using ground-based observations was unsuccessful, possi-</p><p>bly due to inadequate spectral resolution and atmospheric interference. Another</p><p>approach tried unsuccessfully was to look for the argon fluorescent line at about</p><p>3 keV with an X-ray spectrometer on an Earth orbiting observatory. The Cassini–</p><p>Huygens GCMS detection of Argon (see Chapter 6) was a happy ending to this long</p><p>search.</p><p>2.7 Photochemistry</p><p>The discovery of a molecular nitrogen atmosphere on Titan by the combined infrared,</p><p>ultraviolet and radio occultation experiments on Voyager brought new light to our</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>32 Titan: Exploring an Earthlike World</p><p>understanding of photochemistry in Titan’s atmosphere. The formation of ethane and</p><p>acetylene through methane photolysis, as well as the further catalytic dissociation</p><p>of methane by acetylene producing polyacetylenes, had been advocated even before</p><p>the encounter, but the involvement of molecular nitrogen in hydrocarbon chemistry</p><p>had not been widely considered. The first model of Titan’s photochemistry based</p><p>on the Voyager 1 observations and on photochemical reactions in a N2-CH4 atmo-</p><p>sphere was proposed in 1984 by Y. Yung and colleagues of the California Institute</p><p>of Technology. According to this, the atmosphere in the beginning contains only the</p><p>parent molecules: molecular nitrogen, methane (probably contained in the volatiles</p><p>trapped at the time of Titan’s formation) and water (probably from a small but fairly</p><p>regular meteoritic source, raining in from space).</p><p>Photochemistry occurs on Titan because sunlight in the ultraviolet part of the</p><p>spectrum has enough energy to break up the methane molecules in the upper atmo-</p><p>sphere; this is called UV photolysis. Energetic particles, mainly electrons, also rain</p><p>down on Titan from Saturn’s magnetosphere. The satellite has little magnetic field</p><p>to deflect these particles, and they also dissociate molecules, including nitrogen, so</p><p>these additional products become available to join in the photochemical ‘soup’. The</p><p>interaction of these molecules with ultraviolet radiation, energetic particles and cos-</p><p>mic rays in the thermosphere and mesosphere produces species such as CO, CO2,</p><p>C2H2, C2H4, and CH3C2H, as well as hydrogen cyanide (HCN, a prebiotic molecule)</p><p>and other nitriles (HC3N, C2N2, etc.). The major source of HCN according to this</p><p>model is N2 dissociation by magnetospheric electron impacts in the thermosphere,</p><p>while the hydrocarbons mainly result from methane photolysis.</p><p>The photochemical fragments of methane nitrogen and the other simple</p><p>molecules present can recombine as larger molecules, which continue to grow until</p><p>they form quite large and complex molecules. Just how large and how complex is one</p><p>of the big questions that remains outstanding to this day. These heavier molecules</p><p>diffuse downwards to the regions where the lowest temperatures are found and</p><p>further condensation, coalescence, transport and mixing can occur. The resulting</p><p>suspended solid or liquid particles of condensed hydrocarbons and other complex</p><p>molecules are responsible for Titan’s characteristic orange colour, produced by a</p><p>cycle somewhat like that which produces the man-made “smog” which occurs over</p><p>many of the Earth’s cities.</p><p>In parallel with these model predictions, B. Khare, C. Sagan and T. Reid at Cor-</p><p>nell University performed laboratory spectroscopy to discover the optical properties</p><p>(refractive index and absorption coefficient) of the organic compounds that are</p><p>produced in the laboratory when mixtures of methane and other gases are subjected</p><p>to electrical discharges. Of course, energetic sparking for a few days in a jar is not</p><p>the same thing as irradiation by low levels of ultraviolet photons and bombardment</p><p>by energetic particles over millennia, but all of these processes involve breaking</p><p>up</p><p>methane and nitrogen molecules and recombining their products, perhaps many</p><p>times over.</p><p>The result is a mixture of materials of an oily or tarry nature, including liquids</p><p>as well as solids, which the experimenters lumped together for convenience under</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>The Voyager Missions to Titan 33</p><p>the name “tholin”, which, according to C. Sagan, comes from the Greek “θωλóς”,</p><p>meaning “muddy”. The exact composition of the solids made in the laboratory exper-</p><p>iments is difficult to determine, and anyway varies in experiments that use different</p><p>starting mixtures of gases under different conditions of pressure and discharge, as</p><p>discussed further in Chapter 7. Nevertheless, the tholins manufactured in the lab-</p><p>oratory were shown to provide a close match to the colours and light-scattering</p><p>properties of Titan’s orange upper-atmospheric haze. This haze, rather like Earth’s</p><p>ozone layer, absorbs sunlight at the blue end of the spectrum and results in heating</p><p>of the middle regions of the atmosphere.</p><p>As a result of these parallel lines of enquiry, backed by the Voyager results,</p><p>the idea of photochemical production of Titan’s orange haze became well estab-</p><p>lished. It has an interesting and puzzling corollary, however. The production of the</p><p>haze, and the movement downwards of the particles as they become too large to</p><p>stay suspended, must have been going on for all of Titan’s history. Calculations</p><p>suggest that the products that have condensed out and precipitated to the surface</p><p>should have accumulated to a depth of about 1 kilometre over the whole globe in</p><p>the age of the Solar System. At the same time, this process should have reduced</p><p>the amount of methane in Titan’s atmosphere, indeed calculations show that the</p><p>amount of methane present now would be completely used up in about 10 million</p><p>years. In that relatively short time, it would all be converted to heavier hydrocar-</p><p>bons, nitriles and other organics and to spare hydrogen. The last of these diffuses</p><p>upward and escapes to space, the others end up on the surface as frozen or liquid</p><p>deposits. A replenishing source for methane is therefore required in order to account</p><p>for what we see today. The possibilities include the direct delivery of methane</p><p>into the atmosphere from external sources such as comets. More likely, there could</p><p>be a supply of methane inside Titan, which escapes regularly into the atmosphere</p><p>by a kind of volcanic activity or outgassing from Titan’s surface (see Chapter 9).</p><p>Most intriguingly, since the amount of methane in the lowermost part of the atmo-</p><p>sphere is close to saturation, the need for a reservoir supports the idea of liquid</p><p>methane forming part of lakes or seas on the surface, in direct contact with the</p><p>atmosphere.</p><p>According to this scenario, much favoured in the 1980s when Titan’s surface still</p><p>remained virtually unseen, deep seas of liquid ethane, methane, propane and possibly</p><p>other liquid materials probably existed there. The vapour pressures of ethane and</p><p>propane are too low to permit recycling back into the atmosphere, and the larger</p><p>‘tholin’ molecules are probably solid, or at least very thick and viscous, at Titan’s</p><p>surface temperature. However, the most volatile part of the ocean, methane, could</p><p>be released into the atmosphere. As it rises into the cooler upper troposphere, some</p><p>may condense and be cycled back to the surface as liquid methane rain, while some</p><p>passes through the cold trap at the tropopause and diffuse upward to replenish the</p><p>upper atmosphere. There it would be decomposed and eventually precipitated back</p><p>to the surface as higher hydrocarbons and nitriles, falling as a mixture of outlandish</p><p>tar, oil and snow.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>34 Titan: Exploring an Earthlike World</p><p>2.8 Cloud and Haze Properties</p><p>The images taken by the cameras aboard Voyager 1 showed Titan covered by deep</p><p>haze layers that enshroud the entire globe. The droplets appeared to be small and</p><p>well-spaced, a haze rather than a cloud, so it was deduced that visibility inside the</p><p>atmosphere would be quite good over small and medium distances. It is because the</p><p>main layer is so deep — extending from close to the surface to an altitude of about</p><p>200 km — that it obscures our view of the surface from outside. Voyager scientists</p><p>saw the subtle difference in the brightness of the two hemispheres, and hypothesised</p><p>that it was probably caused by seasonal effects in the production of the haze or in</p><p>the dynamical processes which keep the droplets aloft. The local peak in the density</p><p>of aerosol particles about 100 km above the main layer, at an altitude of around</p><p>340–360 km, shows clearly as a separate layer in the Voyager pictures.</p><p>Figure 2.10 A sketch from 1988 of the possible scenario on Titan, with haze, clouds, rain, and a</p><p>variegated surface with an intrepid explorer! This imaginative view is by David Morrison and Toby</p><p>Owen.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>The Voyager Missions to Titan 35</p><p>In 1983 K. Rages and J. Pollack at NASA AMES Research Center investigated</p><p>the properties of the aerosols from high-phase-angle Voyager images and found the</p><p>particle radii to be between 0.2 and 0.5 µm. However, the degree of polarisation</p><p>of the light scattered from the haze was found to be incompatible with spherical,</p><p>liquid drops; they suggested they might be irregular conglomerates of smaller solid</p><p>particles stuck together. The existence of solid material would imply that the pho-</p><p>tochemical processing of methane and its products goes on to the point where quite</p><p>complex organic materials are produced, since these are most likely to be solid rather</p><p>than liquid or gaseous. The first Titan haze models to include fractal aggregate par-</p><p>ticles composed of several tens of small (0.06 µm in radius) monomers were shown</p><p>to produce strong linear polarization. 45 monomers would compose aggregates with</p><p>an effective radius of about 0.35 µm, matching the Voyager observations.</p><p>2.9 Speculations on the Surface and Landscape of Titan</p><p>fromVoyager</p><p>Voyager’s cameras could see no signs of the surface through the continuous blan-</p><p>ket of aerosol in the upper atmosphere. In fact, the early evidence suggested that</p><p>Titan’s atmosphere was too opaque to permit a view of the surface until very</p><p>long (radio or radar) wavelengths were reached. This misapprehension came about</p><p>because Voyager had no near-infrared capability, and was understandable in terms</p><p>of the knowledge available at the time the mission was planned, but turned out</p><p>to be crucial, as Titan’s haze is in fact optically thin at wavelengths just slightly</p><p>longer than visible light. Earth-bound telescopes and the Hubble Space Telescope</p><p>used this post-Voyager discovery to provide spectacular near-infrared views of the</p><p>surface.</p><p>Based on the pre-Cassini knowledge, a number of features were hypothesized</p><p>for Titan’s landscape. However, it was really very difficult to speculate on what</p><p>surface features on Titan might be like before the arrival of Huygens. The usual</p><p>approach was to work on the basis of landforms found throughout the Solar System,</p><p>but because Titan is such a unique object it would be easy to be misled by such</p><p>thinking. Some parallels could be drawn, primarily with other cold, icy satellites,</p><p>but also perhaps with Venus, when we consider the effect of the dense, optically</p><p>thick atmosphere.</p><p>Following Voyager, there was quite a strong case for liquids in some locations</p><p>on the surface of Titan. It seemed equally probable that there would be some craters.</p><p>Other bodies in the Solar System, especially in its outer reaches, have been heavily</p><p>bombarded and Titan should be no exception. However, the craters could have been</p><p>modified, and in some cases virtually obliterated, by a combination of processes</p><p>(see Section 9.5.2).</p><p>Voyager also had little to say with certainty about Titan’s interior, although it</p><p>did provide an improved density estimate, and stimulated the production of more</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>36 Titan: Exploring an</p><p>Earthlike World</p><p>Figure 2.11 Crescent Titan viewed by Voyager 2 in August 1981 from a distance of around 200,000</p><p>kilometres, from a position almost directly looking back toward the Sun. The effect of the thick</p><p>atmosphere can be seen in the refraction of sunlight around the edge of the disk to produce a continuous</p><p>ring of light (NASA/JPL).</p><p>elaborate theoretical models, the most recent of which are described in Chapter 9.</p><p>Titan’s density falls midway between the two largest satellites of Jupiter, Ganymede</p><p>and Callisto. From the abundances of the elements in the solar neighbourhood and the</p><p>composition of primordial material in the present-day Solar System, as represented</p><p>by comets, all three are believed to consist principally of iron, silicates and water ice,</p><p>with smaller amounts of other elements and compounds. The simplest model which</p><p>could be imagined for the solid body of Titan, which explains its low density and</p><p>recognises cosmogonic considerations — that is, what we know about the universe</p><p>and the formation of the Solar System — is one which has a rocky core, covered by</p><p>a thick mantle of water ice. To give the observed mean density of 1.88 gm cm−3, the</p><p>depth of the ice would have be about half the radius of Titan.</p><p>2.10 The Aftermath of Voyager</p><p>One consequence of the success of theVoyager encounters was a sharp increase in the</p><p>level of interest in Titan. Enough was revealed, especially about the atmosphere, to</p><p>stimulate a considerable number of theoretical and modelling studies, most of which</p><p>raised further questions which went on to define the goals for Cassini–Huygens.</p><p>Some of the most intriguing ones were summarised in the first edition of this book,</p><p>after Voyager but before Cassini, as follows:</p><p>• Where did the atmosphere come from, and why is it unique in the outer Solar</p><p>System?</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>The Voyager Missions to Titan 37</p><p>Figure 2.12 A model of the interior of Titan, showing the layering which may be present, and the</p><p>escape of argon from the deep interior. The detection of 40Ar by the Huygens probe is evidence for</p><p>this kind of outgassing, and by implication methane must be escaping also (Atreya et al., 2006).</p><p>• Does the composition vary from place to place? Are there condensable species</p><p>which give rise to clouds and rain?</p><p>• What is the degree of complexity achieved by the chemistry on Titan? Are species</p><p>like amino acids forming?</p><p>• What is the nature of the surface, its composition and topography? Are there seas,</p><p>lakes or underground reservoirs?</p><p>• How much does the temperature vary on the surface of Titan, and in the atmo-</p><p>sphere, across the globe?</p><p>• What is the circulation of the atmosphere? How strong are the winds and storms,</p><p>do thunder and lightning occur? What is the effect of the seasons on weather?</p><p>We will be dealing with these questions in detail in the chapters that follow, and</p><p>the progress made with the Cassini–Huygens mission, a massive investment by the</p><p>US and European space agencies and the world scientific community. Since Voy-</p><p>ager, there has also been an invigorated and successful ground-based observing</p><p>programme, and new observations by Earth-orbiting telescopes, in particular the</p><p>Hubble Space Telescope and the Infrared Space Observatory. Among other achieve-</p><p>ments, these produced the first images of Titan’s surface, showing intriguing details</p><p>on the surface, and identified a number of additional atmospheric species, providing</p><p>more data to stimulate the production of improved atmospheric models.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>CHAPTER 3</p><p>Observations of Titan from the Earth</p><p>“. . .Galileo’s intellectual heir, Christiaan Huygens, the landed son of a Dutch</p><p>diplomat who made science his life. . . , had divined that the “moons” Galileo</p><p>observed at Saturn were really a ring, impossible as that seemed at the time.</p><p>Huygens also discovered Saturn’s largest moon, which he named Titan. . . . But</p><p>Huygens couldn’t be tied to the telescope all the time. He had too many other things</p><p>on his mind. It is even said that he chided Cassini, his boss at the Paris Observatory</p><p>for the director’s slavish devotion to the daily observing.”</p><p>Dava Sobel, Longitude</p><p>3.1 Introduction</p><p>In the two decades that followed the Voyager discoveries on Titan, up to and during</p><p>the development, launch and flight of the Cassini–Huygens mission, astronomers</p><p>turned to ground-based observatories and to Earth–orbiting artificial satellites in</p><p>order to continue to probe what was now seen as a fascinating world. Breakthroughs</p><p>were still to come despite the large distances involved, and these would help to opti-</p><p>mize the return from the new space mission. In particular, new telescopes and pio-</p><p>neering observing techniques allowed a better understanding of the nature of Titan’s</p><p>surface well before the powerful Huygens instruments landed there. We review here</p><p>the observational tools used to acquire data between Voyager and Cassini, and dis-</p><p>cuss how all of this was applied to our enhanced understanding of Titan’s atmosphere</p><p>and surface at the end of the 20th and in the first years of the 21st centuries.</p><p>3.2 Space Observatories</p><p>In the years since Voyager, technological progress has meant that observations from</p><p>space through telescopes orbiting the Earth are now an affordable means of study-</p><p>ing astronomical objects, including the planets and satellites of the Solar System.</p><p>The Hubble Space Telescope is only the first of a long series of such observato-</p><p>ries in space, which allow astronomers to avoid the atmospheric perturbations that</p><p>affect even the highest terrestrial observatory, and to achieve higher quality data.</p><p>From November 1995 until early 1998, another space telescope, the Infrared Space</p><p>38</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 39</p><p>Observatory (ISO), operated in geocentric orbit and yielded information on a great</p><p>variety of astronomical objects. Both telescopes have observed Titan and the data</p><p>have provided a source of valuable information on the satellite in the near-infrared</p><p>and visible range.</p><p>3.2.1 Hubble SpaceTelescope</p><p>The Hubble Space Telescope is a 2.4 m reflecting telescope complemented by sci-</p><p>ence instruments including three cameras and two spectrographs and fine guidance</p><p>sensors primarily used for astrometric observations. These achieve a spatial resolu-</p><p>tion of about 0.1 arcsec, rarely attainable from the Earth.</p><p>The Space Telescope Imaging Spectrograph (STIS) operates across the spec-</p><p>tral range from the ultraviolet (0.115 µm) through the visible red and the near-IR</p><p>(1 µm). As a spectrograph, it spreads out the light recovered by the telescope so</p><p>that it can be analysed to study the chemical composition, the temperature struc-</p><p>ture, radial and rotational velocity and magnetic fields of astronomical objects. STIS</p><p>uses three detector arrays, all with a 1024 × 1024 pixel format: a caesium iodide</p><p>photocathode for wavelengths from 115 to 170 nm, caesium telluride for 165 to</p><p>310 nm, and a Charge Coupled Device (CCD) for 305 to 1000 nm. The field of view</p><p>is 25 × 25 arcsec for the shorter wavelengths and 50 × 50 arcsec for the CCD.</p><p>The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) provides the</p><p>capability for infrared imaging and spectroscopic observations of various targets.</p><p>NICMOS operates in the 0.8 to 2.5 µm range with a resolving power up to 100,000.</p><p>The Faint Object Camera (FOC), built by the European Space Agency, uses two</p><p>complete detector systems and is very sensitive: it can observe objects fainter than</p><p>magnitude 21.</p><p>After the telescope was transported into space in 1990 by the crew of the space</p><p>shuttle Discovery (STS-31) and set onto a 600 km orbit around the Earth, it was</p><p>found to suffer from spherical aberration problems. Its primary mirror is 2 µm —</p><p>only 0.0002 cm — too flat at the edge. Rescue missions in December 1993 and</p><p>February 1997 successfully fitted parts which restored the expected performance of</p><p>the telescope. The original Wide Field/ Planetary Camera was</p><p>replaced by WF/PC2</p><p>(pronounced Wifpic 2), which is actually 4 cameras in one. The kernel consists</p><p>of an L-shaped trio of wide-field sensors and a smaller, high resolution planetary</p><p>camera tucked in the square’s remaining corner. The Corrective Optics Package</p><p>was installed, replacing the High Speed Photometer, its purpose being to correct</p><p>the effects of the primary mirror aberration on the Faint Object Camera. Later</p><p>instruments were designed with their own corrective optics.</p><p>Each HST orbit lasts about 95 minutes, not all of which is spent observing,</p><p>since some is used for housekeeping functions. The observing plans are designed</p><p>by the Space Telescope Science Institute at Johns Hopkins University in Baltimore,</p><p>Maryland. Then the Goddard Space Telescope Operations Control Center takes</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>40 Titan: Exploring an Earthlike World</p><p>Figure 3.1 The Hubble Space Telescope during deployment (NASA).</p><p>over and produces the operations schedule to be sent to the onboard computers</p><p>and executed. Data are broadcast from HST to the ground stations immediately</p><p>or stored on tape and downlinked later. If an observer wishes, they can examine</p><p>images and raw data within minutes for a quick-look analysis. The Science Institute</p><p>is responsible for the data reduction and first processing (calibration, editing, etc.)</p><p>and distribution to the scientific community. Competition is very active to obtain</p><p>time on HST, with an acceptance score of about one proposal out of ten.</p><p>J. Caldwell and colleagues first used the Hubble Space Telescope to observe</p><p>Titan in 1990. Subsequently, in 1994 and 1995 HST was used with the Wide</p><p>Field/Planetary Camera and broad filters in the visible and near-IR range. The</p><p>Titan images were part of an effort to probe Titan’s lower atmosphere and sur-</p><p>face using ‘windows’ in the spectrum where methane is weakly absorbing. The</p><p>1994 HST data in the window at 0.94 µm were the first to show surface features.</p><p>They included a bright equatorial ‘continent’ on Titan’s leading hemisphere, which</p><p>explained the brightness fluctuations previously observed by spectroscopists as the</p><p>satellite rotated. The HST results on Titan are more extensively discussed in the</p><p>following sections.</p><p>3.2.2 The James Webb SpaceTelescope</p><p>NASA’s planned follow-on to the successful Hubble Space Telescope was known</p><p>as The Next Generation Space Telescope before it was named after a former admin-</p><p>istrator of the agency. The new telescope has a stated goal of “finding clues that</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 41</p><p>Figure 3.2 One of the conceptual designs for the James Webb Space Telescope, successor to the</p><p>Hubble. It is likely to be placed in orbit either at the Lagrangian point L2, or in a 1 × 3 astronomical</p><p>unit ellipse (NASA).</p><p>remain hidden by time and discovering how our universe evolved”, to be addressed</p><p>by observing the universe at much earlier times than currently achieved, to find the</p><p>chemical makeup of early galaxies and the shape of the very early universe. The</p><p>search for extra-solar planets will form a further step toward answering the question</p><p>as to whether we are alone in the universe.</p><p>The JWST is scheduled for launch in 2013. The instruments it will carry are</p><p>designed to work primarily in the infrared spectrum, using a primary mirror 6.5 m</p><p>in diameter. Maintained at a temperature of 40 K or lower, NASA says it will be</p><p>up to 1,000 times more sensitive than any existed or planned facility in the near-</p><p>infrared (0.8–5 µm) region. To reduce stray light, it has a large sunshield and orbits</p><p>a million miles from Earth, probably in the second Lagrangian point L2 where the</p><p>gravitational attractions of Earth and Moon are equal, or in an elliptical orbit which</p><p>passes close to the Earth.</p><p>3.2.3 Infrared Space Observatory</p><p>ISO was launched on November 17, 1995, from Kourou in French Guyana. It oper-</p><p>ated for more than two years in Earth orbit, with a 24 hr period, a perigee of 1000 km,</p><p>and an apogee of 70,000 km. It consists of a helium-cooled 60 cm telescope and</p><p>four scientific instruments: a camera, an imaging photopolarimeter and two spec-</p><p>trometers, SWS and LWS, dedicated respectively to short- and long-wavelength</p><p>observations that cover the entire thermal spectrum between them from 2 to 200</p><p>microns, with resolving power ranging from 1,000 to 20,000. Two different observ-</p><p>ing modes are available on each spectrometer: Grating mode (mean resolution of</p><p>about 0.5 cm−1) and Fabry-Pérot mode (resolution between 0.01 and 0.05 cm−1).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>42 Titan: Exploring an Earthlike World</p><p>Figure 3.3 The Infrared Space Observatory (ESA).</p><p>The grating operated in the 2 to 45 µm wavelength range with a resolving power</p><p>λ/�λ of 2000 and in the 45 to 200 µm range (50–220 cm−1) with λ/�λ = 200. The</p><p>Fabry-Pérot acquired data from 15 to 35 µm (285–665 cm−1) with a resolving power</p><p>of 20,000 and observes the 45 to 200 µm range with λ/�λ = 10,000. The noise</p><p>equivalent power of the detectors was very low, so ISO benefited from extremely</p><p>high sensitivity in a part of the spectrum that had not been explored before. However,</p><p>due to the faint signal recovered and to the interference of stray light from Saturn,</p><p>it was essentially the spectral region from 200 to 1500 cm−1 (7 to 50 µm), as with</p><p>Voyager, that proved exploitable in the SWS, and, in addition, the 2.5–4.5 micron</p><p>region.</p><p>The observing programme for ISO included about 20 hours devoted to obser-</p><p>vations of Titan. The first SWS observations were obtained on January 10, 1997 in</p><p>the 220–340 and 600–1500 cm−1 ranges. Titan does not fill the ISO field-of-view</p><p>(which is 100 arcsec for LWS and 30 arcsec for SWS, while Titan is only 0.8 arcsec</p><p>in diameter); therefore the data are all averages over the whole disk. However, ISO</p><p>spectroscopy — thanks to the higher resolution — made it possible to obtain more</p><p>precise abundance and temperature profiles, to detect new molecules and to probe</p><p>the atmosphere at lower levels than Voyager. The Titan SWS and PHT-S ISO data</p><p>were processed and rendered exploitable by an expert team at the ISO Data Center</p><p>of the ESA Satellite Tracking Station at Villafranca del Castillo, led by A. Salama</p><p>and B. Schultz.</p><p>It is not simple to compare ISO and Voyager or Cassini results, since the flyby</p><p>missions made measurements at close approach, therefore acquiring information on</p><p>local parts of the satellite. A space observatory in Earth orbit, such as ISO, sees Titan</p><p>as a small disk in a wider field-of-view, recovering information only as a disk average.</p><p>In spite of that, ISO provided important insights into the satellite’s atmosphere,</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 43</p><p>Figure 3.4 Pre-Cassini space infrared observations of Titan: ISO data from 1997 compared with</p><p>Voyager, from 1980. Where the Voyager /IRIS spectrometer brought local information on Titan’s disk,</p><p>the ISO/SWS spectrometer provided much higher spectral resolution to compensate for the lack of</p><p>spatial resolution.</p><p>mainly because it possesses a much higher spectral resolution than IRIS. At the</p><p>spectral resolution of the ISO SWS/Grating mode, most of the molecular bands are</p><p>resolved, allowing information on the mixing ratio of the constituent as a function</p><p>of altitude to be retrieved. Also, the contributions of different species with bands in</p><p>close locations can be separated because of the high resolving power. This gives more</p><p>precise information on the mean molecular abundances of the atmospheric gases</p><p>and also on the disk-average temperature profile. The ISO measurements mostly</p><p>pertain to low latitudes because the limb contribution is not very important in the</p><p>ISO field-of-view.</p><p>Despite the improved spectral resolution, ISO detected essentially the same</p><p>hydrocarbon and nitrile molecules as seen by Voyager. However, it also detected</p><p>for the first time the presence of water vapour in the higher parts of the atmosphere</p><p>on Titan. This was a very exciting</p><p>discovery because it addresses the mystery of</p><p>the origin of the oxygen seen as CO and CO2 in Titan’s atmosphere and also, at the</p><p>same time, gives an estimate of the water influx value and so some insight into</p><p>the complicated issues involved with the transport of icy grains in the Saturn system</p><p>(see Chapter 6). The observations of H2O concentrated on narrow spectral regions</p><p>at around 40 µm, in the rotational band of water vapour where the strongest lines</p><p>were expected to occur.Another first tentative detection by SWS was that of benzene</p><p>(C6H6) at 674 cm−1. Thus, ISO observations nicely complemented the Voyager data</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>44 Titan: Exploring an Earthlike World</p><p>and offer important additional information to be combined with the recent Cassini</p><p>results, as discussed extensively in the following chapters.</p><p>3.3 Ground-Based Observatories</p><p>Titan has been observed regularly from the Earth since Kuiper made his famous</p><p>detection of methane in 1944, with a surge of new findings in the mid-seventies</p><p>by Trafton, Gillett and others. In more recent years, since 1990 or so, long-term</p><p>observing campaigns in the United States and Europe have concentrated on those</p><p>regions of the spectrum where methane absorption is weak and observations probe</p><p>down to the lower troposphere and surface. Increasingly sophisticated instruments</p><p>operating mainly in the near-infrared, but also in the thermal and millimetre spectral</p><p>ranges, have allowed more precise determinations of temperature and the detection</p><p>of new molecules in the atmosphere. The technique of adaptive optics (AO), which</p><p>corrects for the distorting effect of the atmosphere, allows ground-based near-IR</p><p>observations to resolve Titan’s disc and obtain some spatial resolution on the surface</p><p>by using spectroscopy and imaging. Attempts have been made to follow variations</p><p>in the atmosphere and on the surface as a function of Titan’s position in its orbit, as</p><p>we discuss below.</p><p>The astronomical observatories that have been used to study Titan are located</p><p>all over the world. In the Northern Hemisphere, the most important (and most</p><p>impressive) site for ground-based astronomical observations is located on top of</p><p>the mountain called Mauna Kea on the big island of Hawaii. In the southern hemi-</p><p>sphere, the ESO (European Space Observatory) facilities at La Silla, Chile, offer a</p><p>serious counterpart for Hawaii in the north. Besides its observatories in the Canary</p><p>Islands, Europe maintains a large radio-antenna in the Sierra Nevada, and Russia</p><p>and other Eastern European countries have major sites devoted to astronomy. Much</p><p>of our current knowledge of the Solar System has been provided by these facilities,</p><p>a testimony to the dedication and determination of the teams of engineers and tele-</p><p>scope operators who maintain and improve the instrument capabilities and ensure</p><p>the correct performance of the telescopes.</p><p>3.3.1 Mauna Kea Observatories</p><p>Mauna Kea (“White Mountain”) is a dormant volcano on the Big Island of Hawaii,</p><p>located about 300 km from the Hawaiian capital city, Honolulu. The highest point</p><p>in the Pacific Basin and the highest island-mountain in the world, Mauna Kea rises</p><p>9750 m (32,000 ft) from the ocean floor to an altitude of 4205 m (13,796 ft) above</p><p>sea level, which places its summit above 40 percent of the Earth’s atmosphere. The</p><p>broad volcanic landscape of the summit area is made up of cinder cones on a lava</p><p>plateau.</p><p>The atmosphere above Mauna Kea is extremely dry — an important asset when</p><p>detecting infrared and sub-millimetre radiation through the atmosphere — and</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 45</p><p>Figure 3.5 The Mauna Kea Observatories in Hawaii.</p><p>cloud-free, so that the proportion of clear nights is among the highest in the world.</p><p>Since it is surrounded by thousands of miles of flat and relatively thermally-stable</p><p>ocean, there is less turbulence in the upper atmosphere and less light-reflecting dust</p><p>in the air than at most locations on Earth. Its distance from city lights ensures an</p><p>extremely dark sky, which is also dry and free from atmospheric pollutants.</p><p>Many international telescopes have been built at the highest point of this</p><p>(hopefully) extinct volcano, resulting from world-spanning collaborations between</p><p>nations. There are currently twelve telescopes — more than on any other single</p><p>mountain peak — plus the Hawaii Antenna of the Very Long Baseline Array. For the</p><p>purposes of infrared astronomy, the Canadian French Hawaiian Telescope (CFHT),</p><p>the United Kingdom Infrared Telescope (UKIRT), the Subaru (Japanese National</p><p>Large Telescope), the University of Hawaii (UH), the Infrared Telescope Facility</p><p>(IRTF) and the Keck Telescopes have the instruments best adapted for spectroscopy</p><p>and/or imaging with adaptive optics. Other telescopes on Mauna Kea include the</p><p>Submillimetre Array (SMA), The James Clerk Maxwell Telescope (JCMT), the</p><p>California Institute of Technology 10.4 m Submillimetre Observatory (CSO), and</p><p>the Gemini Northern 8 m Telescope.</p><p>Several of the instruments on Mauna Kea have been used to study Titan, by means</p><p>of spectroscopy and imaging in the search for information on the atmosphere and</p><p>surface of the satellite. They include the Hokupa’a AO system, which consists of a</p><p>36-element curvature wave front sensor and bimorph mirror, TEXES, the 8–25 µm</p><p>high-resolution grating spectrograph at the IRTF, and the 3.6-metre optical/infrared</p><p>CFHT. The CFHT has been used since 1990, first for spectroscopic measurements,</p><p>and then for adaptive optics imaging of Titan. The twin telescopes at the W. M.</p><p>Keck Observatory are among the world’s largest, at eight stories tall and 300 tons</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>46 Titan: Exploring an Earthlike World</p><p>in weight, with a primary mirror ten metres in diameter composed of 36 hexagonal</p><p>segments that work in concert as a single reflector. The Keck I telescope began</p><p>science observations in May 1993; Keck II began in October 1996. The adaptive</p><p>optics system has a wave front sensor with 289 actively controlled subapertures,</p><p>requiring a brighter star for operation than Hokupa’a but providing better image</p><p>quality in poor seeing, ideal for Titan observations.</p><p>3.3.2 The European Southern Observatories</p><p>The telescopes based at La Silla and Paranal in Chile compete with Hawaii in</p><p>offering a large number of clear nights per year and a location away from artificial</p><p>light and dust sources. La Silla is a 2400 m mountain on the southern edge of</p><p>the Atacama desert, about 600 km north of Santiago, in the province of Elqui. Its</p><p>summit houses one part of the European Southern Observatory (ESO), a set of more</p><p>than 15 astronomical instruments devoted mainly to exploring the southern celestial</p><p>hemisphere. The mountain’s name (La Silla) means “The Saddle”, after its shape,</p><p>and by coincidence the sinuous road leading up to the Mauna Kea Observatories</p><p>is also called “The Saddle Road”. Apart from a 15 m diameter parabolic antenna</p><p>devoted to radio observations, the largest telescopes on La Silla include a couple</p><p>of 3.6 m optical reflectors, the rest ranging in aperture from 2.2 m to half a metre.</p><p>Among others, the NTT 3.6 m telescope and the adaptive optics system (ADONIS)</p><p>it houses were used for observing Titan in the imaging mode in the first studies</p><p>related to its surface.</p><p>Figure 3.6 The European Southern Observatory telescopes in the Chilean desert.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 47</p><p>Figure 3.7 Close-up view of the ESO Paranal Observatory, with the control building in the fore-</p><p>ground. Also visible are the railway tracks on which the auxilliary telescopes move, and the individual</p><p>observing stations.</p><p>Several years ago, the ESO Council decided to build the Very Large Telescope at</p><p>Cerro Paranal, in the Atacama desert. The VLT consists of four 8.2 m telescopes and</p><p>several moving 1.8 m Auxiliary Telescopes, working independently or in combined</p><p>mode as an interferometer. In this latter</p><p>case, the total light collecting power of</p><p>the telescope equals that of a 16 m diameter single telescope, making the VLT the</p><p>world’s largest and most advanced optical telescope, with unprecedented optical</p><p>resolution and collecting surface area. Currently, eleven instruments including two</p><p>interferometric instruments are in operation at the VLT and scientific observations</p><p>are being carried out including observations of binary stars, circumstellar disks,</p><p>protostars, brown dwarfs and extrasolar planets.Among theVLT instruments used to</p><p>study Titan is ISAAC, an IR (1–5 µm) imager and spectrograph with two arms, one</p><p>equipped with a 1024 × 1024 Hawaii Rockwell array, and the other with a 1024 ×</p><p>1024 InSb Aladdin array from Santa Barbara Research Center. Another device,</p><p>NAOS-CONICA, provides adaptive optics assisted imaging, imaging polarimetry</p><p>and spectroscopy in the 1–5 µm range. It also includes a Fabry-Perot unit in the</p><p>2–2.5 µm range.</p><p>The adaptive optics system, NAOS, is equipped with both visible and infrared</p><p>wavefront sensors. It contains five dichroics, which split the light from the telescope</p><p>between CONICA and one of the wavefront sensors. CONICA is the infrared camera</p><p>and spectrometer attached to NAOS and is equipped with an Aladdin 1024 × 1024</p><p>pixel InSb array detector. It contains several wheels carrying masks, slits, filters,</p><p>polarizing elements, grisms (devices that combine a diffraction grating and a wedge</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>48 Titan: Exploring an Earthlike World</p><p>prism to disperse the light without deviating the central wavelength), and several</p><p>cameras, allowing diffraction limited operation across the full wavelength range.</p><p>The UVES spectrometer was also used to characterise the zonal winds on Titan.</p><p>3.3.3 The University of Arizona and Steward ObservatoryTelescopes</p><p>The Department of Astronomy at the University of Arizona and its associated</p><p>research division, Steward Observatory, form one of the finest centres for astro-</p><p>nomical studies in the world with some world-class telescopes. New light detectors</p><p>and giant telescope mirrors are a catalyst for breakthroughs in optical and infrared</p><p>astronomy.</p><p>The current telescope list includes the 10 m Sub-Millimetre Telescope and the</p><p>1.8 m Lennon Reflector on Mt. Graham; a 4.5 m equivalent aperture multi-mirror</p><p>telescope (MMT) Reflector on Mt. Hopkins; the 2.3 m Bok Reflector and a 0.9 m</p><p>Reflector on Kitt Peak; the 1.6 Bigelow Reflector and a 0.4 m Schmidt on Catalina;</p><p>the 1.5 m NASA reflector and a 1 m Reflector on Mt. Lemmon; and a 0.5 m reflector</p><p>on the university campus, collectively equipped with a full range of instrumentation</p><p>for photometry and spectroscopy in the visible and the infrared. The Bok and MMT</p><p>reflectors have been used to study Titan’s atmospheric transmittance in the near-</p><p>infrared range.</p><p>Figure 3.8 The multi-mirror telescope MMT on the summit of Mount Hopkins, as photographed by</p><p>H. Lester.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 49</p><p>3.3.4 Radio Astronomy</p><p>Radio astronomy covers the electromagnetic spectrum from the far infrared</p><p>(500 µm) to kilometre wavelengths, a range that includes several terrestrial atmo-</p><p>spheric windows which are relatively free of atmospheric absorption so that radio</p><p>emission from Titan can reach ground-based instruments. In the millimetre and sub-</p><p>millimetre windows, the emission from cool objects such as planets, and molecules</p><p>and dust in space, can be detected. The absorption of radio waves by the ionosphere</p><p>becomes more important as wavelength increases until, at wavelengths longer than</p><p>about 10 m, the ionosphere becomes opaque to incoming signals.</p><p>Before that limit is reached, at wavelengths longer than about 20 cm (1.5 GHz),</p><p>irregularities in the ionosphere distort the incoming signals, causing scintillation. As</p><p>at optical and infrared wavelengths, sophisticated signal processing can be used to</p><p>correct for these effects, so that the effective angular resolution and image quality is</p><p>limited only by the size of the instrument. To derive the physical properties of cold</p><p>objects such as Titan, or to identify new molecules, the instruments at the various</p><p>telescopes described below measure the spectral energy distribution of molecular</p><p>emission using heterodyne receivers that amplify signals in such a way that high</p><p>resolution spectroscopy becomes feasible.</p><p>3.3.4.1 IRAM</p><p>Institut de RadioAstronomie Millimétrique has the world’s largest telescope operat-</p><p>ing at wavelengths between 0.8 and 3.5 mm (frequencies between 350 and 80 GHz).</p><p>Figure 3.9 The IRAM Radio Telescopes with a view of the 15 m antennas located at the Plateau de</p><p>Bures in the French Alps.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>50 Titan: Exploring an Earthlike World</p><p>It received its first millimetric light in May 1984, and was opened to the astronomical</p><p>community the following year. Mainly the responsibility of France, Germany and</p><p>Spain, it operates two major facilities: a 30 m diameter telescope on Pico Veleta in</p><p>the Sierra Nevada of southern Spain, and an array of five 15 m diameter telescopes</p><p>on the Plateau de Bure in the French Alps. The 30 m telescope at Pico Veleta has</p><p>been used to make heterodyne observations of Titan that have allowed A. Marten</p><p>and colleagues the precise determination of the vertical profiles of CO and HCN.</p><p>In 1992 the first detection of a more complex nitrile than any of those detected by</p><p>Voyager, CH3CN (acetonitrile) was reported in the millimetre range around 220.7</p><p>GHz at 0.1–MHz resolution. More recently, several observational campaigns pro-</p><p>duced maps of Titan in the lines of CO, HCN, HC3N and CH3CN with a spatial</p><p>resolution of 0.6", about half the apparent size of Titan’s disk including the extended</p><p>atmosphere. Vertical profiles for these constituents were derived at altitudes above</p><p>400 km. The same data contains information about winds in Titan’s upper atmo-</p><p>sphere, from the Doppler shifting of the spectral lines, thus complementing the wind</p><p>field observations made by Cassini–Huygens.</p><p>3.3.4.2 TheVery Large Array (VLA)</p><p>The National Radio Astronomy Observatory’s (NRAO) Very Large Array is located</p><p>in the plains of SanAgustin, west of Socorro, New Mexico, at an elevation of 2,124 m.</p><p>It has operated since January 1981 at various bands between 300 and 50,000 MHz,</p><p>corresponding to wavelengths of 90 to 0.7 cm. The VLA consists of 27 antennas</p><p>each 25 metres in diameter, arranged in a Y pattern to form the equivalent of a dish</p><p>Figure 3.10 The National RadioAstronomy Observatory (NRAO) Very LargeArray (VLA) antennas</p><p>in New Mexico.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 51</p><p>130 metres in diameter, steerable at a rate of 40◦ per minute in azimuth and 20 in</p><p>elevation. The maximum achievable resolution at the highest frequency of 43 GHz</p><p>is 0.04 arc seconds, the angle subtended by a golf ball 150 km away.</p><p>The VLA normally operates as an interferometer, multiplying the data from</p><p>each pair of telescopes together to form interference patterns that are subsequently</p><p>analysed by taking the Fourier transform of the signal to make maps. It was used as</p><p>the receiver for a signal transmitted to Titan by the NASA Goldstone radio telescope</p><p>in California, in order to recover radar signal returns and study the satellite’s surface</p><p>(see Chapter 9).</p><p>3.3.4.3 TheVery Long Baseline Array (VLBA)</p><p>The VLBA is similar to VLA in that it is made up of a system of radio-telescope</p><p>antennas, each with a dish 25 m in diameter and weighing 240 tons. However, its</p><p>ten dishes are deployed over a baseline of more than 5000 miles, from Mauna Kea</p><p>in Hawaii to St. Croix in the U.S. Virgin Islands. This provides astonishing spatial</p><p>resolution, the equivalent of reading a newspaper in Los Angeles from New York.</p><p>3.3.4.4 The Atacama Large Millimetre Array (ALMA)</p><p>ALMA is a major new facility under construction since 2003 for a mm and sub-</p><p>mm wavelength telescope, the largest and most sensitive in the world, with sixty-</p><p>four 12</p><p>m antennas located at an elevation of 5,000 m (16,400 feet) in Llano de</p><p>Chajnantor, in the ChileanAndes, east of theAtacama Desert. It operates in all atmo-</p><p>spheric windows between 10 mm and 350 microns and achieves a spatial resolution</p><p>of 10 milliarcseconds, 10 times better than theVLA and the Hubble Space Telescope.</p><p>The plan is for ALMA to be completed in 2010. On March 10, 2007, an official</p><p>ceremony took place at 2,900 m altitude on the site of ALMA to mark the comple-</p><p>tion of the structural works. This operations support facility site will become the</p><p>operational centre of one of the most important ground-based astronomical facilities</p><p>on Earth.