The volcanic chert sediments of Kitty's Gap (Pilbara, Australia), a 3.45-billion-year-old rock formation, were deposited in a shallow-water environment. This enabled them to provide an ideal habitat for certain types of microorganisms on the early Earth, such as chemolithotrophs, which use the oxidation of inorganic substrates as a source of energy. The volcanic sediments of ancient rocks, formed under conditions similar to those on Mars, should make it possible to test various analytical techniques necessary to search for traces of life in their counterparts that existed during the Noachian period more than 3.5 years ago Billions of years have been deposited on the Red Planet. However, the small cell size of microorganisms living on the surface and interior of these terrestrial sediments, combined with the diffuse nature of these fossilized colonies, makes the identification and characterization of potential biosignatures in these types of organisms difficult.
Scientists from the Center for Molecular Biophysics (CNRS/University of Orléans) implemented a series of spectroscopic techniques (optical microscopy, Raman spectroscopy, LA-ICP-MS, etc.) to determine the syngeneity and biogenicity of the fossil remains preserved in these sediments.
The results show that the sediments were deposited in a semi-enclosed basin influenced by tides, rivers and hydrothermal fluids, and whose conditions are compatible with the colonization and development of microbial life. Elemental and molecular morphological analyzes of the carbonaceous material associated with the degraded remains of potential microfossils confirmed their syngeneity and biogenicity. In particular, the researchers carried out elemental (X-ray fluorescence and photon-induced X-ray emission – PIXE) and molecular (UV and IR fluorescence) analyses. These analytical techniques revealed an enrichment of trace metals (vanadium, chromium, iron, cobalt, etc.) as well as aromatic and aliphatic molecules associated with carbonaceous material, supporting the hypothesis of a biological origin. Transmission electron microscopy observations also made it possible to identify amorphous nanostructures, which were interpreted as degraded remains of microorganisms and their by-products (e.g. extracellular polymeric substances, filaments).
The study highlights the need to use a broad range of complementary analyzes at multiple levels to search for life in extraterrestrial samples returned to Earth. These results, to which the Physics of Two Infinitys Laboratory (CNRS/University of Bordeaux), the Archaeomaterials Research Institute (CNRS/University of Orléans), as well as the Natural History Museum of London and the University of Bologna, contributed, have just been published in the journal Astrobiology.