Exobiology in the Solar System & The Search for Life on Mars - ESA

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abundances at various locations over Titan’s disc (Coustenis & Bézard, 1995). Seasonal effects caused an enhancement of some of <strong>the</strong> less abundant m<strong>in</strong>or components over Titan’s North Pole at <strong>the</strong> time of <strong>the</strong> Voyager encounter (Fig. I.5.2.1/1). <strong>The</strong> exact degree of complexity of <strong>the</strong> organic chemistry active <strong>in</strong> Titan’s atmosphere is not well known. More complex organic compounds are still be<strong>in</strong>g discovered <strong>in</strong> gaseous or solid <strong>for</strong>m <strong>in</strong> <strong>the</strong> atmosphere. Titan is <strong>the</strong> only o<strong>the</strong>r object <strong>in</strong> our <strong>Solar</strong> <strong>System</strong> to bear a strong resemblance to our own planet <strong>in</strong> terms of atmospheric composition and temperature structure. <strong>The</strong> latter shows an <strong>in</strong>version near <strong>the</strong> tropopause and <strong>in</strong>creases towards <strong>the</strong> surface (T=94K) ow<strong>in</strong>g to a small greenhouse effect. <strong>The</strong> temperature profile <strong>in</strong> <strong>the</strong> stratosphere also varies as a function of latitude (Fig. I.5.2.1/2). Measurements by ESA’s Infrared Space Observatory (ISO) <strong>in</strong> January 1997 have confirmed models based on <strong>the</strong> Voyager data (Coustenis et al., 1993), and are still under analysis. <strong>The</strong>y provide more accurate abundance measurements <strong>in</strong> <strong>the</strong> grat<strong>in</strong>g mode (although on a disc-averaged basis) and even provide vertical distributions <strong>for</strong> some of <strong>the</strong> m<strong>in</strong>or components by use of <strong>the</strong> Fabry-Perot mode. Water vapour was discovered <strong>in</strong> Titan’s atmosphere by ISO/SWS (Coustenis et al., 1998). I.5.2.2 <strong>The</strong> Titan Surface <strong>The</strong> surface of Titan has never been viewed directly because of <strong>the</strong> thick cloud deck surround<strong>in</strong>g <strong>the</strong> satellite. Models have predicted <strong>the</strong> existence of oceans or large lakes of hydrocarbon material on <strong>the</strong> ground. <strong>The</strong>se models have recently been disputed by ground-based and Hubble Space Telescope (HST) spectroscopic observations. Observations of Titan have been per<strong>for</strong>med with <strong>the</strong> Fourier Trans<strong>for</strong>m Spectrometer at <strong>the</strong> CFH Telescope <strong>in</strong> Hawaii. <strong>The</strong> spectra were recorded over a period of 5 years (1991-1996) and cover most of Titan’s orbit (16 days) <strong>in</strong> <strong>the</strong> 1- 2.5 µm region. <strong>The</strong> observations show that, with<strong>in</strong> an orbit, Titan’s geometric albedo exhibits significant variations <strong>in</strong>dicative of a brighter lead<strong>in</strong>g hemisphere (fac<strong>in</strong>g <strong>the</strong> Earth) and a darker trail<strong>in</strong>g one (Fig. I.5.2.2/1). <strong>The</strong> maximum albedo appears near 120±20º LCM (Longitude of Central Meridian) and <strong>the</strong> m<strong>in</strong>imum near 230±20º LCM. <strong>The</strong> variations (25-35%) have been observed <strong>in</strong>dependently by various groups s<strong>in</strong>ce 1989 and <strong>the</strong>y are recurrent. <strong>The</strong>y must <strong>the</strong>re<strong>for</strong>e be correlated with Titan’s potential non-martian sites <strong>for</strong> extraterrestrial life/I.5 Fig. I.5.2.2/1 Titan’s geometric albedo, 1991- 1995 (Coustenis, private communication). 69