</p><p>3.3.4.5 The Square Kilometre Array (SKA)</p><p>SKA is an international project to develop a telescope to provide two orders of</p><p>magnitude increase in sensitivity over existing facilities at metre to centimetre wave-</p><p>lengths, a goal that requires a telescope with 1 million square metres of collecting</p><p>area — one hundred times more than the VLA. The SKA will be an interferometric</p><p>array of 30 stations each with the collecting area equivalent to a 200 m diameter tele-</p><p>scope, and 150 stations each with the collecting area of a 90 m telescope, distributed</p><p>over distances out to 3000 km or more. This high angular resolution capability will</p><p>allow imaging of faint emission from the interstellar medium of distant galaxies, as</p><p>well as the surface of stars, planets, and the active nuclei of galaxies.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>52 Titan: Exploring an Earthlike World</p><p>3.4 Earth-Based Studies of Titan</p><p>Over the last 50 years a wide range of ground-based observatories, techniques, and</p><p>powerful tools have been applied to the study of Titan. Here we review some of</p><p>the most prominent advances in our understanding of the satellite that have resulted</p><p>from these programmes.</p><p>3.4.1 Occultations of Titan</p><p>Stellar occultations are another indirect means to probe Titan’s atmosphere and</p><p>recover information on the thermal structure, winds and other parameters. For</p><p>instance, the atmospheric oblateness due to the zonal winds can be constrained from</p><p>the analysis of the central flash, the increase of the signal at the centre of the shadow</p><p>that forms when the star is behind Titan, due to the focusing of the atmospheric rays</p><p>at the limb.</p><p>On July 3, 1989, Saturn and Titan passed in front of the 5th magnitude star 28</p><p>Sagitarii. The occultation was observed, among other places, from Paris Observatory</p><p>at Meudon, Pic du Midi Observatory, Israel, the Vatican, and Catania Observatory.</p><p>This rare event provided information on Titan’s stratosphere in the 250–500 km alti-</p><p>tude range. A mean scale height of 48 km at 450 km altitude was inferred, allowing</p><p>the mean temperature between 149 and 178 K to be constrained at that level (about</p><p>3 µbar). Titan’s shadow centre passed within about 20 km south of Meudon. The</p><p>central flash observed there provided a unique opportunity to constrain the apparent</p><p>oblateness of Titan at the 0.25 mbar level (about 250 km of altitude), and gave a</p><p>value which may be as high as 0.014. Mean temperatures around 180 K were found,</p><p>in agreement with models of Titan’s mesosphere.</p><p>The occultation allowed an estimate of the optical depth of the haze at heights</p><p>between 300 and 450 km above the surface. Two haze layers were detected, a lower</p><p>layer, present globally, and a higher haze layer present only northward of about</p><p>20◦S. This is the opposite of the asymmetry noted from Voyager and Pioneer data,</p><p>where the high detached haze layer is present in the southern hemisphere and at</p><p>low northern latitudes. The occultation data were also used to estimate the opacity</p><p>of the haze at an altitude of 318 km above the ground: tangential optical depths of</p><p>around 0.1 for the low haze and 2 for the upper haze were derived for a reference</p><p>wavelength of 0.5 µm. The occultation measurements found the haze to be situated</p><p>one pressure scale height higher than what Voyager found, with the scale heights of</p><p>the haze layers themselves both smaller than those deduced from the spacecraft data.</p><p>No discrepancy is necessarily implied — the 28 Sgr occultation occurred almost at</p><p>northern midsummer, while the Voyager encounter occurred during Titan’s northern</p><p>spring, so seasonal effects could account for the difference.</p><p>The stellar occultation data showed that Titan’s atmosphere is not spherically</p><p>symmetric, but has an oblateness of about 0.017 at an altitude of about 250 km</p><p>(0.25 mbar), which may be due to the rapid wind speeds around the equatorial zone</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 53</p><p>Figure 3.11 Occultation of 28 Sagittarius by Titan, 3 July 1989 (Sicardy et al., 1990).</p><p>(see Chapter 5). It also suggested a slight systematic enhancement of the haze opacity</p><p>on the lit side of Titan. Regional variations in haze opacity at small scales may be</p><p>required to satisfy all the observations, which are no doubt associated in some way</p><p>yet to be understood with equator-to-pole temperature gradients, dynamical effects,</p><p>more efficient radiative cooling at higher latitudes, seasonal and spatial variations</p><p>of the solar flux, and the latitudinal variations observed in the composition of some</p><p>of the minor constituents (see Chapters 6 and 7).</p><p>Furthermore, the 28 Sgr occultation revealed fast zonal winds, up to 180 m s−1</p><p>in the South, and close to 100 m s−1 at mid latitudes. Other occultations occurred</p><p>on December 20, 2001 and November 14, 2003. They seem to suggest a seasonal</p><p>variation with respect to 1989. In 2001 a strong 220 m s−1 jet was located at 60◦N,</p><p>with lower winds extending between 20◦S and 60◦S, and much slower motion at</p><p>mid-latitudes. Last but not least, temperature inversions have been detected in the</p><p>stellar occultation data, suggesting a stratified upper atmospheric structure, as con-</p><p>firmed later by the Huygens/HASI experiment. Inversion layers were present close</p><p>to 510 km altitude in the HASI and the 2003 occultation data, and at 425 and 455 km</p><p>in 1989 occultation light curves, with vertical wavelengths on the order of 100 km.</p><p>With the Cassini spacecraft in orbit in the Saturn system, spatial observations of</p><p>Titan occultations are made possible. Solar and stellar occultations are observed by</p><p>the VIMS (Visible and Infrared Mapping Spectrometer) instrument. While Earth-</p><p>based observations are mainly refractive occultations, Cassini observes absorbing</p><p>occultations. Absorption is due to gaseous constituents of the atmosphere and to the</p><p>haze layers. With VIMS, absorption spectrum are available at each altitude sounded.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>54 Titan: Exploring an Earthlike World</p><p>The main features observed in those data are methane and CO bands, while haze</p><p>absorption fixed the continuum level.</p><p>3.4.2 The Radar Search for Oceans, Seas or Lakes</p><p>As we saw in Chapter 2, after Voyager the prevailing view from the sum of observa-</p><p>tions and theoretical considerations was that the processes that give rise to the haze</p><p>and cloud materials, although not understood in detail, are likely to lead to precipi-</p><p>tation onto the surface. Since at least some of these were expected to be liquids, and</p><p>since the process of accumulation had gone on for a long time, the idea of an exten-</p><p>sive ocean was an attractive one. Despite this expectation, well before Cassini and</p><p>Huygens arrived, ground-based data was indicating that Titan has a predominantly</p><p>solid surface. We need to look more closely at the arguments, and the recent history</p><p>of how they have evolved, for clues to resolve this paradox.</p><p>The fundamental driving force in the long-term evolution of Titan’s atmosphere</p><p>is the photolysis of methane in the stratosphere to form higher hydrocarbons and</p><p>aerosols. The photochemical products falling out of the atmosphere act to decrease</p><p>the methane vapour pressure. The predominant photochemical product, ethane, is</p><p>cryovulcanism</p><p>methane</p><p>clathrates</p><p>lakes of methane-</p><p>ethane-nitrogen</p><p>organic</p><p>sediment</p><p>organic</p><p>deposits</p><p>condensation</p><p>to spaceto</p><p>. . . . . . . . . . . . . . . . . . . . . . . 181</p><p>6.5.1 Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . 185</p><p>6.5.2 Nitriles . . . . . . . . . . . . . . . . . . . . . . . . . . 188</p><p>6.5.3 Oxygen Compounds . . . . . . . . . . . . . . . . . . . 189</p><p>6.5.4 Condensation Efficiencies . . . . . . . . . . . . . . . . 192</p><p>6.5.5 Aerosol Production . . . . . . . . . . . . . . . . . . . . 193</p><p>7. Clouds and Hazes 194</p><p>7.1 Introduction and Overview . . . . . . . . . . . . . . . . . . . . 194</p><p>7.2 Terrestrial Clouds and Precipitation . . . . . . . . . . . . . . . 197</p><p>7.3 Visible Aspects of Titan’s Haze . . . . . . . . . . . . . . . . . . 198</p><p>7.4 Size and Vertical Distribution of the Haze Particles . . . . . . . 201</p><p>7.4.1 Haze Vertical Profiles . . . . . . . . . . . . . . . . . . 203</p><p>7.4.2 Haze Opacity Spatial Variations . . . . . . . . . . . . . 205</p><p>7.5 Tropospheric Condensate Clouds . . . . . . . . . . . . . . . . . 206</p><p>7.6 Thermal and Dynamical Interactions with the Haze . . . . . . . 210</p><p>7.7 Observational Evidence on the Aerosol Composition . . . . . . 213</p><p>7.8 Laboratory Simulations of Haze Materials . . . . . . . . . . . . 217</p><p>7.8.1 Chemical Composition of Tholins . . . . . . . . . . . . 218</p><p>7.8.2 Optical Properties of Tholins . . . . . . . . . . . . . . . 221</p><p>7.9 Microphysical Models of Titan’s Haze . . . . . . . . . . . . . . 223</p><p>7.9.1 Organic Haze Production . . . . . . . . . . . . . . . . . 223</p><p>7.9.2 Fractal Models and Scattering Properties of the Haze . . 227</p><p>7.10 Discussion and Conclusion . . . . . . . . . . . . . . . . . . . . 229</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>viii Titan: Exploring an Earthlike World</p><p>8. Atmospheric Dynamics and Meteorology 231</p><p>8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231</p><p>8.2 Dynamics of Planetary Atmospheres . . . . . . . . . . . . . . . 232</p><p>8.3 Titan’s General Circulation . . . . . . . . . . . . . . . . . . . . 237</p><p>8.4 Zonal Motions . . . . . . . . . . . . . . . . . . . . . . . . . . 238</p><p>8.5 The Meridional Circulation . . . . . . . . . . . . . . . . . . . . 243</p><p>8.5.1 The Hemispherical Asymmetry . . . . . . . . . . . . . 244</p><p>8.5.2 The Polar Vortex . . . . . . . . . . . . . . . . . . . . . 245</p><p>8.6 Vertical Motions . . . . . . . . . . . . . . . . . . . . . . . . . 247</p><p>8.7 Waves, Tides and Turbulence . . . . . . . . . . . . . . . . . . . 250</p><p>8.8 The Weather Near the Surface . . . . . . . . . . . . . . . . . . 251</p><p>8.9 Does Lightning Occur on Titan? . . . . . . . . . . . . . . . . . 255</p><p>9. The Surface and Interior of Titan 258</p><p>9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258</p><p>9.2 Remote Sensing of the Surface . . . . . . . . . . . . . . . . . . 259</p><p>9.3 Huygens Takes a Plunge . . . . . . . . . . . . . . . . . . . . . 261</p><p>9.4 Naming Distant New Places . . . . . . . . . . . . . . . . . . . 266</p><p>9.5 Evidence for Geological Activity . . . . . . . . . . . . . . . . . 269</p><p>9.5.1 Albedo Variations . . . . . . . . . . . . . . . . . . . . . 269</p><p>9.5.2 Craters . . . . . . . . . . . . . . . . . . . . . . . . . . 270</p><p>9.5.3 Mountains and Volcanoes . . . . . . . . . . . . . . . . 273</p><p>9.5.4 Dunes . . . . . . . . . . . . . . . . . . . . . . . . . . . 274</p><p>9.5.5 Lakes . . . . . . . . . . . . . . . . . . . . . . . . . . . 275</p><p>9.6 The Nature and Composition of the Surface . . . . . . . . . . . 277</p><p>9.7 The Interior of Titan . . . . . . . . . . . . . . . . . . . . . . . 281</p><p>10. Titan’s Origin and Evolution in the Solar System 286</p><p>10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286</p><p>10.2 Relations Among Solar System Bodies . . . . . . . . . . . . . . 287</p><p>10.2.1 The Formation of the Solar System . . . . . . . . . . . 289</p><p>10.2.2 The Terrestrial Planets and Titan . . . . . . . . . . . . . 290</p><p>10.2.3 Titan and the Outer Planets . . . . . . . . . . . . . . . . 297</p><p>10.2.4 Titan and the Other Saturnian Satellites . . . . . . . . . 304</p><p>10.2.5 Titan and Europa . . . . . . . . . . . . . . . . . . . . . 318</p><p>10.2.6 Nitrogen Atmospheres in the Outer Solar System . . . . 318</p><p>10.3 Titan’s Origin and Evolution . . . . . . . . . . . . . . . . . . . 321</p><p>10.3.1 Evolutionary Models for Titan’s Atmosphere . . . . . . 324</p><p>10.3.2 Origin of the Atmospheric Components . . . . . . . . . 325</p><p>10.4 Titan and Life . . . . . . . . . . . . . . . . . . . . . . . . . . . 329</p><p>10.5 Open Questions . . . . . . . . . . . . . . . . . . . . . . . . . . 330</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>Contents ix</p><p>11. Beyond Cassini/Huygens: The Future Exploration of Titan 332</p><p>11.1 Returning to Titan . . . . . . . . . . . . . . . . . . . . . . . . . 332</p><p>11.2 Titan as a Target of Astrobiological Interest . . . . . . . . . . . 334</p><p>11.3 Science Drivers and Measurements Needed . . . . . . . . . . . 335</p><p>11.4 Advanced Titan Mission Concepts . . . . . . . . . . . . . . . . 337</p><p>11.3 Technology Requirements . . . . . . . . . . . . . . . . . . . . 339</p><p>11.6 Mission Architecture and Design . . . . . . . . . . . . . . . . . 339</p><p>11.7 Getting to Titan: Launch and Propulsion . . . . . . . . . . . . . 340</p><p>11.8 The Voyage of the Titania . . . . . . . . . . . . . . . . . . . . . 341</p><p>11.9 Explorers on Titan . . . . . . . . . . . . . . . . . . . . . . . . 343</p><p>Glossary and Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350</p><p>References and Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . 358</p><p>Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>This page intentionally left blankThis page intentionally left blank</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>xi</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>This page intentionally left blankThis page intentionally left blank</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>Prologue</p><p>Humans have searched for another Earth practically since coming into existence, at</p><p>least since we first began to realise that the points of light in the sky were large objects</p><p>very far away, and the science of astronomy was born. In the modern era, every</p><p>possible approach, from telescopes and space missions to philosophical arguments,</p><p>is being used to probe the existence of Earthlike planets elsewhere in the universe.</p><p>The ultimate question is whether there is another world sufficiently like ours, with</p><p>the same temperature conditions and atmospheric composition, where we could live</p><p>in comfort, or even where life could have developed in a form that we would readily</p><p>recognise.</p><p>In the last thirty years or so, space missions and Earth-based telescopes have</p><p>shown us that our own Solar System is mainly composed of uninhabited, and essen-</p><p>tially uninhabitable, worlds. There is plenty of variety, ranging from giant balls of</p><p>primordial gas with no solid surface (the outer planets) to objects that have no sub-</p><p>stantial atmosphere like Mercury and the minor planets; or like our Moon and almost</p><p>all of the other satellites in the Solar System. Mars has enough atmospheric density</p><p>to exhibit phenomena like polar caps, clouds, and dust storms, but the composition</p><p>is mostly CO2, and therefore unfit for human respiration. Venus and Mercury are</p><p>too hot and dry, and so on. Since the prospects for life are not good in our immediate</p><p>neighbourhood, it is not surprising then that scientists have turned to the rest of the</p><p>Universe, outside the Solar System.</p><p>Recently we have been witnessing the regular discoveries, through modern tech-</p><p>niques ranging from adaptive optics and radio astronomy to gravitational microlens-</p><p>ing, of protostellar disks, and of a rapidly increasing number of new extrasolar</p><p>planets and planetary systems around distant stars like Beta Pictoris, Fomalhaut,</p><p>51 Pegasus, HD209458, and Tau Bootis. As we get better at searching, we increase</p><p>our chances of finding another Earth some day, but it will not be nearby or easy to</p><p>reach, even with robot probes. In our Solar System, in particular, the chances for life</p><p>outside Earth, long ago considered excellent, receded as exploration revealed the</p><p>details of the non-Earthlike</p><p>stratosphere</p><p>meteoritic input</p><p>photolysis</p><p>products</p><p>mantle of dirty ice</p><p>CH4 C2H6</p><p>H2O-NH3 regolith</p><p>CH4</p><p>N2</p><p>N2 C2H6 CH4 N2- -</p><p>C2H6</p><p>rain</p><p>evaporation</p><p>Figure 3.12 A schematic of the relationship between the atmosphere and bodies of liquid on the</p><p>surface of Titan. The main processes involved are methane (CH4) evaporation, photolysis and con-</p><p>densation, and the production, condensation and fallout of ethane (C2H6).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 55</p><p>Table 3.1 Pre-Cassini results for Titan’s surface oceans, for two extreme assumptions about the</p><p>surface temperature and atmospheric composition, compiled from several models.</p><p>Surface temperature 92.5 K 101 K</p><p>Nitrogen 1.8% 6%</p><p>Methane 7.3% 83.4%</p><p>Argon 0 5.6%</p><p>CH4 abundance (lower troposphere) 1.55% 21.1%</p><p>Ocean composition: ethane + propane 90% 5%</p><p>Current depth 695 m 9.4 km</p><p>liquid at 94 K, and has a vapour pressure much lower than methane, allowing the</p><p>two to mix in solution. Nitrogen, carbon monoxide and liquid products of methane</p><p>photolysis other than ethane, such as propane, would also dissolve and accumulate</p><p>on Titan’s surface. Products that are solid at the temperatures on Titan, such as</p><p>acetylene, would sink to the bottom.</p><p>The resulting “ocean” could serve as both the source and the sink of for the</p><p>photolysis cycle. In numerical models, its depth ranges from 500 m to 10 km, and</p><p>it contains a mass of nitrogen comparable to the atmospheric abundance. In the</p><p>absence of outgassing from the crust or an external supply of volatiles, the ocean</p><p>composition evolves to become more ethane-rich, as methane is photolysed over</p><p>geologic time. The atmosphere responds to the change in ocean composition with</p><p>a corresponding change in gaseous composition and spatially averaged cloud com-</p><p>position. The detailed solutions found in modelling studies lie within the extremes</p><p>in Table 3.1.</p><p>The first experimental result which led to reservations about these theoretical</p><p>studies, and the first remote sensing technique to be used to study Titan’s surface,</p><p>came from the detection of radar reflections from Titan by D. Muhleman and his</p><p>colleagues at the California Institute of Technology. The first experiment was con-</p><p>ducted in 1991, with another successful one in 1995. Titan is the most distant object</p><p>detected by radar to this date. Muhleman and colleagues used NASA’s radio tele-</p><p>scope at Goldstone in California to send a signal from Earth, and the National Radio</p><p>Astronomy Observatory’s Very Large Array in Socorro, New Mexico, as a receiver</p><p>at 3.5 cm. These wavelengths are unaffected by the presence of haze or clouds in</p><p>the atmosphere of Titan, but the returning signal is sensitive to the type of materials</p><p>present on the surface from which it bounces. The 1991 high reflectivity values</p><p>reported (which led theoreticians originally to consider Titan as a very bright object,</p><p>analogous to the Jovian satellites) were corrected in an update published in 1995, in</p><p>which Muhleman et al. gave a revised value of 0.15 for the reflectivity of the Titan</p><p>bright terrain. This value was confirmed by 1992 Goldstone radar observations by</p><p>another group. The darker terrain of Titan showed generally lower radar reflectivity,</p><p>albeit with marginal statistical significance.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>56 Titan: Exploring an Earthlike World</p><p>The echoes of the signal sent from Earth showed that Titan is not covered with a</p><p>deep global ocean of ethane-methane, because such an ocean, if devoid of suspended</p><p>particulates and deeper than a few hundred metres, is a very poor reflector of radar</p><p>signals. It should return only a small percentage (about 0.02) of the energy origi-</p><p>nally transmitted, whereas the radar cross sections obtained vary between 5% and</p><p>25%. The higher end of the range is then considerably smaller than the reflectivity</p><p>of the largest Jovian satellites, Europa, Ganymede, and Callisto, which range from</p><p>30% to 90%. Even if large particles of some kind could remain suspended in the</p><p>postulated ocean on Titan, perhaps stirred by the wind, the value of the reflectivity</p><p>could not approach the Callisto value. The lower end of this range indicates that</p><p>some of Titan’s dark terrain could be covered with considerable amounts of solid or</p><p>liquid hydrocarbons. The bright terrain, on the other hand, as far as the radar data</p><p>can tell, has considerably smaller reflectivity than even Callisto, compatible with</p><p>a rocky surface like those found in the inner solar system. Other radar parameters,</p><p>such as the polarisation of the reflected signal, also indicate that Titan’s surface is</p><p>different from those of the Galilean satellites. Microwave emissivity measurements</p><p>for Titan’s surface resemble the Moon’s rocky, silicate-type more than the icy Jovian</p><p>satellites. This seems incompatible with the high surface albedo values found in</p><p>the near-infrared, described in the next section, which are indicative of an icy com-</p><p>ponent, except perhaps for the low surface albedo reported near 0.94 µm by some</p><p>investigators, which could be compatible with the presence of a silicate absorption</p><p>band. Titan appeared then as a unique object, more radar-reflective than if it were</p><p>covered by a global hydrocarbon ocean or by tholin material, but still rather dark</p><p>outside the brighter terrains at 60–160 degrees longitude.</p><p>A mixture of ice and rock on the surface could perhaps reconcile the various</p><p>observations. In 2003, D. Campbell and colleagues collected radar measurements</p><p>with the Arecibo Observatory in Puerto Rico, which showed a specular component</p><p>at 12 of 16 of the regions observed, which was globally distributed in longitude at</p><p>about 26◦S. This was interpreted as indicative of dark, liquid hydrocarbon extends</p><p>on Titan’s surface. However, this was challenged in 2005 by R. West and colleagues</p><p>whose observations failed to find any such signatures and proposed instead that flat</p><p>solid surfaces could be causing the radar evidence.</p><p>As the idea of a global or deep ocean began to appeared to be incompatible with</p><p>the latest observations, new theoretical studies showed that tidal forces would have</p><p>dissipated Titan’s orbital eccentricity of 0.03 long ago if there was such an ocean.</p><p>The most compelling evidence of all against a deep global ocean on Titan with the</p><p>first images of its surface, which showed extensive non-homogeneous and recurrent</p><p>features clearly incompatible with a surface covered by liquid. On the other hand,</p><p>a surface that is completely dry — either exposed ice or ice buried under layers</p><p>of solid photochemical products, soil or rock — would not provide the source of</p><p>methane nor the sink for the liquid products of the photochemistry (notably ethane</p><p>and propane). Some early models attempted to reconcile these two requirements.</p><p>D. Stevenson of Caltech suggested that the hydrocarbon ocean may be stored in</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 57</p><p>Figure 3.13 A radar echo spectrum of Titan taken in 2002 at Arecibo, showing the expected (OC)</p><p>sense of received circular polarization, with a specular component at 0 Hz, interpreted by Campbell</p><p>and colleagues in 2003 as proof for liquid surfaces on Titan.</p><p>a porous, uppermost few kilometres of methane clathrate or water ice “bed rock”.</p><p>The surface in this model is porous enough (about 20% to a depth of about one-half</p><p>kilometre, not unrealistic) to allow all the required ethane and methane to pass into</p><p>a subsurface “aquifer” in which the liquid is stored. The model satisfies the tidal</p><p>constraints because the confined liquid cannot attain large velocities. One argument</p><p>against such a porous surface is that hardened regolith upper layers enriched in</p><p>organics (similar to “caliches” seen in Earth’s deserts) may prevent ethane dribbling</p><p>down into the regolith.</p><p>A qualitatively different model has the hydrocarbons which originate in the</p><p>haze deposited as solids and thus permanently removed from the atmosphere. The</p><p>atmospheric methane supply is replenished by outgassing</p><p>from the interior, gaining</p><p>access to the atmosphere through occasional release during cryovolcanic activity</p><p>or through upward diffusion. The biggest problem with this sort of concept comes</p><p>in explaining how the photochemical products can be buried in the mantle while</p><p>leaving water ice exposed on the surface, which the spectroscopic evidence (see</p><p>next section) seems to require.</p><p>3.4.3 Spectroscopic Measurements of Titan’s Albedo</p><p>Observations in the near-infrared offer an extremely useful tool for attempting to</p><p>solve the ambiguities in the models by obtaining observations that could be inter-</p><p>preted in terms of quantitative models of the structure and composition of Titan’s</p><p>lower atmosphere and surface. The spectral albedo (that is, the reflectivity at differ-</p><p>ent wavelengths, as a function of orbital position, measured as Titan rotates) was</p><p>June</p><p>4,2008</p><p>8:53</p><p>B</p><p>-611</p><p>9.75in</p><p>x</p><p>6.50in</p><p>ch03</p><p>58</p><p>T</p><p>ita</p><p>n</p><p>:E</p><p>x</p><p>plo</p><p>rin</p><p>g</p><p>a</p><p>n</p><p>E</p><p>a</p><p>rth</p><p>like</p><p>W</p><p>o</p><p>rld</p><p>Table 3.2 Microwave properties of Titan and other solar system bodies.</p><p>Property Titan terrain Europa Ganymede Callisto Moon</p><p>Bright Dark</p><p>3.5 cm same sense 0.08 ± 0.08a 0.03 ± 0.0a 1.40 ± 0.23b 0.9 ± 0.10b 0.40 ± 0.04b 0.006b,g</p><p>3.5 cm opp. sense 0.16±0.10a</p><p>0.15±0.05c</p><p>0.15±0.04d</p><p>0.13±0.03a</p><p>0.10±0.04c</p><p>0.91 ± 0.13b 0.6 ± 0.10b 0.32 ± 0.02b 0.07g</p><p>Polarisation ratio 0.5a 0.3a 1.43 ± 0.23b 1.40 ± 0.1b 1.22 ± 0.08b 0.1b</p><p>Microwave emissivity 0.85c 0.85e 0.4f 0.5f 0.8f 0.95c</p><p>aAverages of 60◦ 160◦ (dark region) data from Muhleman et al.</p><p>(1995), by Lorenz and Lunine (1997). bOstro et al. (1992). cMuhleman et al. (1995). dGoldstein and Jurgens (1992). eGrossman</p><p>and Muhleman (1992), f Muhleman et al. (1991). gPettengill (1965).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 59</p><p>Figure 3.14 Titan’s albedo exhibits several strong absorption bands, but also “windows” where the</p><p>methane absorption is weak enough to allow the lower atmosphere and surface to be probed. The data</p><p>shown here come from two observatories in Hawaii and (in the 2.75 micron region, where Earth-based</p><p>observations are difficult) the Infrared Space Observatory, ISO.</p><p>the first quantity observed and is one of the most useful for probing the haze and</p><p>surface properties.</p><p>The near infrared spectrum of Titan (0.8 to 5 µm), like those of the giant planets,</p><p>is dominated by the absorption bands of methane. Where the methane absorption is</p><p>weak, clear regions or “windows” permit the detection of radiation from the deep</p><p>atmosphere and, in some cases, from the surface. Also, the haze scattering effect</p><p>is small in the infrared, compared to the visible. Thus, the principal atmospheric</p><p>windows, through which Titan’s lower atmosphere and surface can be observed,</p><p>are near 4.8, 2.8, 2.0, 1.6, 1.28, 1.07, 0.94 and 0.83 µm in wavelength, or 12,050,</p><p>10,640, 9300, 7810, 6250, 5000, 3500, and 2050 cm−1 in wavenumber. In between</p><p>the windows, unlike the case of the giant planets, the “dark” regions are of interest</p><p>since the solar flux is not totally absorbed but instead is scattered back through</p><p>the atmosphere by the stratospheric aerosols, especially at the shorter wavelengths.</p><p>Modelling of the albedo derived from ground-based spectra of Titan can therefore</p><p>indicate the properties of the haze present in the atmosphere.</p><p>The haze is optically thick at visible wavelengths, and gas opacities are neg-</p><p>ligible, so the haze dominates the albedo and completely hides the surface. At</p><p>wavelengths longer than 0.6 µm, the scattering extinction efficiency of the particles</p><p>decreases due to the increasing ratio of wavelength to particle size. Also, if the data</p><p>derived from tholins produced in laboratory simulations applies, the haze mate-</p><p>rial becomes virtually non-absorbing. Thus, the haze becomes progressively more</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>60 Titan: Exploring an Earthlike World</p><p>transparent at longer wavelengths and the surface properties increasingly affect the</p><p>observed albedo, except in the strong CH4 absorption features which become promi-</p><p>nent longwards of 1 µm. Therefore, in the near-infrared spectral region, the albedo</p><p>is determined mainly by the methane amount present and the surface properties,</p><p>although the contribution of the haze opacity is not completely negligible even at</p><p>far infrared wavelengths.</p><p>Several different groups have independently provided practical demonstrations</p><p>that the surface of Titan can be probed by measuring the geometric albedo in</p><p>the 1–2.5 µm region. First came the observations by U. Fink and H. P. Larson in</p><p>1979, using the Kitt Peak National Observatory 4 m telescope. D. Cruikshank and</p><p>J. Morgan observed Titan in 1980 and looked for a 32-day variation in the albedo.</p><p>Griffith and Owen used Titan’s near-infrared spectrum to investigate Titan’s sur-</p><p>face in 1991, using the first detailed radiative transfer models of the near-infrared</p><p>spectrum produced to deduce that the surface albedo is inconsistent with a global</p><p>ocean, invoking instead the presence of ‘dirty’ water ice on the surface, and leav-</p><p>ing open the possibility that cloud cover could partly account for the observed</p><p>reflectivity in the atmospheric windows. C. Griffith and her co-workers of the</p><p>State University of New York in Stony Brook using the CGAS array spectrometer</p><p>at the IRTF Telescope in Hawaii, M. Lemmon and his colleagues at the Univer-</p><p>sity of Arizona using the GeSpec instrument at the Steward Observatory 2.3 m</p><p>telescope and at the Multiple Mirror Telescope, and A. Coustenis and colleagues</p><p>from Paris Observatory using the Fourier Transform Spectrometer at the 3.6 m</p><p>Canadian French Hawaiian Telescope. An additional window near 2.75 µm, unde-</p><p>tectable from the Earth, was first observed by the Infrared Space Observatory</p><p>in 1997.</p><p>The observations all agreed in showing that the geometric albedo of Titan, mea-</p><p>sured over its 16-day orbit, shows significant variations indicative of a brighter</p><p>leading hemisphere (facing the direction of Titan’s orbital motion) and a darker</p><p>trailing one. The leading side corresponds to Titan’s Greatest Eastern Elongation</p><p>at about 90◦ LCM (Longitude of Central Meridian — a geographical longitude of</p><p>about 210◦), when Titan rotates synchronously with Saturn; the trailing side is near</p><p>270◦ LCM. At conjunctions, that is on the hemispheres facing Saturn and its oppo-</p><p>site, the albedo was similar, of intermediate values between the maximum appearing</p><p>near 120◦ (±20◦) LCM and the minimum near 230◦ (±20◦). These albedo variations</p><p>could have been due to (a) tropospheric cloud variation, if they were uncorrelated</p><p>with the rotation period, or (b) surface properties variations, if correlated with Titan’s</p><p>orbital period of 15.945 days. Because they were measured independently over five</p><p>years and they were recurrent, observers agreed that the features must be correlated</p><p>with Titan’s surface morphology. Titan’s surface was then demonstrated to be het-</p><p>erogeneous, which was already an important result in that it ruled out a global ocean</p><p>or indeed any uniform coverage by a single material, such as water ice. The variable</p><p>brightness behaviour was subsequently identified in images as being mainly due to</p><p>a single large bright area near the equator.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 61</p><p>Figure 3.15 Light curve of Titan’s geometric albedo with data taken between 1991 and 1996 with</p><p>the FTS of the CFHT telescope, showing the hemispheric asymmetry, with a brighter leading (at 90◦</p><p>LCM) than trailing (at 250◦ LCM) side (Negrão et al., 2006).</p><p>The next step — to deduce information on the real nature of the surface from the</p><p>infrared spectrum of sunlight reflected from the surface of Titan — is very model-</p><p>dependent and in particular requires precise knowledge of the non-surface contri-</p><p>bution. This is obtained using a computer code, which includes such atmospheric</p><p>properties as the production rate, and microphysics of the haze particles, the amount</p><p>and distribution</p><p>of methane in the atmosphere, and the optical properties of both.</p><p>The methane absorption coefficients used as one of the model’s input parameters</p><p>have a major impact on the calculated geometric albedo and consequently inferred</p><p>surface albedo. These come from theoretical calculations and laboratory measure-</p><p>ments by a number of different researchers, and are assembled in widely accessible</p><p>compilations known as databases. The STDS database, for instance, is built from a</p><p>theoretical model of the methane (12CH4 and 13CH4) rotation-vibration interactions</p><p>and transition moments, with parameters fitted against laboratory high-resolution</p><p>absorption spectra. It includes all the methane absorption lines between 1.62 and</p><p>7.69 µm, although modelling of the methane absorption spectrum at wavenumbers</p><p>lower than 2.2 µm is still not fully satisfactory because of the large complexity of the</p><p>rotation-vibration interactions in these regions, rendering the analysis very difficult.</p><p>Most recent work has used band models based on laboratory measurements,</p><p>which include all of the multitude of weak lines that cannot be observed individually</p><p>but which make a significant contribution in total. The laboratory measurements</p><p>cannot reproduce the low temperatures and the large path lengths on Titan, but</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>62 Titan: Exploring an Earthlike World</p><p>Figure 3.16 Methane absorption in the 2-micron range from calculations by V. Boudon and</p><p>colleagues (Univ. de Bourgogne, France).</p><p>approximate these with some data using large methane concentrations, to give total</p><p>path lengths typical of those found on Titan, and others at low temperatures, in one</p><p>case down to 100 K. Combining the two provides an acceptable simulation of Titan’s</p><p>atmosphere.</p><p>The surface spectrum for Titan between 0.9 and 2.5 µm typically shows three</p><p>distinct spectral regions: at 1–1.6 µm the surface albedo is about two times higher</p><p>than at 2.0 µm or at 0.94 µm. Recent measurements at 2.9 and 5 µm place the</p><p>associated surface albedos below 0.1, at around 0.03 and 0.07 respectively. If the</p><p>exact values are not completely agreed upon, the various teams who have applied</p><p>models to Titan data in order to model the surface spectrum from a large set of</p><p>spectra, both from the ground or with Cassini–Huygens instruments, agree that there</p><p>seem to exist at least two lower-albedo regions in the spectrum near 1.6 and 2 µm</p><p>with respect to the continuum near 1 µm. The most probable reason that the surface</p><p>albedo of Titan is significantly higher at 1.075–1.28 µm than at 1.6–2.0 µm is the</p><p>presence of water ice on Titan’s surface. Since, so far as we can tell at present, moving</p><p>from one hemisphere to the other changes the total brightness at all wavelengths</p><p>rather than altering the shape of the spectrum, the extra species responsible for the</p><p>orbital variations may be spectrally neutral.</p><p>Laboratory-generated organic tholins like those thought to be present in the haze</p><p>material show a neutral and fairly bright spectrum in the near IR and a tendency to</p><p>get darker towards the visible end of the spectrum, and so represent a plausible cause</p><p>of the high absolute albedos and the 0.94 µm depression. The water ice spectrum</p><p>and the tholin reflectance, if combined, can in fact produce a reasonable simulation</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 63</p><p>of Titan’s surface albedo. However, we would expect the tholins to be distributed</p><p>more or less uniformly with longitude if they are solids which have precipitated onto</p><p>the surface from the global haze, and then they would not produced the observed</p><p>orbital variations. Even so, it is interesting that the spectrum of Titan matches the</p><p>decrease in the tholin spectrum from 1.3 to 0.9 µm most strongly for the darker,</p><p>trailing side. The fact that this decrease is more evident on the dark side may be</p><p>diagnostic of the presence of more tholin/organic material on the surface on that side</p><p>of Titan than on the brighter, leading side. This might mean that there is a second</p><p>source of tholins, not necessarily the same material but with broadly similar spectral</p><p>properties, that originates outside of Titan. One is reminded of the dark, reddish</p><p>coating on the leading side of Iapetus and other airless satellites, that looks like</p><p>dark material swept up from space. Quite how such behaviour would work on Titan,</p><p>with its thick atmosphere, is hard to see, and other possibilities are not rued out.</p><p>For instance, the surface patterns, and hence the orbital variations, may be due to</p><p>longitudinal differences in the ice morphology: high and low regions, covered with</p><p>fresh or old, soft or hard, big or small particles, or even to the physical properties of</p><p>the ice itself, like strength, viscosity, etc.</p><p>Observations of other satellites in the near-infrared range support the presence</p><p>of water ice on Titan. There are depressions observed in the spectra of Hyperion</p><p>and Callisto near 1.6 and 2 µm, due to water ice bands, and the Hyperion spectrum</p><p>is in general consistent with that of Titan over the whole 1–2 µm region. This is</p><p>Figure 3.17 Titan’s surface spectrum. The main features — bright around 1.2 µm and darker around</p><p>0.94 and 2.0 µm — are compared here with several possible surface constituents, and found to be</p><p>compatible with water ice covered with a layer of condensed hydrocarbons (Negrão et al., 2006).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>64 Titan: Exploring an Earthlike World</p><p>reproducible if water ice were the dominant constituent on Titan’s surface, as has</p><p>also been deduced from radar reflectivity and radio emissivity measurements. The</p><p>water ice bands are present in Titan’s surface spectrum at all longitudes.</p><p>However, although water ice must be present in large amounts, a single surface</p><p>component cannot explain the low albedo at 0.94 µm, or the variations in brightness</p><p>which Titan exhibits during its orbit around Saturn. From the other relevant ices</p><p>that could be found on Titan’s surface, CO2 and CH4 ice spectra show a behaviour</p><p>compatible with what is observed in the general shape of the 1–2.5 µm region.</p><p>The NH3 spectrum does not help the fit of the surface data either. Hydrocarbon ice</p><p>(such as CH4 or C2H2) has absorption bands near 1.65 and 2.3 µm and its presence</p><p>cannot be excluded. So far, however, only water ice can be definitely described as a</p><p>reasonable candidate from all the observations, and its presence was confirmed in</p><p>the recent Huygens observations by DISR.</p><p>3.4.4 ImagingTitan’s Atmosphere in the Near-Infrared</p><p>In 1994, two sets of data taken independently and with different methods were the</p><p>first to clearly show in images of Titan’s surface the heterogeneity that had been</p><p>inferred from the near-IR and radar light curves. Extensive quasi-permanent features,</p><p>which were too bright to be hydrocarbon liquid, were graphically revealed by the</p><p>adaptive optics technique using the ADONIS camera at the 3.6 m ESO Telescope in</p><p>Chile, and by the Hubble Space Telescope. The first HST images produced maps at</p><p>0.94 and 1.08 µm, with contrasts due to surface features of about 10%, prompting</p><p>a search to identify spectrally-distinct surface units, which may indicate regions of</p><p>different composition, and to detect and monitor atmospheric features. Prominent</p><p>among the latter is the north-south asymmetry observed by Voyager, believed to be</p><p>due to a planet-wide variability in the haze properties, such as spatial distributions</p><p>of particle size, number density, or optical properties.</p><p>In the season when Voyager observed Titan (northern spring) the southern hemi-</p><p>sphere had an albedo about 25% brighter than the north at blue wavelengths, with</p><p>the contrast between the hemispheres smaller at green and violet wavelengths, and</p><p>the UV and orange albedo asymmetry lower still. More recent Hubble Telescope</p><p>images, using the WFPC-2 camera, clearly showed the north-south asymmetry,</p><p>especially at blue and green wavelengths, to be similar to that observed by Voy-</p><p>ager, two seasons earlier. In the</p><p>near-infrared (between 0.94 and 1.07µm) these</p><p>images showed a strong north-south asymmetry and brightening towards the lower</p><p>limb. Spatially resolved images of Titan from ground-based telescopes, using adap-</p><p>tive optics at longer wavelengths, showed the same north-south asymmetry when</p><p>Titan was observed in the methane bands, for instance at 2.2 µm. At 0.889 µm, in</p><p>a methane band, HST images showed an asymmetry having a structure opposite to</p><p>that observed at visible wavelengths, due to the optical properties of the aerosols,</p><p>accumulated in the south at the current epoch. Indeed, the north-south asymmetry</p><p>appears to have reversed in the methane band, as would be expected from seasonal</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 65</p><p>effects as the sub-solar point moves north and south over a Titan year, due to the</p><p>26.4◦ obliquity of the Saturnian system.</p><p>The HST and Voyager images at visible, UV and near-infrared wavelengths</p><p>allow the determination of limb-darkening coefficients, which measure the fall-off</p><p>in brightness from the centre of Titan’s disk to the edge, as a function of wavelength.</p><p>These coefficients provide important information on the spectral characteristics of</p><p>the haze. Limb darkening is observed to increase from violet to red wavelengths,</p><p>giving the impression that the disk of Titan shrinks at longer wavelengths. In con-</p><p>trast, limb brightening is observed at 0.889 µm, in the strong methane absorption</p><p>band, because the lower atmosphere is opaque here, whereas the optically thin</p><p>haze in the upper atmosphere is bright. The data tell us that a change in the con-</p><p>centration of aerosol particles between 70 and 120 km altitude could explain the</p><p>observed asymmetry on Titan. However, changes in the photochemical production</p><p>of aerosols cannot be responsible, because the time constants associated with the</p><p>aerosol formation and accumulation into optically thick layers are much longer than</p><p>the seasonal period. More likely, the aerosol is being moved by the circulation of</p><p>the atmosphere (meridional and vertical winds), as discussed further below. In this</p><p>case, the boundary where the albedo contrast occurs, currently situated between 10</p><p>and 20◦N, should also move with season. However, there is no conclusive evidence</p><p>that these movements do take place in the expected direction.</p><p>The hemispheric asymmetry can also be measured in the thermal infrared. For</p><p>wavenumbers longer than 600 cm−1, the relatively warm stratospheric haze is the</p><p>main source of the continuum. This has been used in an analysis of Voyager 1 IRIS</p><p>data taken at different latitudes to determine that there is an apparent increase in the</p><p>haze optical depth of about 2.5 near the north pole with respect to the equator. At</p><p>visible wavelengths, the increased opacity in the infrared in the north corresponds</p><p>to darkening in that hemisphere. This stratospheric north-to-south enhancement in</p><p>the haze opacity, associated with an equivalent increase in gaseous abundances, can</p><p>explain the colder temperatures found at high northern latitudes, as this may be</p><p>caused by more efficient radiative cooling, although dynamical effects may also be</p><p>involved.</p><p>Another phenomenon which was first reported in 2001 from AO data taken in</p><p>1998, was an East-West asymmetry, with a brighter morning limb found on Titan</p><p>on several occasions. This dawn haze enhancement could be due to a deposition of</p><p>condensates during the Titan night (8 Earth days, though the super-rotation of Titan’s</p><p>atmosphere would lead to shorter nights for stratospheric clouds), manifesting itself</p><p>in a morning haze enhancement phenomenon at stratospheric altitudes. Indeed,</p><p>most of the images showing this phenomenon are mainly probing the stratosphere</p><p>(between 40 and 311 km). All teams observing Titan with adaptive optics at the time</p><p>of the Huygens descent agree there is evidence of this diurnal effect.</p><p>In addition to the global asymmetries, images of Titan’s atmosphere showed</p><p>additional discrete bright areas, mainly near Titan’s south pole, sometimes above a</p><p>fine bright southern polar limb. Clouds have been invoked to interpret these features,</p><p>including a very bright one, located very close to the south pole, that has attracted</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>66 Titan: Exploring an Earthlike World</p><p>growing attention in the past decade. This feature was first discernible in speckle</p><p>images of Titan published in 1999 and has since been extensively observed in dif-</p><p>ferent filters and by different teams. It is particularly visible in 2.12 µm images,</p><p>where the upper troposphere of the southern limb is probed. Small clouds have been</p><p>reported near mid-latitudes in observations from the Keck and Gemini Observato-</p><p>ries. The clouds cluster near 350◦W longitude, 40◦S latitude and cannot be explained</p><p>by a seasonal effects but may be linked to surface features, the most exciting pos-</p><p>sibility being that they are initiated by the plumes escaping from cryovolcanoes.</p><p>Bright features are relatively rare in the north, apart from the bright northern limb</p><p>due to the north-south anomaly.