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

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<strong>The</strong> different types, naturally, require different analytical approaches. All environments need, however, to be characterised <strong>in</strong> detail <strong>in</strong> order to provide a proper basis <strong>for</strong> estimat<strong>in</strong>g <strong>the</strong> local history of biological activity. Such characterisation has to comprise: • m<strong>in</strong>eralogy, texture and bulk chemistry of primary rocks; • m<strong>in</strong>eralogy and sedimentology of <strong>the</strong> soil-, w<strong>in</strong>d- and water-deposited sediments, regolith, etc (gra<strong>in</strong> sizes, gra<strong>in</strong> shapes, secondary m<strong>in</strong>erals such as clays, carbonates, zeolites, hydrates, chlorites); • m<strong>in</strong>eralogy of mobile phases and hard ground cements: halogenides, sulphates, nitrates, silica, carbonates, Fe oxyhydrites, etc; • search <strong>for</strong> biomarkers: framboidal sulphides or oxides, biophosphates, oxalates, silica, biogenic magnetite, barite, fossils. team II: <strong>the</strong> search <strong>for</strong> chemical <strong>in</strong>dicators of life/II.5 Fig. II.5.2/1. Sediments are <strong>for</strong>med via different processes. Volcanic tuff at Monte Leste, Sal Island, Cabo Verde. Tuff has been cemented and hardened at different degrees depend<strong>in</strong>g on orig<strong>in</strong>al m<strong>in</strong>eralogy and porosity of tuff layers. White crust: late <strong>for</strong>mation of calcium sulphate. (Courtesy G. Kurat) Fig. II.5.2/2. Sediments governed by fluid circulation. Tuff, Monte Grande, Sal Island, Cabo Verde. Harden<strong>in</strong>g and cementation of tuff below <strong>the</strong> surface (here opened <strong>in</strong> a pit) is governed by fluid circulations (down and up). It will not be detectable at <strong>the</strong> surface but will <strong>in</strong>fluence drill<strong>in</strong>g. (Courtesy G. Kurat) 131
SP-1231 132 II.5.3 Isotopic Analysis II.5.2.1.2 Extant <strong>Life</strong> In a search <strong>for</strong> extant life beyond <strong>the</strong> Earth, microorganisms are <strong>the</strong> most likely candidates <strong>for</strong> a biota <strong>in</strong> an extraterrestrial habitat. Microbial communities leave discrete vestiges of <strong>the</strong>ir existence <strong>in</strong> <strong>the</strong> surround<strong>in</strong>g <strong>in</strong>organic habitat. Biom<strong>in</strong>eralisation and biodegradation of m<strong>in</strong>erals are examples of life’s <strong>in</strong>teraction with <strong>the</strong> lithosphere. <strong>The</strong>re<strong>for</strong>e, a search <strong>for</strong> biogenic m<strong>in</strong>erals would provide valuable <strong>in</strong><strong>for</strong>mation about ongo<strong>in</strong>g or past activities of life. <strong>The</strong> biogenic nature is impr<strong>in</strong>ted <strong>in</strong> <strong>the</strong> dist<strong>in</strong>ctive crystallographies, morphologies and/or isotopic ratios that make <strong>the</strong>se m<strong>in</strong>erals dist<strong>in</strong>guishable from <strong>the</strong>ir abiotically-produced counterparts of <strong>the</strong> same chemical composition. Those m<strong>in</strong>erals result<strong>in</strong>g from genetically-controlled m<strong>in</strong>eralisation processes and <strong>for</strong>med with<strong>in</strong> a pre<strong>for</strong>med organic framework would have non-<strong>in</strong>terchangeable characteristics, such as <strong>the</strong> orientation of <strong>the</strong> crystallographic axes and <strong>the</strong> microarchitecture. Terrestrial examples are <strong>the</strong> strontium sulphate skeletons of <strong>the</strong> unicellular mar<strong>in</strong>e Acantharia, <strong>the</strong> amorphous silicate shells of diatoms and <strong>the</strong> biogenic magnetite <strong>for</strong>med by bacteria. It is important to note that m<strong>in</strong>erals produced under biologically-controlled processes are not necessarily <strong>in</strong> equilibrium with <strong>the</strong> extracellular environment. By contrast, those m<strong>in</strong>erals <strong>for</strong>med extracellularly by biologically-<strong>in</strong>duced m<strong>in</strong>eralisation processes can be less easily dist<strong>in</strong>guished from <strong>the</strong>ir abiotically-<strong>for</strong>med counterparts. <strong>The</strong>y are <strong>for</strong>med <strong>in</strong> an open environment and are <strong>in</strong> equilibrium with this environment. II.5.2.2 Geochemistry (Elemental Composition Analysis) <strong>The</strong> quantitative analysis of as many chemical elements as possible will provide essential <strong>in</strong><strong>for</strong>mation on <strong>the</strong> elemental bulk composition of <strong>the</strong> rocks and soil. Major, m<strong>in</strong>or and trace element abundances will allow depiction of <strong>the</strong> location’s geological history and <strong>the</strong> analysis of <strong>the</strong> oxidation state of certa<strong>in</strong> elements (e.g. Fe, Mn, S, N) will provide <strong>in</strong><strong>for</strong>mation such as <strong>the</strong> ratio FeIII/FeII on <strong>the</strong> redox conditions and <strong>the</strong>ir historical development. Knowledge of <strong>the</strong> relative abundances of <strong>the</strong> biologically significant elements C, H, N, O, S and P, and <strong>the</strong>ir distribution between organic and <strong>in</strong>organic matter is particularly important. Also of great <strong>in</strong>terest is <strong>the</strong> determ<strong>in</strong>ation of: • <strong>the</strong> abundance of carbon <strong>in</strong> relation to carbonates (<strong>the</strong> importance of which is drastically <strong>in</strong>creased by <strong>the</strong> problem of <strong>the</strong> carbonates <strong>in</strong> <strong>the</strong> SNC meteorites); • <strong>the</strong> abundance of nitrogen and its oxidation state <strong>in</strong> <strong>the</strong> martian soil, to understand what happened to <strong>the</strong> <strong>in</strong>itial atmospheric nitrogen. In all cases, determ<strong>in</strong><strong>in</strong>g <strong>the</strong> variation <strong>in</strong> abundance with depth is crucial <strong>for</strong> extrapolat<strong>in</strong>g <strong>the</strong> data and estimat<strong>in</strong>g <strong>the</strong> abundances <strong>in</strong> <strong>the</strong> soil’s deeper layers. Isotopic ratios are a very valuable set of chemical biomarkers (Schidlowski et al. 1983; Watanabe et al., 1997). II.5.3.1 Carbon and Hydrogen <strong>The</strong> 13 C depletion produced by <strong>the</strong> Calv<strong>in</strong> cycle of photosyn<strong>the</strong>sis is a clear signature of biological activity. O<strong>the</strong>r carbon-fix<strong>in</strong>g pathways <strong>in</strong>volv<strong>in</strong>g photosyn<strong>the</strong>sis, such as <strong>the</strong> C4 (Hatch Slack) pathway, also fractionate <strong>in</strong> favour of 12 C, but to a lesser extent. <strong>The</strong> δ 13 C values of average biomass are about 20-30‰ more negative than those of <strong>in</strong>organic carbon. As discussed <strong>in</strong> Part I, such negative values have been ma<strong>in</strong>ta<strong>in</strong>ed over <strong>the</strong> whole history of life on Earth. Methanogenic microorganisms generate substantial fractionations that prefer hydrogen to deuterium by sometimes as much as 30% relative to <strong>the</strong> agneous substrate. Carbon is isotopically lighter than that fixed by photosyn<strong>the</strong>sis <strong>in</strong> such systems. Methylotrophic organisms thus produce matter that is isotopically light <strong>in</strong> both carbon and hydrogen; kerogens of this k<strong>in</strong>d have been encountered <strong>in</strong> Precambrian <strong>for</strong>mations.
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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 184 II.7.5 A Possible <stro
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Team IV was asked to examin
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