Experimental clues for detecting biosignatures on Mars

Forthcoming exploration of Mars aims at identifying fossil biosignatures withinancient clay-rich formations. The subsurface of Mars has mostly acted as a giantfreezer for the last 4 Gyr, thereby preserving potential remains of martian life.Yet, volcanism and impactors have periodically triggered the circulation of hydrothermalfluids, inevitably causing alteration of potentially fossilised biogenicorganic materials. It thus appears crucial to quantify the impact of hydrothermalprocesses on organic biogeochemical signals in the presence of clay minerals.Here, we submitted RNA to hydrothermal conditions in the presence of Mg-smectites.Results show heterogeneous organo-mineral residues, with sub-micrometricphosphates, carbonates and amorphous silica particles together withMg-smectites with interlayer spaces saturated by N-rich organic compounds.Although the chemical structure of RNA did not withstand hydrothermal conditions,clay minerals efficiently trapped organic carbon, confirming the relevanceof drilling for organic carbon in ancient martian sediments. In addition, thedegradation of RNA in the presence of Mg-smectites led to the precipitation of a quite uncommon mineral assemblage thatcould be seen as a biosignature per se. Martian targets exhibiting this mineral assemblage will thus constitute high priorityand highly relevant candidates for sample return