</p><p>The large polar vortex, a meteorological system over Titan’s south pole, seemed</p><p>to vary in shape and brightness with time. It was later demonstrated by observations</p><p>from the Cassini orbiter that its shape changed, like a large ring of thin clouds or haze,</p><p>deforming, with little bright clouds trapped within it, appearing and disappearing</p><p>randomly, sometimes disintegrating, something which could not be resolved with</p><p>Earth-based systems. The latter nevertheless allowed some monitoring of this phe-</p><p>nomenon, which has apparently diminished in intensity and brightness, becoming</p><p>quite marginal in the Cassini and ground-based images taken since 2005.</p><p>3.4.5 Imaging the Surface</p><p>Earlier HST observations had hinted at the ability to sound the surface of Titan,</p><p>since a number of its filters sample the 940 nm window, but had been thwarted by</p><p>the spherical aberration in the primary mirror. When the new Wide-Field Planetary</p><p>Camera (WFPC-2) was installed, which corrected this optical problem, Titan could</p><p>be resolved as about 20 pixels across, with the point-spread function about 3 pixels</p><p>across. In the filters used, a large proportion of light still comes from scattering by</p><p>haze, but the near-full longitudinal coverage of the HST dataset allowed this to be</p><p>determined and removed (since the haze is longitudinally-invariant).</p><p>The team led by P. Smith of the University of Arizona that produced the first</p><p>HST images in which features were discernible on Titan’s surface worked in the</p><p>0.94 µm window, observing the bright leading and dark trailing sides, with a large</p><p>bright region, about the size of Australia, at 110◦E and 10◦S, as well as a number of</p><p>less bright regions. A coarse map was also produced at red wavelengths (0.673 µm)</p><p>— albeit somewhat blurred, since the haze optical depth is of the order of 3 at</p><p>this wavelength, so each photon is reflected a number of times as it fights its way</p><p>through the atmosphere. Subsequent HST data with the NICMOS camera have</p><p>confirmed the initial findings with more extensive mapping at 1.6 and 2.0 µm, and</p><p>identified spectrally-distinct surface units, which may indicate regions of different</p><p>composition. The contrast in the HST images was about 10–20%.</p><p>At about the same time, the images taken using the adaptive optics ADONIS</p><p>camera at the 3.6 m ESO Telescope at Chile showed the same bright region at the</p><p>equator and near 120 degrees orbital longitude, but also revealed a north-south hemi-</p><p>spheric asymmetry apparent on Titan’s darker side. Diffraction-limited images were</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 67</p><p>thus obtained regularly since 1994 at 1.3, 1.6 and 2.0 µm with narrow band filters</p><p>centred on the methane windows and in the wings of the absorption bands, which</p><p>allowed for 50–300 independent spatial elements to be distinguished on Titan’s disk</p><p>(for instance at 2.0 µm a typical adaptive optics resolution</p><p>would be 0.1 arcsec).</p><p>Adaptive optics is now a generally-adopted method and such systems exist in</p><p>almost all the large Earth-based telescopes. Prior to the Cassini encounter, the adap-</p><p>tive optics system in Hawaii and its equivalent at the VLT in Chile were used to</p><p>scrutinize Titan and produce surface maps at different wavelengths. The Keck, Sub-</p><p>aru and Gemini AO systems have also been applied to Titan and returned some of the</p><p>most interesting images of the satellite. The contrast in the adaptive optics images</p><p>can achieve 50% under good observing conditions.</p><p>The NACO data on Titan were taken with narrow-band filters in the terrestrial</p><p>atmospheric windows near 1.3, 1.6, and 2.0 µm, which probe the surface of Titan.</p><p>Figure 3.18 Images of Titan taken with the adaptive optics system NACO at the Very large Telescope</p><p>of ESO in Chile in 2002. The different wavelength filters allow probing of various altitude levels from</p><p>the atmosphere to the surface, hence the variable appearance. At 1.08 and 1.28 µm, Titan’s surface is</p><p>observed; elsewhere, the north-south asymmetry in the atmosphere evolves with a brighter northern</p><p>pole at higher altitudes. At 2.12 µm, the bright spot on the south pole is due to a large meteorological</p><p>system present from 2000 until 2005 (Gendron et al., 2004).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>68 Titan: Exploring an Earthlike World</p><p>Figure 3.19 The wavelength dependence of the north/south albedo ratio on Titan from HST mea-</p><p>surements over different periods in time, showing how the North-South asymmetry reverses at green</p><p>and yellow wavelengths (Lorenz et al., 1999).</p><p>Maps of Titan’s surface at different wavelengths show very little variation, and</p><p>always feature a bright equatorial region. At 2.0 µm, the images are less affected by</p><p>scattering (about one-third) than at 1 µm, and the contrast achieved is then higher.</p><p>The spatial resolution, before deconvolution is applied, is at the limit of diffraction</p><p>(0.13 arcsec), and after deconvolution is very similar to that of the HST. Spectro-</p><p>scopically resolved images, recorded with a circular variable filter in adaptive optics,</p><p>at 2.10 µm, were found to be quite similar to the 2.0 µm images, not showing the</p><p>strong absorption by liquid hydrocarbons such as ethylene and ethane (C2H4, C2H6)</p><p>that would have been expected in the presence of large hydrocarbon lakes in the</p><p>dark regions. Of course, as supporters of the lake hypothesis pointed out, the lakes</p><p>could contain impurities that confound this analysis, provided they have the proper</p><p>spectral behaviour.</p><p>Even from the first set of data, it became apparent that the brighter leading</p><p>hemisphere of Titan found in the spectra was associated with a large feature located</p><p>near the equator centred near 114◦ LCM and extending over 30◦ in latitude and 60◦</p><p>in longitude. This bright spot we now know as Xanadu could be due to differences</p><p>in relief, but it has a spectral behaviour that suggests otherwise. Like other bright</p><p>spots visible in the S-W region (near 25◦S) and near 30◦N, it appears bright at all</p><p>investigated wavelengths (0.9, 1.1, 1.3, 1.6 and 2.0 µm). It was also demonstrated</p><p>that Titan’s surface was much more complex than initially thought and that the “dark”</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 69</p><p>hemisphere, was — fortunately because it was soon found out that the Huygens</p><p>probe was not going to land where initially scheduled, close to the bright region, but</p><p>rather on the trailing side — not all that dark, but also showed some fine structure</p><p>with bright areas.</p><p>Early observers were tempted to attribute the bright ‘continents’ to the presence</p><p>of mountainous areas on the surface, or alternatively depressions in the form of</p><p>cratering produced by impacts. Higher regions certainly would look brighter, just by</p><p>virtue of their height above some of the obscuring atmosphere. However, calculations</p><p>show that even the effect of an unrealistically high plateau covering all of Titan’s</p><p>bright side and reaching up to 30 km in altitude, cannot account for the total observed</p><p>albedo increase from one hemisphere to the other. Such a large asymmetry on the</p><p>surface would alter Titan’s centre of mass and the satellite would slowly turn to</p><p>point the feature toward Saturn, which is not where the bright feature is found. This</p><p>could mean that the feature formed relatively recently, in geological terms, but it</p><p>seems much more likely that the brightness difference may be due to a compositional</p><p>variation. Part of the brightness could be caused by moderately high mountains</p><p>Figure 3.20 Titan’s disk in the near-infrared, imaged with the HST, showing north-south asymmetry</p><p>with a bright southern hemisphere, indicative of aerosol concentration in that region (Smith, Lemmon</p><p>et al., 1996).</p><p>Figure 3.21 Keck images of Titan showing the cloud-like system in the south polar region (Roe</p><p>et al., 2002a).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>70 Titan: Exploring an Earthlike World</p><p>Figure 3.22 Images of Titan’s surface at 2 µm (left) and 0.94 µm (right), obtained with the HST</p><p>and ADONIS, respectively, in 1994. Note the large bright equatorial region located on the leading</p><p>hemisphere (Coustenis and Lorenz,1998).</p><p>Figure 3.23 Laboratory spectra of some ices which may occur on Titan, measured by Schmitt and</p><p>colleagues (Coustenis et al., 1999).</p><p>(up to about 4 or 5 km would not be impossible, according to the above analysis)</p><p>and the rest to a difference in their surface covering from the surrounding plains:</p><p>mountains covered with fresh snow of water or methane, perhaps? Methane turns</p><p>to ice at temperatures lower than 90 K, which is possible on top of a mountain, even</p><p>near the equator, where the lapse rate is about 1 degree K per km. Other possible</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch03</p><p>Observations of Titan from the Earth 71</p><p>candidates in their icy form can be found on Titan and might be responsible for</p><p>the observed images and spectra of the surface. Alternatively, methane rain might</p><p>wash the high ground free of the dark hydrocarbon mud that probably coats much</p><p>of Titan’s surface.</p><p>3.5 Ground-Based Observations and Cassini–Huygens</p><p>With the arrival of the Cassini–Huygens mission in 2004, one might ask if ground-</p><p>based measurements are still profitable. Even a powerful mission like Cassini can</p><p>use complementary information from the ground (or from space for that matter:</p><p>post-Cassini missions are already being discussed) and of course the Huygens probe</p><p>explored the surface at one single area. Ground-based measurements acquired simul-</p><p>taneously with the orbiter’s observations mean that the Cassini–Huygens data may</p><p>be extrapolated to the whole disk surface. In particular, ground-based observations</p><p>can provide measurements at solar phase angles not attained by Cassini (e.g., small</p><p>phase angles); complementary observations for regions that are unlit (all flybys</p><p>include global images and spectra before and after the close encounters; regions that</p><p>are unlit can be observed within a few days on the ground); data during times the</p><p>spacecraft is observing other objects to look for time-variable phenomena (cloud</p><p>formation and decay); and measurements at wavelengths that are not included in</p><p>spacecraft instrumentation. Campaigns performed during the descent of the Huy-</p><p>gens probe in Titan’s atmosphere and its landing on the surface on January 14, 2005,</p><p>connected the in situ observations by the probe with the large coverage provided</p><p>by high-technology ground-based runs. The observations covered radio telescope</p><p>tracking of the Huygens signal at 2040 MHz, studies of the atmosphere and surface</p><p>of Titan with spectroscopy and imaging, and some attempts to observe radiation</p><p>emitted during the Huygens probe entry into Titan’s atmosphere, which failed. The</p><p>probe’s radio signal was, on the other hand, successfully captured by a network of</p><p>terrestrial radio telescopes, allowing scientists like D. Luz and colleagues to recover</p><p>a vertical profile of wind speed in Titan’s atmosphere</p><p>from 140 km altitude down to</p><p>the surface. O. Witasse supervised these campaigns for ESA and in his own words:</p><p>“Ground-based observations brought new information on atmosphere and surface</p><p>properties of the largest Saturn’s moon…. The ground-based observations, both</p><p>radio and optical, are of fundamental importance for the interpretation of results</p><p>from the Huygens mission”.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>CHAPTER 4</p><p>Cassini–Huygens: Orbiting Saturn and Landing onTitan</p><p>“You thought I should find nothing but ooze” he said. “You laughed at my explo-</p><p>rations, and I’ve discovered a new world!”</p><p>H.G. Wells, In the Abyss</p><p>4.1 Introduction</p><p>We are still in the era of the preliminary exploration of the Solar System, seek-</p><p>ing to understand the basic nature of the planets and their satellites. Before</p><p>Cassini/Huygens, Saturn had been visited only by fast ‘flyby’ spacecraft that spent</p><p>only a few days in the vicinity of Titan and did not approach very closely, let alone</p><p>land. Cassini is the first artificial satellite of Saturn, following a successful orbit</p><p>insertion manoeuvre after a nearly seven-year journey. It has already spent several</p><p>years carrying out the first detailed survey of the planet, its rings and satellites, and</p><p>made multiple close approaches to Titan. An extended mission, recently approved,</p><p>will assure operations until 2010, provided the spacecraft survives.</p><p>Huygens, a large probe equipped with parachutes, was carried by Cassini to</p><p>Saturn and then ejected on a trajectory that resulted in a near-perfect landing on Titan.</p><p>On the way down, the probe provided the first direct sampling of Titan’s atmosphere</p><p>and the first detailed views of its surface, surviving for several hours after landing.</p><p>Huygens now holds the record for a man-made machine landing the farthest away</p><p>from the Earth. Together, Cassini and Huygens are leading to an explosion in our</p><p>knowledge of the ringed planet and its huge, cloudy moon.</p><p>Cassini–Huygens was always a very ambitious mission, conceived as a collabo-</p><p>ration between the United States and 17 European countries, working through their</p><p>space agencies, NASA and ESA. Although the mission’s objectives span the entire</p><p>Saturnian system, for Cassini (as for Voyager before it) Titan was a priority tar-</p><p>get and the mission was designed to address our principal questions regarding the</p><p>giant moon and its atmosphere. The spacecraft together are equipped with 18 sci-</p><p>ence instruments (12 on the orbiter and 6 on the probe), gathering both remote</p><p>sensing and in situ data. Cassini communicates through one high-gain and two low-</p><p>gain antennas. Because of the great distance from the Sun, electricity to drive the</p><p>72</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 73</p><p>spacecraft systems is provided by nuclear power, in the form of three radioisotope</p><p>thermoelectric generators fuelled by plutonium.</p><p>Weighing about 6 tons (actually 5,712 kg), the Cassini–Huygens spacecraft was</p><p>launched successfully on October 15, 1997, from the Kennedy Space Center at Cape</p><p>Canaveral at 4:43 a.m. EDT. Because of its massive weight, Cassini could not be sent</p><p>directly to Saturn but used the ‘gravity assist’technique to gain the energy required by</p><p>looping twice around the Sun. This allowed it to perform flybys of Venus (April 26,</p><p>1998 and June 24, 1999), Earth (August 18, 1999) and Jupiter (December 30, 2000).</p><p>Cassini–Huygens reached Saturn and performed a flawless Saturn Orbit Insertion</p><p>(SOI) at 10:30 p.m. EDT on June 30, 2004, becoming trapped forever in orbit like</p><p>one of Saturn’s natural moons. As well as imaging the atmosphere and surface, the</p><p>Huygens probe took samples of the haze and atmosphere as it descended. These</p><p>in situ measurements complement the remote-sensing measurements made from</p><p>the orbiter.</p><p>During its four-year nominal mission, the Cassini Orbiter makes about 40 flybys</p><p>of Titan, some as close as 1000 km (Voyager 1 flew by at 4400 km from the surface),</p><p>and takes a huge number of measurements with the visible, infrared, and radar instru-</p><p>ments. The instruments perform remote studies of Saturn, its atmosphere, moons,</p><p>rings and magnetosphere, measuring temperatures in various locations, plasma lev-</p><p>els, neutral and charged particles, compositions of surfaces, atmospheres and rings,</p><p>solar wind, and even dust grains in the Saturn system. Some perform spectral map-</p><p>ping and obtain high-quality images of the ringed planet, its moons and rings. The</p><p>final aim is to better understand the Saturnian system in a unique opportunity to look</p><p>for answers to many fundamental questions about the physical processes that rule</p><p>the origin and evolution of planets and moons, perhaps even — through the study</p><p>of Titan — to the conditions that give rise to life.</p><p>4.2 The Spacecraft and its Systems</p><p>Cassini–Huygens is the most complex interplanetary spacecraft ever built, and the</p><p>18 instruments on board the orbiter and the probe represent the most advanced</p><p>technological efforts of the countries involved in the endeavour. Cassini went into</p><p>orbit around Saturn in a complex, multiple trajectory scheme that allows it to conduct</p><p>nearly a decade of detailed studies of the Kronian system, while also carrying the</p><p>Huygens probe and assuring the relay of its data to Earth. The descent module</p><p>remained dormant for most of the trip to Saturn and only “woke” when, after release,</p><p>it reached the top of Titan’s atmosphere, deploying its parachutes and performing a</p><p>total of more than 5 hours of intensive measurements in the atmosphere and on the</p><p>surface.</p><p>Cassini carries three nuclear power sources or RTGs, which use the heat from the</p><p>radioactive decay of 33 kg of plutonium dioxide to produce 885Watts of power for the</p><p>spacecraft and its payload. In addition, 117 smaller radioisotope heater units — like</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>74 Titan: Exploring an Earthlike World</p><p>Figure 4.1 Detailed layout of the Cassini orbiter showing the main subsystems and the scientific</p><p>instruments (NASA).</p><p>the ones recently used on the Mars Exploration Rovers — are used to keep the</p><p>electronics at their operating temperatures. The generators and the heater units have</p><p>had a long history of safe and reliable performance as part of theVoyager and Galileo</p><p>missions.</p><p>While Cassini was still near the Earth, it used its two low-gain antennas to com-</p><p>municate with the operators on the ground.At the beginning of 2000, as the spacecraft</p><p>entered the cooler outer regions of our Solar System, it turned its high-gained antenna</p><p>toward the Earth and began to conduct all subsequent communications through it.</p><p>The large communications dish is 4 metres across, and capable of relaying data from</p><p>Saturn at rates as high as 140,000 bits per second. The on-board storage of data is</p><p>by solid-state memory with a total capacity of 3.6 Gbytes. Telecommunications use</p><p>a 20 W transmitter, operating at a frequency of 8.4 GHz (X-band).</p><p>Individually, the Cassini orbiter weighs 2,125 kg (4,685 pounds) and the</p><p>Huygens probe 320 kg (705 pounds). The whole thing is 6.7 m (22 feet) tall, 4 m</p><p>(13.1 feet) wide with a mass at launch of 5,712 kg (12,593 pounds), including the</p><p>probe, 335 kg of scientific instruments, and 3,132 kg of propellant. Thus, more than</p><p>half the spacecraft’s total mass at launch was fuel, more than the propellant mass of</p><p>the Voyager and Galileo spacecraft together. Half of it (about 1,500 kg) was required</p><p>just for the orbit insertion at Saturn; the rest is used for manoeuvres during the</p><p>course of the mission. Some of the propellant is hydrazine for the 16 small thruster</p><p>jets, part of the orientation and flight path control system of the spacecraft. This</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 75</p><p>keeps the communications dish pointed towards the Earth most of the time, with</p><p>small excursions from this alignment when necessary to point the scientific instru-</p><p>ments, which are bolted onto the spacecraft, at their targets. When major</p><p>changes</p><p>to Cassini’s trajectory are required, propulsion is provided by one of the two main</p><p>engines, which use monomethylhydrazine as the fuel and nitrogen tetroxide as the</p><p>oxidizer.</p><p>Energy from the RTGs is distributed to the other subsystems, including the sci-</p><p>entific instruments, as a 30V supply. The 12 engineering subsystems in the orbiter</p><p>control the spacecraft functions such as wiring, electrical power distribution, com-</p><p>puters, telecommunications, orientation and propulsion. The subsystems distribute</p><p>commands, and collect and format the data for transmission; they also control the</p><p>propulsion devices, such as the orbit insertion motor and the attitude control jets;</p><p>monitor and control the temperature everywhere on the spacecraft, and operate spe-</p><p>cial devices like that which separates the probe from the orbiter. The spacecraft is</p><p>controlled from the Earth through a sophisticated sequence of software commands.</p><p>A given command sequence can operate on one of the computers on board for more</p><p>than a month without interference from ground controllers.</p><p>The shape of the spacecraft was designed to accommodate the requirements of</p><p>the various instruments and communication systems. The main body of the orbiter</p><p>is cylindrical, in three parts or ‘modules’, two for equipment, with the propulsion</p><p>module in the middle, capped by the high-gain antenna. Four booms protrude: the</p><p>magnetometer is mounted on an 11 m long boom that extends outward from the</p><p>spacecraft, as do three 10 m antenna booms. To protect Cassini against the extreme</p><p>conditions in space and keep the computers, mechanical devices and electronic</p><p>systems safe, the spacecraft and its instruments are covered with a thick shiny amber-</p><p>coloured or black thermal blanket. Mylar is incorporated into this material to protect</p><p>against micrometeorites that could hit the spacecraft. The onboard computers are</p><p>designed to survive even solar flares, which can increase the interplanetary activity</p><p>a thousand times. Cassini has some 22,000 wire connections and more than 12 km</p><p>of cables linking the instruments and subsystems. Inside the spacecraft, insulating</p><p>blankets are used, including kapton, to protect the instruments from the dust and to</p><p>retain the heat.</p><p>The Huygens probe is 2.7 m (8.9 feet) in diameter and weighs 320 kg</p><p>(705 pounds), including 43 kg of scientific instruments. For power, Huygens takes</p><p>advantage of the orbiter while onboard via an umbilical cable. After separation,</p><p>power was provided by five 23-cell lithium sulphite batteries, which were charged</p><p>from the orbiter power supply before release of the probe. Data was radioed to</p><p>the orbiter, for relay to Earth, at a rate of 8 kbps, using two separate transmitters,</p><p>each with its own antenna. The link was one way only; all of the functions of</p><p>the probe were automatic once it was released from the orbiter and the ‘umbili-</p><p>cal’ connection severed. After separation from Cassini, Huygens flew for 22 days</p><p>through the cold regions around Saturn, using 35 heater units to keep the equipment</p><p>operational.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>76 Titan: Exploring an Earthlike World</p><p>Figure 4.2 An exploded view of the Huygens probe showing the main subsystems and the experiment</p><p>platform that carries the scientific instruments (ESA).</p><p>electrostatic</p><p>dischargers (3x)</p><p>pressure and</p><p>temperature</p><p>sensors</p><p>parachute container</p><p>electrical</p><p>sensors (2x)</p><p>heat-shield separation</p><p>mechanisms (3x)</p><p>telemetry</p><p>antennae (2x)</p><p>aperture</p><p>for imaging</p><p>radar altimeter</p><p>antenna (4x)</p><p>inlets for</p><p>scientific instruments</p><p>spin vanes</p><p>Figure 4.3 The Huygens probe under its parachute. The HASI booms are deployed, and the DISR</p><p>sensor head can be seen. To the right, the disposition of the GCMS, ACP and SSP inlets is shown</p><p>(ESA).</p><p>Huygens consists of the descent module, a front shield, an aft cover and a spin-</p><p>eject device, the latter being part of the support structure on the orbiter which</p><p>propelled the probe toward Titan and caused it to spin. The front shield, 2.7 m in</p><p>diameter, consisted of a special thermal protective material called AQ60. This was</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 77</p><p>also used in the aft cover, which ensures a slow and safe descent. During entry</p><p>into Titan’s atmosphere, the front shield had to sustain temperatures above 1,500 ◦C</p><p>(2,700 ◦F), but thanks to the layers of insulation, the equipment inside the probe</p><p>remained at temperatures below 50 ◦C (122 ◦F).</p><p>The probe’s main body consists of two platforms and an aluminium shell. The</p><p>central experiment platform also carries the electrical subsystems, which manage</p><p>the data, keep time, switch on the probe prior to entry, assure telemetry, etc. The top</p><p>platform above it is used to stow the parachute and carries the transmitter that sent</p><p>the data to Cassini.</p><p>4.3 Scientific Objectives</p><p>Cassini–Huygens left Earth equipped with a set of interrelating instruments designed</p><p>to address many of the most important questions about the complex Saturnian sys-</p><p>tem. The general scientific objectives of the mission are concisely summed up in</p><p>Figure 4.4 Lift-off for Cassini on a Titan IVB/Centaur at 4:43 a.m., October 15, 1997 (NASA/JPL).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>78 Titan: Exploring an Earthlike World</p><p>NASA’s official statement of its purpose: to investigate the physical, chemical, and</p><p>temporal characteristics of Titan and Saturn: its atmosphere, rings, icy satellites and</p><p>magnetosphere. For Saturn, this includes discovering the sources and the distribu-</p><p>tion of lightning, by looking visually with television for flashes and listening at radio</p><p>and other frequencies for electrostatic discharges and ‘whistlers’. It also includes</p><p>infrared remote sensing of the temperature field, cloud properties, and the com-</p><p>position of the atmosphere. None of these properties remains constant — Saturn’s</p><p>atmosphere is a very dynamic environment — so repeated measurements are nec-</p><p>essary, to obtain not just the basic atmospheric structure parameters but also some</p><p>indication of their variability. Of particular interest is the global wind field, includ-</p><p>ing its wave and eddy components. Winds are hard to measure directly, but can be</p><p>computed using observations of the movement of cloud features, and calculations</p><p>based on pressure gradients inferred from temperature measurements. This gives</p><p>the dynamics of the observable regions in the region at and above the cloud tops,</p><p>so the internal structure and rotation of the deep atmosphere must be inferred by</p><p>extrapolation from the layers above, while higher levels can be studied by ultraviolet</p><p>spectroscopy and the fields and particles experiments to understand the diurnal vari-</p><p>ations and magnetic control of the charged particle concentrations in the low-density</p><p>region.</p><p>A list of objectives for Titan was drawn up before the mission was launched.</p><p>It will be many years after the observations end before we can finally say in detail</p><p>how well they have all been addressed, but well into the mission new questions have</p><p>already arisen and we already have some ideas about what still needs to be done by</p><p>the extended Cassini or by new and even more sophisticated missions. The initial</p><p>list reads:</p><p>• Determine the abundances of atmospheric constituents (including any noble</p><p>gases).</p><p>• Establish isotope ratios for abundant elements, which will help constrain scenarios</p><p>of formation and evolution of Titan and its atmosphere.</p><p>• Observe vertical and horizontal distributions of trace gases.</p><p>• Search for even more complex organic molecules.</p><p>• Investigate energy sources for atmospheric chemistry.</p><p>• Model the photochemistry of the stratosphere.</p><p>• Study the formation, composition and distribution of aerosols.</p><p>• Determine the winds and map the global temperatures, investigate cloud physics,</p><p>general circulation, and seasonal effects in Titan’s atmosphere.</p><p>• Search for lightning discharges.</p><p>• Determine the physical state, topography, and composition of the surface with</p><p>in situ measurements at</p><p>different locations of the disk.</p><p>• Infer the internal structure of the satellite.</p><p>• Investigate the upper atmosphere, its ionisation, and its role as a source of neutral</p><p>and ionised material for the magnetosphere of Saturn.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 79</p><p>4.4 The Long History of the Cassini–Huygens Mission</p><p>The outer planets have always been difficult targets for exploration, primarily</p><p>because of their vast distances (Saturn is about 30 times as far from Earth as Mars,</p><p>for example) that can only be spanned by using powerful launch vehicles. Another</p><p>reason is the need to use nuclear power to provide electricity to run the spacecraft</p><p>and its scientific instruments since, at Titan’s distance, the Sun is just a big star and</p><p>the use of solar cells is impractical: it would take an array the size of a tennis court.</p><p>Finally, the journey takes a long time, and the operations costs (such as tracking</p><p>station time, or keeping teams together who built and understand the spacecraft, and</p><p>who could detect and solve problems as they arise) as well as the need for reliable</p><p>components and redundant systems on board the spacecraft, all push the cost up</p><p>until it is counted, not in millions, but in billions of dollars. In the future, when</p><p>the initial surveys are over, exploration of the outer Solar System will be feasible</p><p>with smaller spacecraft, with fewer, more focussed objectives, especially as new</p><p>technology comes along so that small, inexpensive missions can do as much or more</p><p>as big spacecraft do now. We may never see big missions to the outer Solar System</p><p>like Cassini again.</p><p>Fortunately, Cassini promises a lot to be going on with. Before it arrived, only</p><p>glimpses of Saturn and Titan had been obtained from the Pioneer and Voyager fast</p><p>flybys of the 1970s and 1980s. As we saw in Chapter 2, these went tearing past</p><p>Saturn in a couple of days, and past Titan in just a few hours. As an orbiter/probe</p><p>mission, Cassini’s heritage is not so much the Voyager missions, which recently</p><p>ended their long odyssey in the Solar System, but the Galileo mission to the Jovian</p><p>system. Galileo was the first artificial satellite of Jupiter, and it deployed the first</p><p>probe into any outer solar system atmosphere.</p><p>From its initial conception to the completion of the mission in 2010, the Cassini–</p><p>Huygens mission is an achievement that spans almost 30 years. Originally known</p><p>as Saturn Orbiter–Double Probe (abbreviated SOPP), in the early days of its design</p><p>when it was to have carried a probe for Saturn itself as well as one for Titan, Cassini</p><p>was conceived as the Saturnian version of Galileo. Table 4.1 summarises the major</p><p>milestones in the development and implementation of Cassini and Huygens.As early</p><p>as 1982, a working group composed of the Space Science Board of the National</p><p>Academy of Science in the U.S. and the Space Science Committee of the European</p><p>Science Foundation was formed to study possible modes of cooperation between</p><p>the U.S. and Europe for their mutual scientific, technological and industrial benefits.</p><p>Among other possible projects, the group proposed a Saturn Orbiter–Titan Probe</p><p>mission in which, it was suggested at this time, Europe would provide the orbiter</p><p>while their colleagues in the USA would develop the probes.</p><p>This proposal was submitted both to the European Space Agency and to the</p><p>National Aeronautics and Space Administration in the USA, and by 1985 the joint</p><p>ESA/NASA assessment of the Saturn Orbiter–Titan Probe mission was completed</p><p>and under development by both agencies in Phase A. This is the stage at which</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>80 Titan: Exploring an Earthlike World</p><p>Table 4.1 Milestones in the history of Cassini–Huygens and timeline for its trip to Saturn.</p><p>1982 Joint Working Group of European Science Foundation and US</p><p>National Academy of Sciences proposes a Saturn</p><p>Orbiter–Titan Probe mission</p><p>1983 US Solar System exploration Committee recommends a Titan</p><p>Probe and Radar Mapper for NASA’s core programme</p><p>1984–5 Joint NASA-ESA Assessment Study of Saturn Orbiter–Titan</p><p>Probe mission</p><p>1987 ESA Science Programme Committee approves Cassini for</p><p>a Phase A study</p><p>1987–88 NASA develops Mariner Mark 2 concept for Cassini and</p><p>Comet Rendezvous-Asteroid Flyby (CRAF)</p><p>1988 ESA selects Cassini mission, names probe Huygens</p><p>1989 US Congress approves CRAF and Cassini</p><p>1992 Cassini restructured and rescheduled; CRAF cancelled</p><p>1995 US House Appropriations Committee targets Cassini for</p><p>cancellation</p><p>1996 Spacecraft and instruments begin testing</p><p>April 1997 Cassini shipped to Cape Canaveral</p><p>October 15, 1997 Launch</p><p>April 1998 First Venus gravity assist flyby</p><p>June 1999 Second Venus gravity assist flyby</p><p>August 1999 Earth gravity assist flyby</p><p>December 2000 Jupiter gravity assist flyby</p><p>December 2001 First gravitational wave experiment</p><p>June 2004 Phoebe flyby at 52,000 km</p><p>July 2004 Spacecraft goes into orbit around Saturn</p><p>December 25, 2004 Release of Huygens Probe on a trajectory to enter Titan’s</p><p>atmosphere</p><p>January 14, 2005 Huygens returns data as it descends through Titan’s atmosphere</p><p>and reaches the surface; Orbiter begins tour of the Saturn</p><p>system</p><p>July 1, 2008 Nominal end of mission</p><p>2010+ End of extended mission</p><p>detailed designs are worked out and a reliable budget established, prior to a final</p><p>commitment to build the hardware. Cassini was developed along with another</p><p>ambitious mission, the Comet Rendezvous/Asteroid Flyby (CRAF), both funded in</p><p>1989 by the U.S. Congress. The all-American SOPP had eventually evolved into</p><p>Cassini/Huygens, now composed of an American orbiter and a European descent</p><p>probe — sometimes referred to as ‘Inissac’ at the time, because it was the origi-</p><p>nal Cassini concept backwards! This made it the first truly international planetary</p><p>mission, in addition to its other breakthroughs. NASA and ESA launched a call for</p><p>instruments for both missions, but, in 1992, CRAF was cancelled and Cassini almost</p><p>was too. In the end, the international collaboration with its contractual obligations</p><p>saved Cassini, but it had to be restructured to reduce the cost by simplifying the</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 81</p><p>spacecraft capabilities. The articulated instrument platform was abandoned and the</p><p>instruments hard-mounted to the spacecraft, which now had to turn in its entirety in</p><p>order to point at the target. Other losses include tape recorders, mechanical gyro-</p><p>scopes and the steerable antenna on the probe, which was replaced with a cheaper</p><p>fixed one. The total cost of the mission was then about $3.26 billion, of which</p><p>$2.6 billion was to be the U.S. contribution while the rest, mainly for the probe and</p><p>amounting to an estimated $660 million, came from Europe. At about this time,</p><p>ESA gave a separate name to the probe, and the combined mission was then called</p><p>Cassini–Huygens.</p><p>Trouble brewed for Cassini at an early stage when NASA developed its “faster,</p><p>better, cheaper” strategy, which sought to replace large expensive missions with</p><p>small ones that did the same thing. Just how this miracle was to be brought about was</p><p>never fully demonstrated, but suddenly Cassini/Huygens was no longer fashionable,</p><p>in the US at least, as a terrestrial ambassador to the planets. The NASAAdministrator</p><p>took to referring to Cassini as ‘Battlestar Galactica’, and it was not meant as a</p><p>compliment. However, it should not be forgotten that the research and development</p><p>for Cassini–Huygens has brought forth new technologies that, while not satisfying</p><p>NASA’s new motto certainly provided innovations that found their way into the new</p><p>low-cost ‘Discovery’class missions, such as Mars Pathfinder. While the instruments</p><p>on board Cassini and Huygens used the knowledge accrued from Pioneer, Voyager,</p><p>and Galileo, they also introduced newer technology to address many of the known</p><p>mysteries of the Saturn system much better than the earlier probes could, even if</p><p>they had not gone by so quickly and</p><p>so far away. For example, CIRS, the infrared</p><p>spectrometer on Cassini, is a much more advanced version of IRIS on Voyager, with</p><p>an optimal spectral resolution that, at 0.5 cm−1, is an order of magnitude better</p><p>than IRIS, and with better sensitivity through the use of cooled infrared detectors.</p><p>It obtains much better spatial resolution, including an improvement of a factor of</p><p>2–3 over Voyager in the vertical (height) dimension, due to its better optics and 6</p><p>times closer approach to Titan thanVoyager, and much better coverage from multiple</p><p>encounters on different orbits of Saturn.</p><p>Somehow Cassini survived; the threat was always political rather than scientific</p><p>or technical, and did not find much of an echo with NASA’s European partner.</p><p>Huygens of course is completely innovative: the first probe from Earth to actually</p><p>enter Titan’s atmosphere and land on the surface, and the most distant lander in our</p><p>Solar System. We now have in situ temperature-pressure-density profiles, details of</p><p>the atmospheric composition as a function of height, and direct data on the surface</p><p>conditions and its enigmatic appearance.</p><p>While NASA produced the main Cassini orbiter spacecraft and ESA provided the</p><p>Huygens probe, the Italian Space Agency (ASI) was responsible for the spacecraft’s</p><p>radio antenna and portions of three scientific instruments. In the United States,</p><p>NASA’s Jet Propulsion Laboratory in Pasadena, California, where the Cassini orbiter</p><p>was designed, assembled, and tested, manages the mission. In Europe, an industrial</p><p>team from all over the continent created the Huygens probe, with contributions</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>82 Titan: Exploring an Earthlike World</p><p>VENUS 1 FLYBY</p><p>26 APR 1998</p><p>VENUS 2 FLYBY</p><p>24 JUN 1999</p><p>VENUS</p><p>TARGETING</p><p>MANEUVER</p><p>3 DEC 1998</p><p>LAUNCH</p><p>15 OCT 1997</p><p>EARTH FLYBY</p><p>18 AUG 1999</p><p>JUPITER FLYBY</p><p>30 DEC 2000</p><p>S</p><p>A</p><p>T</p><p>U</p><p>R</p><p>N</p><p>’S</p><p>O</p><p>R</p><p>B</p><p>IT</p><p>JU</p><p>P</p><p>IT</p><p>E</p><p>R</p><p>’S</p><p>O</p><p>R</p><p>B</p><p>IT</p><p>29.4</p><p>Y</p><p>E</p><p>A</p><p>R</p><p>S</p><p>11.8</p><p>Y</p><p>E</p><p>A</p><p>R</p><p>S</p><p>SATURN ORBIT</p><p>INSERTION</p><p>1 JUL 2004</p><p>SUN</p><p>Figure 4.5 Cassini’s trajectory between Earth and Saturn, showing the close encounters with Venus,</p><p>Earth and Jupiter which were required to give ‘gravity assisted’ boosts to the spacecraft.</p><p>from France, Germany, Italy, UK, Ireland, Sweden, Spain, Denmark, Switzerland,</p><p>Belgium, Austria, Finland, and Norway. Communications with Cassini are carried</p><p>out through the stations of NASA’s Deep Space Network in California, Spain and</p><p>Australia, with the data from the Huygens probe forwarded from the DSN to an ESA</p><p>operations complex ESOC in Darmstadt, Germany.</p><p>The scientific instruments aboard the mission were prepared in numerous</p><p>European and American laboratories, in many cases with a strong collaboration</p><p>between scientists and engineers on both sides of the Atlantic working on an indi-</p><p>vidual instrument. European scientists lead two experiments on NASA’s orbiter</p><p>and participate in all of them, while US-led teams supplied two instruments in</p><p>ESA’s Huygens probe and American experts contribute to three others. The human</p><p>involvement in this enterprise is vast: more than 5,000 people worked on some por-</p><p>tion or other of the mission, and more than 260 scientists in the U.S. alone have</p><p>contributed to the project. All in all, a fine example of international space science</p><p>collaboration.</p><p>4.5 Departure for the Saturnian System</p><p>More than 200 scientists, engineers and others who had been involved in the develop-</p><p>ment of this ambitious space mission gathered at Cape Canaveral Air Force Station,</p><p>Florida to see it off on its seven year, 3.5 billion km trek to the realm of Saturn. They</p><p>gathered first during the early morning of October 13, 1997 to watch from a location,</p><p>as close as the safety officers would allow and giving a perfect view, 5.5 km away</p><p>from the launching site. Unfortunately on that day there were strong winds blowing</p><p>at altitudes of about 15 km, just where the detachment of the booster rockets from</p><p>the spacecraft occurs. These have to fall to the ground in a predictable way, and land</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 83</p><p>Figure 4.6 Cassini’s tour of the Saturnian System is arranged to make it possible to have a close</p><p>approach to Titan on most of the orbits. This view shows the fifth such encounter, which took place</p><p>on March 31, 2005, to scale (NASA/JPL).</p><p>safely in the ocean, so the launch had to be put back to wait for better weather. Only</p><p>two days later, a little before 5 o’clock on the morning of Wednesday, October 15,</p><p>everyone gathered again and this time they watched in awe as the heavy spacecraft</p><p>lifted off on its long journey.</p><p>The designated area for the launch-watchers is quite beautiful, part of a nature</p><p>reserve, and on that warm Wednesday morning the sky was clear, with a huge full</p><p>moon. For those who had worked and waited long years for this occasion, it was a</p><p>magical moment. Screens set in front of the benches allowed a direct view into the</p><p>control room, and loud speakers gave us all an opportunity to follow the procedures</p><p>as all the systems went “GO” to indicate readiness for departure. The launch system</p><p>was a powerful Titan IVB, with a Centaur upper stage. The coincidence turned out</p><p>to be a good omen. The countdown had us all hypnotized and we held our breaths as</p><p>we came to “lift-off”. Many of us had never experienced a launch before and it was</p><p>very much a personal affair, but few if any of the people gathered there managed to</p><p>escape the rise in emotion.</p><p>There followed a loud roar and a bright flame along the ground, as the Cassini</p><p>spacecraft, carrying the Huygens probe, began to rise slowly towards the sky.</p><p>At 15 km, as planned, the boosters were discarded and the spacecraft crossed the</p><p>dark sky like a bright comet and set off in its course to Saturn. The first measure-</p><p>ments showed that the launch and the trajectory were precisely according to plan.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>84 Titan: Exploring an Earthlike World</p><p>Holding hands or arms, many of us stood there and watched what soon became a</p><p>splendid night show, as Cassini pushed its way through a cloud, a perfect image</p><p>of power and phantasmagoria at the same time, offering a fireworks-like display.</p><p>In spite of all the reassuring comments from the loudspeakers that were positioned</p><p>around the site to relay the words from the mission controllers, it took the whole</p><p>two hours before we could stop fretting that something might go wrong, dry our</p><p>eyes and wish our “baby” an uneventful journey. On the way back to the hotels, we</p><p>all agreed that this was a memorable event, something to cherish on our memories</p><p>for the rest of our lifetimes.</p><p>4.6 Journey to Saturn and Orbit Insertion</p><p>Following the launch from Cape Canaveral, the combined orbiter and probe space-</p><p>craft first travelled in a direction away from Saturn, into the inner Solar System,</p><p>where it executed gravity-assist flybys of Venus in late April 1998 and again in</p><p>June 1999, followed by one of Earth, in mid-August 1999 and one of Jupiter in</p><p>late December 2000. These manoeuvres used the gravitational fields of each target</p><p>planet to increase the spacecraft’s velocity (the “slingshot” effect), giving it the boost</p><p>needed to reach Saturn in “only” 6.4 years.</p><p>Without the assistance of Venus’ gravity, such a large spacecraft as Cassini could</p><p>not be injected into the interplanetary trajectory towards the outer Solar System</p><p>at all, at least not without a much larger and more expensive launch vehicle. With</p><p>the multiple flyby approach, the total mass that can get to Saturn is sufficient to</p><p>include a large science payload, and enough fuel to make a number of manoeuvres</p><p>while in orbit at Saturn. Without these in-orbit motor firings, Cassini would not be</p><p>able to alter its path around Saturn to make repeated encounters with the satellites,</p><p>including Titan.</p><p>The spacecraft completed nearly three-quarters of an orbit around the Sun prior</p><p>to the first Venus flyby. The perihelion of this initial orbit was 0.676 AU. Only</p><p>limited science data collection was</p><p>allowed during the Venus flyby, in order not to</p><p>put spacecraft and instruments at risk while they were so close to the Sun, since</p><p>they were designed to survive this hot environment, but mostly not to operate in</p><p>it. Some data collection for calibration was incorporated into the Earth flyby, but</p><p>science observations did not begin in earnest until the time of the Jupiter flyby, the</p><p>fourth and final gravity assist, in December 30, 2000. Cassini arrived at the Jovian</p><p>system while the Galileo mission was still in place, and so could take advantage of</p><p>the two different vantage points to gain improved knowledge on the shape of the</p><p>Jovian magnetosphere and its interaction with the solar wind, to study the aurorae,</p><p>and to check out the performance of all of the instruments.</p><p>Upon arrival at Saturn on June 30, 2004, nearly seven years after leaving Earth,</p><p>the main rocket engine was turned to face the direction in which it was travelling,</p><p>and fired to slow Cassini down. The spacecraft approached Saturn from below the</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 85</p><p>ring plane, flying through the gap between the F and G rings, about 150,000 km</p><p>from the centre of Saturn and just less than 2 hours before the spacecraft’s clos-</p><p>est approach to the giant planet. The spacecraft was oriented with the high-gain</p><p>antenna used as a shield to provide protection from any small particles that could</p><p>present a threat during the ring crossing. The main engine burn began shortly after</p><p>the ring crossing and ended 97 minutes later. Had the motor failed, or not fired for</p><p>long enough, the spacecraft would have hurtled past Saturn and eventually gone</p><p>into a distant orbit around the Sun. Orbit insertion was cause for anxiety for many</p><p>of us who lived through those 97 minutes watching and hoping that all would</p><p>go well.