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

morphological and biochemical signatures of extraterrestrial life: utility of terrestrial analogues/I.4
sediments (notably <strong>in</strong> <strong>the</strong> Tharsis region and <strong>the</strong> associated Valles Mar<strong>in</strong>eris canyon
system, see II.2.7) that are <strong>in</strong>terpreted as lake deposits and believed to <strong>in</strong>clude thick
sequences of carbonates (McKay & Nedell, 1988). If present at all <strong>in</strong> <strong>the</strong>se early
martian <strong>for</strong>mations, morphological relics of fossil life should lend <strong>the</strong>mselves to as
ready a detection as do <strong>the</strong>ir terrestrial counterparts <strong>in</strong> Archaean sediments, provided
<strong>the</strong> host rock is accessible to ei<strong>the</strong>r robotic sens<strong>in</strong>g or direct <strong>in</strong>vestigation follow<strong>in</strong>g a
sample return mission.
In that regard, it seems to be a reasonable conjecture to resort to <strong>the</strong> paleontological
<strong>in</strong>ventory of <strong>the</strong> oldest terrestrial sediments (3.50-3.85 Gyr old) <strong>for</strong> guidance concern<strong>in</strong>g
potential fossil evidence from coeval martian rocks. On Earth, <strong>the</strong> earliest
prokaryotic (bacterial and archaebacterial) microbial ecosystems have basically left
two categories of morphological evidence, namely (i) stromatolite-type biosedimentary
structures (‘microbialites’) and (ii) cellular relics of <strong>in</strong>dividual microorganisms
(‘microfossils’).
I.4.3.1.1 Microbialites
Microbialites or ‘stromatolites’ (cf. Burne & Moore, 1987) are lam<strong>in</strong>ated
biosedimentary structures that preserve <strong>the</strong> matt<strong>in</strong>g behaviour of bacterial and algal
(primarily prokaryotic) microbenthos. Microbial buildups of this type represent stacks
of f<strong>in</strong>ely lam<strong>in</strong>ated lithified microbial communities that orig<strong>in</strong>ally thrived as organic
films at <strong>the</strong> sediment-water <strong>in</strong>terface, with younger mat generations successively
superimposed on <strong>the</strong> older ones (cf. Figs. I.4.3.1.1/1 and I.4.3.1.1/2). <strong>The</strong> structures
derive from <strong>the</strong> <strong>in</strong>teraction of <strong>the</strong> primary biologically active microbial layer with <strong>the</strong>
ambient sedimentary environment, <strong>the</strong> fossilisation of <strong>the</strong> lam<strong>in</strong>ae result<strong>in</strong>g from
ei<strong>the</strong>r trapp<strong>in</strong>g, b<strong>in</strong>d<strong>in</strong>g or biologically mediated precipitation of selected m<strong>in</strong>eral
constituents.
In terrestrial sediments, microbialites represent <strong>the</strong> most obvious (macroscopic)
expression of fossil microbial life, with a record extend<strong>in</strong>g back to <strong>the</strong> Early Archaean
(~3.5 Gyr ago). This constitutes prima facie evidence that benthic prokaryotes were
already widespread <strong>in</strong> suitable aquatic habitats of <strong>the</strong> Archaean Earth. Both <strong>the</strong>
morphological <strong>in</strong>ventory of <strong>the</strong> oldest stromatolites and <strong>the</strong> observed microfossil
content of <strong>the</strong> ambient rock or coeval sequences (see below) allow a fairly elaborate
reconstruction of <strong>the</strong> Earth’s earliest microbial ecosystems, <strong>in</strong>dicat<strong>in</strong>g that Archaean
stromatolite builders were not markedly different from <strong>the</strong>ir geologically younger
counterparts (<strong>in</strong>clusive of contemporary species). It appears well established that <strong>the</strong>
pr<strong>in</strong>cipal microbial mat builders were filamentous and unicellular prokaryotes
capable of phototactic responses and photoautotrophic carbon fixation (Walter, 1983).
<strong>The</strong> unbroken stromatolite record from Archaean to present times attests, fur<strong>the</strong>rmore,
to an astound<strong>in</strong>g degree of conservatism and uni<strong>for</strong>mity <strong>in</strong> <strong>the</strong> physiological
per<strong>for</strong>mance and communal organisation of prokaryotic microbenthos over 3.5 Gyr
of geological history.
I.4.3.1.2 Cellular Microfossils
Apart from <strong>the</strong> macroscopic vestiges of past microbial activity <strong>in</strong> <strong>the</strong> <strong>for</strong>m of
stromatolites and related biosedimentary structures, <strong>the</strong>re is a second (microscopic)
category of paleontological evidence stored <strong>in</strong> <strong>the</strong> sedimentary record. This
microscopic or cellular evidence is currently supposed to go back to at least ~3.5 Gyr
(as <strong>in</strong> <strong>the</strong> case of microbialites), with <strong>the</strong> biogenicity of microfossil-like morphologies
reported from <strong>the</strong> ~3.8 Gyr-old Isua metasedimentary suite from West Greenland
(Pflug, 1978) still be<strong>in</strong>g debated.
While a wealth of au<strong>the</strong>ntic microbial communities has been reported from Early
and Middle Precambrian (Proterozoic) <strong>for</strong>mations, <strong>the</strong> unequivocal identification of
cellular microfossils becomes notoriously difficult with <strong>in</strong>creas<strong>in</strong>g age of <strong>the</strong> host
rock. In Early Precambrian (Archaean) sediments, both <strong>the</strong> progressive diagenetic
alteration and <strong>the</strong> metamorphic reconstitution of <strong>the</strong> enclos<strong>in</strong>g m<strong>in</strong>eral matrix tend to
blur <strong>the</strong> primary morphologies of delicate organic microstructures. This results <strong>in</strong> a
large-scale loss of contours and o<strong>the</strong>r critical morphological detail. At <strong>the</strong> extreme of
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