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

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II.4 Team I: <strong>Exobiology</strong> and <strong>the</strong> Mars Surface Environment <strong>The</strong> task of Team I was to def<strong>in</strong>e <strong>the</strong> environment of Mars at <strong>the</strong> surface and shallow depth, with a view to detect<strong>in</strong>g possible signatures of past life. ‘Signature’ denotes a large spectrum of evidence from microscopic (bacterial to nanobacterial size) to macroscopic fossils, geochemical <strong>in</strong>dicators of biological activity, and o<strong>the</strong>r <strong>in</strong>dicators such as superparamagnetic gra<strong>in</strong>s. <strong>The</strong> evidence of past life might be present at <strong>the</strong> surface but it will more likely occur below <strong>the</strong> wea<strong>the</strong>red (oxidised) upper layer. <strong>The</strong> depth of this upper ‘barren’ layer can be only guessed and <strong>the</strong> only way to determ<strong>in</strong>e its thickness or to construct a reliable model <strong>for</strong> it is to dig <strong>in</strong>to <strong>the</strong> martian surface and collect geochemical and petrophysical data. <strong>The</strong> approach adopted by Team I is based on requirements concern<strong>in</strong>g two k<strong>in</strong>ds of evidence of past life: chemical signatures and bacterial fossils. However, a large part of <strong>the</strong> discussion is also valid <strong>for</strong> <strong>the</strong> search of extant life. In <strong>the</strong> past few years it has become clear that Mars is an extremely diverse planet <strong>in</strong> geological terms. <strong>The</strong> first data from Mars Global Surveyor (MGS) support <strong>the</strong> exist<strong>in</strong>g evidence of an extensive diversification of geological units, features and crustal structures. This geological diversity is of paramount importance <strong>in</strong> <strong>the</strong> exobiological exploration of <strong>the</strong> planet. This new approach is matched by <strong>the</strong> f<strong>in</strong>d<strong>in</strong>g of <strong>for</strong>ms of life <strong>in</strong> a huge variety of geological sett<strong>in</strong>gs, environments and ecosystems on Earth. It has been well known <strong>for</strong> several decades that bacterial <strong>for</strong>ms live <strong>in</strong> <strong>the</strong> Earth’s crust at depths of several kilometres. Many <strong>for</strong>ms of life live near <strong>the</strong> hydro<strong>the</strong>rmal vents <strong>in</strong> <strong>the</strong> deep oceans and, recently, large liv<strong>in</strong>g communities have been discovered <strong>in</strong> deep-sea zones around <strong>the</strong> bodies of dead whales. It is now clear that life is a strongly colonis<strong>in</strong>g factor which, matched with <strong>the</strong> martian geological diversity, suggests that Mars is actually a place where life was probably present and, just possibly, might still exist <strong>in</strong> some specialised niche. <strong>The</strong> basic requirement <strong>for</strong> <strong>the</strong> <strong>for</strong>mation and survival of life is <strong>the</strong> presence of water and energy (nutrients). This need not be true <strong>for</strong> an exotic <strong>for</strong>m of life, but <strong>in</strong> order to establish <strong>the</strong> environmental constra<strong>in</strong>ts <strong>for</strong> <strong>the</strong> search <strong>for</strong> life on Mars it is better to adopt a conservative approach. <strong>The</strong>re<strong>for</strong>e, <strong>the</strong> best candidates are those environments where water has been present <strong>for</strong> a considerable time. From this po<strong>in</strong>t of view, <strong>the</strong> geological environments should be regarded as any geological sett<strong>in</strong>g, regardless of dimension and position, with def<strong>in</strong>ite boundary conditions. Thus, geological environments would be obvious features such as lakes and rivers, but we must also <strong>in</strong>clude pores, fractures, surface roughness, <strong>for</strong> example. II.4.1.1 Lacustr<strong>in</strong>e Environments Lakes are ubiquitous on Earth and display a tremendous variety of sett<strong>in</strong>gs, conditions, shapes, etc. <strong>The</strong>y have also been recognised as hav<strong>in</strong>g existed on Mars. Martian lakes can be def<strong>in</strong>ed as localised areas where water was present as stand<strong>in</strong>g bodies. Probably, lakes were scattered on <strong>the</strong> surface <strong>in</strong> several geological sett<strong>in</strong>gs. However, <strong>the</strong>y are easily recognisable when <strong>the</strong>y occur as water-filled impact craters. <strong>The</strong>re are several <strong>in</strong>stances where channels enter large craters and, as frequently, exit on <strong>the</strong> opposite side. <strong>The</strong>se crater floors probably consist of sediments <strong>for</strong>med <strong>in</strong> ponded water. In some cases, <strong>the</strong>re are terraces l<strong>in</strong><strong>in</strong>g <strong>the</strong> crater <strong>in</strong>terior walls and <strong>the</strong>se may be <strong>in</strong>terpreted as erosional shorel<strong>in</strong>es. Lacustr<strong>in</strong>e deposits of this k<strong>in</strong>d may be expected to consist of bedded coarsegra<strong>in</strong>ed sand, gravel or cobbles near <strong>the</strong> bottom, grad<strong>in</strong>g downstream and upward to more f<strong>in</strong>e-gra<strong>in</strong>ed material as <strong>the</strong> flow slowed and particles came out of suspension. II.4.1 Environments and Rocks with <strong>Exobiology</strong> Potential 109