</p><p>In fact, all did go well and the orbiter, its scientific instruments now operating,</p><p>began its complex dance around the giant planet. The closest approach (periapsis)</p><p>altitude during Saturn orbit insertion was 0.3 Saturn radii, which is the nearest it gets</p><p>to the planet during the whole four-year tour, consisting of some 60 orbits around</p><p>the planet. The arrival period also provides the first and best opportunity to observe</p><p>Saturn’s rings, and the images returned during this event were breathtaking and an</p><p>enormous input for ring science.</p><p>4.7 Huygens Descends ontoTitan</p><p>The orbits are designed so that most of them involve passing close to Titan, when</p><p>observations could be made using the instruments onboard the orbiter. For the</p><p>first two of these, Cassini was still carrying the Huygens probe. After the second</p><p>encounter, which took place on December 13, 2004, the main rocket motor was</p><p>fired again to raise the periapsis and to bring the orbiter and probe, still combined,</p><p>into an orbit that passes close to Titan. On December 17, 2004, small onboard jets</p><p>were fired to fine-tune Cassini’s flight path to align it on a trajectory that would</p><p>actually result in a collision with Titan. This probe targeting maneuver was essential</p><p>since Huygens had no motors of its own and, once released, would simply follow</p><p>the trajectory on which it had been placed by Cassini. Conditions were now right</p><p>for the release of the probe from the mother ship that had brought it all the way</p><p>to Saturn.</p><p>Final commands were sent to Huygens on December 21, 2004 including the</p><p>setting of the Mission Timer Unit, a sophisticated alarm clock that would be activated</p><p>about 5 hours before Huygens reached an altitude of 1270 km above Titan’s surface.</p><p>On December 25, 2004 at 02:00 UTC the Spin/Eject Device separated Huygens from</p><p>Cassini with a relative speed of approximately 0.35 metres per second. This event</p><p>included the firing of the small explosive pyrobolts, engagement of the separation</p><p>push-off springs, ramps and rollers and the separation of the electrical connectors,</p><p>all in the space of approximately 0.15 seconds. Wrapped in its heat shield, Huygens</p><p>was set spinning at seven revolutions per minute in order to remain stable as it</p><p>approached its target. The landing was targeted for a southern-latitude site on the</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>86 Titan: Exploring an Earthlike World</p><p>Figure 4.7 14 January 2005: The descent sequence followed by Huygens onto Titan. Using three</p><p>parachutes, as well as the drag from the heat shield, the probe fell 1250 km in about two and a half</p><p>hours, its velocity decreasing from 6 km per sec at the top of the atmosphere to about 5 metres per sec</p><p>at impact (ESA).</p><p>dayside of Titan, near the large continental feature that had, at that time, not yet been</p><p>named Xanadu. The probe entry angle into the atmosphere was controlled within a</p><p>few degrees of a relatively steep 65◦ with respect to the vertical, this being the best</p><p>compromise between a short descent and maximum deceleration by drag, designed</p><p>to give Huygens the best opportunity to reach the surface alive.</p><p>The release of such a large mass had a considerable effect on the motion of the</p><p>Cassini orbiter it left behind. Cassini also had to be re-targeted to avoid following</p><p>Huygens into Titan’s atmosphere, while remaining close to the probe’s atmospheric</p><p>entry point so it could achieve the radio relay link geometry. On December 28, an</p><p>orbit deflection maneuver corrected the Cassini trajectory to fly past Titan at an</p><p>altitude of 60,000 km and delayed the closest approach to until around two hours</p><p>after Huygens reached the entry interface. The relative position of Cassini, Huygens</p><p>and Titan enabled a theoretical maximum conversation between orbiter and probe</p><p>of 4 hours 30 minutes.</p><p>The electronic alarm clock switched Huygens on, well before it reached Titan’s</p><p>upper atmosphere, on January 14, 2005 at 11:04 UTC. A few hours later, friction</p><p>with the atmosphere acting on its heat shield began to cause the 300 kg probe to</p><p>‘aerobrake’ from its impact velocity of around 21,600 km/h (6 km/s) to less than</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 87</p><p>1,400 km/h at 180 km altitude, experiencing forces of up to 25 g and an entry temper-</p><p>ature of more than 12,000 ◦C (21,600 ◦F). Once the speed had fallen to about Mach</p><p>1.4 (290 km/h at 170 km altitude), the shield was released and the main parachute,</p><p>8.3 m in diameter, was opened. The parachute was discarded 15 minutes later.</p><p>The people gathered in the ESA Control Centre at Darmstadt that January 14,</p><p>2006 received the first signal that Huygens is alive from an Earth-based radio tele-</p><p>scope, as they waited for the first data and the first images to be transmitted by</p><p>Cassini. Suspense was high because it takes about 67 minutes for the data from</p><p>Cassini to get to the Earth, and the first stream of data, due to be transmitted through</p><p>Channel A, failed because it was not turned on due to a programming error. How-</p><p>ever, after a few extra minutes, Channel B took over and the measurements began to</p><p>come through, including the first images returned by the cameras of DISR while still</p><p>falling through the atmosphere, soon stitched together to make the first panoramas</p><p>of a fantastic and long-anticipated landscape.</p><p>The main parachute was discarded automatically after 15 minutes of descent to</p><p>make sure that the orbiter would not pass below the horizon as seen from the landing</p><p>site before all probe touched down, and had transmitted at least an hour’s worth of</p><p>surface data if it survived the impact. It was also necessary to find a compromise</p><p>descent speed between shortening the duration, so Huygens and its payload did not</p><p>freeze, on one hand, and achieving a sufficiently soft landing, so that it had a chance</p><p>of surviving on the surface, on the other. Tests in which prototypes of Huygens</p><p>were dropped through Earth’s atmosphere allowed practical estimates to be added</p><p>to theoretical calculations of the expected descent time; in fact, the descent from</p><p>atmospheric entry to landing took 2 hr 27 m 50 s.</p><p>At around 125 km, falling at a speed</p><p>environments on the most Earthlike planets, Mars and</p><p>Venus. It made the surprise all the greater when we found, only relatively recently,</p><p>that there is another place in the Solar System that does have an Earthlike atmo-</p><p>sphere, albeit a very cold one. Strangely, this is not on any of the planets, but on a</p><p>satellite, the only one among the dozen or so very large moons in the Sun’s family</p><p>for which this is true.</p><p>xiii</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>xiv Titan: Exploring an Earthlike World</p><p>Titan, Saturn’s biggest moon, and (by a narrow margin) the second in size among</p><p>the satellites in our Solar System, has been known for a long time to have a substantial</p><p>atmosphere. The Catalan astronomer José Comas Solà claimed in 1908 to have</p><p>observed limb darkening on Titan. This is the effect whereby the solar light reflected</p><p>back to Earth by Titan’s limb shows a stronger attenuation than that from its centre,</p><p>which usually implies the presence of a substantial atmosphere. Confirmation came</p><p>from spectroscopic observations by Gerard Kuiper in the 1940s, but it was not until</p><p>the Voyager 1 spacecraft visited Titan in 1980 that the composition and the surface</p><p>pressure were found to be so similar to those on Earth. Furthermore, complex organic</p><p>chemistry is active there, producing multiple layers of orange-coloured haze, which</p><p>render the atmosphere opaque in the optical range of wavelengths. It took a long</p><p>time and a lot of effort to get a glimpse of the surface, and we still do not know</p><p>today exactly what its composition is, except that the crust beneath must be mainly</p><p>water ice.</p><p>To address the many questions asked about Titan over the centuries since its</p><p>discovery, a series of space probes has been developed and dispatched towards this</p><p>intriguing body. Pioneer 11 arrived first, in 1979, followed by Voyager 1 a year</p><p>later. The scientific understanding of Titan as a planet-like object that emerged from</p><p>the analysis of Voyager data was improved by ongoing ground-based observations,</p><p>using increasingly more powerful optical and spectroscopic techniques, such as radar</p><p>and adaptive optics, and advanced platforms on Earth-orbiting space observatories.</p><p>At the same time as new data were acquired and studied, new theoretical models</p><p>were being developed to account for these observations, new theories proposed and</p><p>debated, and old or repeated measurements re-analysed. All of these have yielded</p><p>formidable results, and posed new questions, over the past several years.</p><p>The latest envoy to Titan, a large and sophisticated international space mission</p><p>called Cassini/Huygens, was launched in the year 1997. It arrived in July 2004, and</p><p>started gathering new measurements from an orbit around Saturn that was designed</p><p>to permit multiple Titan encounters. The Huygens probe descended in Titan’s atmo-</p><p>sphere on January 14, 2005 and recorded breathtaking data, revealing an astounding</p><p>new world, and the most distant one to be landed on by a human-made machine.</p><p>The aim of this book is to bring together a general overview of our current</p><p>understanding of all aspects of Titan, at a comprehensive and scientific level, but in</p><p>terms basic enough to be mostly accessible to the non-specialist as well. We begin</p><p>with a history of Titan studies, covering Earth-based facilities and programmes and</p><p>leading into the early space missions. The Voyager mission, in particular, engulfed</p><p>the scientific community with large amounts of data nearly twenty years ago, some</p><p>of which is still being analyzed. The torrent of data from Cassini, with 40 times</p><p>as many flybys, not to mention the Huygens in situ science, dwarfs even that from</p><p>Voyager. We describe how, along with concurrent ground-based observations and</p><p>theoretical modelling, Voyager, and now Cassini, have revolutionised our view of</p><p>the outer Solar System. Titan’s place in the Saturnian system, its structure and com-</p><p>position, its unique atmosphere, and the extent to which it really resembles the Earth,</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>Prologue xv</p><p>are the main themes. As a secondary objective, we hope to show the reader that</p><p>the close look astronomers have been afforded since 2004 with their very expen-</p><p>sive Cassini/Huygens space mission to the Saturnian system is paying dividends.</p><p>An up-to-date synthesis of current Titan knowledge, and the remaining big ques-</p><p>tions, should give everyone the possibility to better appreciate the Cassini/Huygens</p><p>discoveries.</p><p>We have tried to cover most of what is known about Titan today, and some</p><p>informed speculation as well, while keeping the account as simple as possible.</p><p>Our experience tells us that interest in Titan and its exploration, as evidenced by</p><p>the huge popular reaction to the Cassini space mission, extends far beyond the</p><p>small international group of professional planetary scientists. Thus, we have tried</p><p>to make the book accessible to all, assuming only a basic familiarity with physics</p><p>and astronomy. In the chapters that deal with more technical subjects, it has been</p><p>necessary to use some more complicated concepts and words, in order not to leave</p><p>out important knowledge or key questions. Where scientific terminology could not</p><p>be avoided, the more specialised terms are defined in the text, in footnotes, and</p><p>in the glossary, for the general reader. We hope that interested non-scientists will</p><p>persevere as far as they can, and then move on to the next chapter where a more</p><p>basic level is again resumed. For those who, on the other hand, want more detail, or</p><p>wish to read about the original research of which this book is a summary, we include</p><p>references and guides to further reading in a comprehensive bibliography at the end</p><p>of the book.</p><p>Finally, we would like to acknowledge friends and colleagues who have helped</p><p>with the text, either directly or by communicating their work on Titan. Particularly</p><p>valuable inputs and comments on the draft manuscript came from David Luz, Patrick</p><p>Irwin, Emmanuel Lellouch, Ralph Lorenz, Conor Nixon, Robert Samuelson,Tetsuya</p><p>Tokano, Daniel Gautier and Thomas Widemann. For help with illustrations, proof-</p><p>reading and other helpful comments, we further thank Iannis Dandouras, Mathieu</p><p>Hirtzig, Tom Krimigis, Panagiotis Lavvas, Alberto Negrão, Hasso Niemann and</p><p>Véronique Vuitton. We are grateful to the EUROPLANET Consortium for funding</p><p>part of our meetings to work on this project, and to the Observatoire de Paris for</p><p>access to historical material.We are also grateful to many Cassini–Huygens scientists</p><p>who provided guidance and information on their work. In particular we thank Dennis</p><p>Matson and Jean-Pierre Lebreton, Cassini–Huyens project managers who kindly</p><p>read the book in advance and wrote a foreword.</p><p>The original figures in this volume were drawn and montages created by</p><p>Dr. D. J. Taylor, to whom we extend our deepest gratitude.</p><p>Athena Coustenis</p><p>Fred W Taylor</p><p>Paris and Oxford, April 2008</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>This page intentionally left blankThis page intentionally left blank</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>Foreword</p><p>Titan is Saturn’s largest moon. It was discovered by the Dutch Astronomer Christian</p><p>Huygens in 1655. For almost three centuries thereafter, Titan remained nothing</p><p>more than a dot of light in the sky. Then, in 1994, Gerard Kuiper discovered that</p><p>its atmosphere contained methane! It immediately became a world to explore and a</p><p>destination in the space era.</p><p>Titan is shrouded by a thick atmosphere with a blanket of organic haze that</p><p>hides the surface. The haze is a product of UV photolysis of methane in the upper</p><p>atmosphere. In the early 80s, Voyager investigated Titan but was unable to see</p><p>through to the surface. However, Voyager confirmed that the atmosphere served as</p><p>a big chemical factory producing many complex organic compounds. This made</p><p>Titan one of the most fascinating bodies in the solar system.</p><p>In 1999, while the international Cassini–Huygens mission was on its voyage to</p><p>Saturn, the authors of this book published their first comprehensive review of</p><p>of 360 km/h, Huygens deployed its smaller,</p><p>3 m diameter ‘drogue’ parachute, designed to stabilise the probe as it descended</p><p>rather than to slow it too much more.Although a slower descent meant more scientific</p><p>data on the atmosphere, it also meant a greater chance of technical failure, or loss of</p><p>the communications relay, before the probe reached the surface. At about 60 km in</p><p>altitude, the surface-sensing radar was turned on, and finally at an altitude of 700 m</p><p>above the surface the descent lamp of the imaging instrument was activated. The</p><p>purpose of this lamp was to enable scientists to accurately determine the reflectivity</p><p>of the surface. For photography, the natural lighting by the Sun was used to illuminate</p><p>the landing site. The light level on the surface of Titan was roughly 1,000 times less</p><p>than we are used to on Earth by day, but 1,000 times stronger than the light of the</p><p>full moon.</p><p>For those in Darmstadt, the most thrilling moment perhaps was the landing.</p><p>Officially, the mission was planned to end when the probe hit the surface, with any</p><p>further data being seen as a bonus should it survive. Huygens was a descent module</p><p>and not a lander, as we were reminded several times during Cassini–Huygens meet-</p><p>ings. At the very best, a few minutes of survival at the surface could be envisioned</p><p>if “all went perfectly well and we didn’t crash or sink”.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>88 Titan: Exploring an Earthlike World</p><p>As it happened, most things went more than just “perfectly well”. Thanks to the</p><p>parachutes, the surface impact speed was only about 20 km/hr (or 5 m/s). On the sur-</p><p>face, the five batteries onboard the probe lasted much longer than expected, allowing</p><p>Huygens to collect surface data for 1hr and 12 m. During its descent, the DISR</p><p>camera returned more than 750 images and numerous spectra, while the probe’s</p><p>other three instruments (HASI, ACP and GCMS) sampled Titan’s atmosphere to</p><p>help determine its composition and structure. The Surface Science Package had</p><p>plenty of time to acquire data after landing. The telemetry data from Huygens was</p><p>relayed at a rate of 8 kilobits per second and stored in Cassini’s solid-state memory</p><p>while the latter was at an altitude of 60,000 km from Titan. Two nearly redundant</p><p>channels, A and B, were planned, but in the event, as the result of a programming</p><p>error, the only serious problem Huygens experienced led to loss of all data from</p><p>channel A. The Doppler wind experiment was the main casualty, since unlike most</p><p>of the instruments it did not have redundancy and unfortunately lost its data that were</p><p>supposed to be transmitted only through Channel A. Happily, in the end all of the</p><p>measurements were recovered because the weak signal from Huygens was captured</p><p>by Earth-based radio telescopes, which became “Channel C”, and the Doppler exper-</p><p>iment was successful. The signal received via radio telescopes lasted 5 h 42 min,</p><p>including 3 h 14 min from the surface. This was so much more than the few minutes</p><p>that had been expected that it seems — even today — quite unbelievable.</p><p>The tremendous technological and scientific achievement of the Huygens mis-</p><p>sion will bear fruit for many years to come. But more than anything else, it once</p><p>more proves the fantastic capabilities brought about by international collaboration.</p><p>Landing on a new world 10 times farther from the Sun than our own planet stands</p><p>Figure 4.8 The Huygens Titan lander on Earth: a prototype in the snow of Northern Europe after a</p><p>test drop from an aircraft (ESA).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 89</p><p>with taking the first step on the Moon. Humanity has taken a huge step there towards</p><p>broadening its horizons. In March 2007 the Huygens landing site was named the</p><p>“Hubert Curien Memorial Station”, in recognition of the former ESA director’s</p><p>contribution to European space.</p><p>4.8 Experiments and Payloads</p><p>Next we take a closer look at the all-important science payload, instrument by</p><p>instrument. Remembering that Cassini was designed to explore the whole Saturn</p><p>system, the emphasis here is on those devices that make measurements relevant to</p><p>the problems and puzzles specific to Titan.</p><p>4.8.1 The Scientific Instruments on the Orbiter</p><p>The remote sensing instruments study their targets by making maps and pictures</p><p>(imaging) them, by measuring at multiple wavelengths (spectroscopy), and by doing</p><p>both at the same time (spectral imaging). The information sought comes from recog-</p><p>nising structures in the images, and measuring how the reflection and emission of</p><p>ultraviolet, visible and infrared radiation from the target varies with wavelength.</p><p>These spectra also vary with position across the map of an inhomogeneous object</p><p>like Saturn’s atmosphere or Titan’s surface and this gives us information about</p><p>differences in composition or physical state (temperature, for example). With high-</p><p>resolution imaging, extra insight comes from studying the morphology of the object</p><p>and, in the case of clouds or other dynamic targets, its rate of movement or change.</p><p>Cassini carries four remote sensing instruments, one high spatial resolution tele-</p><p>vision camera, with low spectral resolution filters, and three spectrometers covering</p><p>the ultraviolet, visible/near infrared, and middle/far infrared ranges of the spectrum.</p><p>The spectrometers are all designed to get spatial coverage, but not at nearly the high</p><p>resolution of the television; this is usually called ‘mapping’as opposed to ‘imaging’,</p><p>to differentiate.</p><p>The Imaging Science Subsystem (ISS)</p><p>The Imaging Science Subsystem consists of a wide-angle television camera, with</p><p>angular resolution of 60 microradians per pixel (3.5◦ × 3.5◦) and a narrow angle</p><p>camera, with angular resolution of 6 microradians per pixel (0.35◦ × 0.35◦). The</p><p>Wide Angle Camera is a 20 cm f/3.5 refractor with 18 filters in the 380–1100 nm</p><p>spectral range, and a field of view of 3.5◦ × 3.5◦. The Narrow Angle Camera is a</p><p>2 m f/10.5 reflector working from 2000–1100 nm in 24 filters; the field of view is</p><p>an area 100 times smaller than that of the Wide Angle Camera (0.35◦ × 0.35◦).</p><p>The sensors are 1024 × 1024 CCD arrays. These devices for solid-state imaging</p><p>are similar to the computerised digital cameras that are displacing film in domes-</p><p>tic photography, and are specially developed to work out into the ultraviolet and</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>90 Titan: Exploring an Earthlike World</p><p>Table 4.2 Physical characteristics of instruments on the Cassini Orbiter, and some details of the</p><p>people in charge of producing and operating them.</p><p>Name Web Site Principal Mass Mean Power Data</p><p>of Instrument Investigator/ (kg) Power Rate</p><p>Institute (W) (kbps)</p><p>Imaging Science</p><p>Subsystem (ISS)</p><p>http://ciclops.org/ C. Porco</p><p>SSI Boulder</p><p>57.83 55.90 365.57</p><p>Composite Infrared</p><p>Spectrometer (CIRS)</p><p>http://cirs.gsfc.na</p><p>sa.gov/</p><p>M. Flasar</p><p>NASA GSFC</p><p>39.24 26.37 6.000</p><p>Ultraviolet Imaging</p><p>Spectrograph (UVIS)</p><p>http://lasp.colorad</p><p>o.edu/cassini/</p><p>L. Esposito</p><p>U.Colorado</p><p>14.46 11.83 32.1</p><p>Visible-IR Mapping</p><p>Spectrometer (VIMS)</p><p>http://wwwvims.l</p><p>pl.arizona.edu/</p><p>R. Brown</p><p>U. Arizona</p><p>37.14 21.83 182.78</p><p>Ion and Neutral Mass</p><p>Spectrometer (INMS)</p><p>sprg.ssl.berkeley.</p><p>edu/inms/</p><p>H.Waite</p><p>SWRI</p><p>9.25 27.70 1.50</p><p>Cassini Plasma</p><p>Spectrometer (CAPS)</p><p>http://caps.space.s</p><p>wri.edu/</p><p>D.T. Young</p><p>SWRI</p><p>12.50 14.50 8.00</p><p>Cosmic Dust</p><p>Analyser (CDA)</p><p>http://www.mpi-</p><p>hd.mpg.de/dustgr</p><p>oup/</p><p>E. Grün</p><p>MPI</p><p>Heidelberg</p><p>16.36 11.38 0.524</p><p>Magnetometer</p><p>(MAG)</p><p>http://www3.imp</p><p>erial.ac.uk/spat/re</p><p>search/missions/</p><p>space_missions/</p><p>cassini/mag_team</p><p>M. Dougherty</p><p>U. London</p><p>3.00 3.10 3.60</p><p>Magnetospheric</p><p>Imaging (MIMI)</p><p>http://sd-</p><p>www.jhuapl.edu/</p><p>CASSINI/</p><p>T. Krimigis</p><p>APL</p><p>16.00 14.00 7.00</p><p>Radar</p><p>(RADAR)</p><p>http://saturn.jpl.n</p><p>asa.gov/spacecraft/</p><p>instruments-</p><p>cassini-radar.cfm</p><p>C. Elachi</p><p>JPL</p><p>41.43 108.40 364.80</p><p>Radio Science</p><p>(RSS)</p><p>http://saturn.jpl.n</p><p>asa.gov/spacecraft/</p><p>instruments-</p><p>cassini-rss.cfm</p><p>A. Kliore</p><p>JPL</p><p>14.38 80.70 N/A</p><p>Radio and Plasma</p><p>Wave (RPWS)</p><p>http://www-</p><p>pw.physics.uiowa.</p><p>edu/plasma-</p><p>wave/cassini/</p><p>home.html</p><p>D. Gurnett</p><p>U. Iowa</p><p>6.80 7.00 0.90</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 91</p><p>infrared, as well as at visible wavelengths. Compared to the vacuum-tube optical-</p><p>only television used by Voyager, they offer much higher spectral and dynamic range,</p><p>and are more linear. The extension of its coverage into the infrared means that ISS</p><p>can image the surface of Titan in the spectral ‘windows’ at wavelengths near 0.94</p><p>and 1.1 µm.A special filter is provided for detecting lightning in the neutral nitrogen</p><p>lines at 0.82 µm.</p><p>The ISS scientific objectives include investigating the composition, distribution,</p><p>and physical properties of clouds and aerosols, including scattering, absorption and</p><p>solar heating, as well as mapping the 3-dimensional structure and motions within</p><p>Saturn’s and Titan’s atmospheres and searching for lightning, aurorae, airglow, and</p><p>planetary oscillations as well. The ISS team is also engaged in mapping the surface</p><p>of Titan in the near infrared windows to study the geology and get clues as to the</p><p>nature and composition of the surface materials.</p><p>Composite Infrared Spectrometer (CIRS)</p><p>The Voyager IRIS instrument described in Chapter 2 demonstrated the value of</p><p>infrared spectroscopy for studying the atmospheres of Saturn and Titan, but its</p><p>relatively wide field-of-view and distant approach meant IRIS could not resolve</p><p>the atmosphere at the limb for either Saturn or Titan. Limb measurements (i.e.</p><p>tangentially viewing at the edge of the planetary disk) are particularly valuable</p><p>because they are not complicated by emission from the surface, and because vertical</p><p>resolution is largely determined by the field of view of the instrument rather than</p><p>solely by properties of the atmosphere itself. Thus, an instrument with a sufficiently</p><p>narrow field of view can obtain high vertical resolution by limb viewing.</p><p>Figure 4.9 The Imaging Science Subsystem Wide Angle Camera. Team-Leader: C. Porco</p><p>(NASA/JPL/SSI, University of Colorado).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>92 Titan: Exploring an Earthlike World</p><p>from telescope</p><p>solar</p><p>blocker</p><p>absorber input polarizer</p><p>collimator</p><p>collimator</p><p>polarizing</p><p>beamsplitter</p><p>focal</p><p>plane 1</p><p>scan</p><p>mechanism</p><p>reference</p><p>interferometer</p><p>focal planes</p><p>3 and 4</p><p>beamsplitter</p><p>and compensator</p><p>shutter</p><p>mid-IR</p><p>amplitude FTS</p><p>far-IR</p><p>polarizing FTS</p><p>Figure 4.10 The Composite Infrared Spectrometer. PI: F. M. Flasar (NASA/JPL/GSFC).</p><p>The Cassini orbiter carries an improved version of IRIS called CIRS, the Com-</p><p>posite Infrared Spectrometer. CIRS repeats the IRIS measurements at ten times</p><p>higher spectral resolution (0.5 cm−1), in an extended spectral range (7 to 1000 µm,</p><p>which is 10–1400 cm−1), with a 4.3 mrad circular field of view and, of course, over</p><p>a much longer period of time than Voyager, allowing the recovery of 40 times as</p><p>many spectra. Built at Goddard Space Flight Center near Washington, DC, CIRS</p><p>measures infrared emission from atmospheres, rings, and surfaces which can be used</p><p>to map the temperature, hazes and clouds, and the chemical structure in Saturn’s and</p><p>Titan’s atmospheres, including the global surface temperatures on Titan, to search</p><p>for the spectral lines of new molecules, and to map the composition and thermal</p><p>characteristics of Saturn’s rings and other icy satellites.</p><p>As the ‘composite’part of the name suggests, CIRS is made up of two interferom-</p><p>eters in a single housing, served by a single 50 cm diameter telescope and contained</p><p>in an overall mass of 39.24 kg. The far-infrared (10 to 600 cm−1) spectrometer has</p><p>a 4.3 mrad circular field of view (FOV), which covers 600 to 1100 cm−1 with a 1 ×</p><p>10 array of 0.273 mrad square fields of view, and a shorter-wavelength spectrometer</p><p>covers the rest of the mid-infrared (1100 to 1400 cm−1) with a similar array.</p><p>The nature of the Cassini orbit leads to particular problems when trying to scan</p><p>the limb of Saturn. The closer approaches to Titan occur at about 1000 km, or</p><p>about 40% of Titan’s radius, whereas a typical closest approach to Saturn is at</p><p>300,000 km, 5 times the planet’s radius. To achieve a vertical resolution comparable</p><p>with the atmospheric variability requires a much smaller field of view for Saturn</p><p>measurements. CIRS has a spatial resolution of 0.3 mrad for wavelengths less than</p><p>17 µm, compared to the 4.3 mrad of IRIS. The smaller field of view, combined with</p><p>the use of ten element detector arrays at these wavelengths, allows extensive limb</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 93</p><p>Figure 4.11 A composite CIRS spectrum taken by Cassini in 2005 compared to a spectrum taken</p><p>by Voyager 1/IRIS in 1980. The difference in the two spectra resides both in the CIRS higher spectral</p><p>resolution and in the larger spectral range.</p><p>observations of temperature and constituent abundances of both Titan and Saturn at</p><p>vertical resolutions that are comparable with the vertical scales of processes within</p><p>each atmosphere.</p><p>The combination of smaller field of view and higher spectral resolution is possible</p><p>because of the use of cooled detectors. Team members at Oxford provided the</p><p>radiative cooler and cold focal plane assembly incorporating mid-infrared detector</p><p>supplied by collaborators in France and the US. When typical spectral measurements</p><p>by CIRS are compared to IRIS data taken 25 years before, the CIRS spectrum</p><p>provides both higher resolution and larger spectral coverage, with the inclusion of</p><p>the 10–200 cm−1 region. Regions of the spectrum where the noise (i.e. the noise</p><p>equivalent spectral radiance) level is higher than the expected signal from Titan can</p><p>be studied by increasing the duration of the observation, to give more signal.</p><p>In addition to improved sensitivity and vertical resolution, the long wavelengths</p><p>accessible to CIRS allow sounding deeper into both atmospheres than was possible</p><p>with IRIS. The high performance of CIRS enables several key objectives, including</p><p>global mapping of the vertical distribution and temporal variation of gaseous species,</p><p>temperature and clouds in both the tropospheric and stratospheric regions of Saturn</p><p>and Titan, with latitude, longitude and height, as well as allowing for the possibility</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>94 Titan: Exploring an Earthlike World</p><p>of discovering previously undetected chemical species in both atmospheres. New</p><p>information on some species in Jupiter’s atmosphere, particularly tropospheric NH3</p><p>and PH3 and stratospheric C2H2, and C2H6, were obtained during the Jupiter flyby</p><p>in December 2000.</p><p>The scientific applications of the compositional data expected may be sum-</p><p>marised under three broad headings. The first of these is evolution, or the attempt</p><p>to infer from information on the relative elemental abundances and their isotopic</p><p>ratios an improved understanding of the formation and evolution of the Saturnian</p><p>system and the Solar System as a whole. Secondly, knowledge of the atmospheric</p><p>composition and its vertical and horizontal variation is important for understanding</p><p>the physical and chemical processes presently active on Saturn and Titan, in particu-</p><p>lar photochemical and radiative processes and those which give rise to the observed</p><p>clouds and hazes. Finally, compositional data contributes to our understanding of</p><p>the circulation and dynamics of the atmosphere, since transport between sources</p><p>and sinks must occur and many species are useful tracers of the dynamical motions</p><p>and timescales.</p><p>Ultraviolet Imaging Spectrograph (UVIS)</p><p>The UV Imaging Spectrograph has a set of detectors to measure the light reflected</p><p>from atmospheres, surfaces and rings over wavelengths ranging from 55 to 190 nm,</p><p>i.e. shorter than visible light, with a resolution of 0.2 to 0.5 nm. UVIS is actually</p><p>made up of four instruments, a far ultraviolet (110 to 190 nm) spectrograph, an</p><p>extreme ultraviolet (55.8 to 118 nm) spectrograph; a high speed photometer (115 to</p><p>185 nm) and a hydrogen-deuterium absorption</p><p>cell operating at 121.5 nm wave-</p><p>length. Standard stars are used to calibrate the spectra, and to obtain solar and stellar</p><p>occultation profiles of the thermosphere as they set behind the planet.</p><p>The data that this instrument acquires are being used to investigate the compo-</p><p>sition, aerosol content, and temperature of the upper reaches of Titan’s atmosphere.</p><p>Species which can be detected in the UV include CH4, C2H6, C2H2, H, H2, N, N2,</p><p>Ar, CO, C2N2, and the D/H ratio can also be measured. These can be mapped to</p><p>get the vertical/horizontal composition, the latitude-longitude variability, and any</p><p>toroidal</p><p>diffration</p><p>grating</p><p>parabolic</p><p>mirror</p><p>electronics</p><p>detector</p><p>ion pump</p><p>variable slit</p><p>from</p><p>target</p><p>Figure 4.12 The Cassini Ultraviolet Imaging Spectrograph. PI: L. Esposito (NASA/JPL/University</p><p>of Colorado).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 95</p><p>interhemispheric differences, to elucidate the atmospheric chemistry occurring in</p><p>Titan’s atmosphere, as well as the distribution and scattering properties of aerosols,</p><p>the nature and characteristics of the circulation, and the distribution of neutrals</p><p>and ions.</p><p>UVIS also studies Saturn’s atmosphere, again at the higher levels; the radial</p><p>structure of the rings, and the surfaces and tenuous atmospheres of the icy satellites.</p><p>Visual and Infrared Mapping Spectrometer (VIMS)</p><p>The Visual and Infrared Mapping Spectrometer spans spectral windows between 0.6</p><p>and 5 µm, allowing the identification of surface materials with high (∼500 m resolu-</p><p>tion) as well as resolved composition measurements. The VIMS instrument consists</p><p>of a pair of imaging grating spectrometers designed to measure the reflected and</p><p>emitted radiation from atmospheres, rings and surfaces over wavelengths from 0.35</p><p>to 5.1 µm, in two bands: 0.35 to 1.07 µm (called the visible subsystem, and having</p><p>96 spectral channels) and 0.85 to 5.1 µm (the infrared subsystem with 256 channels).</p><p>All of these channels have 32 × 32 mrad fields-of-view.</p><p>VIMS is studying the composition and distribution of atmospheric and cloud</p><p>species and the temperature structure on Titan and Saturn and on the icy satel-</p><p>lites and rings. It helps to search for lightning and active volcanism on Titan, to</p><p>observe the surface and map temporal behaviour of winds, eddies and other tran-</p><p>sient features on Saturn and to observe Titan’s surface. Looking at Titan’s night-</p><p>side, the instrument can search for lightning and thermal emission from active</p><p>cryovolcanoes.</p><p>Figure 4.13 The Visible and Infrared Mapping Spectrometer. PI: R. Brown (NASA/JPL/University</p><p>of Arizona).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>96 Titan: Exploring an Earthlike World</p><p>Radar</p><p>The Cassini multimode radar, which completely penetrates the hazy atmosphere of</p><p>Titan, operates as a radiometer (to measure surface temperature or emissivity), a</p><p>scatterometer and altimeter (to measure the reflectivity and topography along the</p><p>orbiter groundtrack) and has a synthetic aperture imager mode (SAR). It makes use</p><p>of the five-beam feed assembly associated with the spacecraft high gain antenna to</p><p>direct radar transmissions toward targets, and to capture blackbody radiation and</p><p>reflected radar signals from targets.</p><p>The hardware, developed at the Jet Propulsion Laboratory, consists of a synthetic</p><p>aperture radar imager (13.78 GHz Ku-band; 0.35 to 1.7 km resolution), an altimeter</p><p>((13.78 GHz Ku-band; 24 to 27 km horizontal, 90 to 150 m vertical resolution),</p><p>and a radiometer (13.78 GHz passive Ku-band; 7 to 310 km resolution). This set of</p><p>frequencies is used to investigate the geologic features and topography of the solid</p><p>surface of Titan, and is providing evidence for whether extensive bodies of liquid</p><p>exist on the surface or not. The radar uses the main data relay antenna for three</p><p>functions: sending the radar pulse, receiving the echo, and transmitting the scientific</p><p>data back to the Earth (of course, when the radar is operating, the data has to be</p><p>stored for later transmission when the dish resumes its Earth-pointing mode).</p><p>The imaging mode, nearest closest approach, maps Titan’s surface at 0.5 to 2 km</p><p>resolution (which is not quite as good as the radar on Magellan achieved at Venus)</p><p>over about 1% of Titan’s surface for each flyby devoted to radar measurements. Thus</p><p>perhaps 20% of Titan’s surface will be imaged — in long thin strips — by the time</p><p>the nominal mission ends. The imager on the orbiter carries filters at 940 nm, tuned</p><p>to the windows between the methane bands, and so — like the HST — is able to</p><p>measure surface contrasts. Additionally, polarizers are carried which remove most</p><p>of the light scattered by the haze at near 90◦ phase angle, so these measurements also</p><p>study the surface. The exact resolution achievable depends on the scene contrast and</p><p>the haze optical depth at the time of the measurement, as well as the image motion</p><p>compensation that can be achieved, but is likely to be better than 100 m. Other filters</p><p>are able to probe different altitudes in the atmosphere.</p><p>Radio Science Subsystem (RSS)</p><p>Using a well-established technique developed on earlier planetary missions, the</p><p>Cassini spacecraft’s communication link is used to track the spacecraft, a task that</p><p>can be done with remarkable precision. The signal is used to follow the bending of</p><p>the radio beam as it passes through Titan’s atmosphere; from this the refractivity of</p><p>the medium can be deduced, and from that its molecular weight, temperature and</p><p>pressure. The team is also engaged in the study of the solar corona, the radial structure</p><p>and particle size distribution within Saturn’s rings, the temperatures and electron</p><p>densities with Saturn’s and Titan’s ionospheres and a search for gravitational waves</p><p>coming from beyond the Solar System.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 97</p><p>Cassini’s capabilities for this sort of study are again much better than those</p><p>available to Voyager. The transmitting and receiving frequencies available are S-</p><p>band (around 2 GHz), X-band (around 8 GHz, and Ka-band (around 34 GHz), usable</p><p>in non-coherent mode, where the frequency reference is the on-board ultrastable</p><p>oscillator, or coherent mode where the Ka-band or X-band uplink is also used. The</p><p>oscillator is stable to 1 part in 1013; in coherent mode, 1 part in 1016 is achievable.</p><p>Since the latter requires a two-way path to the Earth, it is not available during Titan</p><p>occultations, hence the need for the oscillator.</p><p>A total of 15 Titan occultations are obtained during the course of the mission.With</p><p>this data, it is possible to obtain accurate values for the lapse rate in the troposphere,</p><p>thus further constraining the composition, in particular the argon abundance. The</p><p>temperature-pressure field measurements allow zonal wind fields to be deduced, and</p><p>yield temperatures and electron densities within Titan’s ionosphere.</p><p>Because it allows very precise tracking of Cassini, the RSS also reveals how Titan</p><p>affects the orbit of the spacecraft, and hence provides details of Titan’s gravitational</p><p>field, from which models for the shape of the solid body of Titan can be calculated. It</p><p>will tend to be oblate (flattened) as a result of the effect of its rotation, as most large</p><p>planetary bodies are; however, the tidal forces exerted by its large parent, Saturn,</p><p>will tend to have the opposite effect, and induce a prolate shape. Exactly what the</p><p>end result is will depend on the elasticity of the body, which in turn depends on</p><p>factors such as the amount and distribution of volatile material.</p><p>Thus, we gradually obtain insights into questions such as whether Titan is in</p><p>hydrostatic equilibrium, whether the interior is in fact differentiated, perhaps even</p><p>whether there is a sub-surface ocean as some models predict. In conjunction with</p><p>Earth-based Titan stellar observations, the results may also shed some light on the</p><p>question of how Titan’s orbit round Saturn comes to have such a large eccentricity,</p><p>by permitting</p><p>better estimates of the tidal dissipation which should be tending to</p><p>circularise the orbit. If Titan were flexing a lot, as it would if it contains a lot of</p><p>volatiles, then the most likely explanation would be the fairly recent impact of a</p><p>large body. This could have left an observable record on the surface, which the radar</p><p>can pick up as it covers the surface one region at a time.</p><p>Particle and Fields Experiments</p><p>This family of instruments is designed to investigate the magnetosphere of Saturn</p><p>and its interaction with the solar wind, the rings, and the satellites, including of course</p><p>Titan. This interaction is both a source and a sink for charged particles and molecules</p><p>at Titan, and an understanding of the detailed processes involved is probably crucial</p><p>for understanding the evolution of Titan’s atmosphere on long time scales.</p><p>The magnetosphere of Saturn, discovered by Pioneer 11 and explored in more</p><p>detail by the Voyager 1 and 2 spacecraft, has been shown to extend beyond the orbit</p><p>of Titan at local noon, and to be extremely dynamic due to both external (solar wind)</p><p>and internal drivers. The Cassini orbiter carries comprehensive instrumentation that</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>98 Titan: Exploring an Earthlike World</p><p>has provided a comprehensive picture of the magnetic field, radio emissions, plasma,</p><p>and energetic particle environments since orbit insertion on July 1, 2004. In the</p><p>energetic particle and magnetic field data the latter increases close to the planet</p><p>and then decreases with distance, as expected, but also exhibits what appear to be</p><p>periodic variations. These correspond to plasma sheet encounters as marked by the</p><p>vertical bars, and occur roughly at the planetary rotation period. The source of this</p><p>periodicity, observed not only in particle and magnetic field intensities but also in</p><p>planetary radio emissions is not fully understood at present.</p><p>The orbit of Titan is surrounded by a doughnut-shaped torus of charged hydrogen,</p><p>nitrogen and other atoms that escaped from Titan. The Cassini payload is designed to</p><p>characterise the ion flow around Saturn and Titan, measuring the composition of the</p><p>torus, including heavy ions, and to investigate how they originate in the interaction</p><p>between the planetary atmospheres and ionospheres with the magnetosphere. The</p><p>principal loss process for particles leaving the torus may be particle precipitation</p><p>into Titan’s ionosphere; the instruments examine the processes responsible for the</p><p>two-way traffic between the upper atmosphere and the torus.</p><p>The Ion and Neutral Mass Spectrometer (INMS) measures, by direct sampling,</p><p>the distribution of positive ion and neutral species in the upper atmosphere of Titan,</p><p>down to about 950 km, which is the closest Cassini is likely to approach to the surface</p><p>of the satellite. These allow direct comparison with the density profile measured</p><p>by the entry deceleration of the Huygens probe. INMS has recently allowed the</p><p>detection of a rich mixture of hydrocarbons and nitriles which are found with mixing</p><p>ratios that vary from 10−4 to 10−7: acetylene, ethylene, ethane, benzene, toluene,</p><p>cyanogen, propyne, propene, propane, and various nitriles.</p><p>The Cassini Plasma Spectrometer (CAPS) consists of an ion mass spectrom-</p><p>eter, an ion beam spectrometer and an electron spectrometer. These measure the</p><p>flux of ions as a function of mass per charge and the flux of ions and electrons</p><p>as a function of energy per charge and angle of arrival at the instrument. Sur-</p><p>prisingly, given the composition of the neutral atmosphere, CAPS has found that</p><p>nitrogen ions are comparatively rare in Titan’s magnetosphere, which instead is</p><p>dominated by plasma composed almost entirely of ionized water and water products,</p><p>including O+, OH+, H2O+ and H3O+, probably originating from the plumes on</p><p>Enceladus.</p><p>The Dual Technique Magnetometer (MAG) consists of a combination of a vector/</p><p>scalar helium magnetometer and a fluxgate magnetometer, used to determine the</p><p>magnetic field of Saturn and hence its interactions with Titan and the icy satellites,</p><p>the rings and the solar wind. Among other achievements, MAG provided the first</p><p>hints on the water atmosphere around Enceladus.</p><p>The Magnetospheric Imaging Instrument (MIMI) maps the composition, charge</p><p>state and energy distribution of energetic ions, electrons and fast neutral species</p><p>in Saturn’s magnetosphere. At Titan it monitors the composition and loss rate of</p><p>particles from Titan’s atmosphere due to ionisation and pickup, to determine the</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 99</p><p>LEMMS</p><p>Low Energy</p><p>Magnetospheric</p><p>Measurement System</p><p>CHEMS</p><p>Charge Energy</p><p>Mass Spectrometer</p><p>INCA</p><p>Ion and Neutral</p><p>Camera</p><p>Data</p><p>Processing</p><p>Unit</p><p>Spacecraft</p><p>Power</p><p>and</p><p>commands</p><p>MIMI: Magnetospheric Imaging Instrument</p><p>Figure 4.14 The Magnetospheric Imaging instrument. PI: T. Krimigis (NASA/JPL/APL).</p><p>importance of Titan’s exosphere as a source for the atomic hydrogen torus in Saturn’s</p><p>magnetosphere.</p><p>The Radio and Plasma Wave Science (RWPS) instrument has electric and mag-</p><p>netic field sensors, a Langmuir probe, and high, medium and wide band receivers to</p><p>measure the electric and magnetic fields and the electron density and temperature</p><p>in the interplanetary medium, Saturn’s magnetosphere, as well as Titan’s induced</p><p>magnetosphere and ionosphere, and the production, transport, and loss of plasma</p><p>from Titan’s upper atmosphere. It also searches for radio signals characteristic of</p><p>lightning in Titan’s atmosphere (although a similar search by Voyager failed to</p><p>indicate any such emission).</p><p>Finally, Cassini carries an instrument to measure the number mass, velocity</p><p>and composition of solid particles of dust in the environment around Saturn and</p><p>Titan. The Cosmic Dust Analyzer (CDA) is investigating the physical, chemical, and</p><p>dynamical properties of these particles, and their interactions with the rings, satel-</p><p>lites, and magnetosphere of Saturn. This data tells us about the origin of oxygen</p><p>compounds in Titan’s reducing atmosphere, since they are probably due to photo-</p><p>chemical reactions involving meteoric water.