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

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morphological and biochemical signatures of extraterrestrial life: utility of terrestrial analogues/I.4 sediments (notably <strong>in</strong> <strong>the</strong> Tharsis region and <strong>the</strong> associated Valles Mar<strong>in</strong>eris canyon system, see II.2.7) that are <strong>in</strong>terpreted as lake deposits and believed to <strong>in</strong>clude thick sequences of carbonates (McKay & Nedell, 1988). If present at all <strong>in</strong> <strong>the</strong>se early martian <strong>for</strong>mations, morphological relics of fossil life should lend <strong>the</strong>mselves to as ready a detection as do <strong>the</strong>ir terrestrial counterparts <strong>in</strong> Archaean sediments, provided <strong>the</strong> host rock is accessible to ei<strong>the</strong>r robotic sens<strong>in</strong>g or direct <strong>in</strong>vestigation follow<strong>in</strong>g a sample return mission. In that regard, it seems to be a reasonable conjecture to resort to <strong>the</strong> paleontological <strong>in</strong>ventory of <strong>the</strong> oldest terrestrial sediments (3.50-3.85 Gyr old) <strong>for</strong> guidance concern<strong>in</strong>g potential fossil evidence from coeval martian rocks. On Earth, <strong>the</strong> earliest prokaryotic (bacterial and archaebacterial) microbial ecosystems have basically left two categories of morphological evidence, namely (i) stromatolite-type biosedimentary structures (‘microbialites’) and (ii) cellular relics of <strong>in</strong>dividual microorganisms (‘microfossils’). I.4.3.1.1 Microbialites Microbialites or ‘stromatolites’ (cf. Burne & Moore, 1987) are lam<strong>in</strong>ated biosedimentary structures that preserve <strong>the</strong> matt<strong>in</strong>g behaviour of bacterial and algal (primarily prokaryotic) microbenthos. Microbial buildups of this type represent stacks of f<strong>in</strong>ely lam<strong>in</strong>ated lithified microbial communities that orig<strong>in</strong>ally thrived as organic films at <strong>the</strong> sediment-water <strong>in</strong>terface, with younger mat generations successively superimposed on <strong>the</strong> older ones (cf. Figs. I.4.3.1.1/1 and I.4.3.1.1/2). <strong>The</strong> structures derive from <strong>the</strong> <strong>in</strong>teraction of <strong>the</strong> primary biologically active microbial layer with <strong>the</strong> ambient sedimentary environment, <strong>the</strong> fossilisation of <strong>the</strong> lam<strong>in</strong>ae result<strong>in</strong>g from ei<strong>the</strong>r trapp<strong>in</strong>g, b<strong>in</strong>d<strong>in</strong>g or biologically mediated precipitation of selected m<strong>in</strong>eral constituents. In terrestrial sediments, microbialites represent <strong>the</strong> most obvious (macroscopic) expression of fossil microbial life, with a record extend<strong>in</strong>g back to <strong>the</strong> Early Archaean (~3.5 Gyr ago). This constitutes prima facie evidence that benthic prokaryotes were already widespread <strong>in</strong> suitable aquatic habitats of <strong>the</strong> Archaean Earth. Both <strong>the</strong> morphological <strong>in</strong>ventory of <strong>the</strong> oldest stromatolites and <strong>the</strong> observed microfossil content of <strong>the</strong> ambient rock or coeval sequences (see below) allow a fairly elaborate reconstruction of <strong>the</strong> Earth’s earliest microbial ecosystems, <strong>in</strong>dicat<strong>in</strong>g that Archaean stromatolite builders were not markedly different from <strong>the</strong>ir geologically younger counterparts (<strong>in</strong>clusive of contemporary species). It appears well established that <strong>the</strong> pr<strong>in</strong>cipal microbial mat builders were filamentous and unicellular prokaryotes capable of phototactic responses and photoautotrophic carbon fixation (Walter, 1983). <strong>The</strong> unbroken stromatolite record from Archaean to present times attests, fur<strong>the</strong>rmore, to an astound<strong>in</strong>g degree of conservatism and uni<strong>for</strong>mity <strong>in</strong> <strong>the</strong> physiological per<strong>for</strong>mance and communal organisation of prokaryotic microbenthos over 3.5 Gyr of geological history. I.4.3.1.2 Cellular Microfossils Apart from <strong>the</strong> macroscopic vestiges of past microbial activity <strong>in</strong> <strong>the</strong> <strong>for</strong>m of stromatolites and related biosedimentary structures, <strong>the</strong>re is a second (microscopic) category of paleontological evidence stored <strong>in</strong> <strong>the</strong> sedimentary record. This microscopic or cellular evidence is currently supposed to go back to at least ~3.5 Gyr (as <strong>in</strong> <strong>the</strong> case of microbialites), with <strong>the</strong> biogenicity of microfossil-like morphologies reported from <strong>the</strong> ~3.8 Gyr-old Isua metasedimentary suite from West Greenland (Pflug, 1978) still be<strong>in</strong>g debated. While a wealth of au<strong>the</strong>ntic microbial communities has been reported from Early and Middle Precambrian (Proterozoic) <strong>for</strong>mations, <strong>the</strong> unequivocal identification of cellular microfossils becomes notoriously difficult with <strong>in</strong>creas<strong>in</strong>g age of <strong>the</strong> host rock. In Early Precambrian (Archaean) sediments, both <strong>the</strong> progressive diagenetic alteration and <strong>the</strong> metamorphic reconstitution of <strong>the</strong> enclos<strong>in</strong>g m<strong>in</strong>eral matrix tend to blur <strong>the</strong> primary morphologies of delicate organic microstructures. This results <strong>in</strong> a large-scale loss of contours and o<strong>the</strong>r critical morphological detail. At <strong>the</strong> extreme of 51
