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

Similarly, <strong>the</strong> determ<strong>in</strong>ation of 15 N/ 14 N can provide very important <strong>in</strong><strong>for</strong>mation on
possible biological activities <strong>in</strong> <strong>the</strong> analysed samples. <strong>The</strong> same is true <strong>for</strong> <strong>the</strong> 34 S/ 32 S
isotopic composition change between sulphides and sulphates.
II.5.3.2 Sulphur
<strong>The</strong> element sulphur represents an important constituent of <strong>the</strong> terrestrial biosphere.
Its presence and even more so <strong>the</strong> characteristic isotope signatures of sulphur <strong>in</strong>
various oxidation states give testimony to biologically-controlled processes, mostly <strong>in</strong>
low-temperature sedimentary environments.
Sedimentary pyrites often display negative to strongly negative δ 34 S values (e.g.
Chambers, 1982; Habicht & Canfield, 1997; Kaplan & Rittenberg, 1964; Strauss,
1997), and <strong>the</strong>se are correctly <strong>in</strong>terpreted as result<strong>in</strong>g from bacterial sulphate
reduction. However, <strong>the</strong> geological record provides sufficient evidence <strong>for</strong>
sedimentary (biogenic) pyrites with positive or even strongly positive δ 34 S data that
are equally a result of bacterial sulphate reduction (e.g. Lambert & Donnelly, 1990;
Ohmoto, 1992; Ohmoto et al., 1990, 1993; Schidlowski et al., 1983; Strauss, 1993).
In addition, o<strong>the</strong>r biologically-controlled processes with<strong>in</strong> <strong>the</strong> sulphur cycle (e.g.
sulphide oxidation) are associated with only m<strong>in</strong>or displacements <strong>in</strong> <strong>the</strong> isotopic
composition.
Consequently, a full understand<strong>in</strong>g of <strong>the</strong> martian sulphur cycle and, thus, possibly
evidence <strong>for</strong> biological participation, can be obta<strong>in</strong>ed only through a thorough study
of abundances and isotopic compositions of both oxidised and reduced sulphur
species.
II.5.4.1 Inorganics
<strong>The</strong> quantitative analysis of water <strong>in</strong> <strong>the</strong> soil will allow derivation of a water concentration
profile and could give access by extrapolation to <strong>the</strong> water concentration <strong>in</strong> <strong>the</strong>
deeper layers.
This is a key po<strong>in</strong>t <strong>for</strong> study<strong>in</strong>g <strong>the</strong> possibility of extant life <strong>in</strong> <strong>the</strong> present martian
subsurface, water be<strong>in</strong>g one of <strong>the</strong> essential <strong>in</strong>gredients <strong>for</strong> life. <strong>The</strong> analysis of water
is also important <strong>for</strong> <strong>the</strong> estimation of <strong>the</strong> m<strong>in</strong>eralogical composition (hydrate
content). Similarly, it is important to measure <strong>the</strong> martian soil abundances of CO 2
(carbonate content), NO 2/NO 3/N xO y (nitrate content), SO 2/SO 3 (sulphates) and
phosphates.
It is essential to measure <strong>the</strong> abundances of oxidants <strong>in</strong> <strong>the</strong> soil, and determ<strong>in</strong>e <strong>the</strong>ir
concentration gradients with depth (Fig. II.5.4/1). If our models of <strong>the</strong> evolution of
organics on <strong>the</strong> surface and <strong>in</strong> <strong>the</strong> near-subsurface are correct, <strong>the</strong> oxidant concentration
should be anti-correlated with <strong>the</strong> concentration of organics <strong>in</strong> <strong>the</strong> soil. <strong>The</strong>se
oxidants are ma<strong>in</strong>ly: O 2, O 3 and pr<strong>in</strong>cipally H 2O 2 (with eventually organic peroxides).
Fe 3+ , Mn x+ , carbonates, nitrates and sulphates should also be mentioned <strong>in</strong> this context.
II.5.4.2 Organics
<strong>The</strong> discovery of organics <strong>in</strong> <strong>the</strong> martian soil would be of prime importance <strong>for</strong>
exobiology. In <strong>the</strong> very oxidised martian environment, <strong>the</strong> abiotic <strong>for</strong>mation of
organics through atmospheric chemical processes is unlikely. <strong>The</strong> only likely major
abiotic source of organics on <strong>the</strong> surface and <strong>in</strong> <strong>the</strong> near-surface is extra-martian
importation, from <strong>the</strong> impact of carbonaceous meteorites. However, by differentiat<strong>in</strong>g
a meteoritic abiotic orig<strong>in</strong> from a bio-orig<strong>in</strong>, <strong>the</strong> detection of organic carbon <strong>in</strong> <strong>the</strong>
martian soil could provide <strong>the</strong> evidence of <strong>for</strong>mer life processes.
After <strong>the</strong> death of organisms on Earth, <strong>the</strong>ir primary biopolymers (such as prote<strong>in</strong>s
and polysaccharides) undergo a complex process of degradation and condensation
(humification) to give complex and chemically stable macromolecular materials
(kerogens). In addition, stable lipid-rich biopolymers survive this process to
contribute directly to kerogens, which are amorphous condensed aggregates made up
ma<strong>in</strong>ly of aliphatic and aromatic moieties (de Leeuw & Largeau, 1993). Kerogens
typically comprise by far <strong>the</strong> majority (>90%) of sedimentary organic matter and <strong>the</strong>ir
team II: <strong>the</strong> search <strong>for</strong> chemical <strong>in</strong>dicators of life/II.5
II.5.4 Molecular Analysis
Fig. II.5.4/1. A pyrite crystal that oxidised
<strong>in</strong>wards from <strong>the</strong> surface. <strong>The</strong> surface consists
of iron oxides/hydroxides – very different from
<strong>the</strong> <strong>in</strong>side. It demonstrates <strong>the</strong> necessity of
sampl<strong>in</strong>g <strong>the</strong> <strong>in</strong>teriors of m<strong>in</strong>erals and rocks.
(Courtesy G. Kurat)
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