</p><p>4.8.2 The Scientific Instruments on the Probe</p><p>The Huygens probe carried six instruments through Titan’s atmosphere down to</p><p>the surface, to study the lower stratosphere and the conditions in the troposphere</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>100 Titan: Exploring an Earthlike World</p><p>Figure 4.15 Scientific instruments on the Huygens Probe. Views of the payload accommodation on</p><p>the top and bottom parts of the experiment platform (ESA).</p><p>during the descent and to send back information on the satellite’s surface. The probe</p><p>survived the landing and transmitted from the surface for a much longer time than</p><p>expected because it touched down on a relatively soft solid surface, so the period</p><p>of operation on the surface was limited by the uplink to the Cassini orbiter. Once</p><p>that went over the horizon as seen from Huygens, the probe’s useful life was over</p><p>even though we know, from the continued reception of the signal by ground-based</p><p>radio telescopes, that it continued to function for a further few hours. Data on the</p><p>wind field on Titan were recovered by measuring the probe signal on Earth by radio</p><p>telescopes.</p><p>Aerosol Collector Pyrolyser (ACP)</p><p>The Aerosol Collector Pyrolyser under the responsibility of Guy Israel (Service</p><p>d’Aéronomie du CNRS) was designed to identify the composition of the aerosols</p><p>in Titan’s atmosphere by sampling them during the descent of the probe. Once</p><p>inside the instrument, droplets were evaporated and thermally dissociated in an oven,</p><p>and a chemical analysis of the products made with the gas chromatograph- mass</p><p>spectrometer (GCMS) instrument which is also on board the probe. ACP sampled</p><p>the aerosol from two atmospheric layers (150 to 45 km and 30 to 15 km from the</p><p>surface), each followed by a 3-step pyrolysis at temperatures of 20 ◦C, 250 ◦C and</p><p>650 ◦C before injection of the products into the GCMS via a dedicated transfer</p><p>line.</p><p>The latter was performing its own analysis of Titan’s gaseous atmosphere most of</p><p>the time, as discussed below; the ACP experiment made use of its capability for</p><p>approximately 20% of its operating life time.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 101</p><p>Figure 4.16 The Aerosol Collector Pyrolyser instrument. PI: G. Israel (ESA).</p><p>Descent Imager/Spectral Radiometer (DISR)</p><p>The Descent Imager/Spectral Radiometer was built under the responsibility of</p><p>M. Tomasko at the University of Arizona, with German and French collaborators.</p><p>It measures solar radiation using silicon photodiodes, a two-dimensional silicon</p><p>charge-coupled detector and two InGaAs near-infrared linear array detectors. Fibre</p><p>optics collected light from upward and downward visible (480–960 nm) and infrared</p><p>(0.87–1.7 µm) spectrometers with resolutions of 2.4 to 6.3 nm. For surface imaging,</p><p>DISR used downward and side looking cameras with 0.06◦ to 0.20◦ fields-of-view.</p><p>In the event, these were sensitive enough to photograph the surface, despite the fact</p><p>that only 10% of the incident sunlight — already a hundred times weaker than on</p><p>Earth — reaches the ground on Titan, without the help of the floodlight that was</p><p>also carried as a precaution.</p><p>DISR measures at solar and near-infrared wavelengths in both the upward and</p><p>downward direction to look at the scattering properties of the aerosols and to find</p><p>out at which height levels most of the energy from the Sun is deposited in the</p><p>atmosphere. The downward flux minus the upward flux gives the net flux, and the</p><p>difference in the net flux at two altitudes gives the amount of solar energy absorbed</p><p>by the intervening layer of atmosphere. This determines the rate at which the Sun</p><p>is heating the atmosphere, as a function of height, which depends on the absorption</p><p>properties of the gases and aerosols. The result is important for determining the</p><p>temperature structure, and the gradients that drive atmospheric motions.</p><p>Measurements at 550 and 939 nm of small angle scattering in the solar aureole,</p><p>the bright area around the Sun caused by forward scattering by suspended particles,</p><p>of side and back scattering and polarisation, and of the extinction as a function of</p><p>wavelength, all allow the optical properties of the haze particles (quantities such as</p><p>optical depth, single scattering albedo, and the shape of the scattering phase function)</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>102 Titan: Exploring an Earthlike World</p><p>Figure 4.17 The Descent Imager/Spectral Radiometer, with the floodlight prominently featured. PI:</p><p>M. Tomasko (ESA/NASA/JPL/University of Arizona).</p><p>Figure 4.18 The Descent Imager/Spectral Radiometer, with the different directions of imaging</p><p>(ESA/NASA/JPL/University of Arizona).</p><p>to be worked out. These properties together with determinations of size and shape</p><p>can yield the imaginary part of the refractive index, and possibly constrain the real</p><p>part also. Knowledge of the refractive index can be a strong, with luck, unique, clue</p><p>to the composition of the particles. The DISR spectra of the downward streaming</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 103</p><p>sunlight also showed the absorption bands of methane, allowing the determination</p><p>of the profile of the mixing ratio of methane gas during the descent, analogous to a</p><p>relative humidity profile on the Earth.</p><p>DISR also took spectacular pictures of the surface on the way down, observ-</p><p>ing in three different directions during its descent and on the ground to allow</p><p>for the recovery of stereographic information. The many questions about the sur-</p><p>face of Titan which are being addressed with the DISR pictures, including looking</p><p>for channels, craters, lakes, glacial flows, frost and ice coverings, and active gey-</p><p>sers and volcanoes, are addressed in later chapters. The descent imager pictures</p><p>started at a height of 160 km, when the spatial resolution was about 1 km. They</p><p>gave many clues about the topography of the surface, the reflection spectra of sur-</p><p>face features, the composition of the different types of terrain observed, and the</p><p>interactions of the surface and the atmosphere. They also provided the horizontal</p><p>wind direction and speed calculated from images of the surface obtained every few</p><p>kilometres in altitude from the drift of the probe over the surface of Titan as it</p><p>descended.</p><p>DopplerWind Experiment (DWE)</p><p>The primary scientific goal of the Doppler Wind Experiment led by M. Bird of the</p><p>Univ. of Bonn in Germany, with Italian and American involvement, was to measure</p><p>the direction and strength of the winds in the atmosphere of Titan, through Doppler</p><p>tracking of the probe from the orbiter. The DWE used ultra stable oscillators within</p><p>the radio transmitter on the probe and in the receiver on the orbiter to improve the</p><p>stability of the relay link to make accurate Doppler measurements, the first time</p><p>these devices have been used in deep space. They are compact, atomic resonance</p><p>frequency-controlled oscillators whose output signal is obtained from a 10 MHz</p><p>voltage controlled crystal oscillator, which in turn is frequency-locked to the atomic</p><p>resonance frequency (6.834 GHz) of the ground-state hyperfine transition of the</p><p>element 87Rb.</p><p>The DWE data were lost due to the problem with Channel A in the commu-</p><p>nications link between Huygens and Cassini. Thankfully, the Huygens signal was</p><p>captured and monitored from several large radio telescopes on Earth, and the exper-</p><p>iment was saved. The Earth-based antennas that received the signal were the NRAO</p><p>Robert C. Byrd Green Bank Telescope in WestVirginia, USA, and the CSIRO Parkes</p><p>Radio Telescope in Australia. From the detection of the incredibly weak Huygens</p><p>radio signal, winds on Titan were deduced from the surface to a height of 160 km.</p><p>The winds are weak near the surface and increase slowly with altitude up to about</p><p>60 km, and are from west to east at nearly all altitudes. Above 60 km large fluctua-</p><p>tions sin the Doppler signal were observed, indicating a turbulent atmosphere with</p><p>large values of wind shear. The science and engineering data recorded on board</p><p>Huygens confirmed that the probe was buffeted by the winds in this region. A max-</p><p>imum wind speed of 120 m s−1 (430 km hr−1) was found at an altitude of about</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>104 Titan: Exploring an Earthlike World</p><p>120 km. DWE also monitored the spin of the probe, and the amplitude of its swing</p><p>under the parachute as it descended.</p><p>Gas Chromatograph/Mass Spectrometer (GCMS)</p><p>The Gas Chromatograph/Mass Spectrometer was developed by Hasso B. Niemann</p><p>and his team from NASA Goddard Space Flight Center in Greenbelt, Maryland</p><p>to measure the composition of Titan’s atmosphere. The gaseous atmosphere was</p><p>sampled directly during the descent, while the aerosol droplets were first vaporised</p><p>by the pyrolysis experiment. The GCMS also had a heated inlet, so that the volatile</p><p>component of the surface material at the landing site could be determined.</p><p>As Titan’s atmosphere has so many components, these were separated in two</p><p>dimensions by chromatography as well as mass spectroscopy. The instrument could</p><p>handle an atomic mass range of 2 to 146 amu with a sensitivity of one part in 1012 and</p><p>a mass resolution of about 10−6 at 60 amu. The gas chromatograph used H2 as the</p><p>carrier gas in 3 parallel columns. Its function is to increase the measurement capabil-</p><p>ity of the instrument by time separation of species with different chemical properties</p><p>for detection and identification and analyses by the mass spectrometer. The mass</p><p>spectrometer system works by ionising the incoming gas using ion sources, then</p><p>making species concentration measurements by passing them through a magnetic</p><p>field which separates them by mass, and focuses them onto an ion detector.</p><p>Figure 4.19 The Gas Chromatograph/Mass Spectrometer instrument on the Huygens probe. PI:</p><p>H. Niemann (ESA/GSFC).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 105</p><p>The three gas chromatograph columns were optimised for individual objectives.</p><p>The first was devoted to discriminating N2 and CO, which have the same atomic</p><p>mass and which therefore cannot be distinguished by mass spectroscopy alone. The</p><p>second and third focussed on hydrocarbons and nitriles, and heavy hydrocarbons,</p><p>respectively. The GCMS measured the major isotopes of carbon, nitrogen, hydrogen,</p><p>oxygen and argon, and detected neon and the other noble gases to levels of 10–</p><p>100 ppb. The GCMS collected data from an altitude of 146 km to ground impact. The</p><p>Probe and the GCMS survived impact and collected data for 1 hour and 9 minutes</p><p>on the surface. The major constituents of the lower atmosphere were confirmed to</p><p>be N2 and CH4, and the vertical profile of the latter measured.</p><p>Huygens Atmospheric Structure Instrument (HASI)</p><p>The entry deceleration was measured by the Huygens Atmospheric Structure Instru-</p><p>ment, built by the Italian Space Agency for a team led by M. Fulchignoni (LESIA,</p><p>Paris Observatory). HASI was the only probe instrument to operate prior to parachute</p><p>deployment on the probe. The deceleration is proportional to density, and from</p><p>the density profile and the assumption of hydrostatic equilibrium, a temperature</p><p>profile of the upper atmosphere was derived. HASI carried on making direct mea-</p><p>surements throughout the atmosphere, obtaining atmospheric temperature, pressure,</p><p>density and conductivity profiles from 170 km down to the surface as the probe</p><p>descended by parachute. In addition to determining the atmospheric structure, these</p><p>data help to identify the composition of the condensates in the regions where hazes</p><p>or clouds form.</p><p>HASI also included a Permittivity, Wave, and Altimetry Experiment (PWA)</p><p>which measured the electrical properties of the atmosphere (important in determin-</p><p>ing haze charging and coagulation physics) and searched for thunder and lightning.</p><p>Figure 4.20 The pressure profiling sensor, part of the Huygens Atmospheric Structure Instrument.</p><p>PI: M. Fulchignoni (ESA/Paris Observatory).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>106 Titan: Exploring an Earthlike World</p><p>The probe also carried a radar altimeter, which estimated radar reflectivity and sur-</p><p>face topography. The radar altimeter, part of the probe system itself, passed its signal</p><p>to the PWA for science data processing.</p><p>On the surface, the conductivity was measured over a dynamic range from 10−15</p><p>to infinity, the relative permittivity in the range 1 to infinity, and acoustic density</p><p>measurements in the 0–5 kHz range, with a sensitivity of 90 dB at 5 mPa. These data</p><p>characterise the dielectric properties, conductivity and permittivity of the surface</p><p>material.</p><p>Surface Science Package (SSP)</p><p>The material making up the surface of Titan at the landing site was directly inves-</p><p>tigated by the Surface Science Package, which was developed at the University</p><p>of Kent at Canterbury, England, by J. Zarnecki. SSP was equipped to measure</p><p>the properties of a liquid surface, which was considered probable at the time</p><p>the experiment was designed, although we now know that Huygens landed on a</p><p>soft, solid surface like sand or snow. During descent, tilt sensors measured the</p><p>probe’s attitude, and an acoustic sounder measured the speed of sound in the atmo-</p><p>sphere, which depends on temperature and molecular mass. Prior to impact, the</p><p>sounder also probed the surface roughness at the landing site. During impact,</p><p>SSP’s accelerometer and penetrometer measured the mechanical properties of the</p><p>surface material, from which particle size and stickiness can be estimated. The</p><p>full complement of sensors used by the Surface Science Package is listed in</p><p>Table 4.3.</p><p>Figure 4.21 The Surface Science Package — see Table 4.3 for the meaning of the acronyms. PI:</p><p>J. Zarnecki (ESA/Open University).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 107</p><p>Table 4.3 The individual sensors that make up the Huygens Surface Science Package.</p><p>Acronym Function Description</p><p>ACC Impact accelerometer Piezoelectric type</p><p>TIL Tilt sensor (X and Y axes) Electrolytic type</p><p>THP Thermal properties Hot wire</p><p>API-V Velocity of sound Piezoelectric transducers</p><p>API-S Acoustic sounder Piezoelectric transmitter/receiver</p><p>PER Fluid permittivity Capacitance sensor</p><p>DEN Density of fluid Archimedes sensor</p><p>REF Refractive index Critical angle refractometer</p><p>4.9 Touring the Saturnian System</p><p>Once communication with Huygens was lost, the orbiter continued on its tour of the</p><p>Saturnian system, returning to make additional close flybys to acquire more remote</p><p>sensing data on Titan and its atmosphere. The geometry of each flyby is calculated</p><p>to give the best compromise between Titan observations and getting the right gravity</p><p>assist to send the spacecraft on the path to its next target. Huygens, of course, has</p><p>been silent on these subsequent Titan encounters. In a typical timeline for one of</p><p>the Titan encounters, remote sensing starts 12 hours before, and ends 4 hours after,</p><p>encounter, with a concentrated burst of other measurements when the spacecraft is</p><p>at its closest approach.</p><p>Besides Titan, flybys of selected icy satellites are used to determine their surface</p><p>compositions and geologic histories, and some orbits are aligned in an anti-solar</p><p>direction to permit studies of the magnetic tail (Table 4.4). Near the end of the four-</p><p>year tour, the orbital inclination will be increased to approximately 85◦, passing</p><p>over the polar regions of Saturn. Then it can investigate the field, particle, and wave</p><p>environment at high latitudes, and try to identify the source of very long radio</p><p>waves (kilometric radiation) from the planet, a phenomenon unique to Saturn. High</p><p>inclinations also permit high-latitude Saturn radio occultations, viewing of Saturn</p><p>polar regions, and more nearly vertical viewing of Saturn’s rings.</p><p>The nominal mission was scheduled to end on July 1, 2008, for a total mission</p><p>duration since launch of 10.7 years, but it is to be extended since enough propellant</p><p>is left to support the function of the orbiter’s systems and science instruments until</p><p>at least 2010.</p><p>4.9.1 Observations of Saturn</p><p>Measurements by Cassini have been essential for improving our understanding of</p><p>the giant planet, which in turn bears important consequences in the studies of com-</p><p>parative planetology, key to our comprehension of the origin and evolution of the</p><p>Solar System and, by extension, of the exoplanetary systems. Among the various</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>108 Titan: Exploring an Earthlike World</p><p>Table 4.4 Cassini flybys of Titan and the icy satellites in the nominal mission.</p><p>Orbit Satellite Flyby date Altitude at closest approach</p><p>0 Phoebe June 11, 2004 1,997 km (1,241 mi)</p><p>A Titan October 26, 2004 1,200 km (746 mi)</p><p>B Titan December 13, 2004 2,358 km (1,465 mi)</p><p>B Probe Release December 24, 2004 n/a km (n/a mi)</p><p>C Iapetus January 1, 2005 65,000 km (40,398 mi)</p><p>C Titan January 14, 2005 60,000 km (37,290 mi)</p><p>3 Titan February 15, 2005 950 km (590 mi)</p><p>3 Enceladus February 17, 2005 1,179 km (733 mi)</p><p>4 Enceladus March 9, 2005 500 km (311 mi)</p><p>5 Titan March 31, 2005 2,523 km (1,568 mi)</p><p>6 Titan April 16, 2005 950 km (590 mi)</p><p>11 Enceladus July 14, 2005 1,000 km (622 mi)</p><p>12 Mimas August 2, 2005 45,100 km (28,030 mi)</p><p>13 Titan August 22, 2005 4,015 km (2,495 mi)</p><p>14 Titan September 7, 2005 950 km (590 mi)</p><p>15 Tethys September 24, 2005 33,000 km (20,510 mi)</p><p>15 Hyperion September 26, 2005 990 km (615 mi)</p><p>16 Dione October 11, 2005 500 km (311 mi)</p><p>17 Titan October 28, 2005 1,446 km (899 mi)</p><p>18 Rhea November 26, 2005 500 km (311 mi)</p><p>19 Titan December 26, 2005 10,429 km (6,482 mi)</p><p>20 Titan January 15, 2006 2,042 km (1,269 mi)</p><p>21 Titan February 27, 2006 1,812 km (1,126 mi)</p><p>22 Titan March 18, 2006 1,947 km (1,210 mi)</p><p>23 Titan April 30, 2006 1,853 km (1,152 mi)</p><p>24 Titan May 20, 2006 1,879 km (1,168 mi)</p><p>25 Titan July 2, 2006 1,911 km (1,188 mi)</p><p>26 Titan July 22, 2006 950 km (590 mi)</p><p>28 Titan September</p><p>7, 2006 950 km (590 mi)</p><p>29 Titan September 23, 2006 950 km (590 mi)</p><p>30 Titan October 9, 2006 950 km (590 mi)</p><p>31 Titan October 25, 2006 950 km (590 mi)</p><p>35 Titan December 12, 2006 950 km (590 mi)</p><p>36 Titan December 28, 2006 1,500 km (932 mi)</p><p>37 Titan January 13, 2007 950 km (590 mi)</p><p>38 Titan January 29, 2007 2,776 km (1,725 mi)</p><p>39 Titan February 22, 2007 953 km (592 mi)</p><p>40 Titan March 10, 2007 956 km (594 mi)</p><p>41 Titan March 26, 2007 953 km (592 mi)</p><p>42 Titan April 10, 2007 951 km (591 mi)</p><p>43 Titan April 26, 2007 951 km (591 mi)</p><p>44 Titan May 12, 2007 950 km (590 mi)</p><p>45 Titan May 28, 2007 2,425 km (1,507 mi)</p><p>46 Titan June 13, 2007 950 km (590 mi)</p><p>47 Tethys June 27, 2007 16,200 km (10,068 mi)</p><p>(Continued)</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 109</p><p>Table 4.4 (Continued )</p><p>Orbit Satellite Flyby date Altitude at closest approach</p><p>47 Titan June 29, 2007 1,942 km (1,207 mi)</p><p>48 Titan July 19, 2007 1,302 km (809 mi)</p><p>49 Rhea August 30, 2007 5,100 km (3,170 mi)</p><p>49 Titan August 31, 2007 3,227 km (2,006 mi)</p><p>49 Iapetus September 10, 2007 1,000 km (622 mi)</p><p>50 Titan October 2, 2007 950 km (590 mi)</p><p>52 Titan November 19, 2007 950 km (590 mi)</p><p>53 Titan December 5, 2007 1,300 km (808 mi)</p><p>54 Titan December 20, 2007 953 km (592 mi)</p><p>55 Titan January 5, 2008 949 km (590 mi)</p><p>59 Titan February 22, 2008 959 km (596 mi)</p><p>61 Enceladus March 12, 2008 995 km (618 mi)</p><p>62 Titan March 25, 2008 950 km (590 mi)</p><p>67 Titan May 12, 2008 950 km (590 mi)</p><p>69 Titan May 28, 2008 1,316 km (818 mi)</p><p>measurements that can contribute to constraining planetary formation models, the</p><p>definition of the chemical composition of the giant planets and in particular the</p><p>content in heavy elements in Saturn, is most critical. The budgetary restrictions that</p><p>eliminated the Saturn probe that was initially part of the Cassini–Huygens mission,</p><p>prevented in situ measurements similar to those performed by the Galileo probe</p><p>in Jupiter in 2005, and thus the determination of the elemental composition in the</p><p>well-mixed region in Saturn.</p><p>Some of the most recent equilibrium thermodynamics models developed for the</p><p>giant planets predict a region where water is well-mixed on Saturn below 20 bars, for</p><p>the nominal case where the condensable volatile abundances found on the planet are</p><p>10 times solar. However, because Saturn, like Jupiter, is a convective and turbulent</p><p>planet, this level may, in fact, lie well below the 20 bar level, at higher pressures.</p><p>Storms on Saturn have been observed by the Cassini ISS in the visible and by Cassini</p><p>VIMS at 5 µm, locating them near the 6–8 bar pressure level, and consequently</p><p>pushing estimates for the well-mixed regions for water to as low as 50–100 bars,</p><p>where the temperatures are in the order of 400–500 K.</p><p>4.9.2 The Icy Satellites, and Saturn’s Rings</p><p>It is of interest to look also at the lists of objectives for Saturn’s rings, and for the</p><p>Saturnian satellites other than Titan, usually referred to as the ‘icy’satellites because</p><p>they show icy surfaces to our telescopes and cameras (although there is plenty of ice</p><p>on Titan as well, of course, hidden below the haze). As well as the indirect bearing</p><p>that these have on understanding Titan, which formed as one of a family of satellites,</p><p>and in an environment that also produced the rings, it has to be born in mind that</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>110 Titan: Exploring an Earthlike World</p><p>Figure 4.22 A typical Titan encounter sequence. The bar in the middle shows the time from closest</p><p>approach to Titan, when the spacecraft is just 800 km above the cloud tops and the satellite’s diameter</p><p>subtends more than 90◦, filling half the sky. The panel at the top of the figure shows how the data</p><p>accumulates with time in the spacecraft memory, before the playback period starts four hours after</p><p>closest approach. The bars at the bottom show the distance of Cassini from the centre of Titan, and</p><p>the angular size of the satellite as seen from the spacecraft (NASA/JPL).</p><p>Figure 4.23 Saturn’s rings photographed edge-on by Cassini, showing how thin they are. The bright</p><p>spot is the moon Enceladus (NASA/JPL).</p><p>Titan is not the only objective of Cassini/Huygens. Some of the instrumentation was</p><p>selected with these other objectives in mind, and even a big spacecraft can carry</p><p>only a limited number of experiments.</p><p>High-resolution imaging can determine the thickness of Saturn’s rings and gather</p><p>information about the sizes, composition, and physical nature of the individual</p><p>particles. With this data it is possible to determine the rate and nature of energy and</p><p>momentum transfer within the rings, and study gravitational interactions between</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 111</p><p>Figure 4.24 The surfaces of Rhea, left, and Hyperion, right, illustrating the extraordinary detail</p><p>of the topography being captured by Cassini’s cameras, exceeding anything achieved previously</p><p>(NASA/JPL/SSI).</p><p>the rings and Saturn’s satellites. Other issues being worked out now include:</p><p>• Studying the configuration of the rings and dynamical processes (gravitational,</p><p>viscous, erosional, and electromagnetic) responsible for ring structure.</p><p>• Mapping the composition and size distribution of the ring material.</p><p>• Investigating the interrelation of rings and satellites, including imbedded satellites.</p><p>• Determining the dust and meteoroid distribution in the vicinity of the rings.</p><p>• Studying interactions between the rings and Saturn’s magnetosphere, ionosphere,</p><p>and atmosphere.</p><p>A similar set of goals is being worked on for the airless satellites (some of which,</p><p>we now know, do in fact have very tenuous atmospheres). These include:</p><p>• Determining the general characteristics and geological histories of the satellites,</p><p>from their cratering record for instance.</p><p>• Defining the mechanisms of crustal and surface modifications, both external and</p><p>internal, particularly for major features like cliffs and rifts.</p><p>• Investigating the compositions and distributions of surface materials, particularly</p><p>dark, organic rich materials and low melting point condensed volatiles, primar-</p><p>ily by spectroscopy (which is much easier for the ices than it is for the com-</p><p>plex, tarry organics; a complete understanding of the latter needs the analysis of</p><p>samples in a laboratory, but some defining characteristics can be divined from</p><p>spectra).</p><p>• Constraining models of the satellites’ bulk compositions and internal structures,</p><p>from their densities and the effects of their gravitational fields on the spacecraft</p><p>as it passes close by, and also from their shape, where that departs from spherical.</p><p>• Studying the interactions of the satellites with the magnetosphere and ring systems</p><p>of Saturn, including material injected into the magnetosphere from the satellites,</p><p>as for example by the plumes on Enceladus.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>112 Titan: Exploring an Earthlike World</p><p>Phoebe was the first satellite to be encountered by Cassini, from 2000 km away,</p><p>a rather unsatisfactorily large distance that was dictated by the requirements for the</p><p>rest of the trajectory. Among the most impressive observations by Cassini are the</p><p>images of Hyperion with its heavily cratered surface and Iapetus with its unexplained</p><p>dichotomy. Also, the first detection of water geysers on Enceladus, give a new</p><p>dimension to our understanding of the “habitable zones” and in any event push back</p><p>our beliefs of where liquid water could be found in the Solar System.</p><p>4.9.3 Saturn’s Magnetosphere andTitan</p><p>Like Jupiter, Saturn has a strong internal magnetic field and hence an extensive</p><p>magnetosphere. This contains neutral gas, some of which can become ionized and</p><p>contribute to the charged particle populations of the radiation belts. The mechanism</p><p>is probably sputtering by charged particle and meteoroid impact on the material in the</p><p>rings and on the surfaces of Saturn’s icy moons. This is believed to create an extensive</p><p>neutral cloud of water molecules</p><p>and water dissociation products, including ionized</p><p>species, around the planet in the inner magnetosphere.</p><p>Titan and Saturn’s magnetosphere interact: Titan’s substantial atmosphere and</p><p>ionosphere act as both a source and a sink of neutrals and ions to Saturn’s outer</p><p>magnetosphere. The primary processes involve energetic ions that enter Titan’s</p><p>atmosphere and sputter off neutral atoms or molecules. Certain photochemical reac-</p><p>tions can achieve the same effect, if they energize the neutrals beyond the escape</p><p>speed of Titan. These ejected neutrals will end up orbiting Saturn, producing a torus</p><p>composed mainly of hydrogen and nitrogen. These neutral atoms and molecules</p><p>can become ionized and picked up by the local magnetic field, producing a plasma</p><p>torus around Saturn. Alternatively, neutrals in Titan’s atmosphere can be ionized</p><p>by impact with Saturn’s magnetospheric electrons or by solar extreme ultraviolet</p><p>photons. Ions created at lower altitudes may flow out of the atmosphere, down the</p><p>wake, while ions created at higher altitudes are picked up by the external plasma</p><p>and magnetic field, leading to a Titan plasma plume. This wraps all the way around</p><p>Saturn, so as Titan orbits it can encounter the plasma again, leading to some complex</p><p>interactions.</p><p>As is evident form the data taken by MIMI, energetic particle intensities in the</p><p>vicinity of Titan’s orbit are highly variable and fluctuate in response to changes in</p><p>solar wind pressure and internal magnetospheric activity. In fact, Titan often finds</p><p>itself outside the magnetopause near local noon, and exposed to the heated solar</p><p>wind in the magnetosheath. This variability in energetic particle input has important</p><p>consequences on Titan’s ionosphere and upper atmosphere. The energetic neutral</p><p>atoms around Titan that originate as fast ions trapped in the magnetic field can</p><p>charge-exchange with the upper atmosphere. The maximum intensity occurs at the</p><p>relatively high altitude of ∼3000 km, surrounding the entire moon.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 113</p><p>The Cassini teams are working to clarify this picture, with the overall goal</p><p>being to understand what role these interactions have had, and may have in the</p><p>future, for the evolution of Titan’s atmosphere. The first task is to measure the</p><p>configuration of the planetary magnetic field, globally and in the vicinity of Titan,</p><p>and determine the composition and the sources and sinks of the various types of</p><p>magnetosphere particles. These will eventually form the basis for improved models</p><p>of the interactions of Titan’s atmosphere and exosphere with the solar wind and</p><p>Saturn’s magnetospheric plasma.</p><p>4.10 Being Involved: Scientists and Instrument Providers</p><p>Missions like Cassini do not just happen. They are conceived and brought into being,</p><p>not so much by the space agencies who manage them but by visionary people in the</p><p>world scientific community. Acting through their institutions, learned societies and</p><p>through committees of various scientific and research organisations, it is generally</p><p>a committed few who take the initial steps to define and lobby for a project, often in</p><p>the form of a very rudimentary concept, which may not be the same as that which</p><p>finally evolves. We have already seen how the ‘SOPP’ version of Cassini, meaning</p><p>a Saturn orbiter carrying two Probes, one for Saturn and one for Titan, was scaled</p><p>down to save money.</p><p>Mission proposals gather momentum by attracting funds for studies, which</p><p>define the scientific objectives and the details of what will be required to accom-</p><p>plish them. Usually, the anticipated cost, compared as objectively as possible with</p><p>the new knowledge and other gains anticipated, is the crucial thing that emerges</p><p>as the ‘bottom line’. If a mission concept attracts enough support from the com-</p><p>munity, as sampled and evaluated by the agency’s committees (themselves usually</p><p>staffed by working scientists from universities and research centres), it can then get</p><p>approved as a flight project and the requisite funds become part of the agency’s</p><p>budget. Cassini required this process to be undergone in the USA and in Europe</p><p>in tandem.</p><p>Once approved, the work to build the spacecraft, acquire the launcher and all</p><p>of the other mired requirements of such a complicated venture, are set in motion.</p><p>Many of them require contracts with large industrial companies. Plans to track</p><p>and control the spacecraft are put in place and time reserved on the giant anten-</p><p>nae of the Deep Space Network, the only ones large enough to pick up the high-</p><p>speed data link from Saturn. By the time Cassini becomes a memory, around the</p><p>year 2020 or so, many thousands of people will have contributed to it in a variety</p><p>of ways.</p><p>The really exciting role to play, for a scientist, is that of a scientific investigator.</p><p>This means you have been selected by the space agency, advised by peer review</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>114 Titan: Exploring an Earthlike World</p><p>groups, to carry out a key scientific role in the mission (and, incidentally, for a long</p><p>mission like Cassini, have been given a job for life!). There are three main kinds of</p><p>investigators: Principal Investigator (abbreviated to ‘PI’), Co-Investigator, or Co-I,</p><p>and Interdisciplinary Scientist, or IDS. These people take the lead in planning,</p><p>guiding, analysing and reporting the scientific aspects of the mission, which of</p><p>course underlie everything else. How does one get such an exciting and rewarding</p><p>job on a mission, an adventure, like Cassini?</p><p>The answer is that you write a proposal, having first made enough of a reputation</p><p>that the proposal will be taken seriously. The space agency requests proposals when it</p><p>announces its plans for the mission, and makes arrangements to have them reviewed</p><p>and evaluated. Principal Investigators lead teams which provide instruments, so their</p><p>proposals have to show what their instrument would measure, what technique they</p><p>would use, and how they would get it all done (and at what price!). Co-Investigators</p><p>write part of the same proposal, showing how they would bring specific expertise to</p><p>the design or building of the instrument, or perhaps to some aspect of planning the</p><p>observations or analysing the data.</p><p>Table 4.5 The Interdisciplinary Scientists (IDSs) on Cassini–Huygens.</p><p>Interdisciplinary scientist Tasks</p><p>Dr. Michel Blanc,</p><p>Observatoire de Midi-Pyrenees,</p><p>France</p><p>Transport of mass, linear and angular momentum, and energy</p><p>in the magnetosphere/ionosphere/thermosphere system of</p><p>Saturn</p><p>Dr. Jeffrey N. Cuzzi,</p><p>NASA Ames Research Center,</p><p>USA</p><p>Chemical composition, structure, particle sizes, origin, and</p><p>evolution of Saturn’s rings</p><p>Dr. Tamas I. Gombosi,</p><p>University of Michigan, USA</p><p>Molecular composition, structure, and dynamical behaviour</p><p>of the plasma within Saturn’s magnetosphere</p><p>Dr. Tobias C. Owen,</p><p>University of Hawaii, USA</p><p>Abundances and isotopic ratios of atmospheric constituents</p><p>on Titan and Saturn, and their implications for their evolu-</p><p>tionary paths</p><p>Dr. Laurence A. Soderblom,</p><p>United States Geological Survey</p><p>Flagstaff, USA</p><p>Geologic processes, evolutionary history, composition, and</p><p>physical state of the surfaces of Saturn’s icy satellites</p><p>Dr. Daniel Gautier,</p><p>Observatoire de Paris-Meudon,</p><p>France</p><p>Titan’s atmospheric aerosols, photochemistry, general circu-</p><p>lation, and upper atmosphere</p><p>Dr. Jonathan I. Lunine,</p><p>University of Arizona, USA</p><p>Titan’s atmospheric and surface evolution and the stability of</p><p>the present atmosphere</p><p>Prof. François Raulin,</p><p>LISA, Univ. Paris 12, France</p><p>Organic chemistry in Titan’s environment and its implications</p><p>for exobiology and the origins of life</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>Cassini–Huygens: Orbiting Saturn and Landing on Titan 115</p><p>IDSs usually write their own proposals, explaining how they have special knowl-</p><p>edge or skills that would blend in with and enhance the efforts of the experiment</p><p>teams. This might take the form of an advanced computer model of the dynamics</p><p>of Titan’s atmosphere, for</p><p>example, which could be used to analyse the data from</p><p>many instruments. Eight ‘interdisciplinary’ scientific investigations are part of the</p><p>Cassini/Huygens mission (5 on Cassini and 3 on Huygens). In these, named scien-</p><p>tists lead teams to look at specific problems using a combination of models and data</p><p>from as many of the different instruments as apply.</p><p>Finally, the Cassini Project Scientist, D. Matson of the Jet Propulsion Laboratory</p><p>and the Huygens Project Scientist, J.-P. Lebreton of the European Space Science</p><p>and Technology Centre, are the overall spokespersons for the orbiter and probe parts</p><p>of the mission respectively.</p><p>Once selected, the teams meet regularly and plan and supervise the development</p><p>of their experiment. They also contribute members to teams that develop particular</p><p>aspects of the mission, such as the observing sequences. This can be painful at times,</p><p>especially for the PI whose instrument falls behind schedule, or goes over budget,</p><p>with launch only a year or two away. The ‘window’ from Earth to Saturn is only</p><p>open for a few weeks, when the two planets are aligned correctly, so the launch</p><p>cannot wait (especially one with such a complicated trajectory as Cassini, with no</p><p>less than four planets needed in the right place at the right time!). Teams then work</p><p>all night, relax some of the requirements on their hardware, or any of a dozen other</p><p>options to get back on track. It is a rare experiment, or a simple one, which does not</p><p>have to make compromises at some stage, usually to save money when it runs short.</p><p>4.11 Reaping the Benefits</p><p>Even before launch, the Cassini project produced valuable technology, manage-</p><p>ment tools, and knowledge, resulting in new applications for aerospace, business,</p><p>communications, computing, and education. Many of Cassini’s key technologi-</p><p>cal innovations are now in use by other low-cost, high-efficiency space mission</p><p>programmes.</p><p>As examples one can cite the use of solid-state data recorders with no moving</p><p>parts, new electronic chips on the orbiter computer which directs the operations,</p><p>a new integrated circuit on the computer system, an innovative solid-state power</p><p>switch, a new X-band radio transponder, and a new hemispherical resonator gyro-</p><p>scope. The Cassini project also develops curricula tools and classroom supplements</p><p>to enhance science teaching in schools. A European Network, called EuroPlanet</p><p>(http://europlanet.cesr.fr/), has been activated recently and focuses on the promo-</p><p>tion of planetary exploration from missions like Cassini. The presence of a Cassini</p><p>signature disc, comprising more than 616,400 handwritten signatures from 81 coun-</p><p>tries all over the world, marks the interest of the public for this enterprise and is</p><p>reminiscent of the souvenir that Voyager was carrying in the 1970s.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch04</p><p>116 Titan: Exploring an Earthlike World</p><p>Cassini and Huygens have already provided a wealth of data. The analysis is still</p><p>far from over, as we write, and the mission promises to unveil yet more of Titan’s</p><p>secrets in the months and years to come. The following chapters provide an account</p><p>of the latest information we have on Titan’s environment, from all available means</p><p>of investigation, shortly before the end of Cassini’s nominal mission.</p><p>June 4, 2008 8:54 B-611 9.75in x 6.50in plate</p><p>Colour Plates 117</p><p>Plate 1 Saturn’s satellite and ring system (NASA/JPL).</p><p>Plate 2 Two views of Titan’s disk. Left: taken by the Voyager 2 cameras in Aug. 1981 and showing</p><p>Titan’s North to South asymmetry, with the southern hemisphere appearing lighter and a well-defined</p><p>band near the pole. The extended haze is also clearly visible around the satellite’s limb, but no glimpses</p><p>of the surface were possible. Right: 23 years later, Titan’s complex surface is at last revealed by</p><p>Cassini/VIMS, by imaging at a longer wavelength in the near infrared (NASA/JPL).</p><p>June 4, 2008 8:54 B-611 9.75in x 6.50in plate</p><p>118 Titan: Exploring an Earthlike World</p><p>Plate 3 A schematic representation of the atmospheric structure and surface interactions on Titan.</p><p>Plate 4 Views of Titan at various wavelengths, from Earth-based telescopes, in a montage by</p><p>M. Hirtzig. From upper-left to bottom-right, the images are sorted in descending order of the altitude</p><p>probed. The diameter of the image reflects the size of the telescope used: 10 m at the Keck (large),</p><p>8.2 m at the VLT (medium), and 2.4 m for the HST (small). The colour code corresponds to the wave-</p><p>length, from blue (near IR around 1 µm) to red (longward of 2 µm). Several atmospheric features can</p><p>be detected, including a bright northern limb in the upper stratosphere due to a local excess of aerosols,</p><p>a bright polar collar in the lower stratosphere, and clouds above the south pole in the troposphere.</p><p>Surface features are also clearly visible in the right-hand images, with high contrast and up to 300 km</p><p>per pixel spatial resolution, while the four HST images show the rotation of Titan’s surface, made</p><p>obvious by the movement of the bright continent Xanadu.</p><p>June 4, 2008 8:54 B-611 9.75in x 6.50in plate</p><p>Colour Plates 119</p><p>Plate 5 The Cassini spacecraft, showing the instruments and subsystems attached to the main struc-</p><p>ture (NASA/JPL).</p><p>June 4, 2008 8:54 B-611 9.75in x 6.50in plate</p><p>120 Titan: Exploring an Earthlike World</p><p>Plate 6 Close-ups of the Huygens probe configuration (left) and the technical and mechanical struc-</p><p>ture of the combined Cassini–Huygens spacecraft (right) (ESA/NASA/JPL).</p><p>Plate 7 The launch of the Cassini–Huygens mission from Cape Canaveral on October 15, 1997, on</p><p>an appropriately named Titan rocket (NASA/JPL).</p><p>June 4, 2008 8:54 B-611 9.75in x 6.50in plate</p><p>Colour Plates 121</p><p>Plate 8 The trajectory followed by Cassini–Huygens through the Solar System (NASA/JPL).</p><p>Plate 9 An artist’s concept of Cassini during the Saturn Orbit Insertion manoeuvre on July 1, 2004.</p><p>The orbit insertion motor fired for 90 minutes, allowing Cassini to be captured by Saturn’s gravity into</p><p>a five-month orbit. The spacecraft’s proximity to the planet at this time offered a unique opportunity</p><p>to observe Saturn and its rings at extremely high resolution (NASA/JPL).</p><p>June 4, 2008 8:54 B-611 9.75in x 6.50in plate</p><p>122 Titan: Exploring an Earthlike World</p><p>Plate 10 Density profiles of several important ion species measured by the INMS versus altitude,</p><p>time from CA, and solar zenith angle for the outbound T5 encounter of Cassini with Titan. An electron</p><p>density profile measured by the RPWS experiment is also shown (Cravens et al., 2005).</p><p>Plate 11 Cassini MIMI observations of magnetosphere dynamics near Titan, showing the emis-</p><p>sion from hydrogen atoms in Titan’s exosphere, about 2500 km above the surface, stimulated by the</p><p>energetic plasma flow in Saturn’s magnetic field (NASA/APL).</p><p>June 4, 2008 8:54 B-611 9.75in x 6.50in plate</p><p>Colour Plates 123</p><p>Plate 12 Christmas Day, 2004: Cassini releases the Huygens probe, seen inside its flat, conical heat</p><p>shield, in the direction of Titan, seen below in the foreground with Saturn in the distance (NASA/JPL).</p><p>June 4, 2008 8:54 B-611 9.75in x 6.50in plate</p><p>124 Titan: Exploring an Earthlike World</p><p>Plate 13 An artistic view of Huygens landed on the Titan surface (ESA).</p><p>Plate 14 A montage illustrating the Doppler Wind Experiment, which used observations of the</p><p>Huygens signal from several Earth-based radio telescopes.</p><p>June 4, 2008 8:54 B-611 9.75in x 6.50in plate</p><p>Colour Plates 125</p><p>Plate 15 Views of Titan’s surface from the Huygens/DISR cameras during the descent through Titan’s</p><p>atmosphere on January 14, 2005. This montage shows flattened (Mercator) projections of the scene</p><p>taken at four different altitudes (ESA/NASA/JPL/University of Arizona).</p><p>June 4, 2008 8:54 B-611 9.75in x 6.50in plate</p><p>126 Titan: Exploring an Earthlike World</p><p>Plate 16 Titan and Earth atmospheres compared (NASA/JPL).</p><p>Plate 17 An imaginative view of sunrise over a hydrocarbon lake on Titan, with Saturn visible through</p><p>a gap in the cloud (© Kees Veenenbos).</p><p>June 4, 2008 8:54 B-611</p><p>9.75in x 6.50in plate</p><p>Colour Plates 127</p><p>Plate 18 Synopsis of current observations for Titan’s surface from Cassini and from the Huygens</p><p>probe. The background map is from a mosaic of Cassini ISS images (NASA/ESA/JPL/University</p><p>Arizona; LPG/Nantes & Sigal@LESIA).</p><p>Plate 19 Venus, Mars and Titan, roughly to scale.</p><p>June 4, 2008 8:54 B-611 9.75in x 6.50in plate</p><p>128 Titan: Exploring an Earthlike World</p><p>Plate 20 The future exploration of Titan by instrumented platforms floating in the atmosphere: above,</p><p>a Zeppelin-type dirigible over a varied landscape and below, a hot-air balloon over the shore to a</p><p>hydrocarbon lake (artistic views by T. Balint).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>CHAPTER 5</p><p>Titan’s Atmosphere and Climate</p><p>Scientific materialism, which is commoner among lay followers of science than</p><p>among scientists themselves, holds that what is composed of matter or energy and</p><p>is measurable by the instruments of science is real. Anything else is unreal, or at</p><p>least of no importance.</p><p>Robert M. Pirsig, Zen and the Art of Motorcycle Maintenance</p><p>5.1 The Climate onTitan</p><p>In planetary science, ‘climate’ usually refers to the structure of the atmosphere,</p><p>especially the global and annual mean distribution of pressure and temperature as a</p><p>function of height, and how that affects conditions at the surface.Apart from distance</p><p>from the Sun, the most important factors determining the climate on any planetary</p><p>body are the total mass of the atmosphere (determined from the surface pressure and</p><p>acceleration due to gravity), and its composition. Of course, the average conditions</p><p>are a very complex derivative of these basic parameters, in particular they are the</p><p>result of some very dynamic meteorology and cloud chemistry. Both of these topics</p><p>are complex enough to be the subjects of separate chapters later in the book. The</p><p>long-term evolution of Titan’s atmosphere, within the context of the Solar System</p><p>as a whole, will also be the subject of a separate chapter.</p><p>The data from Cassini and Huygens resulted in an explosion of knowledge about</p><p>Titan’s atmosphere, particularly the dynamical processes mentioned above, and the</p><p>near-surface environment. They have also permitted some refinement of the basic</p><p>facts about Titan that affect its climate, including key atmospheric parameters, which</p><p>we summarise in Table 5.1.