SP-1231 Fig. II.4.1.1/1. A Gilbert-type delta at <strong>the</strong> mouth of a short-headed stream. <strong>The</strong> Gilbert-type delta bodies suggest <strong>the</strong> presence of stand<strong>in</strong>g bodies of water. (Vik<strong>in</strong>g image, NASA.) Fig. II.4.1.1/2. Terraces; Left: <strong>in</strong> an emabyment of Hydroates Chaos (MGS-MOC image, NASA/MSSS). Right: around <strong>the</strong> rim of an impact crater with an <strong>in</strong>flow channel (Vik<strong>in</strong>g image, NASA.) 110 Crater lakes can be isolated, without channels enter<strong>in</strong>g <strong>the</strong> bas<strong>in</strong>. In this case, only <strong>the</strong> terraces on <strong>the</strong> craters’ <strong>in</strong>ner rims are found. However, most <strong>in</strong>stances show <strong>in</strong>put channels with delta-like structures (Fig. II.4.1.1/1). <strong>The</strong> morphology of <strong>the</strong>se deltas resembles terrestrial Gilbert-type deltas. <strong>The</strong>se <strong>in</strong>dicate stand<strong>in</strong>g bodies of water: when a stream flows <strong>in</strong>to a water-filled bas<strong>in</strong> with steep marg<strong>in</strong>s, it <strong>for</strong>ms a delta (Gilbert-type) composed of a flat upper surface (topset) correspond<strong>in</strong>g to <strong>the</strong> water level and a steep delta front. This type of delta and wave-dom<strong>in</strong>ated terraces (Fig. II.4.1.1/2) are <strong>the</strong> best evidence <strong>for</strong> bodies of stand<strong>in</strong>g water. <strong>The</strong> sedimentation pattern could be quite simple, with <strong>the</strong> coarser deposits near <strong>the</strong> crater rims and mostly concentrated at <strong>the</strong> channel mouths (<strong>in</strong> <strong>the</strong> deltaic area). <strong>The</strong> f<strong>in</strong>e sediments (very f<strong>in</strong>e silt or clay m<strong>in</strong>erals) probably rema<strong>in</strong> <strong>in</strong> <strong>the</strong> central area. Evaporitic deposits, ma<strong>in</strong>ly carbonate, can be present <strong>in</strong> this zone. II.4.1.2 Sebkha Environments ‘Sebkha’ is <strong>the</strong> Arabic word <strong>for</strong> dry lake. Sebkha on Earth are lakes only occasionally
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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
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I AN EXOBIOLOGICAL VIEW OF THE SOLA
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SP-1231 18 surface pressure of CO 2
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SP-1231 20 10 bar befor</st
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SP-1231 22 kilometres in</s
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SP-1231 24 Laboratory Investigation
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I.3 Limits of Life
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temperatures of 95-106ºC. This sug
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(mainly found <str
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cannot exclude fin
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Responses to Cosmic Radiation <stro
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acid to identify the</stron
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Horneck, G. (1993). Responses of Ba
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SP-1231 Fig. I.4.2.2/1. Size distri
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SP-1231 Fig. I.4.2.2/4. Evaporite w
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SP-1231 Fig. I.4.2.3/1A (left). Net
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SP-1231 48 A detailed chemical anal
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SP-1231 Fig. I.4.3.1.1/1. Scheme of
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SP-1231 Fig. I.4.3.1.2/1. A: Compar
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SP-1231 54 I.4.3.2.1 Sedimentary Or
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Page 61 and 62: SP-1231 64 ities in</strong
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Page 75 and 76: SP-1231 78 I.7.3 Organic Chemistry
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Page 79 and 80: II.2 The Planet Ma
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Page 93 and 94: principal atmosphe
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Page 117 and 118: SP-1231 122 II.4.6 Rovers and Drill
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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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