For several years, thanks in particular to the sophistication of onboard analysis and observation instruments, astrobiology has been a booming field. More and more automated devices (probes, rovers) carry biochemical study instruments with the aim of detecting biosignatures, that is to say elements — molecules, gases or other phenomena — testifying to the presence, past or present, of life on a planet.
The search for biosignatures is focused on the knowledge we have today of life on Earth. That is, a complex carbonaceous life form whose biomolecular structure is common to all known living beings. Due to its unique properties, carbonaceous life is the source of a number of biosignatures such as the consumption and release of gases into the atmosphere, the dispersion of chemical elements or the production of biomass and organic waste. .
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This study of biosignatures is of particular importance on Earth since it helps scientists to reconstruct the "organic past" of our planet. This field of research is mainly dominated by geomicrobiology, which aims to better understand the appearance, development and evolution of life, particularly concerning bacteria and other unicellular organisms, by analyzing sediments and rocks.
Scientists are looking for biogeochemical clues through the study of microfossils (fossilized organic micro-residues), stromatolites (sedimentary piles produced by cyanobacteria), molecular biomarkers (amino acids, nitrogenous bases, nucleic acids, lipids, etc.), isotopic ratios (carbon, nitrogen, hydrogen, sulfur) and enantiomeric excesses (excess of certain enantiomers in a sample).
Although some scientists have suggested the possibility of biochemistries based on atoms other than carbon, the search for life in the universe today is based exclusively on biosignatures corresponding to carbonaceous biochemistry.
Indeed, without presuming carbon chauvinism (the theory that carbon is the only element capable of structuring life), the search for terrestrial biomarkers on other planets is currently the best way to effectively detect the presence of extraterrestrial life.
Thus, biosignatures in astrobiology have a double advantage. First, they provide strong clues about the presence of extraterrestrial life. Secondarily, they make it possible to invalidate a possible abiotic (non-living) origin of these signatures. Of course, in this context, the scientific method requires that confirmation of biosignatures be given only when all other explanations have been ruled out.
To do this, scientists have a wide range of possible biosignatures:complex biomolecules of necessarily biotic origin, cell morphologies, biogenic minerals, stable isotopic ratios in minerals and organic components, molecular chirality, atmospheric gases, photosynthetic pigments, etc According to NASA's Department of Strategic Astrobiology Research, these extraterrestrial biosignatures are classified into ten distinct categories.
Isotopic composition of the medium
Some organisms, such as archaea, induce changes in the isotopic ratio of a medium through specific oxidation-reduction reactions. This is particularly the case with photosynthetic bacteria. An example on Earth is given by phototrophic sulphur-oxidizing bacteria — such as purple sulfur bacteria — which reduce sedimentary sulphate, thereby decreasing the amount of sulfur isotope 32 and increasing that of isotope 34. Many other imbalances isotopic signs the presence of living organisms.
Organic chemical residues
The detection of a single biochemical compound cannot alone testify to the presence, past or present, of life on a planet. Indeed, some abiotic phenomena can cause chemical reactions involving organic substances. The study of such residues must necessarily go through a systematic search for complex arrangements, a particular chirality or a precise chemical degradation to identify the presence of biosignatures. For example, on Earth, life only uses levorotatory amino acids and dextrorotatory carbohydrates.
Biochemical compounds and organic degradation
Organic degradation products represent a… (continued on next page)