</p><p>5.1.1 Atmospheric Pressure Profile</p><p>The accelerometer on Huygens first detected the aerodynamic drag forces due to</p><p>Titan’s atmosphere at a height of about 1,500 km, about ten times higher than would</p><p>have been the case if its target had been the Earth. The pressure scale height (the</p><p>vertical height in which the pressure falls by a factor of e−1, or by about one-third</p><p>of its value at some baseline) is proportional to temperature, so it varies with height,</p><p>129</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>130 Titan: Exploring an Earthlike World</p><p>Table 5.1 Titan’s orbital and body parameters, and atmospheric properties from</p><p>recent Cassini–Huygens measurements.</p><p>Surface radius 2575 km</p><p>Mass 1.35 × 1023 kg (= 0.022 × Earth)</p><p>Atmospheric Mass 6.25 × 1018 kg (= 1.19 × Earth)</p><p>Mean density 1880 kg m−3</p><p>Surface gravity 1.354 m s−2</p><p>Distance:</p><p>from Saturn 1.23 × 109m (= 20 Saturn radii)</p><p>from Sun 9.546 AU</p><p>Orbital period</p><p>around Saturn 15.95 days</p><p>around Sun 29.5 years</p><p>Obliquity 26.7◦</p><p>Surface temperature 93.65 K</p><p>Surface pressure 1.467 bar</p><p>Atmospheric composition near surface:</p><p>Nitrogen, N2 95.1%</p><p>Methane, CH4 4.9%</p><p>Argon, Ar 0.004%</p><p>Isotopic ratios:</p><p>14N/15N 183 (0.67 × Earth)</p><p>12C/13C 82.3 (0.915 × Earth)</p><p>36Ar/40Ar 0.0065 (2 × Earth)</p><p>and inversely proportional to the product of the mean molecular weight, which is</p><p>nearly the same on Earth and Titan (about 28) since the atmospheric compositions</p><p>are similar, and the acceleration due to gravity, which of course is much smaller on</p><p>Titan by almost an order of magnitude. The scale height is typically about 8 km in</p><p>the troposphere of the Earth, 37 km on Saturn, with its low mean molecular weight,</p><p>and 20 km in the lower atmosphere of Titan. In the stratosphere, Titan’s scale height</p><p>is closer to 40 km.</p><p>The atmospheric pressure as a function of height was measured by Huygens from</p><p>the velocity as a function of time, determined by integrating the measured decel-</p><p>eration while it was descending under its parachutes, using the altitude obtained</p><p>by integrating the vertical component of the velocity using tracking data obtained</p><p>by the Cassini navigation team. Pressures were obtained from the density profile</p><p>under the assumption of hydrostatic equilibrium using the known values for Titan’s</p><p>mass, radius and surface gravity. Once the heat shield had been discarded and the</p><p>parachute deployed, the on-board pressure and temperature sensors were exposed to</p><p>the atmosphere. Stable readings commenced at about 150 km altitude and continued</p><p>down to, and on, the surface. The pressure at the landing site (at 15◦S, 192◦W) was</p><p>1, 4767 ± 1 hPa, remaining steady within this range during the short time measure-</p><p>ments were made on the surface.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>Titan’s Atmosphere and Climate 131</p><p>806040 100 120 140 180 200 220160</p><p>Temperature (K)</p><p>Surface temperature</p><p>94 K (-179 C) Lakes on surface</p><p>1400</p><p>1000</p><p>500</p><p>250</p><p>50</p><p>0</p><p>10-8</p><p>10-6</p><p>10-4</p><p>10-2</p><p>100</p><p>102</p><p>H</p><p>e</p><p>ig</p><p>h</p><p>t</p><p>(k</p><p>m</p><p>)</p><p>P</p><p>re</p><p>ss</p><p>u</p><p>re</p><p>(</p><p>m</p><p>b</p><p>)</p><p>Oily drizzle</p><p>Haze</p><p>Model</p><p>Huygens</p><p>UV &</p><p>blue light</p><p>IR &</p><p>red light</p><p>Troposphere</p><p>Stratosphere</p><p>Mesosphere</p><p>Thermosphere</p><p>Methane</p><p>clouds & rain</p><p>Figure 5.1 A model of Titan’s atmosphere showing the temperature profile, clouds and haze layers,</p><p>precipitation, and surface, and some of the important radiation fluxes. The jagged curve is the temper-</p><p>ature profile derived from Huygens measurements (Fulchignoni et al., 2005) and the smooth curve is</p><p>a pre-Cassini (but post-Voyager) theoretical model. The two coincide closely below the haze layers.</p><p>5.1.2 AtmosphericThermal Structure</p><p>Huygens also obtained a temperature profile in the upper atmosphere as a function</p><p>of altitude from about 1,400 km down to 150 km, using the measured decelerations,</p><p>the inferred pressures and densities, and the equation of state for a perfect gas,</p><p>and assuming a value for the atmospheric mean molecular weight. Below 160 km,</p><p>measurements with higher accuracy (better than 1 K) were obtained by direct mea-</p><p>surement, using resistance thermometers exposed to the atmosphere.</p><p>By analogy with the Earth, Titan’s atmosphere is subdivided into regions defined</p><p>by the way temperature varies with height above the surface. On both planetary</p><p>bodies, the temperature profile is characterised by inversions (locations above which</p><p>the temperature profile switches from increasing with altitude to decreasing, or vice</p><p>versa). This is usually produced by a localised region of heating, such as occurs</p><p>in the middle atmosphere of the Earth due to the stratospheric ozone layer, and</p><p>also in Titan’s haze layers. Both of these absorb solar energy and heat the region,</p><p>producing a local temperature maximum. The regions between the temperature</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>132 Titan: Exploring an Earthlike World</p><p>Figure 5.2 The pressure profile of the lower atmosphere as measured by the HASI experiment on</p><p>Huygens (solid line), corrected for dynamical effects. These new data have an estimated uncertainty</p><p>of 1% at all altitudes, and are not very different from the pressure values obtained by Voyager 1 radio</p><p>occultation, which are shown as circles (ingress) and crosses (egress) (Fulchignoni et al., 2005).</p><p>Figure 5.3 Temperature profile of the lower atmosphere as measured by HASI on Huygens.</p><p>The estimated uncertainty is ±0.25 K in the range from 60 to 110 K, and ±1 K above 110 K. Like</p><p>the pressure profile, they provide a good match to temperatures calculated from the Voyager radio</p><p>occultation experiment assuming a pure nitrogen atmosphere, shown as circles (ingress) and crosses</p><p>(egress) (Fulchignoni et al., 2005).</p><p>maxima and minima are given the corresponding names in both atmospheres, while</p><p>the boundaries</p><p>the</p><p>knowledge of Titan. They are two of the scientists most knowledgeable about this</p><p>fascinating moon of Saturn. As a result of Cassini–Huygens’ arrival around Saturn</p><p>in 2004, and the Huygens probe landing on Titan’s surface on January 14, 2005,</p><p>a giant advance has been made in our understanding of Titan. Thus, it is timely to</p><p>issue a new textbook. Athena Coustenis and Fred Taylor have come up with a superb</p><p>revised version of their book. The new edition gives an excellent and up to date</p><p>account of our knowledge about Titan. It provides a comprehensive review of this</p><p>peculiar solar system object which bears many similarities to Earth albeit under very</p><p>different conditions, where methane plays the role that water plays on Earth. The</p><p>latest results are described and include Earth-based observations, laboratory work</p><p>and modelling in addition to the Cassini–Huygens observations.</p><p>The book tells the history of the exploration of Titan. It includes the most recent</p><p>ideas about the processes that govern Titan. It shows extremely well the synergy</p><p>of in situ robotic observations and space-based and Earth-based telescopic obser-</p><p>vations. It also shows the importance of laboratory work and modelling. All of</p><p>these approaches are needed to make progress in our understanding of Solar System</p><p>objects. The book is an excellent reference for students with some general back-</p><p>ground in the field of planetary sciences and for new planetary scientists looking</p><p>for a comprehensive book on Titan. After reading the book, you may decide to stick</p><p>with Titan for the rest of your career! The authors of the book are currently active</p><p>xvii</p><p>June 5, 2008 11:26 B-611 9.75in x 6.50in fm</p><p>xviii Titan: Exploring an Earthlike World</p><p>in the Cassini–Huygens mission and have been involved in Titan research for sev-</p><p>eral decades. They have mentored young scientists who are now Titan researchers</p><p>themselves.</p><p>Titan has become a well-known object in the Solar System. It is the most Earthlike</p><p>world we know of. This book puts this extraordinary solar system object on our Solar</p><p>System map for additional exploration. The closing chapter describes new concepts</p><p>for the future exploration of Titan and gives us an irresistible invitation to return.</p><p>Jean-Pierre Lebreton</p><p>ESA Huygens Project Scientist</p><p>Dennis Matson</p><p>NASA Cassini Project Scientist</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>CHAPTER 1</p><p>Introduction</p><p>For more than a hundred years, those of us in the speculative fiction business have</p><p>been speculating like mad about Titan, Saturn’s largest moon. Back in 1894, in</p><p>Journey in Other Worlds, John Jacob Astor wrote of a group of travelers whose</p><p>spacecraft crossed the orbit of Titan on its way to Saturn. (On Saturn, the travelers</p><p>wore their winter clothes, as it was rather cold.) Over the passing years, science</p><p>fiction authors have written of Titan as a mining colony (Arthur C. Clarke’s Imperial</p><p>Earth, Alan E. Nourse’s Trouble on Titan) and as a source of aliens (Philip K.</p><p>Dick’s The Game-Players of Titan). These days, authors seem more inclined to</p><p>consider the possibilities of life on Saturn’s largest moon (Stephen Baxter’s Titan</p><p>and Michael Swanwick’s Hugo Award-winning novelette “Slow Life.”)</p><p>Pat Murphy & Paul Doherty, Fantasy & Science Fiction</p><p>1.1 Early History</p><p>The discovery that the giant planets of the outer Solar System had large moons came</p><p>quickly following the invention of the telescope in 1610. Galileo observed Jupiter</p><p>and found the four biggest Jovian satellites, which he called the Medici stars, now</p><p>known to us as the Galilean satellites: Io, Europa, Ganymede and Callisto. In the</p><p>four decades from 1610 to 1651, Italian, French and English astronomers identified</p><p>seventeen giant planet satellites, or more than half of the total known before the</p><p>space age began. Then, from 1671 to 1684, Cassini found four more in the Saturnian</p><p>system, the satellites we now know as Iapetus, Rhea, Tethys and Dione.</p><p>Next came Herschel, discovering in two years (1787–88) two Uranian satellites</p><p>(Titania and Oberon) and two more Kronian (i.e. Saturnian) satellites, Mimas and</p><p>Enceladus. Finally, four more moons were discovered thanks to the observational</p><p>skills of Lassell from 1846 to 1851: Triton, the large satellite of Neptune, Hyperion</p><p>of Saturn (a co-discovery with Bond from the USA), and two moons of Uranus,</p><p>Ariel and Umbriel.</p><p>Thus, European astronomers developed a trend by which they discovered, every</p><p>sixty to one hundred years or so, in short intense periods, four at a time, the satellites</p><p>of the Solar System, until in the mid-nineteenth century they mostly lost interest</p><p>and turned to new astronomical topics, leaving the rest of the satellite discoveries</p><p>to their American colleagues. Near the beginning of that first period of satellite</p><p>1</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>2 Titan: Exploring an Earthlike World</p><p>discoveries, one of them did not conform to the four-by-four rule and was for many</p><p>years believed to be the biggest satellite of all. They called it Titan. It was not, in</p><p>fact, until Voyager measured Ganymede’s radius with precision that Titan lost its</p><p>crown as the largest moon in the Solar System, and then only by a very small margin.</p><p>It still looks bigger than Ganymede, however, because its disk is extended more than</p><p>100 km by the thick, hazy atmosphere.</p><p>The story of the first detection of Titan is a classic of its kind. On the night</p><p>of March 25, 1655, a novice Dutch astronomer, Christiaan Huygens, pointed his</p><p>telescope at Saturn. It was the first professional telescope he had ever had in his</p><p>possession, and was by no means an exceptional one, with just a 4 m long refractor</p><p>and an enlarging capacity factor of 50. He saw a small star 3 arc minutes away from</p><p>the planet. This was not even the first time someone had noticed this object: Hevelius</p><p>from Poland, and Wren in England had perceived it previously in the night sky, but</p><p>not knowing what to make of it — and star catalogues being scarce at the time —</p><p>they had believed it to be just another star.</p><p>Huygens did not make this mistake: he almost immediately guessed it was a</p><p>satellite, and confirmed his guess a few days later when the ‘star’ had moved. Thus,</p><p>a Dutchman discovered, right in the middle of the period of Italian, French and</p><p>English supremacy, a Saturnian satellite. It is true that Huygens was not one of</p><p>those who discovered four satellites, and in fact he never even noticed Tethys and</p><p>Dione. But what he did discover was enough to make him famous, because Titan</p><p>was soon realised to be an important object by virtue of its great size. Huygens</p><p>believed Titan to be the biggest satellite of all, greater even than Ganymede, the</p><p>Figure 1.1 Giovanni Domenico Cassini, 1625–1712 (Observatoire de Paris).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>Introduction 3</p><p>Figure 1.2 Christiaan Huygens, 1629–1695 (Observatoire de Paris).</p><p>Figure 1.3 Huygens at his telescope (Observatoire de Paris).</p><p>largest satellite of Jupiter, the king of planets. The error was understandable: Titan’s</p><p>atmosphere increases the apparent size of the object, so the disc as seen from Earth</p><p>is larger than any other satellite, the others being airless, or nearly so. This attribute</p><p>is the reason for the name it was given, following a proposition by Herschel, who</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>4 Titan: Exploring an Earthlike World</p><p>Figure 1.4 Huygens’ first recorded observations of Saturn in March 1655 (above) and a sketch by</p><p>Cassini made on January 18, 1691 (below) (Observatoire de Paris).</p><p>Figure 1.5 Saturn — the original (Observatoire de Paris).</p><p>had suggested using the names of gods associated with Saturn for his satellites,</p><p>according to mythology:</p><p>As Saturn devoured his children, his family could not be assembled around him,</p><p>so that the choice lay among his brothers and sisters, the Titans and Titanesses.</p><p>The name of Iapetus seemed indicated by the obscurity and remoteness of the</p><p>exterior satellite, Titan by the superior size of the Huyghenian, while the three</p><p>female appellations</p><p>between regions are identified by terms ending with -pause (from</p><p>the Greek παυση = end). However, the vertical extent of each region is different,</p><p>with Titan’s atmosphere generally being more extensive than the Earth’s. In fact, the</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>Titan’s Atmosphere and Climate 133</p><p>extent of Titan’s atmosphere is comparable to the radius of the solid body (2,575 km),</p><p>while that of the Earth is sometimes likened to the skin on a grape.</p><p>5.1.3 Troposphere</p><p>Much of the electromagnetic radiation emitted by the Sun and reaching a planet</p><p>is at wavelengths in or near the visible part of the spectrum. The haze and cloud</p><p>layers scatter the photons and reflect a portion of the radiation back to space, while</p><p>some of it is absorbed by the aerosols and gases in the atmosphere. The rest of the</p><p>energy, about 10% on Titan, reaches the ground where it is absorbed. The lower</p><p>atmosphere is then heated by the ground, and becomes unstable against convection.</p><p>This simply means that, if the temperature gradient is too steep, lower layers will</p><p>be more buoyant than those above and they will tend to rise, forcing more powerful</p><p>convection. If the gradient is too shallow, the motion will stop until the temperature</p><p>gradient builds up again. The net result is that the gradient is usually close to a</p><p>value called the adiabatic lapse rate, at which convection is just possible. Assuming</p><p>that hydrostatic equilibrium applies, and that there is no net exchange of energy</p><p>between a parcel of air and its surroundings, then the perfect gas law and some</p><p>elementary thermodynamics tell us that the temperature gradient with height is</p><p>just the acceleration due to gravity divided by the specific heat of the air. Thus it</p><p>is constant for a given composition, and works out to be equal to approximately</p><p>10 K km−1 for dry terrestrial air. For moist air, it is less (about 6.5 K km−1 typically)</p><p>and can be as small as 3 K km−1. Since Titan has the same main constituent as on</p><p>Earth, the difference in lapse rate is due mainly to the difference in gravity, and its</p><p>value on Titan works out to be only about 1.4 K km−1. Again, this varies depending</p><p>on the condensable vapour present in the form of methane, although the temperature</p><p>lapse rate measured by Huygens shows that the temperature gradient, or lapse rate,</p><p>is shallower (more stable) than the dry adiabatic lapse rate, the rate for the buoyancy</p><p>of a dry air parcel to just balance the vertical pressure gradient.</p><p>On the Earth, the region where convection dominates vertical heat transport is</p><p>known as the troposphere (‘turning-region’). The upper boundary occurs near the</p><p>level where the overlying atmosphere is of such a low density that a substantial</p><p>amount of radiative cooling to space can occur in the thermal infrared region of</p><p>the spectrum. At the tropopause, radiation cools rising air so efficiently that the</p><p>temperature tends to become constant with height and convection ceases. The Earth’s</p><p>tropopause varies in height over about 6 km with latitude, being highest (around</p><p>16 km) in the tropics, where solar heating is greatest, but it is generally quite a distinct</p><p>feature in the temperature profile everywhere, and usually occurs only slightly below</p><p>the temperature minimum.</p><p>On Titan, the temperature falls with height from the surface up to about 40 km, so</p><p>this level is also referred to as the tropopause. At the Huygens entry site, the probe’s</p><p>sensors found that the temperature minimum, and hence the tropopause temperature,</p><p>was 70.43 K at about 44 km where the pressure was 115 ± 1 hPa. This is about 1 K</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>134 Titan: Exploring an Earthlike World</p><p>150</p><p>100</p><p>50</p><p>25</p><p>0</p><p>H</p><p>e</p><p>ig</p><p>h</p><p>t</p><p>(k</p><p>m</p><p>)</p><p>Methane cloud</p><p>and rain</p><p>Hydrocarbon haze</p><p>Surface Lakes &</p><p>oceans?</p><p>Oily drizzle</p><p>80 100 120 140 180160</p><p>Temperature (K)</p><p>Figure 5.4 The mean temperature-height profile for Titan represented by a simple radiative-</p><p>convective equilibrium model (heavy line), compared to the profile measured by Huygens. The model</p><p>fails completely at higher levels because it takes no account of the heating due to aerosol, which</p><p>greatly modifies the temperature. However, the profiles compare quite well in the middle to lower</p><p>atmosphere, and at the surface.</p><p>10-810-9 10-610-7 10-4</p><p>10-5</p><p>Cooling rate (K s-1)Cooling rate (K s-1)</p><p>P</p><p>re</p><p>ss</p><p>u</p><p>re</p><p>(</p><p>m</p><p>b</p><p>)</p><p>100</p><p>10-810-9 10-610-7 10-4</p><p>10-5</p><p>1000</p><p>10</p><p>1</p><p>0.1</p><p>0.01</p><p>Collision-induced</p><p>+</p><p>molecular band</p><p>+</p><p>particle</p><p>opacity</p><p>Collision-induced</p><p>opacity</p><p>Collision-induced</p><p>+</p><p>molecular band</p><p>+</p><p>Collision-induced</p><p>+</p><p>molecular band</p><p>opacity</p><p>Collision-induced</p><p>opacity</p><p>Figure 5.5 Calculated cooling rates in the atmosphere of Titan, showing the effects of considering</p><p>the two different kinds of gaseous absorption, separately and together, and adding in the effects of</p><p>infrared emission from aerosol particles (Bézard et al., 1995).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>Titan’s Atmosphere and Climate 135</p><p>colder than Voyager radio occultation measurements, but most of this difference can</p><p>be ascribed to the assumption by Voyager scientists of an atmospheric composition</p><p>of pure N2. The addition of 1.5% of methane brings the two measurements into</p><p>agreement, a remarkable result considering they were made at different locations</p><p>on Titan, and more than 20 years apart.</p><p>5.1.4 Stratosphere</p><p>By analogy with this philosophy for naming the troposphere, the lower stratosphere</p><p>on Titan is then the shallow, quasi-isothermal region near 50 km, while the upper</p><p>stratosphere is the much deeper region from about 50 to 300 km altitude where tem-</p><p>perature increases with height, i.e. the region between the first temperature minimum</p><p>and the first maximum above the surface.</p><p>The Earth’s stratosphere originally got its name because it is the region where</p><p>convection stops and the air forms layers that tend to stay put, i.e. the atmosphere is</p><p>stratified. The absence of enough absorption above the tropopause to stop emitted</p><p>photons from reaching space causes the lapse rate to tend to zero (i.e. to a constant</p><p>temperature with height). Each layer is heated by radiation from the optically thick</p><p>atmosphere below, and cooled by radiating to space, so height is no longer important</p><p>to first order. The fact that the stratospheric temperature is soon observed to increase</p><p>is, as we have seen, a consequence of the absorption of solar ultraviolet radiation by</p><p>photochemically-produced ozone in the Earth’s stratosphere. Ozone absorbs in the</p><p>Hartley band, which forms a continuum from 0.2 to 0.3 µm. Below 70 km, virtually</p><p>all of the energy absorbed is converted to heat. The ozone concentration in the</p><p>stratosphere peaks near 25 km, but a calculation of the heating rate finds that this</p><p>peaks at a height of about 50 km. The temperature is also a maximum at this level,</p><p>which forms the stratopause.</p><p>On Titan, the corresponding effect is due to the absorption and thermalization of</p><p>UV solar radiation by different gases and aerosols in the stratosphere, the tempera-</p><p>ture again rising with altitude starting from a minimum of 70.43 K at 44 km altitude</p><p>(where the first temperature inversion occurs) up to 186 K at 250 km as found in</p><p>the measurements by Huygens. Prior to Huygens entry measurements, M. Flasar</p><p>and colleagues working with data from the Composite Infrared Spectrometer on</p><p>Cassini provided information on the lower and upper stratosphere from roughly 70</p><p>to 400 km in altitude, indicating the presence of a stratopause at around 310 km of</p><p>altitude with the same maximum temperature of 186 K.</p><p>If the lower stratosphere, just above the tropopause, is modelled as an optically</p><p>thin slab, i.e. one with emissivity much less than one, its temperature can be related</p><p>to the effective radiative, or equivalent black body, temperature of the planet by using</p><p>well-known radiation laws due to Planck, Boltzmann and Stefan. The stratosphere</p><p>is heated from below by a flux of infrared radiation from the planet, and cooled by</p><p>identical emitted fluxes in the upward and downward directions, each proportional</p><p>to the fourth power of its temperature. By balancing the two, we find that the mean</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>136 Titan: Exploring an Earthlike World</p><p>stratospheric temperature is about 210 K for Earth and about 70 K for Titan, both</p><p>values being close to what is observed.</p><p>5.1.5 Mesosphere</p><p>Above the stratopause, the temperature declines again, reaching a minimum at the</p><p>mesopause, where the second temperature inversion occurs, signifying the end of</p><p>the mesosphere. On Earth, the mesosphere begins at around 50 km altitude, whereas</p><p>on Titan it runs through the 300–600 km altitude range. This is the coldest level</p><p>anywhere in the Earth’s atmosphere. Huygens observed an inversion region corre-</p><p>sponding to the mesopause, less contrasted than that inferred from Voyager 1 data,</p><p>at 490 km with a temperature of 152 K.</p><p>5.1.6 Thermosphere</p><p>The pressure at the mesopause is only a few microbars. With such low densities of</p><p>gas above, solar photons in the extreme ultraviolet, and very energetic particles too,</p><p>penetrate into the region, ionising and dissociating molecules and releasing kinetic</p><p>energy. The heating thus produced causes the temperature to increase rapidly with</p><p>height, leading to the name thermosphere (θερµó = warm), the most extensive part</p><p>of the atmosphere, in which the energy is transported by thermal conduction. The</p><p>Earth’s thermosphere begins around 85 km altitude and, as might be expected, is</p><p>much warmer at more than 1,000 K. On Titan, the thermospheric temperatures were</p><p>expected to increase steadily with altitude from about 140 K at the mesopause at</p><p>around 600 km, up to around 190 K above 1,200 km. Huygens found rather higher</p><p>temperatures than the models predicted (average temperature of 170 K), with vertical</p><p>waves of 10–20 K in amplitude, attributed by the science team to gravity waves and</p><p>tidal phenomena, recorded above 500 km.</p><p>5.1.7 Exosphere</p><p>Between the tropopause, where large-scale vertical convection ceases, and the</p><p>mesopause, the atmosphere remains fairly well mixed by turbulence produced by a</p><p>variety of instabilities in wave motions and the mean flow. A fairly small distance up</p><p>into the thermosphere, at around the 100 km altitude level on Earth, diffusion takes</p><p>over as the dominant process and the atmosphere starts to separate into its lighter</p><p>and heavier components. For many practical purposes, this level (the homopause)</p><p>may be considered to be the effective top of the atmosphere. The very tenuous region</p><p>above is often called the exosphere (έξω = out) since here light elements escape</p><p>the planet’s grip and are lost to space. Titan’s atmosphere is generally much more</p><p>extended than Earth’s, as we have seen, and the exosphere begins much higher above</p><p>the surface on the lower-gravity world. According to estimates based on Voyager</p><p>data, the homopause on Titan falls at an altitude of around 1,000 km; the Cassini</p><p>mass spectrometer team recently refined this to 1,200 km.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>Titan’s Atmosphere and Climate 137</p><p>5.2 Radiation inTitan’s Atmosphere</p><p>5.2.1 Solar andThermal Radiation</p><p>The most energetic solar photons at UV wavelengths and shorter are removed at</p><p>various, mostly high, levels in the atmosphere, where they participate in the dissoci-</p><p>ation and ionisation of atmospheric gases. The longer solar infrared wavelengths (1</p><p>to 5 µm) are absorbed in some regions of the near infrared spectrum by molecular</p><p>vibration-rotation bands, particularly those of water vapour and carbon dioxide on</p><p>Earth and methane on Titan. The remainder, radiation at or near visible wavelengths,</p><p>mainly propagates to the ground unless reflected or absorbed by clouds. On a global</p><p>average, about 30% of the energy from the Sun falling on Titan is reflected back to</p><p>space, 60% is absorbed in the atmosphere, and 10% at the ground; for the Earth, the</p><p>corresponding numbers are 30%, 25% and 45%.</p><p>Because of its low temperature relative to that of the Sun, the planet’s surface re-</p><p>emits the absorbed energy at a much longer wavelength than it was received. Some</p><p>of this thermal infrared radiation is transmitted directly to space by the atmosphere,</p><p>but at most wavelengths — substantially longer than visible — the atmosphere is</p><p>very opaque. The main source of this opacity is not the main constituent, nitrogen,</p><p>and oxygen does not contribute much opacity on Earth, nor argon on Titan. Rather, it</p><p>is the minor and trace constituents, especially water vapour, carbon dioxide, ozone,</p><p>methane and nitrous oxide on Earth and methane on Titan which are the principal</p><p>absorbing gases.</p><p>The reason for this is that molecules made up of identical atoms, like N2, and</p><p>monatomic ones like Ar, usually do not interact with infrared radiation. Molecules</p><p>of mixed composition like methane and carbon dioxide, on the other hand, have</p><p>very rich infrared spectra because their internal charge distribution produces a net</p><p>dipole moment. They absorb photons at many wavelengths, leaving only a number</p><p>of ‘window’regions where clouds and haze are the dominant opacity sources. Nitro-</p><p>gen does contribute a significant opacity on Titan at longer wavelengths, through</p><p>its collision-induced spectrum. During collisions, the molecule is distorted and a</p><p>temporary dipole moment results. This effect is also present on Earth, but is much</p><p>less important. The reason is the low temperature on Titan, the almost complete</p><p>absence of water vapour, which dominates the long-wavelength spectrum on Earth,</p><p>and the presence of a large abundance of methane, which turns out to be a particu-</p><p>larly effective collision partner for nitrogen. In fact, N2–CH4 absorption is the main</p><p>opacity source on Titan at wavelengths where most of the flux is emitted.</p><p>5.2.2 Energy Balance and SurfaceTemperature</p><p>What would we expect the temperature to be on Titan’s surface? This important</p><p>question can be addressed with some simple theory.Applying the Stefan–Boltzmann</p><p>law, which states that the energy emitted as radiation by any body is proportional to</p><p>the fourth power of its temperature, we can calculate how much energy the Sun is</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>138 Titan: Exploring an Earthlike World</p><p>emitting, at its known temperature of approximately 5750 K. The answer is about</p><p>4 × 1026 W (400 yottawatts in SI units). Then it is easy to work out how much solar</p><p>energy is falling on Titan from the inverse square law of distance from the Sun (just</p><p>228 megawatts, a solar constant of about 14 W m−2).</p><p>In order for Titan to be in equilibrium overall, the energy it intercepts, minus</p><p>the amount reflected (about 30%), must be equal to the energy it emits as infrared</p><p>heat radiation. If this were not the case, Titan would heat up, or cool down, until a</p><p>balance was achieved. We assume this happened long ago on Titan, unlike its parent,</p><p>Saturn, which is still emitting nearly twice as much heat as it receives, and so is</p><p>gradually cooling. Gas giants, with their deep atmospheres, do this in general; Titan</p><p>is smaller and its relatively thin atmosphere reaches equilibrium much faster, like</p><p>that of the Earth.</p><p>Using the Stefan–Boltzmann law again, we find that the emitting temperature</p><p>of Titan has to be approximately 82 K for that balance to be achieved (most of the</p><p>uncertainty in this number, which is a few degrees, is due to the albedo). This is</p><p>the temperature of a solid sphere the same size as Titan at the same distance from</p><p>the Sun, but as a value for the surface temperature it is too low by 12 degrees or</p><p>so. The reason for the discrepancy is the atmosphere. Its partial transparency to</p><p>significant amounts of sunlight, and its high opacity to longer wavelengths, has</p><p>the effect of trapping solar energy near the surface. Some of the energy from the</p><p>Sun reaches the surface of Titan because solar radiation consists mostly of photons</p><p>at near-visible wavelengths and the atmosphere is partially transparent at those</p><p>wavelengths. The balancing, cooling radiation, however, is in the infrared part of</p><p>the spectrum — because the source is so much cooler,</p><p>near one hundred degrees</p><p>instead of a few thousand. On Titan, 90% of the incoming solar radiation is absorbed</p><p>and back-scattered in the higher atmosphere allowing only about 10% to reach the</p><p>surface. Part of it is absorbed by the ground, which warms up and re-radiates infrared</p><p>heat that is absorbed by the overlying atmosphere. Then it is the atmosphere, rather</p><p>than the surface, which cools by radiating to space, and it is the atmosphere that</p><p>has the characteristic temperature of 82 K. Of course, the atmosphere will radiate</p><p>downwards as well as up, so the surface gets an extra contribution of radiated energy</p><p>in addition to the input directly from the Sun, enough to increase the temperature by</p><p>about twelve degrees, to 94 K (82 + 12 = 94), close to that observed by Huygens</p><p>when it landed.</p><p>5.2.3 ModelTemperature Profile</p><p>Using the surface temperature derived in the previous section, simple model pro-</p><p>files of temperature vs. height can be constructed for Titan by assuming convective</p><p>equilibrium in the troposphere and radiative equilibrium in the stratosphere. The</p><p>principal assumptions are that the temperature gradient is constant, and equal to the</p><p>adiabatic lapse rate, in the troposphere, and constant, and equal to zero, in the strato-</p><p>sphere. Then, three parameters (surface temperature, tropospheric lapse rate and</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>Titan’s Atmosphere and Climate 139</p><p>stratospheric temperature) provide a complete specification of the model climate as</p><p>defined above. Despite their approximate nature, such models provide remarkably</p><p>good approximation to the observed profile, in the lower atmosphere.</p><p>To formulate the model, the perfect gas law, the first law of thermodynamics and</p><p>the hydrostatic equation are used to obtain the vertical gradient of temperature T with</p><p>height z in the troposphere. For a ‘dry’atmosphere, in which latent heat effects can be</p><p>ignored, this is just dT /dz = −g/cp, where cp is the specific heat at constant pressure</p><p>and g is the acceleration due to gravity. A more complicated but still straightforward</p><p>expression can be obtained by introducing the Clausius–Clapeyron equation to allow</p><p>for the latent heat effects of condensable species, primarily methane on Titan. Under</p><p>the assumption of optically thin layers in radiative balance, the temperature Ts in</p><p>the stratosphere is constant with height and given by Ts = Te/21/4, where Te is the</p><p>equivalent blackbody temperature at which the planet cools to space. This, and the</p><p>height of the tropopause at which the radiative and convective regimes change over,</p><p>can be calculated either precisely by the detailed application of radiative transfer</p><p>theory, or estimated by any of a range of approximate methods.</p><p>5.2.4 Radiative EquilibriumTemperature Profile</p><p>The exact solution of the problem of calculating how the temperature of the atmo-</p><p>sphere is expected to vary with height involves proceeding from this very simple</p><p>model to one that includes all of the complex physics of the inhomogeneous, vari-</p><p>able atmosphere, with an assumed composition and other factors incorporated. The</p><p>effects of clouds and hazes can be important in calculating the energy balance of</p><p>individual atmospheric layers, so the number density, size distribution and compo-</p><p>sition of aerosols must also form an input to the calculation.</p><p>The calculated rate of cooling from Titan’s atmosphere as a function of height,</p><p>in units of K s−1, shows that, when all of the cooling processes are included, the</p><p>atmosphere at around the 1 mbar pressure level (for example) would cool down by</p><p>one degree K in about a day, if there were no heating to compensate. The next step</p><p>is to calculate how much solar energy is absorbed, how much is transmitted, and</p><p>how much scattered into the forward and backward direction. The same is done</p><p>for thermal energy, as a function of the temperature of the layer. The exchange of</p><p>energy as radiation between every level in the atmosphere, and every other level, has</p><p>to be taken into account, making the whole computation quite complex and time-</p><p>consuming. Eventually, by iteration, we find the equilibrium temperature for every</p><p>layer, i.e. the temperature when every layer emits as much energy as it absorbs.</p><p>This is still not the end of the computation, however, because some vertical</p><p>temperature profiles, as we have seen, are unstable with respect to vertical motions.</p><p>Whenever the calculated vertical gradient exceeds the adiabatic lapse rate, between</p><p>1 and 2 K per km, convection sets in which reduces the gradient and pulls it back to the</p><p>adiabatic value. The temperature profile calculation has to include this adjustment.</p><p>The result is called a radiative-convective equilibrium model of the atmosphere.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>140 Titan: Exploring an Earthlike World</p><p>The set of such model calculations published by C. McKay and colleagues of</p><p>the NASA Ames Research Center in 1989, include representations of what they</p><p>deemed to be the most likely composition and aerosol properties, with different</p><p>assumptions tried out where these properties are uncertain. They could then select</p><p>the model that gives the closest agreement with the temperature profile measured</p><p>by the Voyager radio occultation experiment. They could also see how varying one</p><p>parameter at a time (for example the methane abundance), would be expected to</p><p>affect the atmospheric temperature. They found that a ‘nominal’ model for Titan’s</p><p>atmospheric composition and haze properties, in other words, a model derived from</p><p>other data such as the IRIS spectra, when used in the radiative-convective equilibrium</p><p>model, produced a good match to the measured temperature profile, except that</p><p>the model surface temperature comes out about 5 to 10 K cooler than observed.</p><p>Interestingly, they also found that including methane or ethane condensation clouds</p><p>in the model made very little difference to the temperature profile, when pressure-</p><p>induced absorption by gaseous nitrogen, methane, and hydrogen are properly taken</p><p>into account. The fraction of solar energy absorbed at the surface in the model is about</p><p>10%; this is the longer wavelength solar visible radiation, the UV component having</p><p>been absorbed mostly in the stratosphere, giving rise to the positive temperature</p><p>gradient above the minimum (70.5 K) at the tropopause (44 km).</p><p>60</p><p>40</p><p>30</p><p>20</p><p>10</p><p>50</p><p>200</p><p>50</p><p>100</p><p>0</p><p>250</p><p>150</p><p>0</p><p>H</p><p>e</p><p>ig</p><p>ht</p><p>(</p><p>km</p><p>)</p><p>H</p><p>e</p><p>ig</p><p>ht</p><p>(</p><p>km</p><p>)</p><p>60 80 100 120 140 160 180 65 70 75 80 85 90 180</p><p>Temperature (K) Temperature (K)</p><p>Figure 5.6 Titan’s atmospheric temperature as a function of height. The solid line is the measured</p><p>profile, from the Voyager radio-occultation measurements; the points are from a radiative-convective</p><p>equilibrium model, with the different symbols representing different assumptions about the compo-</p><p>sition of the atmosphere. The curve on the right is an expanded version of the bottom part of the one</p><p>on the left (McKay et al., 1989).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>Titan’s Atmosphere and Climate 141</p><p>P</p><p>re</p><p>ss</p><p>ur</p><p>e</p><p>(</p><p>m</p><p>b)</p><p>100</p><p>1000</p><p>10</p><p>1</p><p>0.1</p><p>0.01</p><p>0.001</p><p>0.0 0.2 0.4 0.6 0.8 1.0 1.2</p><p>Nitrogen</p><p>Methane</p><p>Normalized kernal</p><p>Figure 5.7 Contribution functions for atmospheric temperature sounding of the atmosphere of Titan</p><p>by Cassini/CIRS. The curves show where in the atmosphere the measured radiation originates, calcu-</p><p>lated for nadir (downward) viewing (heavy lines) and limb (tangential) viewing of the atmosphere, in</p><p>selected spectral intervals 0.5 cm−1 wide in the absorption band of methane near 7.7 µm and in the</p><p>pressure-induced far-infrared band of nitrogen (After Flasar et al., 2004).</p><p>5.3 Remote AtmosphericTemperature Sounding</p><p>Satellites and planetary probes employ infrared and microwave molecular spec-</p><p>troscopy techniques to obtain information about the physical properties of atmo-</p><p>spheric gases. This is known as remote sensing or, where quantitative measurements</p><p>of, for example, temperature profiles are concerned, remote sounding. Most of our</p><p>current knowledge of</p><p>Titan and its atmosphere was derived from remote sounding</p><p>from Voyager or from the Earth, and the same approach is an important part of the</p><p>Cassini mission.</p><p>The infrared flux emitted by Titan, and measured by instruments like IRIS and</p><p>CIRS, depends on the profiles of temperature and absorbers in the atmosphere, so,</p><p>in principle at least, measurements of the former can be analysed to estimate the</p><p>temperature and composition profiles, since they are related by the radiative transfer</p><p>equation. The radiance leaving the top of the atmosphere at a particular wavelength</p><p>is given by a quantity called the contribution function, which has a maximum value</p><p>at the height where the optical depth of the atmosphere is unity, which depends on the</p><p>choice of wavelength. A selection of wavelengths in the wing of a strong absorption</p><p>band of a species such as carbon dioxide for Earth, and methane for Titan, whose</p><p>mixing ratio is known and approximately constant up to high levels, allows a family</p><p>of contribution functions to be calculated which will cover a range of levels in the</p><p>atmosphere.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>142 Titan: Exploring an Earthlike World</p><p>A set of measurements of radiance can then be turned into a temperature profile by</p><p>the inversion of a set of radiative transfer equations. The derived profile is a smoothed</p><p>version of the actual atmospheric temperature profile, because of the averaging effect</p><p>of the weighting functions. Some of the detailed structure can be extracted using</p><p>many spectral channels corresponding to overlapping weighting functions; finding</p><p>the optimum solution for the profile from a given set of measurements with their</p><p>associated errors is therefore a complicated process.</p><p>The vertical resolution of remote sounding measurements is limited by the fact</p><p>that, in order to get a measurement of radiance that is not dominated by instru-</p><p>ment noise, narrow spectral intervals, each corresponding effectively to a single</p><p>wavelength, cannot be used in practice. Instead, spectral bands containing many</p><p>molecular vibration-rotation lines must be employed. This has the effect of further</p><p>broadening the weighting function relative to that for an idealised, monochromatic</p><p>observation. Typical widths at half maximum amplitude are in the range one to two</p><p>atmospheric scale heights, or on Titan not less than about 30 km. To improve on</p><p>this, and to allow higher levels to be sounded, has led to the development of limb</p><p>scanning techniques, in which the instrument views the atmosphere tangential to the</p><p>surface and images a thin slab of atmosphere onto the detector. This can improve the</p><p>vertical resolution to a few km, depending on how close the spacecraft approaches</p><p>to the limb. However, the gain is at the expense of poorer horizontal resolution, and</p><p>the opacity of clouds and hazes is around a hundred times greater than when view-</p><p>ing vertically. This problem can be compensated for by using longer wavelengths,</p><p>where aerosols tend to be less opaque, to probe the deeper levels.</p><p>The temperature profiles retrieved from infrared measurements made by CIRS</p><p>include individual profiles and global maps. The spatial resolution of CIRS is such</p><p>that it can observe Titan’s atmosphere either vertically or tangentially, on the limb. In</p><p>limb-viewing mode, where the line-of-sight extends through the atmosphere to deep</p><p>space, the size of the individual detectors in the mid-infrared detector arrays, and the</p><p>distance of the spacecraft from Titan, determine the altitude coverage and vertical</p><p>resolution. On orbits offering a very close approach, the vertical resolution is better</p><p>than 10 km. The coverage and repeatability of temperatures made by remote sensing</p><p>allows the study of dynamical activity (see Chapter 8); the CIRS temperatures exhibit</p><p>a high degree of spatial variability, including effects possibly due to the influence</p><p>of vertically propagating waves.</p><p>The accuracy of the retrieved temperatures decreases below about 300 km,</p><p>because of the uncertain effect of aerosol absorption on the measured radiances.</p><p>Vertical profiles from about 10 mbar (∼125 km) up to the 0.01 mbar level (∼430 km)</p><p>are feasible. A well-defined stratopause is evident near 0.07 mbar (310 km) with</p><p>a temperature of 186 K. In this region, infrared emission to space by C2H6 is the</p><p>dominant cooling mechanism.</p><p>Because the radiative relaxation time in the upper stratosphere is short (∼1yr)</p><p>compared to Saturn’s orbital period (29.5 yr), its temperatures and winds there should</p><p>vary seasonally. Venus has little seasonal variation, because its spin axis is nearly</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>Titan’s Atmosphere and Climate 143</p><p>Figure 5.8 Vertical temperature profile in Titan’s stratosphere near 15◦S, retrieved from CIRS nadir-</p><p>and limb-viewing spectra. The dashed portions of the curve represent the regions where temperatures</p><p>are not well constrained by the spectra and are more influenced by the Voyager radio-occultation</p><p>profiles and radiative mesospheric models used as the initial guess (Flasar et al., 2005).</p><p>Figure 5.9 A meridional cross section, with latitude as the horizontal coordinate and pressure as</p><p>the vertical coordinate, of zonally averaged stratospheric temperatures (K) from CIRS measurements</p><p>(Achterberg et al., 2008).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>144 Titan: Exploring an Earthlike World</p><p>normal to its orbital plane. CIRS mapped stratospheric temperatures over much of</p><p>Titan in the latter half of 2004, when it was early southern summer on Titan (solstice</p><p>was in October 2002). The warmest temperatures are near the equator. Temperatures</p><p>are moderately colder at high southern latitudes, by 4–5 K near 1 mbar, but they are</p><p>coldest at high latitudes in the north, where it is winter.</p><p>These results can be compared to the predictions of three-dimensional general</p><p>circulation models of the temperature structure, developed as part of studies of the</p><p>global circulation since temperature and pressure gradients are associated with the</p><p>large-scale motions. Radiative transfer calculations, based on an assumed compo-</p><p>sition and, where appropriate, its global variations, with the results constrained to</p><p>be consistent with remote sensing and in situ measurements, are used to predict the</p><p>temperature field and its seasonal variations.