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

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SP-1231 100 potential levels of contam<strong>in</strong>ation (Wright et al., 1989). Un<strong>for</strong>tunately, it is not yet so clear that <strong>the</strong> carbonate <strong>in</strong> E79 is martian. E79 appears to conta<strong>in</strong> some modern 14 C, conta<strong>in</strong>ed <strong>in</strong> terrestrially produced carbonate. Wherever large deposits of carbonate exist <strong>the</strong>re is a suggestion that 14 C correlates with 13 C abundance and, if so, that a high δ 13 C identifies <strong>the</strong> carbonate as <strong>in</strong>digenous (Wright et al., 1997a). A study of <strong>the</strong> carbonate-rich samples, <strong>in</strong>clud<strong>in</strong>g some extracted from completely glass-sealed <strong>in</strong>clusions, shows <strong>the</strong>y are high <strong>in</strong> organics. If most of <strong>the</strong> carbonate is <strong>in</strong>deed <strong>in</strong>digenous, <strong>the</strong>n it can be argued that <strong>the</strong> organic matter came to Earth with <strong>the</strong> sample. Indeed, <strong>the</strong> amounts of organic residues encountered are a factor of five greater than <strong>for</strong> any o<strong>the</strong>r martian meteorite, some of which are known to be heavily contam<strong>in</strong>ated. Carbonate-poor fractions of E79 are also among <strong>the</strong> least contam<strong>in</strong>ated that have been studied. Thus critics of <strong>the</strong> work on E79 have developed models <strong>for</strong> selective contam<strong>in</strong>ation. For example, flush<strong>in</strong>g <strong>the</strong> meteorite with large amounts of Antarctic melt water conta<strong>in</strong><strong>in</strong>g low levels of am<strong>in</strong>o acids and PAHs (Becker et al., 1997). <strong>The</strong>re are detailed arguments that can be presented but <strong>the</strong> most obvious observation aga<strong>in</strong>st such a mechanism is <strong>the</strong> fact that <strong>the</strong> sample does not show any evidence of <strong>in</strong>undation by <strong>the</strong> huge quantities of Antarctic melt water required. Wright et al. (1997a) conclude on <strong>the</strong> contrary that whatever process added <strong>the</strong> carbonates to E79 also deposited <strong>the</strong> large quantities of organic compounds measured. Moreover, <strong>the</strong> event occurred at low temperatures and it <strong>in</strong>volved aqueous fluids. If <strong>the</strong> organic compounds <strong>in</strong> E79 are Martian, <strong>the</strong>n <strong>the</strong> significance of <strong>the</strong>ir carbon isotopic composition (δ 13 C~-25‰), as compared to a 13 C-enriched atmosphere, estimated from trapped gases and carbonate measurements, cannot be overstated. On Earth, biological activity has been remarkably consistent <strong>in</strong> terms of its isotopic effects over a period of some 3.5 Gyr. Schidlowski has exam<strong>in</strong>ed <strong>the</strong> carbon isotopic composition of more than 1600 samples of fossil kerogenous organic matter of all ages and recorded an average δ 13 C of -25±7‰, attribut<strong>in</strong>g <strong>the</strong> uni<strong>for</strong>mity to <strong>the</strong> extensive <strong>in</strong>volvement of <strong>the</strong> RUBP carboxylase biosyn<strong>the</strong>tic pathway (Schidlowski, 1987). <strong>The</strong> spread of results is not dist<strong>in</strong>guishable from recent mar<strong>in</strong>e sediments even though more modern photosyn<strong>the</strong>tic pathways now contribute to <strong>the</strong> sedimentary biomass. Over <strong>the</strong> complete geologic record, mar<strong>in</strong>e carbonates vary by no more than ±2-3‰ around a mean value. Schidlowski has argued successfully (see Section I.3) that a fractionation of ~20-35‰ <strong>in</strong> carbon isotopes between dissolved and fixed carbon is <strong>the</strong>re<strong>for</strong>e a diagnostic signature of life on Earth right back to <strong>the</strong> earliest Precambrian times. This hypo<strong>the</strong>sis is so well accepted that even when <strong>the</strong> fractionation is less or greater than <strong>the</strong> required value, e.g. <strong>in</strong> Isua metasediments or <strong>the</strong> Fortescue group, respectively, special plead<strong>in</strong>gs are allowed and <strong>the</strong> concept rema<strong>in</strong>s unviolated. <strong>The</strong> possibility of us<strong>in</strong>g isotopic fractionation as an <strong>in</strong>dicator of life on Mars has previously been considered (Rothschild & DesMarais, 1989) without reach<strong>in</strong>g a def<strong>in</strong>ite conclusion or evaluat<strong>in</strong>g <strong>the</strong> possibility of us<strong>in</strong>g data from a martian meteorite. Schidlowski (1992), however, has po<strong>in</strong>ted out that his pr<strong>in</strong>ciple could apply to meteorites from <strong>the</strong> planet, cit<strong>in</strong>g Wright et al. (1989) as a source of data, but with <strong>the</strong> caveat that various undef<strong>in</strong>ed uncerta<strong>in</strong>ties mitigate aga<strong>in</strong>st mak<strong>in</strong>g a direct l<strong>in</strong>kage to life processes. At <strong>the</strong> time, this caution was rightly related to <strong>the</strong> undef<strong>in</strong>ed provenance of <strong>the</strong> carbonate, <strong>the</strong> organic matter or both. It is now more likely that both are <strong>in</strong>digenous, <strong>in</strong> which case it is a reasonable assumption that <strong>the</strong> carbon isotopic difference of ~40‰ between <strong>the</strong> components could be <strong>in</strong>dicative of a biologically controlled process. Of course, it is un<strong>for</strong>tunate that δ 13 C of <strong>the</strong> organic matter <strong>in</strong> E79 is measured at about -25‰, <strong>in</strong> <strong>the</strong> range of terrestrial contam<strong>in</strong>ants. Even more confidence might have been placed on a potential biological orig<strong>in</strong> if some extreme values had been determ<strong>in</strong>ed, ak<strong>in</strong> to <strong>the</strong> values found <strong>in</strong> some Precambrian samples which have δ 13 C
<strong>in</strong>dicated by carbonates and <strong>the</strong> organics <strong>in</strong> E79 are <strong>in</strong>dicative of life, just as <strong>the</strong> nanofossils might be <strong>for</strong> A84, <strong>the</strong>n <strong>the</strong> conclusion that has to be drawn is of major importance. We do not know <strong>the</strong> actual age of <strong>the</strong> carbonate but because it is secondary, younger than <strong>the</strong> rock of age 180 million years, <strong>the</strong>n one has to accept that life processes are comparatively recent and could be as recent as 0.6 million years, when 79001 was blasted off Mars. It is of great importance to measure emplacement dates <strong>for</strong> E79 carbonates (and <strong>in</strong>deed all o<strong>the</strong>r secondary features <strong>in</strong> martian meteorites, especially A84) to restrict <strong>the</strong> relevant time <strong>in</strong>terval. Until this has been done, it can only be speculated that dur<strong>in</strong>g <strong>the</strong> span of <strong>the</strong> Cenozoic and Mesozoic eras on Earth, a time when d<strong>in</strong>osaurs lives and died, <strong>the</strong> first flower<strong>in</strong>g plants evolved, and even perhaps when <strong>the</strong> first hom<strong>in</strong>ids appeared, our sister planet may have had primitive life <strong>for</strong>ms. It may be that some of <strong>the</strong> publicity attached to recent f<strong>in</strong>d<strong>in</strong>gs concern<strong>in</strong>g martian meteorites will be proved to be misplaced. However, us<strong>in</strong>g Occam’s Razor (‘<strong>the</strong> simplest explanation is <strong>the</strong> most likely’), <strong>the</strong> <strong>in</strong>terpretation of <strong>the</strong> martian meteorite results on <strong>the</strong> basis that biology was somehow <strong>in</strong>volved may well stand. Almost all meteorites conta<strong>in</strong> carbon <strong>in</strong> some <strong>for</strong>m, but it is mostly <strong>in</strong>organic. Of special <strong>in</strong>terest <strong>for</strong> <strong>the</strong> study of exobiology are <strong>the</strong> subgroups of <strong>the</strong> chondrite class, <strong>the</strong> CI and CM carbonaceous chondrites and <strong>the</strong> unequilibrated type 3.0 to 3.1 ord<strong>in</strong>ary chondrites. All <strong>the</strong>se groups are ‘primitive’: <strong>the</strong>y show evidence of <strong>the</strong> passage or coexistence of water on <strong>the</strong>ir parent bodies. O<strong>the</strong>r groups of carbonaceous chondrites might be even more primitive, but <strong>the</strong>y seem to have little extractable organic matter. <strong>The</strong>ir carbon is elemental or <strong>in</strong>organic and <strong>the</strong>y are anhydrous. <strong>The</strong>re are a small number of unique or rare samples, many of which have relatively high amounts of carbon, but little or no work has been done to characterise it. CI and CM samples, typified