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

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pores, affect<strong>in</strong>g <strong>the</strong> <strong>in</strong>terstitial water. <strong>The</strong> evaporation decreases pore pressure which thus draws water from below. This ‘evaporitic pump<strong>in</strong>g’ can produce massive and thick concentrations of gypsum or limestone, embedd<strong>in</strong>g silicoclastic or argilliferous material. Water can be found <strong>in</strong> shallow ponds even at <strong>the</strong> surface and direct deposition of salts can occur <strong>in</strong> <strong>the</strong>se cases. <strong>The</strong> sebkha usually exhibit a flat surface with a small supply of w<strong>in</strong>d-blown detritus. <strong>The</strong>y have not been recognised on <strong>the</strong> martian surface, but some putative deposits have been identified (Figs. II.4.1.2/1 and II.4.1.2/2). Fur<strong>the</strong>r m<strong>in</strong>eralogical <strong>in</strong>vestigation from orbiters needs to be per<strong>for</strong>med <strong>in</strong> order to establish <strong>the</strong> presence of sebkha-like environments on Mars. On Earth, <strong>the</strong>se extreme areas are rich <strong>in</strong> bacterial life – dispersed <strong>in</strong> <strong>the</strong> water or concentrated <strong>in</strong> microbial mats. <strong>The</strong> fossilisation potential of <strong>the</strong>se structures, as well as biomolecules, could be quite high if <strong>the</strong>re is not pervasive recrystallisation of <strong>the</strong> deposits or solubilisation. Microbial fossils would be also preserved by replacement with more stable m<strong>in</strong>erals such as silica. II.4.1.3 <strong>The</strong>rmal-Spr<strong>in</strong>g Deposits On Earth, subaerial <strong>the</strong>rmal spr<strong>in</strong>g deposits are localised areas where heated gasbear<strong>in</strong>g water reaches <strong>the</strong> surface or <strong>the</strong> near-surface zone. <strong>The</strong>se areas are strictly l<strong>in</strong>ked with volcanic edifices or activity. Several putative <strong>the</strong>rmal spr<strong>in</strong>gs have been identified on Mars (Fig. II.4.1.3/1), but none has been confirmed by a detailed geomorphologic and geochemical study. Usually, areas near volcanic edifices, hav<strong>in</strong>g evidence of <strong>the</strong> generation of channels, are thought to be hydro<strong>the</strong>rmal sites. <strong>The</strong>se terrestrial environments are rich <strong>in</strong> microbial material and, if at all present on Mars, could be one of <strong>the</strong> best targets <strong>for</strong> exobiological exploration. <strong>The</strong> large quantity of m<strong>in</strong>eral deposition associated with fast and effective molecule-bymolecule recrystallisation allow <strong>the</strong> preservation of microbial material. Rocks derived from spr<strong>in</strong>g deposits may be rich <strong>in</strong> fluid <strong>in</strong>clusions reta<strong>in</strong><strong>in</strong>g <strong>the</strong> orig<strong>in</strong>al liquid and gas as well as microorganism and biomolecules. II.4.1.4 Duricrusts In <strong>the</strong> presence of surface water, sediments <strong>in</strong> arid lands tend to <strong>for</strong>m particular soils, generically called duricrusts. <strong>The</strong>se deposits occur when <strong>the</strong> surface water <strong>in</strong>filtrates down and br<strong>in</strong>gs up soluble compounds such as calcium carbonates, silica, gypsum and sesquioxides (particularly iron). <strong>The</strong> water can also move upwards <strong>in</strong> a manner similar to evaporitic pump<strong>in</strong>g of sebkha and, <strong>in</strong> fact, duricrusts are soil types sometimes associated with sebkha deposits. <strong>The</strong> evaporation of <strong>the</strong> <strong>in</strong>terstitial water leads to a large concentration of cement <strong>in</strong> <strong>the</strong> <strong>for</strong>m of laterally extensive layers. On Earth, duricrusts are extremely hard and can be several metres thick. Bacterial activity is known to be associated with this type of deposit, which has a strong preservation potential. On Mars, duricrusts have been not observed directly. However, <strong>the</strong>ir presence can be <strong>in</strong>ferred because <strong>the</strong>re is evidence of superficial humidity that can percolate downwards. <strong>The</strong> extreme dryness and gravity of <strong>the</strong> planet