SP-1231 Fig. I.5.2.2/2. Modelled Titan surface spectrum (Coustenis et al., 1999, <strong>in</strong> preparation). 70 surface morphology, ra<strong>the</strong>r than atmospheric cloud structures. <strong>The</strong> Titan surface spectrum, as modelled from <strong>the</strong> observations (Fig. I.5.2.2/2), is remarkably consistent with a mostly solid surface dom<strong>in</strong>ated by water ice and it <strong>the</strong>re<strong>for</strong>e resembles that of o<strong>the</strong>r satellites, such as Hyperion or Callisto (Coustenis et al., 1995). Spatially-resolved images of Titan, us<strong>in</strong>g <strong>the</strong> European Sou<strong>the</strong>rn Observatory (ESO) 3.6 m. telescope and ADONIS optics, were obta<strong>in</strong>ed <strong>in</strong> October 1993, September 1994, October 1995 and November 1996 (Coustenis et al., 1997). <strong>The</strong> images were recorded through a set of narrowband filters (K1, H1, J1), def<strong>in</strong>ed <strong>in</strong> order to maximise <strong>the</strong> contributions of <strong>the</strong> light reflected by Titan’s surface <strong>in</strong> <strong>the</strong> total signal recorded at <strong>the</strong> centre of <strong>the</strong> 2.0, 1.6 and 1.28 µm transparency w<strong>in</strong>dows of Titans atmosphere. At those three wavelengths, Titan’s atmosphere is transparent because of very low molecular methane absorption. In order to allow <strong>the</strong> subtraction of <strong>the</strong> stratospheric contribution and hence improve <strong>the</strong> visibility of ground features, images were recorded <strong>in</strong> adjacent narrowband filters (K2, H2, J2) <strong>in</strong> <strong>the</strong> w<strong>in</strong>gs of <strong>the</strong> strong methane bands. <strong>The</strong> 2.2 µm ADONIS images show <strong>the</strong> expected north-south asymmetry <strong>in</strong> <strong>the</strong> stratosphere, <strong>the</strong> South Pole brighter than <strong>the</strong> North Pole, reversed with respect to Voyager and due to seasonal effects. After deconvolution, correction of centre-limb effects and subtraction of <strong>the</strong> atmospheric component, <strong>the</strong> 2.0 µm surface images show a bright well-def<strong>in</strong>ed equatorial spot at around 120º LCM (lead<strong>in</strong>g hemisphere) extend<strong>in</strong>g over 60º <strong>in</strong> longitude and 30 º <strong>in</strong> latitude, and some smaller and less contrasted features near <strong>the</strong> poles, all rotat<strong>in</strong>g over six consecutive nights at <strong>the</strong> expected rotation rate of Titan’s solid body. <strong>The</strong> area attributed to ground features is <strong>in</strong> agreement with HST images (Smith et al., 1996) taken near 1 µm, both as to <strong>the</strong> location and <strong>the</strong> shape. <strong>The</strong> spatial resolution is at <strong>the</strong> limit of diffraction (0.13 arcsec at 2.0 µm), very similar to <strong>the</strong> HST images. Moreover, <strong>the</strong> ADONIS images of Titan’s trail<strong>in</strong>g hemisphere show high-latitude bright zones with a north-south asymmetry; <strong>the</strong> nor<strong>the</strong>rn latitudes appear brighter by about a factor two with respect to <strong>the</strong> south (Fig. I.5.2.2/3). <strong>The</strong>se bright zones might exist over Titan’s entire disc. <strong>The</strong> entire surface of Titan appears quite dark <strong>in</strong> <strong>the</strong> water ice bands, <strong>in</strong> agreement with spectroscopic measurements by Coustenis et al., 1997. <strong>The</strong> model of a global ocean cover<strong>in</strong>g <strong>the</strong> surface must be ruled out. It is <strong>in</strong>ferred that large cloud structures
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SP-1231 Exobiology
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Cover Fossil coccoid bacteria, 1 µ
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4 I.5 Potential Non-Martian Sites <
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6 6.2 Imaging of F
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The Exobio
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pyrolysis, or similar techniques, f
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Ignasi Casanova, Institute
Page 15 and 16: I AN EXOBIOLOGICAL VIEW OF THE SOLA
Page 17 and 18: SP-1231 18 surface pressure of CO 2
Page 19 and 20: SP-1231 20 10 bar befor</st
Page 21 and 22: SP-1231 22 kilometres in</s
Page 23 and 24: SP-1231 24 Laboratory Investigation
Page 25 and 26: I.3 Limits of Life
Page 27 and 28: temperatures of 95-106ºC. This sug
Page 29 and 30: (mainly found <str
Page 31 and 32: cannot exclude fin
Page 33 and 34: Responses to Cosmic Radiation <stro
Page 35 and 36: acid to identify the</stron
Page 37 and 38: Horneck, G. (1993). Responses of Ba