</p><p>5.4 Titan’s Ionosphere and its Interaction with the</p><p>Magnetosphere of Saturn</p><p>Bodies with atmospheres also have ionospheres — electrically conducting regions</p><p>with a high concentration of ionized gases and charged particles. These are produced</p><p>both by electromagnetic radiation from the Sun, primarily in the ultraviolet spec-</p><p>trum, and by energetic particle bombardment. The latter originate in the solar wind,</p><p>in cosmic rays from outer space, and, in the case of Titan, as particles precipitated</p><p>from the magnetosphere of Saturn. All of these act on the neutral atmosphere, result-</p><p>ing in ionization, charge exchange and secondary electron impact. The principal</p><p>ionospheric layer on Titan, produced by precipitating electrons, lies between about</p><p>700 and 2700 km above the surface, while a secondary layer, produced by the more</p><p>penetrating galactic cosmic rays, was detected by Huygens between 140 km and</p><p>40 km, with electrical conductivity peaking near 60 km, somewhat deeper than pre-</p><p>dicted by theoretical models, near the region of strong wind shear that was detected</p><p>by Huygens during its descent.</p><p>Measurements of the attenuation and frequency dispersion of the radio signal</p><p>from Voyager 1 during occultation by Titan were the first to confirm that Titan</p><p>has an extensive ionosphere. Electron densities of approximately 3,000 cm−3 at</p><p>an altitude of about 1,200 km, on the evening terminator, and 5,000 cm−3 on the</p><p>morning terminator were detected. Theoretical models have matched these data, and</p><p>predicted the abundance profiles of the other species expected to be present. The</p><p>most abundant ion is H2CN+, followed by various hydrocarbons (CnH+</p><p>m) and nitriles</p><p>(CnHmNp+). These form as the products of chain reactions</p><p>which begin with the</p><p>ionization of molecular nitrogen to give N+</p><p>2 and N+, which reacts with both H2 and</p><p>CH4, and methane to give CH+</p><p>4 . Numerous other ions form by charge exchange,</p><p>atom/molecule-ion interchange, and other processes. The terminal ions are removed</p><p>by electron recombination, leading to the formation of a range of neutral species,</p><p>including the hydrogen cyanide (HCN) first detected by Voyager, as well as heavier</p><p>molecules.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>Titan’s Atmosphere and Climate 145</p><p>Some of the Cassini flybys of Titan have been at altitudes less than 1,000 km,</p><p>close enough to be inside the ionosphere. The instruments on board have found that</p><p>more than 10 percent of the ionosphere is made up of ionized hydrocarbon molecules</p><p>chemically similar to compounds such as ethylene, propyne and diacetylene. These</p><p>data are being used to put together a detailed picture of the composition and variabil-</p><p>ity of the ionosphere by refining models until, eventually, a clearer understanding of</p><p>the photochemistry and ion chemistry involved in the formation of Titan’s orange</p><p>haze will emerge.</p><p>Investigations of the properties of the ionosphere are also important for gaining</p><p>an understanding of the processes by which gases are lost from the atmosphere</p><p>of Titan over time. Because Titan has no strong intrinsic global magnetic field,</p><p>charged particles in the rarified upper atmosphere are exposed to bombardment by</p><p>the solar wind and by particles precipitated from Saturn’s magnetosphere, as well as</p><p>by cosmic rays from outer space. These interactions result in the ionosphere being</p><p>drawn into a comet-like tail, with its population of ions and neutrals being lost to</p><p>space at an unknown rate.</p><p>Determining, or at least modelling, these loss rates, is complicated because of</p><p>the unique situation of Titan as the only satellite with a substantial atmosphere. Titan</p><p>differs from, say, Venus, another planetary body with a thick atmosphere and no</p><p>intrinsic magnetic field, in that it orbits Saturn, which has a very large field. Not</p><p>only that, but at 20 Saturn radii from its parent, Titan orbits very close to Saturn’s</p><p>magnetopause, the boundary between the magnetosphere and the outer region where</p><p>the solar wind flows around the planetary obstacle. Since changes in the solar wind</p><p>dynamic pressure affect the position of the magnetopause, Titan can be inside or</p><p>outside, depending on the level of solar activity. This leads to a widely varying</p><p>particle flux onto Titan, both in intensity and direction, and to a variety of interaction</p><p>scenarios.</p><p>Voyager 1 and Cassini observations established an upper limit of 4 nT at the</p><p>surface and at the equator for Titan’s intrinsic field, at least 10,000 times less than</p><p>that of Saturn. As a result, when Titan is outside Saturn’s magnetosphere, the solar</p><p>wind interacts directly with the satellite’s atmosphere. The incoming flow picks up</p><p>the ions created from the ionization of Titan’s exospheric neutrals and decelerates, as</p><p>the magnetic field lines pile up in front of the satellite and drape around it. The heavy</p><p>ions (N+</p><p>2 , H2CN+, etc.) end up in Titan’s ionosphere between 700 and 2700 km</p><p>above the surface, resulting in an asymmetric plasma flow, and a magnetotail with</p><p>four lobes.</p><p>Below 700 km altitude, galactic cosmic rays penetrate down to the tropopause and</p><p>cause some ionisation. At these altitudes, ionisation by meteorites also contributes.</p><p>These sources of ions occur throughout the same altitude range as the methane</p><p>photochemistry, and particle charge is known to strongly influence the coagulation</p><p>of haze particles. Thus, Titan’s atmosphere may be more closely linked with its space</p><p>environment than on other planets. The exospheric ions are eventually picked up by</p><p>the incoming magnetized plasma flow and carried away. The amount of atmospheric</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>146 Titan: Exploring an Earthlike World</p><p>1</p><p>10</p><p>100</p><p>1000</p><p>-90 -60 -30 0 30 60 90 90</p><p>P</p><p>re</p><p>ss</p><p>u</p><p>re</p><p>(</p><p>m</p><p>b</p><p>a</p><p>r)</p><p>Latitude</p><p>1</p><p>10</p><p>100</p><p>1000</p><p>P</p><p>re</p><p>ss</p><p>u</p><p>re</p><p>(</p><p>m</p><p>b</p><p>a</p><p>r)</p><p>-90 -60 -30 0 30 60</p><p>Latitude</p><p>165 165</p><p>160 160</p><p>155 155</p><p>150</p><p>150</p><p>130</p><p>14</p><p>0</p><p>14</p><p>0</p><p>145 145</p><p>120</p><p>110</p><p>100</p><p>175 175170 170</p><p>75</p><p>75</p><p>80</p><p>80</p><p>85</p><p>90</p><p>90</p><p>130</p><p>120</p><p>110</p><p>10090</p><p>75</p><p>75</p><p>80</p><p>80</p><p>85</p><p>90</p><p>Figure 5.10 Model-calculated zonally averaged temperature in K, showing the seasonal variation in</p><p>the rarefied upper atmosphere. The left frame shows northern winter solstice and the right the spring</p><p>equinox (Hourdin et al., 1995).</p><p>Figure 5.11 Major ion density profiles for Titan’s ionosphere based on the neutral atmosphere from</p><p>the photochemical model by Yung et al. (1984), as computed by Keller et al., (1998).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>Titan’s Atmosphere and Climate 147</p><p>constituents lost via this non-thermal escape process has not been estimated, but it</p><p>is believed to be important.</p><p>The precipitation of electrons from the magnetosphere is energetic enough to</p><p>excite nitrogen molecules in the upper atmosphere. They then emit light, the phe-</p><p>nomenon which on the Earth is known as ‘dayglow’. Some of the energy gets</p><p>converted to heat, so that nitrogen atoms can have enough kinetic energy to escape</p><p>from Titan. Hydrogen atoms are light enough to be lost at the normal temperatures</p><p>that prevail in Titan’s outer atmosphere. This means that the photochemical destruc-</p><p>tion of methane is irreversible, as the hydrogen produced is lost from Titan. The</p><p>hydrogen does not have enough energy to escape from Saturn’s gravity well, and</p><p>instead it forms a toroidal cloud around Titan’s orbit. Calculations of the loss rate of</p><p>nitrogen suggest that, at the present rate, less than 1% of the present atmosphere will</p><p>have escaped over the age of the Solar System, while that of hydrogen is, of course,</p><p>much greater, with complete removal of all of the present atmospheric inventory of</p><p>methane in just a few million years.</p><p>5.5 Climate Change onTitan</p><p>Like everything else in the Solar System, Titan has been subject to varying solar</p><p>luminosity and may have experienced climate change in the past and face it in the</p><p>future. Titan’s present surface temperature is elevated above the radiative equilib-</p><p>rium value by about 12 K, due to the greenhouse effect from the combined effect of</p><p>the collision-induced infrared opacity of nitrogen, methane and hydrogen and the</p><p>radiative properties of the haze layers. Changes in atmospheric composition, surface</p><p>pressure, haze and cloud cover, and solar input all have consequences for the sur-</p><p>face temperature, with possible complex feedback processes introducing nonlinear</p><p>effects.</p><p>On Titan, the main amplifying factor for climate change is the evaporation and</p><p>condensation of volatiles, most notably methane. For large negative temperature</p><p>excursions the main atmospheric constituent, nitrogen, can condense and, in the</p><p>other, warmer, direction, significantly increased greenhouse gas contributions from</p><p>higher hydrocarbons, ammonia, and even water vapour are possible. However, the</p><p>most likely fate for the planet-sized moon is an eventual decline in the emission</p><p>of methane into the atmosphere from surface lakes or from the interior. Then the</p><p>greenhouse will collapse in a few million years and most of the atmosphere will</p><p>freeze on the surface, as already happened on Neptune’s large satellite Triton. If the</p><p>absorptivity of the surface subsequently increases with time due to the photochemical</p><p>conversion of material in the ice or accumulation of dark material from space,</p><p>perhaps organics similar to those seen on the surfaces of some of the other Saturnian</p><p>moons, the surface temperature will increase, perhaps to the point where nitrogen</p><p>and methane could re-evaporate and build up the atmosphere again.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>148 Titan: Exploring an Earthlike World</p><p>bo</p><p>w</p><p>sh</p><p>oc</p><p>k</p><p>magnetopause</p><p>m</p><p>ag</p><p>netosh</p><p>eath</p><p>Titan</p><p>Saturn</p><p>solar</p><p>wind</p><p>solar</p><p>radiation</p><p>Figure 5.12 During its orbital motion around Saturn, the angle between the solar photon radiation</p><p>and the magnetospheric flow</p><p>direction at Titan varies through 360◦.At times, the magnetopause moves</p><p>inside Titan’s orbit, removing the magnetospheric particle flux, while around the equinoxes there are</p><p>times when the solar flux is eclipsed by Saturn.</p><p>Figure 5.13 Titan’s atmosphere obstructs the flow of charged particles in Saturn’s magnetic field,</p><p>producing a bow shock and distorting the ionosphere, from which charged particles can escape into</p><p>space.</p><p>Conversely, if the methane supply is abundant, a perturbation such as a small</p><p>increase in insolation over the present value or a large cryovolcanic episode adding</p><p>greenhouse gases to the atmosphere could produce a dramatic warming of Titan’s</p><p>climate. For example, if the methane reservoir referred to above was in the form of</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch05</p><p>Titan’s Atmosphere and Climate 149</p><p>liquid methane seas on the surface, as seems possible from recent Cassini observa-</p><p>tions, small increases in heating would raise the atmospheric mixing ratio of methane</p><p>rapidly and produce an enhanced greenhouse effect that would further increase</p><p>the temperature of the surface, leading to more evaporation. The greenhouse effect</p><p>makes Titan fairly sensitive to increases in tidal heating, for example, such that a</p><p>Titan-like moon in orbit around a giant planet might be rather warmer than if it</p><p>were itself a planet in a heliocentric orbit. If Titan were ∼80 K warmer, its surface</p><p>and near subsurface might be liquid (the ammonia-water peritectic melting point is</p><p>∼176 K). The co-existence of liquid water and the organics produced by photolysis</p><p>allows the easy synthesis of prebiotic molecules such as amino acids.</p><p>As the Sun becomes a red giant, its luminosity will increase by over an order of</p><p>magnitude for several hundred million years. However, as the atmosphere is warmed,</p><p>it expands. Production of aerosol is tied to the absorption of UV at a given pressure</p><p>(typically the aerosol production altitude in scattering models is 0.1 mbar) so that</p><p>the altitude of haze production increases as the solar luminosity increases. Thus, for</p><p>a fixed production rate, since the haze has much further to fall, the column optical</p><p>depth increases. This largely compensates for the increased luminosity. A small</p><p>offsetting effect is the change in the solar spectrum — as the solar surface cools,</p><p>more red light is produced which penetrates deeper into the atmosphere than the</p><p>present Sun. Some models suggest that, while Titan may get somewhat warmer</p><p>when the Sun becomes a red giant and destroys the terrestrial planets, it still does</p><p>not offer a strong prospect as a suitable abode for the survival of humanity.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>CHAPTER 6</p><p>Chemistry and Composition</p><p>Suddenly I was aware of something new. The air in front of me had lost its crystal</p><p>clearness. …. I was aware of a faint taste of oil upon my lips, and there was a</p><p>greasy scum upon the woodwork of the machine. There was no life there. It was</p><p>inchoate and diffuse, extending for many square acres and then fringing off into</p><p>void. No, it was not life. But might it not be the remains of life? Above all, might it</p><p>not be the food of life, a monstrous life, even as the humble grease of the ocean is</p><p>the food for the mighty whale?</p><p>Arthur Conan Doyle, The Horror of the Heights</p><p>6.1 Titan’s Chemical Composition</p><p>What we knew about Titan’s chemical composition until about a decade ago was</p><p>based primarily on the data recovered by Voyager in 1980. More recently, measure-</p><p>ments from the ground and by the ISO space observatory have complemented this</p><p>knowledge with a number of additional atmospheric compounds and more refined</p><p>abundance values. Finally, since 2004, our understanding of the atmospheric compo-</p><p>sition has been greatly enhanced using the data returned by the Cassini and Huygens</p><p>instruments described in Chapter 4, not least by the mass spectrometer on the probe.</p><p>The main constituent in Titan’s atmosphere is molecular nitrogen, just like the</p><p>Earth. Nitrogen is difficult to observe by remote sensing at the usual infrared and</p><p>microwave wavelengths; as a homopolar molecule (both atoms the same), it has</p><p>no permanent dipole moment and cannot interact with photons in the normal way.</p><p>A similar problem exists with argon, which eluded detection until in situ Cassini–</p><p>Huygens data showed the presence of small amounts of 36Ar and 40Ar. Nitrogen and</p><p>argon do register in the ultraviolet spectrum, and nitrogen was hence first firmly</p><p>detected by the Voyager 1 UV spectrometer. However, because the UV spectrum</p><p>originates in the rarefied upper atmosphere, it does not give a reliable indication of</p><p>the bulk composition of the atmosphere. After nitrogen, methane, which has a rich</p><p>infrared spectrum, is the most abundant molecule to have been definitely detected,</p><p>followed by traces of H2 and other more complex organics.</p><p>For a long time, our knowledge of the bulk composition relied heavily on the</p><p>determination of the mean molecular weight m, recovered byVoyager data through a</p><p>150</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>Chemistry and Composition 151</p><p>combination of the radio-occultation method and infrared spectroscopy. The occul-</p><p>tation method is enabled when a spacecraft flies behind the planet so that its radio</p><p>communication beam passes through the atmosphere and the effect on the signal can</p><p>be used to derive a T/m profile. The temperature was thus retrieved in 1980 from</p><p>the analysis of the emission of the methane band at 7.7 µm, recorded by the IRIS</p><p>spectrometer, and the combined data, with their associated uncertainties, yielded</p><p>a range of possible mean molecular weights for the atmosphere. In a preliminary</p><p>study, which assumed the perfect gas law (which is not quite correct), m fell in the</p><p>range from 27.8 to 29.3 amu. Since the value for nitrogen is 28, this was consistent</p><p>with its role as the predominant molecule, but allowed for a few percent of a heavier</p><p>molecule like argon (36 amu), or a smaller amount of a lighter molecule like methane</p><p>(16 amu). None of the other noble gases has been detected on Titan to date, and the</p><p>amount of argon present is limited to a fraction of one part per million.</p><p>The methane abundance is extremely important for its effect on the atmo-</p><p>spheric thermal structure and contribution to the material on the surface, but this</p><p>too remained uncertain until recently. The stratospheric abundance of methane is</p><p>a balance between destruction by photochemical processes and vertical transport</p><p>through the cold trap at the tropopause. Photolysis of methane takes place much</p><p>higher (∼700 km) and catalytic destruction by radicals — such as C2H — are the</p><p>main depletion mechanisms in the stratosphere. Prior to Cassini, the value was</p><p>thought to lie between 1.7 and 3.0%, depending on the argon abundance assumed.</p><p>The mole fraction of CH4 near the equatorial surface was estimated to be higher,</p><p>with probable values in the range 4.5–8.5%, the uncertainty depending primarily on</p><p>the degree of supersaturation present. The near-surface abundance of N2 was then</p><p>likely to be between 95% (for zero argon and 5% methane) and 85% (7% argon, 8%</p><p>methane). Studies based on the Voyager occultation data combined with the IRIS</p><p>spectra between 200 and 600 cm−1, tended to support the idea of methane supersat-</p><p>uration, where the species remains gaseous even though it is below the temperature</p><p>where it can condense, in the troposphere. On the other hand, methane can condense</p><p>to form clouds but it was not known until quite recently whether or not this actually</p><p>happened. The evidence for clouds and aerosols of methane and other condensates,</p><p>and the possible sizes and distributions of suspended particles, are discussed in</p><p>Chapter 7. The exact altitude and frequency of occurrence of the low-level clouds</p><p>is not precisely known yet, but the fact of their formation, which may depend on</p><p>the availability of condensation nuclei for the droplets, means supersaturation at the</p><p>same time is less likely.</p><p>Before Cassini, beyond the compositional uncertainties,</p><p>corresponding uncer-</p><p>tainties on the thermal profile also existed, since the measured spectrum depended</p><p>on both in ways that cannot be completely separated. The state of play even after</p><p>ISO, at the beginning of the 21st century, left room for uncertainties of about 2 K at</p><p>the tropopause and 4 K between 150 and 200 km altitude. The surface temperature at</p><p>the occultation point near the equator was estimated to lie between 92.5 and 95.5 K;</p><p>of course, this is likely to vary with location and time.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>152 Titan: Exploring an Earthlike World</p><p>Table 6.1 Constituents detected in Titan’s atmosphere and first-time references. V1</p><p>stands for Voyager 1, IRIS for the Infrared Radiometer Interferometer Spectrometer,</p><p>GCMS is the Huygens Gas Chromatograph Mass Spectrometer and ISO/SWS is the</p><p>Infrared Space Observatory Short Wavelength Spectrometer.</p><p>Constituent First detection/range/means Refs. of first detection</p><p>Major</p><p>Molecular nitrogen, N2 Voyager radio occultation; UV 1,2</p><p>Nitrogen, N Voyager, 1134 Å multiplet 2</p><p>Methane, CH4 Ground-based, UV and IR:</p><p>6190 & 7250 Å, 1.1 &</p><p>7.7 µm</p><p>3, 4, 5</p><p>Ionosphere with Cassini/INMS 6</p><p>Monodeuterated methane,</p><p>CH3D</p><p>Ground-based at 1.65 and</p><p>8.6 µm</p><p>7, 8</p><p>Hydrogen , H V1, 1216 Å 2</p><p>Hydrogen, H2 Ground-based, 3–0 S(1) 4</p><p>Ionosphere, Cassini/INMS 6</p><p>Argon (Ar36, Ar40) Cassini–Huygens/GCMS 9</p><p>Minor</p><p>Ethane, C2H6 Ground-based, 822 cm−1 10, 11</p><p>Acetylene, C2H2 Ground-based, 729 cm−1 7, 12</p><p>Ionosphere, Cassini/INMS 6</p><p>Monodeuterated acetylene,</p><p>C2HD</p><p>Cassini/CIRS, 678 cm−1 13</p><p>Propane, C3H8 V1/IRIS, 748 cm−1 5, 14</p><p>Ethylene, C2H4 Ground-based, 950 cm−1 7</p><p>Methylacetylene, CH3C2H V1/IRIS, 328, 633 cm−1 5, 14</p><p>Diacetylene, C4H2 V1/IRIS, 220, 628 cm−1 15</p><p>Benzene, C6H6 ISO and Cassini/CIRS,</p><p>674 cm−1</p><p>Huygens/GCMS</p><p>9, 13, 16</p><p>Hydrogen cyanide, HCN V1/IRIS, 712 cm−1 5</p><p>Cyanoacetylene, HC3N V1/IRIS, 500, 663 cm−1 15</p><p>Cyanogen, C2N2 V1/IRIS, 233 cm−1 15</p><p>Dicyanogen, C4N2 V1/IRIS, solid form at</p><p>474 cm−1</p><p>17</p><p>Acetonitrile, CH3CN 220.7 GHz multiplet 18</p><p>Carbon monoxide, CO Ground-based, mm, submm,</p><p>microwave, infrared</p><p>19</p><p>Carbon dioxide, CO2 V1, 667 cm−1 20</p><p>Water, H2O ISO/SWS, 237, 243 cm−1 21</p><p>Ammonia, NH3, C2H3CN,</p><p>C2H5CN, CH2NH</p><p>Suggested indirectly by</p><p>modelling Cassini/INMS</p><p>ionospheric data</p><p>22</p><p>1Lindal et al. (1983); 2Broadfoot et al. (1981a); 3Kuiper (1944); 4Trafton (1972); 5Hanel et al.</p><p>(1981); 6Waite et al. (2005); 7Gillett (1975); 8Lutz et al. (1981); 9Niemann et al. (2005); 10Gillett</p><p>et al. (1973); 11Danielson et al. (1973); 12Caldwell et al. (1977); 13Coustenis et al. (2007);</p><p>14Maguire et al. (1981); 15Kunde et al. (1981); 16Coustenis et al. (2003); 17Samuelson et al. (1997);</p><p>18Bézard et al. (1993); 19Lutz et al. (1983); 20Samuelson et al. (1983); 21Coustenis et al. (1998);</p><p>22Vuitton et al. (2006).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>Chemistry and Composition 153</p><p>The chemical processes in Titan’s present-day atmosphere are dominated by</p><p>reactions between molecules containing H, C, and N, giving rise to a suite of hydro-</p><p>carbons and nitriles in the stratosphere, in varying abundances. The known oxygen</p><p>chemistry involves small amounts of H2O, CO and CO2. The neutral species are</p><p>produced through photochemical reactions, whereas ions are formed primarily as a</p><p>result of charged particle precipitation, although these can form neutral species later</p><p>in the reaction chain. Note however that solar extreme ultraviolet (EUV) photons</p><p>are responsible for the photoionization of neutral species that produce the observed</p><p>ionosphere in the upper atmosphere. Photoelectrons produced by the photoioniza-</p><p>tion of neutrals, enhance the ion production, while cosmic rays and other external</p><p>energetic particles produce the low altitude observed ions. The energetic particles</p><p>from Saturn’s magnetosphere have a smaller contribution than EUV photons and</p><p>their contribution is mainly in the mesosphere. We have a rudimentary understand-</p><p>ing of these processes, as they apply to Titan, and models have been developed to</p><p>fill in the gaps where measurements are still needed. The organics combine in the</p><p>atmosphere to produce aggregates of the materials called tholins, producing aerosols</p><p>that grow chemically until a certain size beyond which they start to coagulate. At</p><p>this point their growth depends on the collision rate. When they are large enough,</p><p>gravitational settling will bring them to the surface; on the way, they may act as con-</p><p>densation nuclei for condensable species such as methane and lead to the formation</p><p>of clouds and rain.</p><p>Titan’s gaseous composition is undoubtedly even more complex than the cur-</p><p>rently measured inventory of gases, summarised in the following tables, suggests.</p><p>Laboratory simulations and chemical models imply that Earth- and spacecraft-based</p><p>spectroscopy of the neutral atmosphere is picking up only the simplest molecules,</p><p>and may not be revealing the full chemical complexity of the satellite’s atmo-</p><p>sphere.Very large or ‘macro’molecules, especially more complex hydrocarbons and</p><p>nitriles than those observed from Earth or by Voyager, could be present in significant</p><p>amounts on Titan. The fact that we did not have evidence for these species prior to</p><p>Cassini–Huygens was attributed to them being present only in very small quantities,</p><p>or having spectral bands that are weak or obscured by more abundant species, or all</p><p>three reasons. However, in very recent Cassini observations from a combination of</p><p>mass/charge and energy/charge spectrometers performed and analysed by Hunter</p><p>Waite and his group, evidence was found for tholin material (as negatively charged</p><p>massive organic molecules) and their formation at high altitudes (about 1,000 km)</p><p>in Titan’s atmosphere.</p><p>The formation and growth of solid and liquid particles in the atmosphere must,</p><p>unless they evaporate in the relatively warm lower atmosphere, which is unlikely</p><p>at least for the organics, eventually result in precipitation as rain or drizzle on</p><p>Titan’s surface, now explored by Cassini–Huygens. However, the possibilities for</p><p>the composition of the surface on Titan remain numerous: water and other ices, rocky</p><p>material, and accumulations of hydrocarbon snow and rain may all be present. Pre-</p><p>liminary analyses of the properties of any liquid areas on Titan’s surface, discovered</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>154 Titan: Exploring an Earthlike World</p><p>Figure 6.1 Above: the atmospheric composition at Titan’s surface from the Huygens GCMS.</p><p>Below: the GCMS report on the mole fraction of methane with respect to nitrogen versus altitude in</p><p>Titan’s atmosphere (Niemann et al., 2005).</p><p>mainly in the north polar region, suggest that they could be mostly composed of</p><p>liquid ethane and liquid methane, in unknown proportions, with a few percent of</p><p>dissolved nitrogen (2 to 6%). If the liquid is warm, relatively speaking, it could be</p><p>an efficient CO reservoir. More recent studies show however that if the surface and</p><p>the atmosphere are in equilibrium (which is not certain), any liquid on the surface</p><p>is likely to be about 60% CH4 near the equator and 30% near the poles, with the</p><p>remainder mostly liquid C2H6. A full discussion and speculation about the surface</p><p>properties is deferred to Chapter 9.</p><p>6.2 The Bulk Composition of the Atmosphere</p><p>Interestingly enough, the proportions of the two main constituent of Titan’s atmo-</p><p>sphere were more difficult to determine than the abundances of the trace constituents.</p><p>The methane stratospheric abundance, for instance, was not well constrained at the</p><p>time of Voyager or ISO, but varied significantly among different estimates, while the</p><p>thermospheric abundance was overestimated. The precise nitrogen content, although</p><p>we now know that it is more than 90%, proved very elusive for many years.</p><p>Cassini/Huygens finally produced firm determinations for the major constituents,</p><p>nitrogen, methane and hydrogen. The CH4 mole fraction is 1.41 × 10−2 in the</p><p>stratosphere, confirming methane as the second most abundant molecule on Titan.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>Chemistry</p><p>and Composition 155</p><p>It begins increasing below 32 km, until at about 8 km, where it reaches a plateau of</p><p>about 4.9×10−2. The data shows an increase of methane at 16 m/z, when compared</p><p>to nitrogen (in this case 14N+) at m/z = 14, near 16 km. This is probably due to</p><p>condensates evaporating in the inlet system of the mass spectrometer as the Huygens</p><p>probe passed through a layer of methane haze. The Huygens GCMS measurements</p><p>in the lower atmosphere are in good agreement with the stratospheric CH4 value</p><p>inferred by CIRS on the Cassini orbiter (1.6 ± 0.5 × 10−2) and the surface estimate</p><p>given by the Huygens DISR spectra (also about 5% at the surface).</p><p>The GCMS also witnessed a rapid increase of the methane signal after the probe’s</p><p>landing heated the surface, which suggests that liquid methane exists on the surface</p><p>or right underneath, together with other trace organic species, including cyanogen,</p><p>benzene, ethane and carbon dioxide. The methane abundance measurement in the</p><p>upper atmosphere by Cassini ended a long controversy over the value measured, and</p><p>later revised, by the Voyager UVS team. The analysis by H. Waite and colleagues of</p><p>the Cassini mass spectrometer measurements showed that methane’s mole fraction</p><p>is 2.71 ± 0.1% at 1,174 km, which led to significantly improved constraints on the</p><p>whole methane profile.</p><p>Nitrogen and methane, the most abundant constituents in Titan’s atmosphere, are</p><p>both photodissociated by solar ultraviolet radiation, energetic particles from Saturn’s</p><p>magnetosphere and galactic cosmic rays, leading to the initiation of a complex</p><p>organic photochemistry, which finally produces the haze. In the atmosphere, atomic</p><p>hydrogen is transformed to molecular hydrogen in the presence of aerosol particles.</p><p>The abundance of H2 is estimated from Cassini and Voyager data to be in the 0.1–</p><p>0.2% range. With the exception of trace amounts of hydrocarbons, nitriles and a few</p><p>oxygen species, the rest of the atmosphere is almost entirely dominated by molecular</p><p>nitrogen, more than about 97% in the stratosphere.</p><p>Argon was also expected to be a major atmospheric constituent, based on cos-</p><p>mogonical considerations. However, although it is indeed the only noble gas detected</p><p>to date, it is found in small amounts only, mostly in the form of primordial 36Ar</p><p>(2.8 × 10−7) or its radiogenic isotope 40Ar (4.32 × 10−5). The low abundance of</p><p>primordial noble gases on Titan has been interpreted as meaning that nitrogen was</p><p>originally captured in a relatively volatile form, i.e. as NH3 rather than N2. Subse-</p><p>quent photolysis could then have created the N2 atmosphere we see today.</p><p>6.3 Ionospheric Chemistry</p><p>Prior to Cassini–Huygens, the most reliable information we had about Titan’s atmo-</p><p>sphere related to the middle atmosphere, the part at altitudes between roughly 100</p><p>and 500 km from the surface, because this is the region probed by the infrared</p><p>spectroscopy measurements conducted by space missions and observatories like</p><p>Voyager and ISO. The lowest part of the atmosphere, the troposphere, remained</p><p>largely unexplored, except for a few long-wavelength ground-based measurements</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>156 Titan: Exploring an Earthlike World</p><p>Figure 6.2 The Cassini/INMS-derived densities of methane, molecular nitrogen, and argon. The</p><p>solid black lines represent the best-fit densities, which correspond to a 149 K isothermal temperature</p><p>profile. The methane mixing ratio from this fit to the data is 2.7% at 1174 km (Waite et al., 2005).</p><p>Figure 6.3 INMS measurements ofApril 2005 (dots) and modelled spectrum (lines). The prominence</p><p>of NH3, CH2NH, CH3CN, C2H3CN, C2H5CN was not predicted by pre-Cassini models (Vuitton</p><p>et al., 2006).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>Chemistry and Composition 157</p><p>Table 6.2 Neutral mole fractions at an altitude of</p><p>1100 km, used in the model by Vuitton et al. (2006)</p><p>for the species detected in the ionosphere of Titan by</p><p>Cassini/INMS. Adapted from Vuitton et al. (2006).</p><p>Ionospheric species</p><p>detected by INMS</p><p>Neutral mole fractions</p><p>H2 4 × 10−3</p><p>CH4 3 × 10−2</p><p>C2H2 3 × 10−4</p><p>C2H4 6 × 10−3</p><p>C2H6 1 × 10−4</p><p>C4H2 6 × 10−5</p><p>HCN 2 × 10−4</p><p>HC3N 2 × 10−5</p><p>CH3CN 1 × 10−5</p><p>C2H3CN 1 × 10−5</p><p>C2H5CN 5 × 10−7</p><p>NH3 7 × 10−6</p><p>CH2NH</p><p>vertical distributions of the constituents. The</p><p>ISO/SWS spectra provided the first detection of water vapour in Titan’s atmosphere</p><p>from 2 emission lines around 40 µm (the mole fraction derived at 400 km of altitude</p><p>is about 10−8), as well as the first hint of the presence of benzene (C6H6) at 674 cm−1</p><p>for a mole fraction on the order of a few 10−10. Since then, the benzene detection</p><p>has been confirmed by Cassini at several locations on Titan’s disk.</p><p>However, apart from the dedicated ISO/SWS observation around 40 µm</p><p>(250 cm−1) where water vapour was detected, the spectral range at wavenumbers</p><p>shorter than 200 cm−1 escaped detection. The Cassini/CIRS instrument fills the gap,</p><p>with spectra in the 10–1,500 cm−1 range at resolutions ranging from 0.5 or 2.5 cm−1</p><p>depending on the objective, and this has confirmed the presence of the species</p><p>observed by IRIS (except for solid C4N2 at the moment), while adding those found in</p><p>the FP1 sub-mm region: CO, H2O, and CH4. In addition, the CIRS spectra have so far</p><p>allowed the detection of several new isotopes, new vertical distributions, and a better</p><p>characterisation of the haze. Pre-Cassini knowledge of the chemical composition of</p><p>Titan’s stratosphere as derived by Voyager and ISO data is summarised in Table 6.3.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>Chemistry and Composition 159</p><p>Table 6.3 Atmospheric gaseous abundances for Titan from</p><p>Voyager 1 equatorial data and ISO disk-average spectra. The</p><p>observations were taken more than 2 Titanian seasons apart.</p><p>Molecule Voyager (1980) ISO (1997)</p><p>C2H6 1.3 × 10−5 1.3 × 10−5</p><p>C2H2 3.0 × 10−6 2 × 10−6</p><p>C3H8 5.0 × 10−7 5 × 10−7</p><p>C2H4 1.5 × 10−7 8 × 10−8</p><p>C3H4 5.0 × 10−9 8 × 10−9</p><p>C4H2 1.4 × 10−9 1.5 × 10−9</p><p>C3H4 (allene)</p><p>Plane 1 encompasses the 10–600 cm−1 range in which the rota-</p><p>tional and vibrational signatures of CO, HCN, CH4, H2O, and their isotopes have so</p><p>far been detected. Focal Plane 3 includes the spectral bands which yield the molecu-</p><p>lar abundances for C2H2, its deuterated isotope (C2HD), C2H4, C2H6, C3H4, C3H8,</p><p>C4H2, HCN, HC3N and CO2, in the 600–1,000 cm−1 spectral region. Methane and</p><p>its monodeuterated isotope CH3D, as well as propane, ethane and their isotopes,</p><p>are observed in Focal Plane 4 between 1,000 and 1,400 cm−1. CH3D is important</p><p>because it allows us to obtain a value of the D/H ratio in methane, which is a key</p><p>parameter in cosmological models, since deuterium is destroyed in stars. The ν4</p><p>CH4 band at 1,304 cm−1 serves as an atmospheric thermometer, particularly for the</p><p>stratosphere, and analysis of its emission gives the temperature profile at different</p><p>locations of Titan’s disk. Once the thermal profile is known from the inversion of</p><p>radiances measured in the methane ν4 fundamental, the radiative transfer equation</p><p>can be used again to derive the mixing ratios of the other gases whose features can</p><p>be seen in the spectra. An iterative procedure is used, which can be described in</p><p>outline as follows.</p><p>A line-by-line radiative transfer program, incorporating the temperature pro-</p><p>file and using the abundances of the absorbers as parameters, generates synthetic</p><p>spectra in the desired spectral range. These spectra are compared to the observa-</p><p>tions until the best agreement is obtained by trial and error. This gives the mixing</p><p>ratios of the molecules exhibiting emission bands in the spectra at a given latitude;</p><p>the procedure is repeated for measurements at different locations and times. The</p><p>atmospheric opacity in the model must be correctly calculated including all of the</p><p>important contributions from molecular sources and from the haze aerosols. Because</p><p>the pressure on Titan is quite high, the radiative transfer calculations have to include</p><p>the collision-induced absorption in the troposphere (with some contribution in the</p><p>lower stratosphere at wavenumbers lower than 250 cm−1) between the more abun-</p><p>dant molecules, i.e. N2–N2, N2–H2, N2–CH4, CH4–CH4, as well as the vibration-</p><p>rotation bands from the molecular species with permanent dipole moments. The</p><p>most important of these are CH4, CH3D, C2H2, HCN, C3H4, HC3N, C4H2, C2H4,</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>162 Titan: Exploring an Earthlike World</p><p>C3H8, C2H6, C2N2 and CO2. The clouds and aerosols contribute continuum opacity,</p><p>that is absorption which varies slowly over a wide spectral range. Since the com-</p><p>position is unknown, but the opacity tends to vary slowly with wavelength for solid</p><p>and liquid absorbers, their effect can be simulated by a cloud model, fitted to the</p><p>measurements outside the bands of the gaseous species. This tends to be the least</p><p>reliable part of the procedure.</p><p>The synthetic spectra, generated by the radiative transfer program, are convolved</p><p>with the properties of the relevant instrument to give the appropriate spectral reso-</p><p>lution of 2.5 or 0.5 cm−1 prior to comparison with the measured spectra. When the</p><p>instrument was viewing in the nadir direction, i.e. vertically downwards, or approx-</p><p>imately so, the derived abundances are relative to some stratospheric level on Titan</p><p>and contain relatively little information as to the vertical distribution of the compo-</p><p>nent. The gas bands of the most abundant hydrocarbons like C2H2, C2H4, C3H8 and</p><p>C2H6 probe levels in the atmosphere peaking around 3–5 mbar. The main emission</p><p>observed in the bands of higher order and less abundant hydrocarbons (C3H4, C4H2)</p><p>comes from lower altitudes (around 9 mbar), whereas the other molecules mainly</p><p>probe intermediate pressure levels (5–6 mbar). Calculations indicate that the regions</p><p>probed are similar for southern and mid-latitudes, but shift to lower altitudes (higher</p><p>pressures) for northern regions.</p><p>However, when limb viewing (approximately tangential to the surface) is pos-</p><p>sible — as for example in a special north pole sequence obtained by Voyager —</p><p>then vertical distributions can be derived, as will be explained below. Voyager limb</p><p>data yielded vertical distributions for some of the hydrocarbons and nitriles. The</p><p>vertical distributions generally showed an increase with altitude, confirming the pre-</p><p>diction of photochemical models that these species form in the upper atmosphere</p><p>and then diffuse downwards in the stratosphere. Below the condensation level of</p><p>each gas, the distributions were assumed to decrease following the respective vapour</p><p>saturation law.</p><p>6.4.1.1 Hydrocarbons</p><p>After methane, the most abundant hydrocarbons in Titan’s stratosphere are ethane,</p><p>acetylene and propane, in that order. Ethane is one of the major trace gases in Titan’s</p><p>stratosphere (the most abundant hydrocarbon, along with CH3D, after CH4) and</p><p>showed no significant variation as a function of latitude in the Voyager spectra. From</p><p>CIRS data we also find little variation in the C2H6 abundance from north to south</p><p>(Table 6.4), with the most common value at lower latitudes around 1.3±0.3×10−5,</p><p>although some subtle increase to the north is suggested by the fits. This ethane</p><p>abundance is in excellent agreement with the 1.3±0.1×10−5 mean equatorial value</p><p>inferred from IRIS.</p><p>The best fit for acetylene, using a constant-with-height stratospheric mix-</p><p>ing ratio, requires a mole fraction of about 3.7 ± 0.8 × 10−6 near the</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>Chemistry and Composition 163</p><p>equator, which holds throughout all latitudes within error bars. When this value</p><p>is used the calculated synthetic spectrum satisfies the emission observed in the</p><p>centre of the Q-branch at 729 cm−1 for 5◦S, but does less well in the right and</p><p>left wings of the band, for both the medium- and the high-resolution selections.</p><p>Given the CIRS nominal temperature profile used for 5◦S, the contribution function</p><p>for the C2H2ν5 Q-branch at 729 cm−1 probes a large pressure range between 0.3</p><p>and 10 mbar, peaking at around 4 mbar, the wings of the band probing generally</p><p>(except for the hot bands) somewhat lower atmospheric levels (around 5 mbar at</p><p>738 cm−1 and 10 mbar at 675 cm−1), so information can be inferred on the acetylene</p><p>Table 6.4 Chemical composition of Titan’s neutral atmosphere, as found in ground-based observa-</p><p>tions or by spatially-resolved Cassini–Huygens measurements. The species are listed in decreasing</p><p>abundance within a family. The stratospheric values pertain to pressure levels in the 3–9 mbar range.</p><p>The ‘North pole’ values correspond to about 50◦N, and can be higher at higher latitudes.</p><p>Gas Mole fraction Comments (Refs.)</p><p>Major components</p><p>Nitrogen N2 0.97 Inferred indirectly</p><p>Methane CH4 1.4 × 10−2 Stratosphere (1,2)</p><p>4.9 × 10−2 Surface (2,3)</p><p>Monodeuterated</p><p>methane CH3D 8 × 10−6 (4)</p><p>Hydrogen H2 0.0011 (5)</p><p>Argon 36Ar 2.8 × 10−7 (2)</p><p>40Ar 4.32 × 10−5 (2)</p><p>Equator North pole</p><p>Hydrocarbons</p><p>Ethane C2H6 7 × 10−6 1.1 × 10−5 (4)</p><p>Acetylene C2H2 2.5 × 10−6 3 × 10−6 (4)</p><p>Monodeuterated</p><p>acetylene C2HD 6 × 10−10 2 × 10−9 (4)</p><p>Propane C3H8 3.5 × 10−7 6 × 10−7 (4)</p><p>Ethylene C2H4 1.5 × 10−7 5 × 10−7 (4)</p><p>Methylacetylene C3H4 5.2 × 10−9 2 × 10−8 (4)</p><p>Diacetylene C4H2 1.1 × 10−9 2 × 10−8 (4)</p><p>Benzene C6H6 2.0 × 10−10 3.8 × 10−9 (4)</p><p>Nitriles</p><p>Hydrogen cyanide HCN 7.7 × 10−8 7.8 × 10−7 (4, 6)</p><p>Cyanoacetylene HC3N 3.0 × 10−10 4.4 × 10−8 (4)</p><p>Cyanogen C2N2 5 × 10−10 9 × 10−10 (6)</p><p>Dicyanogen C4N2 Solid form only (7)</p><p>Acetonitrile CH3CN 1.5 × 10−9 (8)</p><p>(Continued )</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>164 Titan: Exploring an Earthlike World</p><p>Table 6.4 (Continued )</p><p>Gas Mole fraction Comments (Refs.)</p><p>Oxygen</p><p>compounds</p><p>Water vapour H2O 8 × 10−9 (9) at 400 km</p><p>Carbon dioxide CO2 1.1 × 10−8 1.3 × 10−8 (4)</p><p>Carbon monoxide CO (2−4) × 10−5 Troposphere (10, 11)</p><p>(2−6) × 10−5 Stratosphere (1, 12, 13)</p><p>Isotopic ratios</p><p>13C/14C 82.3 ± 1 (2)</p><p>14N/15N in HCN 67 (11)</p><p>in N2 183 ± 5 (2)</p><p>D/H in CH3D 1.2 × 10−4 (4)</p><p>in HD 2.3 × 10−4 (2)</p><p>in C2HD 1−3 × 10−4 (4)</p><p>1. Flasar et al. (2005) from Cassini/CIRS</p><p>[Rhea, Dione, and Tethys] class together the three intermediate</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>Introduction 5</p><p>Cassinian satellites. The minute interior ones seemed appropriately characterised</p><p>by a return to male appellations [Enceladus and Mimas] chosen from a younger</p><p>and inferior (though still superhuman) brood.</p><p>This proposal was favourably received by the astronomical community. Titan has</p><p>certainly turned out to deserve its important name, not only because of its size but</p><p>also because of its substantial atmosphere similar to the Earth’s. This latter, unique</p><p>property remained undiscovered, however, until almost 300 years after Titan itself</p><p>was first observed.</p><p>After Solà’s doubtful claims of having observed an atmosphere around Titan in</p><p>1908 (doubtful because he had ascertained the same thing for the Galilean satel-</p><p>lites), Sir James Jeans decided in 1925 to include Titan and the biggest satellites</p><p>of Jupiter in his theoretical study of escape processes in the atmospheres around</p><p>solar system objects. His results showed that Titan could have kept an atmosphere,</p><p>in spite of its small size and weak gravity, if low temperature conditions that he</p><p>evaluated as between 60 and 100 K (kelvins∗) or –213 to –173 C (Centigrade) pre-</p><p>vailed. In this case, a gas of molecular weight higher than or equal to 16 could not</p><p>have escaped Titan’s atmosphere since the satellite’s formation. The constituents</p><p>which could have been present in non-negligible quantities in the mix of gas and</p><p>dust particles that condensed to form the Solar System and which, at the same time,</p><p>satisfy Jean’s criterion, are: ammonia, argon, neon, molecular nitrogen and methane.</p><p>Ammonia (NH3) is solid at the estimated Titan temperature and could therefore not</p><p>substantially contribute to its atmosphere. The others, however, are gaseous within</p><p>this same temperature range. Methane (CH4), unlike argon, neon and molecular</p><p>nitrogen, exhibits strong absorption bands in the infrared spectrum, which make it</p><p>relatively easy to detect.</p><p>This analysis obviously invited a spectroscopic search for methane absorption</p><p>bands in the light reflected from Titan. We shall see in due course that it took about</p><p>20 years to get around to doing that; but first let us dwell a moment more on the</p><p>background to the name invited by Herschel’s words.</p><p>1.2 Titan in Mythology</p><p>Hesiodos, a Greek epic poet and historian, Homer’s contemporary, relates in his great</p><p>poem “Theogony” (�εoγoνια·: Gods’ birth) the hierarchy among the ancient Greek</p><p>Gods and develops the cosmogonical and theogonical conceptions that prevailed in</p><p>ancient times.</p><p>According to him, in the beginning all was Chaos (χάoς: nothing). Then came</p><p>Gaia (γαια·: Earth), universal mother and nurse of all beings, firm and offering</p><p>unshakeable eternal support. Eros (έρως: love) followed, the most handsome among</p><p>immortals, he who in his sweetness brings happiness to Gods and humans. He is the</p><p>moving force that brings together and unites all elements. Gaia gave birth first to</p><p>Ouranos (oυρανóς: the sky), who covered her with his celestial sphere and became</p><p>∗For definitions of this and other technical terms, see the Glossary at the end of the book.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>6 Titan: Exploring an Earthlike World</p><p>Figure 1.6 Fragments of letters from Cassini to Huygens, written in 1686 (left) in French and 1691</p><p>(right) in Italian (Observatoire de Paris).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>Introduction 7</p><p>Figure 1.6 (Continued )</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>8 Titan: Exploring an Earthlike World</p><p>Enceladus</p><p>Tethys</p><p>Dione</p><p>Hyperion</p><p>Titan</p><p>Rhea</p><p>Figure 1.7 Saturn and the largest of its satellites, roughly to scale; Titan is about twenty Saturn radii</p><p>from the planet, and has about one-twentieth the diameter of the gas giant.