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

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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.