can enhance evaporation of <strong>the</strong> <strong>in</strong>terstitial water, with <strong>the</strong> consequent <strong>for</strong>mation of duricrusts. Iron is largely present at <strong>the</strong> surface and can assist <strong>in</strong> duricrust <strong>for</strong>mation. II.4.1.5 Glacial Deposits <strong>The</strong>re is a wide variety of associated land<strong>for</strong>ms on Mars with characteristics resembl<strong>in</strong>g glacial features produced by <strong>the</strong> Pleistocene ice age on Earth. <strong>The</strong>se features are generally restricted to high latitudes, and <strong>the</strong>y <strong>for</strong>m ordered sequences almost identical to those observed <strong>for</strong> terrestrial glacial land<strong>for</strong>ms. <strong>The</strong>y resemble eskers, mora<strong>in</strong>es, kettles, glacial gouges, kames, druml<strong>in</strong>s, tunnel channels, outwash pla<strong>in</strong>s and glaciolacustr<strong>in</strong>e pla<strong>in</strong>s. Of <strong>the</strong>se features, only eskers, mora<strong>in</strong>es, outwash pla<strong>in</strong>s and glaciolacustr<strong>in</strong>e pla<strong>in</strong>s represent sedimentary deposits. <strong>The</strong> o<strong>the</strong>rs are glacial erosion features that occur mostly <strong>in</strong> glacial deposits. <strong>The</strong>se features will be described <strong>in</strong> some detail because <strong>the</strong>y are not widely known. <strong>The</strong>ir existence is <strong>the</strong> subject of debate, mostly because of <strong>the</strong> important implications <strong>for</strong> paleoclimatic reconstructions (see below). team I: exobiology and <strong>the</strong> mars surface environment/II.4 113
SP-1231 114 S<strong>in</strong>uous ridges resembl<strong>in</strong>g eskers are found at various locations on Mars. <strong>The</strong>y are seen best developed on <strong>the</strong> floor of <strong>the</strong> Argyre bas<strong>in</strong>, <strong>the</strong> south polar region and at several locations <strong>in</strong> <strong>the</strong> nor<strong>the</strong>rn pla<strong>in</strong>s. Like terrestrial eskers, martian s<strong>in</strong>uous ridges cross topographic divides, have very low and high tributary junction angles and rectil<strong>in</strong>ear patterns, and show conspicuous longitud<strong>in</strong>al discont<strong>in</strong>uities and variations <strong>in</strong> width and height, produc<strong>in</strong>g a segmented or beaded appearance. Also like terrestrial eskers, <strong>the</strong>y sometimes pass longitud<strong>in</strong>ally <strong>in</strong>to, or reside with<strong>in</strong> eroded valleys that are probably glacial tunnel channels. Fur<strong>the</strong>rmore, <strong>the</strong>y also show esker-like subhorizontal layer<strong>in</strong>g and ridge crests that are sharply crested, rounded, flat-topped or doublecrested. Individual ridges range from 10 km to 200 km long, 0.3-3 km wide and 20- 160 m high. This is near <strong>the</strong> upper range <strong>for</strong> terrestrial eskers, but <strong>the</strong>y could not be seen at <strong>the</strong> resolution of <strong>the</strong> Vik<strong>in</strong>g images. If <strong>the</strong> martian s<strong>in</strong>uous ridges are eskers, <strong>the</strong>y should consist of f<strong>in</strong>e- to course-gra<strong>in</strong>ed bedded material with occasional boulders. Smooth pla<strong>in</strong>s <strong>in</strong> <strong>the</strong> Hellas and Argyre bas<strong>in</strong>s might be proglacial lake deposits. In Hellas, <strong>the</strong>y occur at <strong>the</strong> term<strong>in</strong>us of <strong>the</strong> mora<strong>in</strong>es and are associated with fluvial channels and a possible shorel<strong>in</strong>e. <strong>The</strong>re are numerous o<strong>the</strong>r areas, <strong>in</strong>clud<strong>in</strong>g crater floors, that could also be lacustr<strong>in</strong>e deposits. Glacio-lacustr<strong>in</strong>e deposits may have bedded well-sorted f<strong>in</strong>e- to course-gra<strong>in</strong>ed material, with possible scattered boulders deposited by <strong>the</strong> glacier near <strong>the</strong> lake’s edge. In several areas, particularly near <strong>the</strong> Argyre bas<strong>in</strong>, <strong>the</strong>re are relatively small-scale braided delta-like fluvial erosional and/or depositional complexes. In one <strong>in</strong>stance, a braided delta-like region beg<strong>in</strong>s