</p><p>her husband. From this very prolific union, the first dynasty of Gods was born, in</p><p>spite of the fact that Ouranos sent his children down the abyss of the Earth for fear</p><p>that they might steal his power.</p><p>The couple’s first children were the twelve Titans: six males and six females.</p><p>Their brothers and sisters were the three Cyclops with one eye, the Ecatochires, with</p><p>a hundred hands and fifty heads, and the Giants. Ouranos’ fears came true when the</p><p>youngest male Titan, Kronos (Saturn to the Romans), assassinated his father, with</p><p>Gaia’s help, and took his place on the throne of Gods. But Kronos inherited his</p><p>father’s curse: for fear of losing his place, he devoured the children his wife Rhea,</p><p>the youngest of the female Titans, brought into the world. Rhea was not pleased</p><p>by this procedure. She asked Gaia’s and Ouranos’ help to save her last son, Zeus</p><p>(known as Jupiter to Latins). She managed to hide him in a cave in Crete where</p><p>a goat, by the name of Amalthea, fed him. When the goat died, Zeus put on the</p><p>animal’s impenetrable skin to protect his body, and decided to go against his father,</p><p>after freeing the rest of the Titans from the Abyss to help. He was surprised to find</p><p>that they preferred to fight on Kronos’ side. With the Ecatochires and the Cyclopes,</p><p>Zeus managed to beat the Titans and Kronos, after 12 years of violent battle. He</p><p>finally overcame his father and took the first place in the “modern” Greek Pantheon.</p><p>Titan then was not one mythological figure, but a group of twelve. Their mighty</p><p>power kept them in key positions in ancient religions and the literary and historic</p><p>community never lost interest in them. Nowadays, the satellite Titan is a new object</p><p>of awe, challenging the astronomical community to uncover its mysteries.</p><p>1.3 Space Exploration of the Solar System</p><p>The Sun is a small yellow star at a distance of 300,000 trillion kilometres from the</p><p>centre of the Milky Way Galaxy. It appeared relatively recently, at a time when many</p><p>of the first stellar generations were already extinct, and, ever since its formation, has</p><p>followed an orbit around the Galactic centre, like hundreds of millions of others. It</p><p>needs 225 million years to complete the circuit, so since its birth 4.5 billion years</p><p>ago, the Sun has completed a full orbit around the centre of the Galaxy only 20 times,</p><p>accompanied on its trek by 8 planets and thousands of smaller bodies. We now know</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>Introduction 9</p><p>that many other planetary systems exist in the cosmos, but among these, only the</p><p>third planet in the Sun’s family is known to have experienced the formation of life.</p><p>During billions of years of evolution, increasingly complex beings filled the</p><p>oceans, invaded the air, and walked the continents of the Earth. A few million years</p><p>ago, some of these beings stood upright, and used their hands to manufacture tools</p><p>and machines that allowed them to invent a better, more comfortable way of liv-</p><p>ing. These men and women developed efficient means of communication between</p><p>themselves and eventually were established as the dominant beings on the planet.</p><p>They soon became aware of the existence of thousands of millions of bright points</p><p>of light in the night sky, turning around the centre of the universe, which was to them</p><p>the Earth itself. The nature of these lights was a mystery to them, and excited their</p><p>curiosity for a long time. Were they signs from the Gods, luminous stones, or holes</p><p>in a black celestial sphere? Were they other worlds, and if so might they bear inhab-</p><p>itants, strange, extraterrestrial forms of life that turned their eyes to their own skies?</p><p>Little by little, people saw beyond the myth and the fear inspired by natural</p><p>phenomena, eventually realising that neither their Earth, nor their Sun, was at the</p><p>centre of the Universe. They also understood that the bright spots on the sky were</p><p>distant luminous suns, much farther away than anything our ancestors could imagine.</p><p>In the fullness of time, the inhabitants of Earth began to realise their dreams of</p><p>travelling into space. It was less than 50 years ago that they managed to send the</p><p>first man into space, and about 40 years ago that they succeeded in landing</p><p>data.</p><p>2. Niemann et al. 2005 from Huygens/GCMS data.</p><p>3. Tomasko et al. (2005) from Huygens/DISR data.</p><p>4. Coustenis et al. (2007, 2008a) from Cassini/CIRS data.</p><p>5. Samuelson et al. (1997a) from V1/IRIS data.</p><p>6. Teanby et al. (2006) from Cassini/CIRS data.</p><p>7. Samuelson et al. (1997b) from V1/IRIS data.</p><p>8. Bézard et al. (1993) from disk-averaged ground-based heterodyne mm observations.</p><p>9. Coustenis et al. (1998) from ISO/SWS data.</p><p>10. Lellouch et al. (2003) from ground-based VLT data at 5 micron.</p><p>11. Marten et al. (2002) from disk-averaged ground-based heterodyne mm observations.</p><p>12. Gurwell and Muhleman (2000) from disk-averaged mm heterodyne data.</p><p>13. Baines et al. (2006) from Cassini/VIMS data.</p><p>abundance as a function of altitude from the study of this band at roughly two</p><p>pressure levels: one around 3 mbar and one around 7 mbar.</p><p>The abundance of propane is, in general, difficult to extract from the Titan spec-</p><p>trum because the propane ν21 band at 748 cm−1 overlaps the R-branch of a strong</p><p>acetylene band. The best fits are obtained for values around 6.0 ± 1.8 × 10−7 near</p><p>the equator, which, despite the uncertainty is in good agreement with the results</p><p>from ground-based observations made at higher resolution, which allows easier</p><p>separation of the lines of the two species.</p><p>The diacetylene ν8 and the methylacetylene ν9 emission bands near 630 cm−1,</p><p>which appeared blended at the IRIS spectral resolution, have clearly resolved con-</p><p>tributions in the CIRS data because of the better spectral resolution, up to 9 times</p><p>higher. This allows their abundances to be inferred with better precision from CIRS</p><p>than from IRIS. These two constituents show the highest increase towards the north</p><p>among all the hydrocarbons, starting in the south with mole fractions of about</p><p>1×10−9 for C4H2 and 4.5×10−9 for C3H4 up to about 40◦N. These molecules then</p><p>increase by factors of 10 and 3 respectively by 70◦N. The derived ethylene mole</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>Chemistry and Composition 165</p><p>Figure 6.4 Ion chemistry photochemical scheme leading to the production of tholin material and</p><p>negative organic ions in the upper atmosphere of Titan (After Waite et al., 2007).</p><p>fractions also show considerable variability with latitude, from 1.5 × 10−7 in the</p><p>south to 5 × 10−7 in the north.</p><p>No contribution by benzene (C6H6) was found in the IRIS spectra in 1980 at a</p><p>resolution of 4.3 cm−1, but its presence was first suggested in ISO data. Benzene</p><p>had been predicted to exist in Titan’s atmosphere from laboratory simulations and</p><p>photochemical models. In the models, the primary mechanism for the production</p><p>of benzene on Titan involves the recombination of propargyl radicals (C3H3) and</p><p>aromatic chemistry that may hold considerable significance in the formation of</p><p>hazes. Since that first indication, benzene has been regularly detected on Titan, in</p><p>particular from Cassini–Huygens data, both in the atmosphere and on the surface.</p><p>CIRS spectra near 674 cm−1 show an emission feature that increases with latitude</p><p>north of the equator, at the 10-σ level at 50◦N and present at levels of a few 10−10</p><p>at lower latitudes. The benzene feature is affected, however, by the presence of a</p><p>nearby additional emission observed at about 678 cm−1and not reproduced by the</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>166 Titan: Exploring an Earthlike World</p><p>Figure 6.5 Comparison of spectral ranges and resolutions of a Voyager/IRIS spectrum (above) taken</p><p>in 1980 near Titan’s equator and its counter-part from Cassini/CIRS as observed in 2005 (below).</p><p>initial theoretical model. It turned out, after further modelling, that this feature is</p><p>due to C2HD, an acetylene isotopomer, present in amounts of a few ppb.</p><p>Benzene was also reported in Titan’s higher atmosphere as a result of Cassini</p><p>mass spectrometer measurements. The CIRS data yielded C6H6 abundances, up to</p><p>3.8 × 10−9 at 70◦N, corresponding to atmospheric levels between 0.5 and 20 mbar</p><p>(the C6H6 contribution function peaks near 6 mbar at the equator and 7.5 mbar in the</p><p>north). This constant abundance assumed above the 30 mbar level yields a column</p><p>density for Titan of about 4 × 1015 molecules cm−2.</p><p>6.4.1.2 Nitriles</p><p>Nitrile detections are limited at present to HCN, HC3N, C2N2, CH3CN and C4N2,</p><p>the last only in its solid form. The constant-with-height mixing ratio of HCN that</p><p>best matches the CIRS data increases from around 5.7×10−8 in the south to an order</p><p>of magnitude or so higher in the north. The value of 7.7 × 10−8 inferred from the</p><p>emission observed in the ν2 713 cm−1 band near the equator is relevant to altitudes</p><p>around 130 km (5–6 mbar).</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>Chemistry and Composition 167</p><p>Figure 6.6 Example of fitting the CIRS spectra in part of the FP3 focal plane where various emis-</p><p>sion features of gases appear (C2H2 at 730, HCN at 713, CO2 at 667, C3H4 at 633 and C4H2 at</p><p>628 cm−1). The observations (grey envelope) are compared to theoretical spectra (black lines); the</p><p>spectral resolution is 0.5 cm−1 (left) and 2.5 cm−1 (right) (Teanby et al., 2007).</p><p>Voyager IRIS spectra had already detected cyanoacetylene, HC3N, near</p><p>500 cm−1 and also through its stronger ν5 band at 663 cm−1, albeit only at high</p><p>northern latitudes (>50◦N) at levels of a few 10−8. No HC3N emission was observed</p><p>in Titan’s thermal infrared spectrum at other locations, where only an upper limit</p><p>of ∼10−9 was obtained. The second band of HC3N was blended with CO2 in the</p><p>case of the IRIS resolution. More recently, the excess in emission observed at</p><p>663 cm−1 in the ISO spectra was attributed to HC3N for an averaged abundance</p><p>of 5.0 ± 3.5 × 10−10. The HC3N contribution functions peak around 6 mbar for</p><p>mid latitudes and around 9 mbar for high northern latitudes, always assuming a</p><p>constant-with-height mixing ratio.</p><p>HC3N emission is clearly visible in the CIRS nadir spectra taken at northern</p><p>latitudes and shows variations from south to north. It can be detected even in the low-</p><p>and mid-latitude CIRS nadir spectra at 663 cm−1 (where the 3-σ noise level is about</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>168 Titan: Exploring an Earthlike World</p><p>Figure 6.7 CIRS contribution functions for some species in Titan’s stratosphere (Coustenis et al.,</p><p>2007).</p><p>2 × 10−9 Wcm−2sr−1/cm−1), with associated abundances around 3 × 10−10, but is</p><p>more easily discernible in the higher-northern latitude spectra, where the mixing</p><p>ratio can reach 4.4 × 10−8.</p><p>C2N2 and C4N2 were found in the Voyager IRIS data, the first as a gaseous</p><p>emission at 234 cm−1, the second only in its solid form at 478 cm−1. The first has</p><p>been identified in CIRS spectra also, only at higher northern latitudes, but so far</p><p>the second has not. C2N2 and HC3N have lifetimes of less than a year in Titan’s</p><p>atmosphere. This leads to a large variation in abundance with altitude. In turn,</p><p>any subsidence becomes extremely evident as a sharp increase in mixing ratio at</p><p>lower altitudes. Short lifetimes mean that these molecules probe short time scale</p><p>atmospheric motions.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch06</p><p>Chemistry and Composition 169</p><p>6.4.1.3 Oxygen-Bearing Molecules: CO, CO2 and H2O</p><p>Carbon monoxide (CO) and carbon dioxide (CO2) were for some time the only</p><p>oxygen-bearing gases known in Titan’s atmosphere, until the 1998 discovery of</p><p>water vapour by ISO. CO2 was the first molecule containing oxygen to be detected</p><p>on Titan, when it was identified in the Voyager IRIS spectra from its emission in the</p><p>ν2 fundamental band at 667 cm−1. In those data, the mean mixing ratio of carbon</p><p>dioxide in Titan’s atmosphere above the 110 mbar level was found to be about</p><p>1.3 × 10−8, with some uncertainty depending on the vertical distribution assumed.</p><p>IRIS did not resolve the emission bands found in the spectra and therefore contained</p><p>little information on the vertical distribution of CO2. It also did not cover any spectral</p><p>region containing carbon monoxide bands, and so was unable to search for that</p><p>species. Cassini/CIRS observed the same CO2 band and</p><p>on our</p><p>closest companion in space, the Moon. Today, the heirs to these pioneers, we can</p><p>actively explore our Solar System in depth and are actively embarked upon the</p><p>continuation of our cosmic adventure. Stimulated by the images returned by the</p><p>early space missions, in the process of placing a permanent settlement of people on</p><p>an orbital station around the Earth, and soon on the Moon and Mars, our generation</p><p>is mostly not aware that the first logical argument for the support of space travel was</p><p>given in the second centuryAD by a Syrian philosopher who wrote in ancient Greek.</p><p>Lucian of Samosata was broadly travelled for his day, and wrote caustic essays</p><p>that criticised the important figures of his time. For many centuries, his ‘Dialogues’</p><p>caused great tension between him and the powerful men of his country.</p><p>“Icaromenippos” (or “A journey through the clouds”) was written in 167AD, and</p><p>consists of dialogue in which the hero, Menippos, tells his friend how he explored</p><p>the sky looking for the truth and what he discovered. His explanation as to why he</p><p>attempted this adventure is extremely modern. He begins by saying that one morning</p><p>he opened up his eyes and realised that earthly goods were only temporary, and that</p><p>important things in life had to do with the mysteries of the Universe. He therefore</p><p>tried to learn something about the birth and the meaning of the Cosmos. But the</p><p>more he tried to think it out, the more he ran into great difficulties, until he reached</p><p>the conclusion that he had to ask for help and consult with the wise philosophers.</p><p>Menippos: “Thus I chose the best men I could find, judging as well as possible</p><p>by the sharpness in their eye, the paleness of their skin and the length and thickness</p><p>of their beard. I have to admit that these men inspired confidence, had a grand air of</p><p>wisdom about them and seemed to be knowledgeable of all the secrets in the sky. So,</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>10 Titan: Exploring an Earthlike World</p><p>I put myself in their hands and paid a handsome sum in advance for their wonderful</p><p>teaching. I sat there and waited for them to impart with me their science of subtle</p><p>reasoning, hoping that in the end I would understand how the Universe works”.</p><p>Soon enough though, Menippos is disappointed by the philosophers who can</p><p>offer him no valuable information, as he admits to his friend: “Well, my friend, you’ll</p><p>laugh when I tell you what sort of crooks these so-called ‘philosophers’ are, what</p><p>kind of strange fantasies they have in their heads, and how cunning they are in setting</p><p>up fictional stories. My teachers, who are nothing more than you and I after all, can</p><p>see no farther than their own noses! Most of them are blind, either because they are</p><p>so old, or because they do nothing all day long. This does not stop them from talking</p><p>about what lies at the end of the Universe, or about the circumference of the Sun.</p><p>They are capable — or so they say — of describing in detail the shapes and sizes</p><p>of the Moon and the other stars farther away, and even say they can reach them!</p><p>These same people who know not what the distance between Athens and Megara</p><p>might be, discuss seriously about the distance of the Sun to the Moon.[...] One hears</p><p>them swear that the Sun is a ball of fire, that the Moon is inhabited and that the</p><p>celestial bodies must be covered with water, because the Sun absorbs the humidity</p><p>from the sea lifting it up in the air, like water from a well, and redistributing it to all</p><p>the world, thus providing us with drinking water!”</p><p>“Such stupidity!” exclaims his friend, after these fantastic descriptions of the</p><p>Solar System and the beautifully (and incidentally correctly) defined cycle of the</p><p>water and clouds in the Earth’s atmosphere. “They really are crooks, all these</p><p>astronomers”.</p><p>Menippos: “And this is not all. Imagine, my friend, that these gentlemen manage</p><p>to have arguments about whether the Universe has borders, therefore being closed,</p><p>or whether it may have no limits. Some of them even insist that there is an infinite</p><p>number of other worlds and fight with those who claim there is only the Earth”.</p><p>After these incredible announcements of the crazy ideas of the philosophers of</p><p>the second century AD, Menippos reaches the inevitable conclusion:</p><p>Menippos: “And so it was that, almost driven to madness and having lost all hope</p><p>to find final answers to my questions here on Earth, I decided that the only solution</p><p>was to get a pair of wings and to go and see for myself what happens in the sky”.</p><p>In these words, somewhat liberally translated by the authors, we find the best</p><p>and most valid reason for space travelling: the scientific approach of searching and</p><p>discovering the reality in nature.</p><p>Besides “Icaromenippos”, Lucian made another trip to space in his “True Story”</p><p>(which, despite the name, was a pure creation of his imagination), where he describes</p><p>how he sailed through the Atlantic and — crossing a storm — found himself on the</p><p>Moon. There he is mixed up in a battle between the Moon people and the inhabitants</p><p>of the Sun. The Moon loses the war and Lucian is carried away to be a prisoner on</p><p>the Sun, where he lives through some more wild and eccentric adventures, worthy</p><p>precursors of modern sci-fi, before returning finally back to Earth.</p><p>Lucian was the pioneer among ancient astronauts in literature. Significantly, the</p><p>idea of space travel has never seemed to present an insurmountable obstacle for</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>Introduction 11</p><p>ancient philosophers or their descendants, who include present-day science fiction</p><p>writers, and even a good number of professional mathematicians and physicists.</p><p>Still, tempting though the idea was of reaching strange worlds, a trip beyond the</p><p>Earth was not an easy venture in practice. How could one attempt it? The ancient</p><p>philosophers thought perhaps that with bird-like wings, by the force of formidable</p><p>storms, through dreams or with the help of demons, man could hope to discover</p><p>other worlds. More recently, the trip has been made, in the imagination, in balloons,</p><p>with strange gravity-defeating machines, or by using great cannons.</p><p>Today we believe that the only practical way to launch a person or a machine into</p><p>space is with a rocket. But this method has only fairly recently been adopted and</p><p>never appeared in literature before the 20th century, when Constantin Tsiolkovski,</p><p>a Russian theoretical physicist, published his famous book on “The exploration of</p><p>space by action-reaction machines”. Tsiolkovski once said: “Our planet is the birth-</p><p>place of humanity, but one doesn’t live one’s whole life in the cradle”. The practical</p><p>realisation of rocket travel is however due to pioneers like Robert Goddard, who</p><p>launched his first prototype in 1926 and continued with the construction of liquid-</p><p>fuelled rockets, which, towards the end of 1930 reached a height of a few kilometres.</p><p>The large-scale construction of rockets devoted to space exploration might have</p><p>remained the hobby of a few inspired inventors, working almost alone and with few</p><p>resources, if the Second World War had not intervened. Despite the great misery</p><p>and disaster it brought, this global madness produced the V2 rocket of Werner von</p><p>Braun and his associates. This terrible weapon of revenge was later to become a</p><p>powerful resource in the service of the space programme.</p><p>The first day of glory in real space flight belongs to the Russians: on October 4,</p><p>1957, the hundredth anniversary of the birth of Tsiolkovski, the legendary team of</p><p>Sergei Korolev sent the first artificial satellite, Sputnik 1, into orbit. On April 12,</p><p>1961, Yuri Gagarin became the first man to leave the human cradle, spending 108</p><p>minutes, the time required by the Vostok spacecraft to perform a complete orbit</p><p>around the Earth, at a height of a few hundred kilometres.</p><p>“Man”, says American anthropologist Ben Finney, “is an exploring animal”.</p><p>Exploration is important to us. We are ready to confront dangers and disasters,</p><p>disappointments and the lessons of humility, in order</p><p>to learn something about the</p><p>appearance and evolution of life on our planet. It is mainly for this reason that</p><p>manned missions are pursued. The Americans made a great leap forward when Neil</p><p>Armstrong became the first man to step on the Moon on July 21, 1969. This was</p><p>the first of seven trips to the Moon, which made up the challenging but also costly</p><p>Apollo programme. Apollo ended in 1973, epoch of the Skylab. The Russians, in the</p><p>meantime, developed space stations like Salyut (1971) and Mir (1986), primarily</p><p>to study the reaction of man to life in space. During the 1980s, the Space Shuttle</p><p>made its debut with great success, a programme that continues into the era of the</p><p>International Space Station, having transcended the tragic accident with Challenger</p><p>in 1986. In spite of the vagaries of economics and war, plans are well advanced to</p><p>send more spacecraft to the Moon and beyond, eventually following in the steps of</p><p>the Voyager missions which have now left the Solar System altogether.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>12 Titan: Exploring an Earthlike World</p><p>The exploration of the planets began, logically, with missions to our closest</p><p>neighbours in the 1960s. Since then, unmanned spacecraft have visited all of the</p><p>planets, some of them many times. Mercury, the closest to the Sun, was studied by the</p><p>NASA spacecraft Mariner 10. Mercury is named after the Greek god of commerce</p><p>and travel, the Roman messenger of the gods. It is slightly bigger than our Moon,</p><p>and in many ways quite similar to it, because it has no atmosphere and the surface is</p><p>heavily cratered by impacts by meteorites. The difference in temperature between</p><p>the sunlit and dark sides is enormous — a blistering 450◦C on the bright side, while</p><p>in the shade of the other hemisphere the temperature plummets to −170◦C. Future</p><p>human explorers will need more than breathing gear if they are to live on Mercury</p><p>without burning or freezing.</p><p>Venus was explored by NASA with Mariners 2, 5 and 10 and the orbiter and</p><p>multiprobes of PioneerVenus. The Soviet Union sentVeneras 7 to 16, as well asVega</p><p>1 and 2. NASA’s Magellan, launched in 1989, returned astounding radar images of</p><p>the surface of Venus. ESA’s Venus Express arrived near the planet in April 2006</p><p>and is returning new views of the planet, including the first close views of the huge</p><p>double vortex at the south pole. Long admired for its beauty as the morning, and</p><p>the evening, star, the high temperatures and densities (nearly 500◦C and 100 bars)</p><p>prevailing in the choking carbon dioxide atmosphere render the surface a hellish</p><p>prospect for humans.</p><p>Not surprisingly, Earth’s Moon has received special attention, with, among oth-</p><p>ers, the Russian Luna sample return missions, the manned Apollo 11–17 landings,</p><p>and prospect of a permanent base sometime in the next few decades. The Moon of</p><p>course has no atmosphere, and offers a desert-like, hostile surface to travellers, so</p><p>the base when it comes will resemble a land-bound version of the space station,</p><p>replete with life support systems.</p><p>Mars was visited by the American Mariner 4, 6, 7, 9 and Viking 1 and 2 missions,</p><p>and more recently Odyssey, Reconnaissance Orbiter and Mars Express, which are</p><p>addressing basic questions about the geology, atmosphere, surface environment,</p><p>history of water and potential for life on Mars. We know today that there are no</p><p>artificial channels on the surface of Mars, and no Martians, although microbial life</p><p>below the surface is still not ruled out. Missions such as Pathfinder have confirmed</p><p>that conditions on the red planet were once very different, warmer and wetter, but it is</p><p>also certain that today Mars is hostile to most forms of terrestrial life. Its atmosphere</p><p>is very thin, cold and unbreathable, while its surface is constantly bombarded by</p><p>cosmic and ultraviolet rays. To last long on the surface of Mars, a person would</p><p>have to use a special heated suit with breathing gear, like the ones used by Apollo</p><p>astronauts on the Moon.</p><p>The outer gas giant planets were visited by the Voyager missions in the 1980s.</p><p>Jupiter, Saturn, Uranus and Neptune formed from the primitive gas nebula, which</p><p>also formed the Sun. Because they are massive and cold, they have thick atmo-</p><p>spheres, in which even the lightest molecules, hydrogen and helium, are retained.</p><p>Thus, they are very different from the terrestrial planets, not only in having atmo-</p><p>spheres of different composition in which the pressures are very high, but also in</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>Introduction 13</p><p>having no surfaces in the usual sense. The system formed by Pluto and its satellite</p><p>Charon, both small, light and distant bodies, does not belong in either the terrestrial</p><p>or giant planet categories, but instead resemble more the icy satellites of the outer</p><p>Solar System or the planetesimals of the Kuiper belt. On August 24, 2006, Pluto</p><p>was demoted from a planet to a “dwarf planet” according to IAU’s resolution 5A.</p><p>The giant planets have many large satellites between them, several of them larger</p><p>than the smaller planets. However, all but one are without significant atmospheres.</p><p>They are mainly composed of ices, and have surfaces that are heavily marked by</p><p>meteoritic impacts. The exception of course is Titan, in the Saturnian system, which</p><p>not only is nearly Earth-sized but also has an atmosphere that has the same major con-</p><p>stituent as ours and much the same surface pressure. In many ways, Titan is like a fifth</p><p>terrestrial planet; cold, and lifeless almost certainly, but nevertheless one of the more</p><p>relevant places in the known universe as a seductive place for humans to explore.</p><p>1.4 The 20th Century, BeforeVoyager</p><p>Before the Voyager missions to the outer Solar System, ground-based observations</p><p>of the satellites of the outer planets had produced only fairly scanty information,</p><p>so much so that the smaller of those known today had not been found at all, and</p><p>virtually nothing definite could be said about those whose existence had been known</p><p>for three hundred years.</p><p>With respect to Titan, the main scientific concern was focused on its atmosphere.</p><p>The first formal proof of its existence came only after the Second World War, in</p><p>1944, when Gerard Kuiper, of the University of Chicago (and originally a Dutchman,</p><p>like Huygens) discovered spectral signatures on Titan at wavelengths longer than</p><p>0.6 µm (microns), among which he identified two absorption bands of methane at</p><p>6190 Å and 7250 Å. By comparing his observations with methane spectra taken at</p><p>low pressures in the laboratory, Kuiper derived an estimate of the amount of methane</p><p>on Titan: 200 metre-amagats.</p><p>Kuiper searched for similar behaviour in the spectra of other Saturnian satellites.</p><p>But his data, obtained in 1952, showed differences between Titan and the other</p><p>satellites in the intensity observed in the ultraviolet and visible continuum, as well</p><p>as in the methane bands, which were absent except on Titan. Kuiper concluded Titan</p><p>was a unique case in the Saturnian system due to the presence of an atmosphere, of</p><p>such a composition that it gave the satellite an orange colour.</p><p>In the years that followed, in spite of much interest, it proved difficult to make</p><p>significant further progress in exploring or comprehending Titan’s atmosphere. By</p><p>1965, a consensus had still not been reached on a value for the ground temperature,</p><p>in the presence of contradictory radio and infrared measurements that ranged from</p><p>165 to 200 K. From 1972 to 1979, a number of scientists concentrated their efforts</p><p>on seeking a better estimate for the methane abundance and the surface pressure,</p><p>using observations made in the 1-to-2 µm infrared spectral region. Limb darkening</p><p>was finally unambiguously observed in 1975, consistent with an optically thick</p><p>atmosphere.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>14 Titan: Exploring an Earthlike World</p><p>At about this time, Laurence Trafton, from the University of Texas, conducted</p><p>observations of the 3ν3 spectral band of methane at 1.1 µm, in which he found unex-</p><p>pected strong absorption,</p><p>indicating either a methane abundance at least 10 times</p><p>higher than that inferred by Kuiper, or a broadening of the CH4 bands induced by</p><p>collisions with molecules of another as yet undetected, but quite abundant, gas in the</p><p>atmosphere. In either case, the intensity of the absorption band is a function of the</p><p>methane abundance and of the local atmospheric pressure. By comparing the weak</p><p>absorption bands of methane in Titan’s visible spectrum with spectra of Jupiter and</p><p>Saturn, in which these bands have almost identical absorption strengths, Lutz and</p><p>his colleagues derived from Trafton’s measurements a 320 m-amagat abundance for</p><p>methane, and an estimate for the effective pressure on Titan of about 200 mbar. The</p><p>immediate consequence of this result was that methane suddenly became just a minor</p><p>atmospheric component, since even 1.6 km-amagat (i.e. 1600 m-amagats, Trafton’s</p><p>highest estimate) could only correspond to a surface pressure of about 16 mbar.</p><p>By 1973, observations of the satellite’s low albedo and of the positive polari-</p><p>sation of the reflected light, confirmed the presence of a thick, cloudy atmosphere,</p><p>with the cloud particles present up to high altitudes. Theoretical considerations sug-</p><p>gested that two sorts of aerosols were expected to co-exist in Titan’s atmosphere:</p><p>clouds of condensed CH4, and a photochemical fog of more complex condensates.</p><p>The latter would arise as a result of methane photolysis, that is dissociation by</p><p>sunlight, mostly at ultraviolet wavelengths. The fragments of methane, CH2, CH3,</p><p>etc., combine, leading to the production of a variety of polymers that condense</p><p>to form oily droplets. Something similar happens on Earth in the photochemical</p><p>smog engendered by terrestrial road traffic. In 1975 again, Gillett found evidence</p><p>in Titan’s thermal emission spectrum not only of methane (CH4), but also of ethane</p><p>(C2H6) at 12.2 µm, monodeuterated methane (CH3D, at 9.39 µm), ethylene (C2H4,</p><p>at 10.5 µm) and acetylene (C2H2, at 13.7 µm).</p><p>Trafton had also announced in 1975 a tentative identification of a spectral feature</p><p>of molecular hydrogen, H2, in the spectrum of Titan, for which he had evaluated an</p><p>abundance of 5 km-amagat. In spite of much effort directed at the detection of NH3,</p><p>the observers of the time failed to produce more than upper limits, which got lower</p><p>and lower with successive measurements, suggesting that if any of this gas existed</p><p>on Titan, it must either have been photodissociated, with subsequent production of</p><p>N2 and H2, or else trapped on the surface as ammonia ice.</p><p>Close examination of Titan’s spectrum had already revealed at this time that</p><p>the continuum absorption decreased with frequency, suggesting that the aerosol</p><p>became more transparent at longer wavelengths. This led to the assumption that</p><p>it might be possible, at certain frequencies in the near infrared, to probe all the</p><p>way down to the satellite’s surface. At short wavelengths, down to about 2200 Å,</p><p>the brightness remains nearly constant, suggesting that the aerosol is uniformly</p><p>mixed at high altitudes. The measurements say little about the nature of the aerosols,</p><p>their composition for example, but the fact that they are present in the atmosphere</p><p>makes all attempts to interpret spectroscopic observations extremely dependent on</p><p>assumptions about the cloud properties.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch01</p><p>Introduction 15</p><p>Figure 1.8 One of the first ground-based observations of Titan’s infrared spectrum (Gillett, 1975).</p><p>A detailed set of assumptions, used to provide the context in which new data can</p><p>be understood, is what scientists call a ‘model’. Before the Voyager encounter, there</p><p>were two principal models in contention for explaining the observations as they</p><p>then stood. The first, suggested by Danielson in 1973 and completed by Caldwell</p><p>in 1977, favoured methane as the main component (about 90%) of the atmosphere,</p><p>and predicted surface conditions of T = 86 K at a pressure of 20 mbar, with a</p><p>temperature inversion in the higher atmospheric levels demonstrated by the presence</p><p>of emission features of hydrocarbon gases in the infrared spectrum of Titan. The</p><p>second model, based on work by Lewis in 1971 and developed by Hunten in 1977,</p><p>started with the assumption that dissociation of ammonia should produce molecular</p><p>nitrogen, which is transparent in the visible and infrared spectrum, in large quantities.</p><p>In this model the surface temperature and pressure would be quite high (200 K and</p><p>20 bars). These high temperatures on the ground could be explained by a pronounced</p><p>greenhouse effect, resulting essentially from pressure-induced opacity in hydrogen</p><p>at wavelengths longer than 15 µm. As on the Earth, and other planets (most notably</p><p>Venus), this opacity blocks the thermal emission from the lower atmosphere and</p><p>surface, creating a build-up of heat in the lower part of the atmosphere.</p><p>Just prior to the Voyager encounter, Owen and Jaffe made radio telescope obser-</p><p>vations with the newly-completedVery LargeArray in New Mexico, and obtained the</p><p>emission temperature of the surface finding a value of 87±9 K, a range that includes</p><p>the modern value. They even suggested that conditions on Titan might support oceans</p><p>of methane, an idea that was ahead of its time, but the paper failed to get the attention</p><p>it deserved as it was published during the excitement of the Voyager encounter.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>CHAPTER 2</p><p>TheVoyager Missions toTitan</p><p>An amoeba of blackness leaked out from the zenith, obscuring Sun and blue sky….</p><p>The same sandy beach was beneath her feet; she dug her toes in. Overhead… was</p><p>the cosmos. They were, it seemed, high above the Milky Way Galaxy, looking down</p><p>on its spiral structure and falling toward it at some impossible speed…… A network</p><p>of straight lines appeared, representing the transportation system they had used.</p><p>It was like the illuminated maps in the Paris Metro.</p><p>Carl Sagan, Contact</p><p>2.1 Space Missions to the Saturnian System</p><p>The previous chapter gives an idea about where the situation stood before Voyager 1</p><p>flew by Titan in 1980. Voyager was not the first visitor from Earth to the Saturnian</p><p>system, as the ringed planet had been visited by small, unmanned Pioneer probes</p><p>in 1979. Nor was Voyager to be the last such visitor, but a quarter of a century</p><p>elapsed before Cassini/Huygens picked up the quest, with nothing but Earth-based</p><p>observations to fill the gap.</p><p>Pioneer 11 was a spin-stabilised spacecraft designed primarily to carry out a</p><p>first reconnaissance of Jupiter in the early 1970s. The 258 kg craft was launched on</p><p>April 5, 1973, and encountered the largest planet in the Solar System in December</p><p>1974. This encounter was such a success that Pioneer, still functioning well, was</p><p>re-targeted to use the gravity assist from Jupiter to send it on to a Saturn encounter</p><p>five years later, in September 1979.</p><p>One of the goals of the Pioneer Saturn encounter was to check the environment</p><p>in the vicinity of the planet’s extensive system of rings. This was not just a scientific</p><p>objective, but aimed partly to blaze the trail that the larger and more sophisticated</p><p>Voyager spacecraft were to follow. It was not known, until Pioneer survived a passage</p><p>through the ring plane, whether there were any particles in the plane outside the</p><p>visible rings that might cause a hazard to spacecraft. Pioneer 11 was targeted at the</p><p>distance from Saturn — 2.9 planetary radii — that Voyager 2 would have to follow</p><p>if it were to go on to Uranus, which in the event it successfully did.</p><p>The Pioneer 11 trajectory carried it across the orbit of Titan one day after its</p><p>closest approach to Saturn, on September 2, 1979, at a distance from the satellite</p><p>of 363,000 km, much too far for Pioneer’s relatively simple instruments to gather</p><p>16</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>The Voyager Missions to Titan 17</p><p>Trapped Radiation Detector</p><p>Cosmic Ray Telescope</p><p>Infrared Radiometer</p><p>Charged Particle Instrument</p><p>Radioisotope Thermoelectric Generator</p><p>Plasma Analyzer</p><p>Main Antenna</p><p>Imaging</p><p>Photopolarimeter</p><p>Geiger Tube Telescope</p><p>Meteoroid Detector Sensor Panel</p><p>Helium Vector</p><p>Magnetometer</p><p>Ultraviolet Photometer</p><p>Asteroid–Meteoroid</p><p>Detector Sensor</p><p>Figure 2.1 Pioneer 11 encountered Saturn on September 1, 1979. It flew within 13,000 miles of</p><p>Saturn and took the first close-up pictures of the planet (NASA).</p><p>Table 2.1 Characteristics of the Pioneer and Voyager missions.</p><p>Pioneer 11 Voyager 1 Voyager 2</p><p>Launch April 5, 1973 Sept. 5, 1977 Aug. 20, 1977</p><p>Titan encounter Sept. 2, 1979 Nov. 12, 1980 Aug. 27, 1981</p><p>Closest approach to Titan (km) 363,000 4,394 663,385</p><p>images or much useful data of any kind. However, it was the first man-made object</p><p>to enter the realm of Saturn, and it showed the way was safe for Voyager. (Although,</p><p>in fact, Voyager 1 made its closest encounter with Titan before passing through the</p><p>ring plane.)</p><p>The Voyager 1 and 2 missions were the first large, stabilised spacecraft to travel</p><p>to the outer Solar System. Both were launched in 1977, Voyager 2 actually a few</p><p>days before Voyager 1, but the faster trajectory of the latter got it to Jupiter first.</p><p>Each of the two spacecraft executed several thousand instructions without error,</p><p>controlled from theVoyager operations centre at the Jet Propulsion Laboratory (JPL)</p><p>in Pasadena, California, USA, which received tracking, engineering and science data</p><p>via the stations of the Deep Space Network. The Voyager 1 encounter with Jupiter</p><p>took place on March 5, 1979, while Voyager 2 swung past the giant planet on July</p><p>9 of that same year. In November 1980, Voyager 1 encountered Saturn and Titan.</p><p>Voyager 2 arrived in the Saturnian system in August 1981, some nine months later.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>18 Titan: Exploring an Earthlike World</p><p>Although not without interest, the data relative to Titan obtained by Voyager 2 are</p><p>not as extensive as those taken by Voyager 1, because the closest approach distance</p><p>of Voyager 2 was more than 100 times greater.</p><p>From an intellectual point of view, the encounter with Titan had been inevitable</p><p>since the time of Titan’s discovery by Huygens more than three centuries earlier. It</p><p>became a step closer in practice in 1918, when the Soviet academicYu.V. Kondratyuk</p><p>devised the principle of using gravity assistance to space flight for the first time. The</p><p>idea in itself is simple enough: by flying past a planet from behind, a spacecraft can</p><p>acquire some extra velocity with respect to the Sun, which allows it to accelerate</p><p>on to another destination. By a series of gravity assists, space missions can go</p><p>farther and faster than would otherwise have been possible with our limited rocket</p><p>technology. Half a century later, two JPL engineers discovered an opportunity for a</p><p>launch in the year 1977, which coincided with a rare alignment of the outer planets</p><p>which would not occur again before the middle of the twenty-second century. By</p><p>grasping this opportunity to use gravity assists at Jupiter, NASA was able to send</p><p>mission to the more distant giant planets, including Saturn, much sooner than it had</p><p>otherwise planned.</p><p>The trajectory of Voyager 1 around Saturn allowed extensive coverage of the</p><p>planet’s atmospheric characteristics, of the rings and the icy satellites. The closest</p><p>approach to Saturn at a distance of 126,000 km took place on November 12, 1980, the</p><p>same day as the closest approach to Titan, 6969 km (4394 miles) from the satellite’s</p><p>centre. The orbital plane of Titan was crossed from north to south, the spacecraft</p><p>trajectory inclined with respect to the orbital plane at about 8.7◦, at a speed with</p><p>respect to the satellite of 17.3 km s−1.</p><p>The Voyager 2 spacecraft, destined originally to travel only to Jupiter and Saturn,</p><p>was performing so well that its mission was dramatically extended, again using the</p><p>gravity assist technique to make the journey feasible. Its trajectory was directed past</p><p>Saturn in such a way that it was accelerated towards Uranus, where it arrived on</p><p>January 24, 1986, and then again on to Neptune, where the encounter took place on</p><p>August 24, 1989. The spacecraft then dived below the ecliptic at an angle of about</p><p>48◦, leaving the Solar System behind at a speed of about 470 million kilometres a</p><p>year. Voyager 1 ended its planetary mission with Titan, and has also left the Solar</p><p>System, rising above the ecliptic plane at a rate of about 520 million kilometres a</p><p>year, at an angle of about 35◦. As of April 2006, Voyager 1 is at 12.32◦ declination</p><p>and 17.114 h right ascension, in the constellation of Ophiuchus, while Voyager 2 is</p><p>at −52.51◦ declination and 19.775 h right ascension, placing it in the constellation</p><p>Telescopium.Voyager 1, the most distant human-made object in the cosmos, reached</p><p>100 astronomical units from the Sun on Tuesday, August 15, 2007 at 12.13 UT,</p><p>meaning it is over a hundred times more distant from the Sun than Earth is.</p><p>The Voyager spacecraft itself is a three-axis stabilised platform with a total mass</p><p>of 800 kg, including the science instruments at about 105 kg. A 3.7 m antenna is</p><p>used for telecommunications and radio science.A very advanced machine for its era,</p><p>Voyager was capable of operating with a high degree of autonomy at vast distances</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>The Voyager Missions to Titan 19</p><p>Figure 2.2 Voyagers 1 and 2 followed similar courses to Saturn and then separated. Because of its</p><p>trajectory, designed to reach Uranus, Voyager 2 did not encounter Titan (NASA/JPL).</p><p>from the Earth. Its high-resolution television camera could read the headlines of a</p><p>newspaper from a distance of one kilometre, with the gas jets controlling its position</p><p>and alignment providing remarkable stability. Three radioisotope thermoelectric</p><p>generators (RTGs) use nuclear power to produce 7000 W of heat, which is converted</p><p>into 370 W of electricity and used to operate the spacecraft, its transmitter, and the</p><p>scientific payload. Before leaving the Earth, the fuel tank was filled with more than</p><p>100 kilos of liquid hydrazine. This propellant was used for attitude control, course</p><p>corrections, and to position Voyager so that the instrument pointing platform can</p><p>observe its targets. Three onboard computers carry out instructions from Earth but</p><p>can also operate the spacecraft autonomously for long periods of time, with what</p><p>now seems like the very limited capability to store about 538 million bits of data</p><p>on-board.</p><p>Each Voyager carries the same eleven scientific instruments, four of them</p><p>mounted on the movable scan platform so they can be pointed at specific targets. The</p><p>latter are the imaging experiment, consisting of boresighted narrow- and wide-angle</p><p>cameras; the infrared interferometer spectrometer and radiometer (IRIS); the ultravi-</p><p>olet spectrometer (UVS); and a photopolarimeter-radiometer. Six other instruments</p><p>are used to study fields, particles and waves in interplanetary space and near planets,</p><p>including magnetometers, a plasma detector, a low-energy charged particle detector,</p><p>a plasma wave detector, a planetary radio astronomy instrument, and a cosmic ray</p><p>detector. In addition, the spacecraft’s radio antenna doubles as a radio telescope for</p><p>scientific investigations.</p><p>June 4, 2008 8:53 B-611 9.75in x 6.50in ch02</p><p>20 Titan: Exploring an Earthlike World</p><p>Figure 2.3 TheVoyager spacecraft: the optical instruments are on the articulated scan platform shown</p><p>at the top; the cylinder below and to the left of the long magnetometer boom is one of the radioisotope</p><p>power generators (NASA/JPL).</p><p>TheVoyagers may be the most prolific space probes ever launched, having visited</p><p>all four of the giant planets and their many satellites and having returned a mass of</p><p>data taken by the cameras and other scientific instrument onboard. This Planetary</p><p>Grand Tour, as it was called, was possible thanks to a rare geometric arrangement</p><p>of the outer planets that only occurs once every 176 years.</p><p>2.2 Voyager Observations of Titan</p><p>Titan’s visible appearance at the time was unexciting — an orange ball, completely</p><p>covered by thick haze which allowed no visibility</p>
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