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

SP-1231
Fig. II.6.4.1/1. Left: Filaments of probable microbiological orig<strong>in</strong>, Faeroe Islands. Micron-thick filaments, partly gravity-oriented, are encrusted with
chalcedony to <strong>for</strong>m up to 1 mm-thick ‘rods’. From a cavity <strong>in</strong> Tertiary basalt. Typical example of macroscopically visible microbial fossil of subsurface
orig<strong>in</strong>. Sample width 40 mm. Right: an enlargement (field of view 5.6 mm). Probable microbiological filaments encrusted by chalcedony. Sample
immersed <strong>in</strong> glycer<strong>in</strong>e, show<strong>in</strong>g extremely f<strong>in</strong>e filamentous cores of ‘chalcedony rods’.
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been observed <strong>in</strong> o<strong>the</strong>r m<strong>in</strong>erals, such as magnetite, haematite, limonite and
magnesioferrite. Some of <strong>the</strong> latter might be oxidation products (magnetite, limonite)
or precursors (greigite) of pyrite.
Based on terrestrial occurrences, pyrite <strong>for</strong>mation can be purely <strong>in</strong>organic as well
as biologically catalysed. Prerequisites <strong>for</strong> <strong>the</strong> <strong>for</strong>mation of pyrite framboids are <strong>the</strong>
availability of iron and sulphur. While <strong>the</strong>se conditions might be available <strong>in</strong> a whole
range of environmental sett<strong>in</strong>gs, <strong>the</strong> frequent presence of framboidal pyrite <strong>in</strong>
organic-rich sediments suggests that organic matter plays an important role <strong>in</strong> its
<strong>for</strong>mation. Carbonaceous sediments appear to be an ideal host <strong>for</strong> pyrite <strong>for</strong>mation.
<strong>The</strong>y conta<strong>in</strong> abundant reactive iron (Canfield, 1989) and organic material that may
serve as an energy source <strong>for</strong> sulphate-reduc<strong>in</strong>g bacteria produc<strong>in</strong>g H 2S. This
biologically-produced H 2S, <strong>in</strong> turn, reacts with <strong>the</strong> iron to <strong>for</strong>m (framboidal) pyrite.
While, from a stochiometrical po<strong>in</strong>t of view, organic carbon is unnecessary,
experimental data <strong>in</strong>dicate only limited <strong>for</strong>mation and stability of framboidal pyrite
without <strong>the</strong> presence of organic matter (<strong>for</strong> a review see Sawlowicz, 1993). <strong>The</strong>
relationship between framboidal pyrite and organic matter (and as such evidence <strong>for</strong>
biological activity) is two-fold. Apart from serv<strong>in</strong>g as a substrate <strong>for</strong> <strong>the</strong> sulphate
reduction, framboidal pyrite frequently replaces fossil organic matter on a micro- or
macroscale (Briggs et al., 1991; 1996).
In summary, sedimentary pyrite and, <strong>in</strong> particular, framboidal texture are
frequently observed <strong>in</strong> recent and ancient sediments on Earth. Although examples
exist <strong>for</strong> a purely <strong>in</strong>organic orig<strong>in</strong> of pyrite framboids (e.g. laboratory experiments:
Sweeney & Kaplan, 1973), its presence <strong>in</strong> sedimentary systems is considered overall
as evidence <strong>for</strong> a biological orig<strong>in</strong>. Additional support <strong>for</strong> this conjecture can best be
obta<strong>in</strong>ed from <strong>the</strong> sulphur isotopic composition, show<strong>in</strong>g a characteristic signature
(<strong>for</strong> a review see Strauss, 1997). In that respect, close exam<strong>in</strong>ation of martian surface
and, <strong>in</strong> particular, subsurface material <strong>for</strong> pyrite as a potential biogenic m<strong>in</strong>eral might
provide important <strong>in</strong>sight as part of <strong>the</strong> search <strong>for</strong> life. Microscopic exam<strong>in</strong>ation
would have to provide a spatial resolution compatible with expected size ranges down
to