by <strong>the</strong> Argal and Murchison meteorites, have been <strong>the</strong> subject of many studies to characterise <strong>the</strong>ir extractable organic compounds: hydrocarbons. Alipathic, alicyclic, isoprenoid and aromatic structures have all been found. Many o<strong>the</strong>r compound classes, normally considered to be related to biological activity when encountered <strong>in</strong> <strong>the</strong> terrestrial sedimentary record, have also been recognised. In this category come acids (carboxylic and am<strong>in</strong>o), nitrogen heterocycles, am<strong>in</strong>es, amides, alcohols etc. <strong>The</strong> early work has been reviewed by Nagy (1975) and more recent work summarised by Cron<strong>in</strong> et al. (1988). Close to 500 <strong>in</strong>dividual compounds have been identified. <strong>The</strong> study of extractable organic molecules <strong>in</strong> samples collected from lowtemperature environments and stored <strong>in</strong> museum collections is fraught with problems related to contam<strong>in</strong>ation dur<strong>in</strong>g <strong>the</strong> time <strong>the</strong> samples have been on Earth. This is particularly true <strong>for</strong> <strong>the</strong> Argal meteorite, which fell <strong>in</strong> 1864. While <strong>the</strong> Murchison meteorite was recovered <strong>in</strong> more enlightened times, not all samples have been collected, stored and preserved with equal care. Surpris<strong>in</strong>gly, samples from <strong>the</strong> somewhat cleaner environment of Antarctica have been less well considered, part of <strong>the</strong> problem be<strong>in</strong>g that a suitable CI meteorite has not yet been located on <strong>the</strong> ice cap. Indeed, none of <strong>the</strong> samples from polar locations is available <strong>in</strong> <strong>the</strong> relatively large amounts needed <strong>for</strong> <strong>the</strong> study of extractable organics. Specimens found <strong>in</strong> ‘sterile’ hot deserts tend to be larger but appear to have been leached of <strong>the</strong>ir organic content (Ash & Pill<strong>in</strong>ger, 1995). Early work (e.g. by Nagy et al., 1961) suggest<strong>in</strong>g <strong>the</strong> presence of biogenic molecules was severely questioned. S<strong>in</strong>ce <strong>the</strong>n only very specific problems have been exam<strong>in</strong>ed <strong>in</strong> detail. <strong>The</strong> best-studied case concerns <strong>the</strong> am<strong>in</strong>o acids. <strong>The</strong> discovery (Kvenvolden et al., 1970) that <strong>the</strong>se existed as racemic mixtures <strong>in</strong> <strong>the</strong> Murchison samples precluded an orig<strong>in</strong> from contam<strong>in</strong>ation (all terrestrial biological am<strong>in</strong>o acids are of <strong>the</strong> L-type). <strong>The</strong> idea that <strong>the</strong>se am<strong>in</strong>o acids were <strong>in</strong>digenous to <strong>the</strong> meteorite was confirmed when several non-prote<strong>in</strong> species (Kvenvolden et al., 1971) were found among <strong>the</strong> extracts, and recently <strong>the</strong>y have been shown to conta<strong>in</strong> more 13 C and 15 N than <strong>the</strong>ir terrestrial counterparts (Engel et al., 1990). <strong>The</strong> distribution of <strong>in</strong>di- <strong>the</strong> martian meteorites/II.3 II.3.5 Carbon Compounds <strong>in</strong> O<strong>the</strong>r Meteorites (Carbonaceous Chondrites) 101
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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
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
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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
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Page 91 and 92: II.3 The Martian M
Page 93 and 94: principal atmosphe
Page 95: 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
Page 117 and 118: SP-1231 122 II.4.6 Rovers and Drill
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Page 121 and 122: SP-1231 126 Site 4: Chasma area, ea
Page 123 and 124: SP-1231 128 Sharp, M., Parkes, J.,
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Page 127 and 128: SP-1231 132 II.5.3 Isotopic Analysi
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Page 133 and 134: SP-1231 138 optical microscope. Bot
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Page 141 and 142: SP-1231 146 soils. In this approach
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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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