SP-1231 Fig. I.4.3.1.2/1. A: Comparison of Huroniospora sp. from <strong>the</strong> ~2.0 Gyr-old Gunfl<strong>in</strong>t iron <strong>for</strong>mation, Ontario (a-c) with Isuasphaera sp. from <strong>the</strong> ~3.8 Gyr-old Isua metasedimentary suite, West Greenland (d-f). <strong>The</strong> optically dist<strong>in</strong>ctive marg<strong>in</strong>al rim may be expla<strong>in</strong>ed as a relic of <strong>the</strong> orig<strong>in</strong>al cell wall. B: Laser Raman spectra obta<strong>in</strong>ed from Huroniospora sp. as an isolated particle (a) and <strong>in</strong> th<strong>in</strong> sections (b) compared with those from Isuaphaera sp. (c, d) obta<strong>in</strong>ed under <strong>the</strong> same conditions. <strong>The</strong> close resemblance of <strong>the</strong> spectra suggests similarities <strong>in</strong> <strong>the</strong> composition of <strong>the</strong> residual organic component of <strong>the</strong> two types of microstructures (<strong>the</strong> prom<strong>in</strong>ent peak close to 1610 cm -1 is <strong>in</strong>dicative of aromatic double bonds among <strong>the</strong> carbon atoms of <strong>the</strong> molecular structure). Adapted from Pflug (1987). 52 such alteration series lie <strong>the</strong> so-called ‘dubiofossils’, with variable and sometimes questionable confidence levels. To ascerta<strong>in</strong> <strong>the</strong> biogenicity of possible cellular morphotypes <strong>in</strong> Archaean rocks, a hierarchical set of selection criteria has been proposed, postulat<strong>in</strong>g that genu<strong>in</strong>e microfossils (i) be au<strong>the</strong>ntic constituents of <strong>the</strong> rock as testified by <strong>the</strong>ir exposure <strong>in</strong> petrographic th<strong>in</strong> sections, (ii) occur <strong>in</strong> vast multiples, (iii) be associated with residual carbonaceous matter, (iv) equal or exceed <strong>the</strong> m<strong>in</strong>imum size of viable cells and display a central cavity plus structural detail <strong>in</strong> excess of that result<strong>in</strong>g from <strong>in</strong>organic processes. Moreover, it has been proposed that, as a matter of pr<strong>in</strong>ciple, putative evidence from metamorphosed sediments should not be considered. In spite of <strong>the</strong> evident impoverishment of <strong>the</strong> Archaean record, <strong>the</strong>re are, however, s<strong>in</strong>gle reports of well-preserved microfossils that, <strong>for</strong> <strong>the</strong> most part, comply with <strong>the</strong> above criteria. Most prom<strong>in</strong>ent among <strong>the</strong>se assemblages are chert-embedded microfloras from <strong>the</strong> Warrawoona Group of Western Australia, which closely approach <strong>the</strong> 3.5 Gyr age mark. After an <strong>in</strong>itial controversy about <strong>the</strong> au<strong>the</strong>nticity of <strong>the</strong>se fossil communities on grounds of imprecisely constra<strong>in</strong>ed petrographic background parameters <strong>for</strong> <strong>the</strong> host lithologies (Awramik et al., 1983; Buick, 1991), Schopf & Packer (1987) and notably Schopf (1993) have <strong>for</strong>warded evidence prompt<strong>in</strong>g an acceptance of <strong>the</strong> observed morphotypes as bona fide microfossils. Conspicuous with<strong>in</strong> <strong>the</strong> Warrawoona microbial community are both <strong>the</strong> coccoidal and filamentous (trichomic) micromorphologies that have been found to abound <strong>in</strong> <strong>the</strong> cyanobacterial precursor floras of Proterozoic <strong>for</strong>mations. While <strong>the</strong> septate filaments stand <strong>for</strong> fossil trichomes that could be attributed to ei<strong>the</strong>r filamentous cyanobacteria or more primitive prokaryotes (such as flexibacteria), <strong>the</strong> coccoidal aggregates described by Schopf & Packer (1987) have been claimed to strictly exclude o<strong>the</strong>r than cyanobacterial aff<strong>in</strong>ities. Given <strong>the</strong> remarkable degree of diversification of <strong>the</strong> Warrawoona microflora, we must, of necessity, <strong>in</strong>fer that <strong>the</strong> genetic l<strong>in</strong>eages of <strong>the</strong> pr<strong>in</strong>cipal microbial species had emerged well be<strong>for</strong>e Warrawoona times. We may, <strong>the</strong>re<strong>for</strong>e, reasonably assume
Page 1 and 2: SP-1231 Exobiology
Page 3 and 4: Cover Fossil coccoid bacteria, 1 µ
Page 5 and 6: 4 I.5 Potential Non-Martian Sites <
Page 7 and 8: 6 6.2 Imaging of F
Page 9 and 10: The Exobio
Page 11 and 12: pyrolysis, or similar techniques, f
Page 13 and 14: 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
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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
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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
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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
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.
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Page 87 and 88: plate tectonic evidence has been ob
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Page 93 and 94: principal atmosphe
Page 95 and 96: Lines of evidence
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quest for
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Anders, E. (1989). Prebiotic organi
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Urey, H.C. (1968). Origin</
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SP-1231 Fig. II.4.1.1/1. A Gilbert-
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SP-1231 Fig. II.4.1.3/1. Th
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SP-1231 114 Sinuou
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SP-1231 Fig. II.4.3/1. Ages of seve
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SP-1231 118 orbital characteristics
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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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