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

<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
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