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

TABLE I.6.2/2 EVIDENCE OF EXTINCT LIFE
<strong>The</strong> chances of f<strong>in</strong>d<strong>in</strong>g evidence of exist<strong>in</strong>g life on a planet at this po<strong>in</strong>t are remote.
We cannot reach deep below <strong>the</strong> surface with any equipment transportable on current
lander missions. Nor would we know just now where to look. Perhaps a future
detailed survey will identify an active surface hydro<strong>the</strong>rmal system by means of water
vapour emission and a <strong>the</strong>rmal signature. If so, a specialised Lander to <strong>in</strong>vestigate that
region would be warranted. At <strong>the</strong> moment, however, we must concentrate <strong>the</strong> search
on evidence of ext<strong>in</strong>ct life. To that end, it will be necessary to provide <strong>the</strong> follow<strong>in</strong>g:
i. clear images of groups of fossil ‘organisms'’ and high-resolution studies of
<strong>in</strong>dividual structural features;
ii. accompany<strong>in</strong>g geochemical and m<strong>in</strong>eralogical evidence, <strong>in</strong> <strong>the</strong> <strong>for</strong>m of
biosignatures, which are related both spatially and temporally (<strong>in</strong> geological
terms) with <strong>the</strong> imaged structures;
iii. organic residuals, shown to be of biological orig<strong>in</strong>.
O<strong>the</strong>r than <strong>the</strong>se small-scale features, we may also seek evidence of ext<strong>in</strong>ct
microbial matt<strong>in</strong>g communities through <strong>the</strong> observation of macroscopically visible
lam<strong>in</strong>ations, <strong>the</strong> so-called stromatolite structures. However, confirmation of a
biological orig<strong>in</strong> <strong>for</strong> any such observed structures will still require, as on Earth,
accompany<strong>in</strong>g <strong>in</strong><strong>for</strong>mation of <strong>the</strong> type <strong>in</strong>dicated <strong>in</strong> (ii) and (iii) above.
<strong>The</strong> ma<strong>in</strong> po<strong>in</strong>ts are summarised <strong>in</strong> Tables I.6.2/1 and I.6.2/2.
science and experiment strategy/I.6
1. Structural Indicators:
Microscopic Observations: of groups of possible microfossil structures <strong>in</strong> sedimentary deposits, <strong>in</strong>clud<strong>in</strong>g petrological analysis
of <strong>the</strong> associated m<strong>in</strong>erals and <strong>the</strong> study of any residual carbonaceous matter.
Electron Microscopy: to provide nm-resolution study of <strong>the</strong> structure of possible microfossils detected by optical microscopy
and to search <strong>for</strong> ‘nanofossils’.
Atomic Force Microscopy: to obta<strong>in</strong> 3D images of <strong>the</strong> structure, at nm-resolution, of possible microfossils and associated
material.
Macroscopic Observation: of stromatolite-type biosedimentary structures <strong>in</strong> freshly exposed scarps or <strong>in</strong> rock sections, us<strong>in</strong>g
a telemicroscope or telescope with IR or Raman spectroscopic capability to confirm <strong>the</strong> m<strong>in</strong>eral and carbonaceous
constituents.
2. Biogeochemical Indicators: from <strong>the</strong> determ<strong>in</strong>ation of <strong>the</strong> elemental abundances <strong>in</strong> reduced (organic) carbon remnants
us<strong>in</strong>g a Gas Chromatograph/Mass Spectrometer (GCMS) and Alpha-Proton-X-ray Spectrometer (APX), to give e.g. H/C
ratio as <strong>in</strong>dex of aromaticity by assay of <strong>the</strong> ‘organic’ to ‘carbonate’ carbon content of fossil-conta<strong>in</strong><strong>in</strong>g sediments from
an analysis of <strong>the</strong> volatile (hydrocarbon) component of <strong>the</strong> reduced carbon constituents <strong>in</strong> associated sedimentary
material, us<strong>in</strong>g pyrolysis/gas chromatography/mass spectroscopy.
3. Isotopic Indicators: <strong>the</strong> enhancement of <strong>the</strong> 12C isotope to 13C dur<strong>in</strong>g conversion of <strong>in</strong>organic carbon to organic <strong>in</strong>
autotrophic fixation is reta<strong>in</strong>ed <strong>in</strong> biogenic carbon residues. It provides a dist<strong>in</strong>ct biomark when compared with <strong>the</strong>
<strong>in</strong>organic carbon pool. <strong>The</strong> Pyrolyser/GCMS is used. An Ion Microprobe is used <strong>for</strong> microstructures. Similarly, D/H
fractionation and 34S/ 32S isotope composition change between sulphides and sulphates can be <strong>in</strong>dicators of earlier
biological activity.
4. Molecular Indicators: certa<strong>in</strong> dist<strong>in</strong>ct biochemical compounds, e.g. lipid type and pigments, may withstand degradation
over very long periods as part of <strong>the</strong> kerogen residue. <strong>The</strong>y may be extracted by organic solvents from residues and
fossils <strong>for</strong> analysis (<strong>in</strong>clud<strong>in</strong>g chiral and isotopic) to obta<strong>in</strong> an <strong>in</strong>dication of <strong>the</strong> nature of <strong>the</strong> orig<strong>in</strong>al body.
5. Chirality Indicator: homochirality is characteristic of life and this structural feature is reta<strong>in</strong>ed on death of <strong>the</strong> organism.
Racemisation proceeds very slowly under cold dry conditions such as now exist on Mars. An optical rotation detector is
used.
6. Spectral Observations: vibrational spectra of organic compounds provide a valuable analytical tool which can be applied
to help unravel <strong>the</strong> nature of sedimentary carbonaceous samples of biological and abiotic orig<strong>in</strong>. It also is able to
identify <strong>the</strong> m<strong>in</strong>eralogical content of <strong>the</strong> associated sediment. IR and Raman spectroscopy predom<strong>in</strong>ate and can be
<strong>in</strong>cluded <strong>in</strong> <strong>the</strong> microscopy.
I.6.2 What to <strong>Search</strong> For?
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