Photodegradation of the biomarker L-histidine induced by edge sites of a clay mineral in the Martian harsh UV environment

Abstract

Clay minerals are thought to play a crucial role in preserving organics on Mars due to their high surface area and expandable interlayers, ideal for adsorbing and intercalating organic molecules. Our study simulated the adsorption of L-histidine, an amino acid potentially diagnostic of life, on the clay mineral saponite, in a slightly acidic aqueous environment as possibly present on Mars during the Hesperian era, followed by a desiccation event and then high UV irradiation typical of the subsequent Amazonian era. For comparison, the same experiments were performed also at alkaline pH. X-Ray Diffraction and Infrared and Deep Ultraviolet Raman and fluorescence spectroscopies were used to characterize the mineral-molecule interactions, indicating that molecules are partly intercalated in the interlayers of saponite at acidic pH and partly co-precipitated with saponite at alkaline pH, while the remainder is adsorbed on lateral edges in both cases. Surprisingly, UV irradiation experiments revealed faster degradation of L-histidine when adsorbed on saponite compared to its pure form at both pHs, suggesting a photocatalytic behaviour of this nano-sized clay mineral, driven by active surface edge sites, despite potential shielding by interlayer sites for the part of L-histidine molecules intercalated in the interlayers at acidic pH. Degradation/alteration of L-histidine adsorbed on saponite occurs in a few hours under Martian UV flux, suggesting that photocatalysis induced by active surface edge sites of clays might have contributed to the depletion of organics in clays at the highly UV-irradiated surface of Mars on very short timescales.