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

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SP-1231 Fig. II.6.2.2/2. Fossil rod-shaped bacteria (sausage-shaped structures) embedded <strong>in</strong> gra<strong>in</strong>y biofilm. From <strong>the</strong> Hooggenoeg Formation, Onverwacht Group, South Africa (age 3445-3472 Myr). 160 II.6.2.2 Microscopic Scale On a microscopic scale, biogenic structures that can be seen <strong>in</strong>clude: • undulat<strong>in</strong>g biofilm lam<strong>in</strong>ae of <strong>the</strong> order of 100 µm; • microfossils with a maximum dimension
<strong>Search</strong><strong>in</strong>g <strong>for</strong> signs of extant or ext<strong>in</strong>ct life <strong>in</strong>cludes <strong>the</strong> evaluation of direct evidence <strong>in</strong> <strong>the</strong> <strong>for</strong>m of preserved fossils, ei<strong>the</strong>r m<strong>in</strong>eralised or as organic rema<strong>in</strong>s, as well as a more <strong>in</strong>direct approach utilis<strong>in</strong>g geochemical and/or isotopic proxies <strong>for</strong> past biologically-driven processes and <strong>the</strong>ir metabolic products, as discussed <strong>in</strong> Section II.5. As such, <strong>the</strong> presence of organic compounds and/or certa<strong>in</strong> chemical elements that represent important build<strong>in</strong>g blocks of life (i.e. C, H, N, S, P), <strong>the</strong>ir specific isotopic compositions and <strong>the</strong>ir presence as m<strong>in</strong>eral matter of presumed biological orig<strong>in</strong> might provide evidence <strong>for</strong> (or aga<strong>in</strong>st) <strong>the</strong> presence or past presence of life. In addition to <strong>the</strong> exclusive search <strong>for</strong> evidence of life, a thorough characterisation of <strong>the</strong> present and past environmental conditions should be a prerequisite. In comparison to terrestrial sett<strong>in</strong>gs, a wide variability <strong>in</strong> habitats do show flourish<strong>in</strong>g life. However, certa<strong>in</strong> physical and chemical parameters provide boundary conditions that limit its survival. Of particular <strong>in</strong>terest are m<strong>in</strong>erals that presumably <strong>for</strong>med through biological processes, i.e. biom<strong>in</strong>erals. Pyrite (FeS 2) is a ubiquitous m<strong>in</strong>eral <strong>in</strong> terrestrial sediments and frequently of biogenic orig<strong>in</strong>. A key process <strong>in</strong> its <strong>for</strong>mation is <strong>the</strong> bacterial reduction of sulphate to H 2S and subsequent reaction with iron m<strong>in</strong>erals, to <strong>for</strong>m iron sulphide. <strong>The</strong> <strong>for</strong>mation of sedimentary pyrite, specifically of biological orig<strong>in</strong>, and <strong>the</strong> relationship between pyrite morphology, orig<strong>in</strong> and depositional environment, has been studied <strong>for</strong> many decades (e.g. Berner, 1970; Goldhaber & Kaplan, 1974; Berner, 1984; Morse et al., 1987; Schoonen & Barnes, 1991; Sawlowicz, 1993; Wilk<strong>in</strong> et al., 1996, Wilk<strong>in</strong> & Barnes, 1997). Fur<strong>the</strong>rmore, pyrite has been considered as a potential orig<strong>in</strong> of life (e.g. Wächtershäuser, 1988; Russel et al., 1989; 1990). Of all <strong>the</strong> possible biogenic morphologies, framboidal pyrite consist<strong>in</strong>g of m<strong>in</strong>ute pyritic gra<strong>in</strong>s resembl<strong>in</strong>g raspberries (Rust, 1935) has received most attention. Generally, <strong>the</strong> sizes of <strong>the</strong>se framboids range from 5 mm to 20 mm, but framboids as large as 250 mm have occasionally been observed. Alternatively, framboids as small as 1 mm were also seen. Agglomeration of <strong>in</strong>dividual framboids might result <strong>in</strong> <strong>the</strong> so called polyframboids with sizes rang<strong>in</strong>g from 35 mm to 900 mm (average 50- 200 mm). Thus, a cont<strong>in</strong>uous spectrum of sizes, microframboids–framboids– polyframboids, exists (e.g. Sawlowicz, 1993). In terrestrial sett<strong>in</strong>gs, framboidal pyrite is most common <strong>in</strong> recent and ancient sedimentary environments, but has also been reported from magmatic rocks or hydro<strong>the</strong>rmal occurrences (e.g. Sassano & Schrijver, 1989). <strong>The</strong> framboidal texture is usually associated with pyrite, but has also team III: <strong>the</strong> <strong>in</strong>spection of subsurface aliquots and surface rocks/II.6 Fig. II.6.2.2/3. Fossil coccoid bacteria 1 µm <strong>in</strong> diameter show<strong>in</strong>g cellular division. <strong>The</strong>se structures have a size typical <strong>for</strong> terrestrial bacteria. High magnifications are needed to image <strong>the</strong>m (>100 times with resolutions of at least 0.25 µm per pixel). II.6.3 Investigation of Biom<strong>in</strong>erals 161
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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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SP-1231 60 References regard to non
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SP-1231 62 Klein,
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SP-1231 64 ities in</strong
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SP-1231 Fig. I.5.1.1/1. A 34×42 km
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SP-1231 Fig. I.5.2.1/1. Titan’s a
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SP-1231 Fig. I.5.2.2/2. Modelled Ti
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SP-1231 72 Galileo: images availabl
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SP-1231 74 TABLE I.6.2/1 EVIDENCE O
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SP-1231 I.6.3 What to Searc
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SP-1231 78 I.7.3 Organic Chemistry
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II.1 Introduction The</stro
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II.2 The Planet Ma
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cover the range 4.
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The depletion of c
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valley networks (Fig. II.2.5/2) are
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plate tectonic evidence has been ob
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Table II.2.7.4/1. Potential Mars <s
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II.3 The Martian M
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principal atmosphe
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Lines of evidence
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indicated by carbo
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quest for
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Anders, E. (1989). Prebiotic organi
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Page 161 and 162: SCIENTIFIC METHOD AND REQUIREMENTS
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