Udecanoic acid in vermiculite: detection, characterization and UV degradation studies for biosignature identification on Mars
Résumé
Laboratory simulations of the Martian conditions are essential to interpret results collected by Mars exploratory missions. Among them, it is crucial to study the degradation and/or the evolution of possible organic biomarkers adsorbed on minerals. It is known that mid-UV radiation is among the main degradation agents on Mars. Laboratory simulations of the harsh Martian conditions can evaluate the preservation likelihood of potential biomarkers on different minerals on Mars1, which in turn support rover missions to select the most appropriate environment to perform in situ analysis.
In this work, the interaction and stability of undecanoic acid in vermiculite and in the presence of chloride salts are studied. The vermiculite mineral is a phyllosilicate phase widespread on the Martian surface and characterized by high organic preservation potential2. Moreover, undecanoic acid was chosen as a biomarker as it belongs to the fatty acid family, constituents of cell membranes, and crucial sources of metabolic energy for life as we know it. Finally, geologic environments containing saline minerals are potential areas of biological activity3. Therefore, it is interesting to study the influence of chloride salts on the photodegradation of organic molecules.
The samples were prepared using the equilibrium adsorption method. In particular, natural vermiculite, previously pyrolyzed at 500°C to eliminate organic contaminants, was doped with a solution of undecanoic acid in ethanol. To study the effect of the salts, sodium or magnesium chloride were added to some of the samples. The suspensions were kept for 3 h on a stirring plate to reach the equilibrium state during molecular adsorption and then were dried at 50 C°.
The samples were analyzed by Diffuse Reflectance InfraRed Fourier Transform Spectroscopy and processed under Martian-like UV irradiation conditions using an experimental setup that allows to monitor the degradation kinetics in situ by infrared spectroscopy analysis. These experiments allowed us to investigate the catalytic/protective behavior of vermiculite and chloride salts on undecanoic acid and the half-lifetime degradation under Martian UV flux.