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

I.4 Morphological and Biochemical Signatures of
Extraterrestrial <strong>Life</strong>: Utility of Terrestrial
Analogues
Any veneer of life cover<strong>in</strong>g <strong>the</strong> surface of a planet will likely <strong>in</strong>teract with <strong>the</strong> solid
and fluid phases with which it is <strong>in</strong> contact. Specifically, it is bound to impose a
<strong>the</strong>rmodynamic gradient on all planetary near-surface environments (<strong>in</strong>clusive of <strong>the</strong>
atmosphere and hydrosphere), which ultimately stems from <strong>the</strong> accumulation of
negative entropy by liv<strong>in</strong>g systems. Consequently, life acts as a driv<strong>in</strong>g <strong>for</strong>ce <strong>for</strong> a
number of globally-relevant chemical trans<strong>for</strong>mations. On Earth, typical examples of
such life-<strong>in</strong>duced chemical <strong>in</strong>equilibria are <strong>the</strong> glar<strong>in</strong>g redox imbalance at <strong>the</strong>
terrestrial surface caused by photosyn<strong>the</strong>tic oxygen, or <strong>the</strong> release of large quantities
of hydrogen sulphide by sulphate-reduc<strong>in</strong>g bacteria <strong>in</strong> <strong>the</strong> mar<strong>in</strong>e realm. Also, <strong>the</strong>
dynamic persistence of metastable atmospheric gas mixtures (such as O 2, N 2 and CH 4
<strong>in</strong> <strong>the</strong> terrestrial atmosphere), and of isotopic disequilibria (e.g. between water-bound
oxygen of <strong>the</strong> hydrosphere and atmospheric O 2), is ultimately susta<strong>in</strong>ed by <strong>the</strong>
<strong>the</strong>rmodynamic imbalance imposed by <strong>the</strong> biosphere on its environment.
Conspicuous <strong>the</strong>rmodynamic <strong>in</strong>equilibria with<strong>in</strong> <strong>the</strong> gaseous and liquid envelopes of
a planet may, <strong>the</strong>re<strong>for</strong>e, be taken as a priori evidence of <strong>the</strong> presence of life (cf.
Hitchcock & Lovelock, 1967; Lovelock, 1979). Apply<strong>in</strong>g this criterion to <strong>the</strong> present
composition of <strong>the</strong> martian atmosphere (Owen et al., 1977), <strong>the</strong> latter gives little, if
any, <strong>in</strong>dication of contemporary biological activity.
Moreover, a planetary biosphere is apt to leave discrete vestiges <strong>in</strong> <strong>the</strong> surround<strong>in</strong>g
<strong>in</strong>organic habitat. Rely<strong>in</strong>g on terrestrial analogues, it is safe to say that organisms
commonly generate a morphological and biochemical record of <strong>the</strong>ir <strong>for</strong>mer existence
<strong>in</strong> sedimentary rocks. Though <strong>in</strong> part highly selective, this record may survive, under
favourable circumstances, over billions of years be<strong>for</strong>e be<strong>in</strong>g annealed dur<strong>in</strong>g <strong>the</strong>
metamorphic and anatectic reconstitution of <strong>the</strong> host rock. This is particularly true <strong>for</strong>
relics of multicellular life (Metaphyta and Metazoa), but also – albeit with restrictions
– <strong>for</strong> microorganisms that held dom<strong>in</strong>ion over <strong>the</strong> Earth dur<strong>in</strong>g <strong>the</strong> first 3000 million
years of recorded geological history.
In <strong>the</strong> follow<strong>in</strong>g, an overview is presented of <strong>the</strong> pr<strong>in</strong>cipal morphological and
biogeochemical evidence <strong>in</strong>dicat<strong>in</strong>g <strong>the</strong> presence of life on a planetary surface.
Section 4.2 summarises this evidence <strong>for</strong> an extant biosphere, while Section 4.3
reviews <strong>the</strong> respective signatures of fossil life (with special reference to <strong>the</strong> oldest
terrestrial record because Early Martian scenarios should have largely resembled
those on <strong>the</strong> Archaean Earth).
I.4.2.1 <strong>The</strong> Microbial World
Microbial prokaryotes have flourished on Earth <strong>for</strong> more than 3.5 Gyr. <strong>The</strong>y
dom<strong>in</strong>ated <strong>the</strong> Earth’s biosphere dur<strong>in</strong>g <strong>the</strong> first 2 Gyr of its history be<strong>for</strong>e <strong>the</strong> first
unicellular mitotic eukaryotes appeared. <strong>The</strong>re<strong>for</strong>e, <strong>in</strong> a search <strong>for</strong> extant life beyond
<strong>the</strong> Earth, microorganisms are <strong>the</strong> most likely candidates <strong>for</strong> <strong>the</strong> biota of an extraterrestrial
habitat. Structural, functional and chemical characteristics that have been
frequently used to identify terrestrial microbial communities are discussed below.
This <strong>in</strong>cludes signatures <strong>for</strong> actively metabolis<strong>in</strong>g or even proliferat<strong>in</strong>g life<strong>for</strong>ms
(active state) as well as <strong>for</strong> rest<strong>in</strong>g life<strong>for</strong>ms, such as dehydrated or frozen organisms
or bacterial spores (dormant state). Special attention is given to methods to detect
microbial biota of extreme environments, such as hot vents, permafrost, permanent
ice, subsurface regions, high atmosphere, rocks and salt crystals. <strong>The</strong>se environmental
extremes may be considered as terrestrial analogues of possible econiches on Mars
and on o<strong>the</strong>r selected planets and moons of our <strong>Solar</strong> <strong>System</strong>.
I.4.1 Introduction
I.4.2 Evidence of
Extant <strong>Life</strong>
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