at a breached crater about 50 km <strong>in</strong> diameter. <strong>The</strong> crater is <strong>the</strong> obvious source of <strong>the</strong> braided complex and may have been <strong>the</strong> site of a glacier from which meltwater flowed to produce outwash-like pla<strong>in</strong>s. Similar braided complexes occur to <strong>the</strong> west and north. If <strong>the</strong>se features represent fluvial deposits from a melt<strong>in</strong>g glacier, <strong>the</strong>n <strong>the</strong>y should consist of f<strong>in</strong>e- to course-gra<strong>in</strong>ed bedded material. Glacial deposits are usually associated with large quantities of water, both liquid and solid, and <strong>the</strong>y satisfy <strong>the</strong> first requisite <strong>for</strong> <strong>the</strong> existence of life. If life was present on Mars and glaciation also occurred, it is quite probable that microbial life flourished <strong>in</strong> glaciated zones (Sharp et al., 1999). <strong>The</strong> potential <strong>for</strong> preservation is, however, quite low. II.4.1.6 Polar Deposits <strong>The</strong>re are two types of polar deposits of probable aeolian orig<strong>in</strong>: layered and massive. <strong>The</strong> layered deposits overlie <strong>the</strong> more massive deposits and are <strong>the</strong>re<strong>for</strong>e younger. As <strong>the</strong>se deposits have <strong>the</strong> lowest impact crater density of any terra<strong>in</strong> on Mars, <strong>the</strong>y are <strong>the</strong> youngest deposits on <strong>the</strong> planet. Both deposits are be<strong>in</strong>g eroded by aeolian action. In some cases, older impact craters are be<strong>in</strong>g exhumed as <strong>the</strong> overly<strong>in</strong>g deposits are stripped away. If <strong>the</strong>se deposits are aeolian <strong>in</strong> orig<strong>in</strong>, <strong>the</strong>y are surely f<strong>in</strong>e-gra<strong>in</strong>ed, well-sorted sediments. <strong>The</strong> orig<strong>in</strong> of <strong>the</strong> layer<strong>in</strong>g <strong>in</strong> <strong>the</strong> youngest deposits is not well understood, but <strong>the</strong>y could be <strong>for</strong>med by climate cycles. <strong>The</strong>re is some evidence <strong>for</strong> angular non-con<strong>for</strong>mities between subjacent bed sets. <strong>The</strong>re is no obvious exobiological potential <strong>in</strong> <strong>the</strong>se polar layered terra<strong>in</strong>s. However, <strong>the</strong>ir high-frequency cyclicity might be a good record of global changes and <strong>the</strong>ir study can improve, by analogy with terrestrial global-change studies, our knowledge on <strong>the</strong> geosphereatmosphere-biosphere <strong>in</strong>teractions. II.4.1.7 Ground Ice-Permafrost On Earth, permafrost is found widely <strong>in</strong> <strong>the</strong> polar areas, with associated extensive bacterial activity. Permafrost is thought to be present extensively <strong>in</strong> <strong>the</strong> martian subsurface. If life was present at sometime on <strong>the</strong> surface, it is quite probable that it is now segregated <strong>in</strong> <strong>the</strong> ground ice. <strong>The</strong> location of <strong>the</strong> permafrost layer is, however, quite deep and probably out of reach of any direct <strong>in</strong>vestigation <strong>in</strong> <strong>the</strong> near future. Direct evidence of bacteria <strong>in</strong> <strong>the</strong> permafrost could be ga<strong>the</strong>red by <strong>the</strong> discovery of methane released from <strong>the</strong> subsurface to <strong>the</strong> surface. Bacterial activity produces large quantities of methane and this could be released to <strong>the</strong> surface. It is possible that <strong>the</strong>
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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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SP-1231 56 The iso
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SP-1231 Fig. I.4.3.2.3/1. Cholester
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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
Page 81 and 82: cover the range 4.
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Page 141 and 142: SP-1231 146 soils. In this approach
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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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