Page 39 and 40: SP-1231 Fig. I.4.2.2/1. Size distri
Page 41 and 42: SP-1231 Fig. I.4.2.2/4. Evaporite w
Page 43 and 44: SP-1231 Fig. I.4.2.3/1A (left). Net
Page 45 and 46: SP-1231 48 A detailed chemical anal
Page 47 and 48: SP-1231 Fig. I.4.3.1.1/1. Scheme of
Page 49 and 50: SP-1231 Fig. I.4.3.1.2/1. A: Compar
Page 51 and 52: SP-1231 54 I.4.3.2.1 Sedimentary Or
Page 53 and 54: SP-1231 56 The iso
Page 55 and 56: SP-1231 Fig. I.4.3.2.3/1. Cholester
Page 57 and 58: SP-1231 60 References regard to non
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Page 61 and 62: SP-1231 64 ities in</strong
Page 63 and 64: SP-1231 Fig. I.5.1.1/1. A 34×42 km
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Page 69 and 70: SP-1231 72 Galileo: images availabl
Page 71 and 72: SP-1231 74 TABLE I.6.2/1 EVIDENCE O
Page 73 and 74: SP-1231 I.6.3 What to Searc
Page 75 and 76: SP-1231 78 I.7.3 Organic Chemistry
Page 77 and 78: II.1 Introduction The</stro
Page 79 and 80: II.2 The Planet Ma
Page 81 and 82: cover the range 4.
Page 83 and 84: The depletion of c
Page 85 and 86: valley networks (Fig. II.2.5/2) are
Page 87 and 88: plate tectonic evidence has been ob
Page 89 and 90: Table II.2.7.4/1. Potential Mars <s
Page 91 and 92: II.3 The Martian M
Page 93 and 94: principal atmosphe
Page 95 and 96: Lines of evidence
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Page 101 and 102: Anders, E. (1989). Prebiotic organi
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Page 113 and 114: SP-1231 118 orbital characteristics
Page 115 and 116: SP-1231 120 II.4.5 The</str
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SP-1231 122 II.4.6 Rovers and Drill
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SP-1231 124 Site 2: Apolin<
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SP-1231 126 Site 4: Chasma area, ea
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SP-1231 128 Sharp, M., Parkes, J.,
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SP-1231 Fig. II.5.1/1. The<
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SP-1231 132 II.5.3 Isotopic Analysi
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SP-1231 134 chemical composition de
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SP-1231 Fig. II.5.7.1/1. Th
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SP-1231 138 optical microscope. Bot
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SP-1231 140 protons of discrete ene
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SP-1231 142 atures should be per<st
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SP-1231 Fig. II.5.7.7/1. Example of
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SP-1231 146 soils. In this approach
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SP-1231 148 discrimin</stro
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SP-1231 150 determin</stron
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SP-1231 Fig. II.5.10.2/2. Enantiome
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SP-1231 154 References achiral colu
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II.6 Team III: The
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in width and heigh
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Searchin</
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minerals or oxidat
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A second group of rocks, however, m
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SCIENTIFIC METHOD AND REQUIREMENTS
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The sample materia
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surprising. To ach
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The main</
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Consideration of a diamond wire-saw
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Schoonen, M.A. & Barnes, H.L. (1991
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SP-1231 180 II.7.2 The</str
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SP-1231 Fig. II.7.4/1. Look
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SP-1231 184 II.7.5 A Possible <stro
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Team IV was asked to examin
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Exobiology in the Solar System & The Search for Life on Mars - ESA

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