L-histidine in nontronite: stability under Martian-like UV irradiation to assist life detection on Mars

Detection of organic molecules on Mars is challenging due to a variety of degradation processes occurring at the Martian surface, including UV irradiation. Nevertheless, the NASA's Curiosity rover found evidence of organic molecules in clays, suggesting that these minerals might be particularly suitable to preserve organics on Mars. In this work, the photostability of L-histidine adsorbed at different pHs on nontronite under Martian-like UV irradiation was investigated in order to assess the preservation potential of this clay in the Martian environment. The interactions between L-histidine and nontronite were investigated via Infrared spectroscopy and X-Ray Diffraction, in order to understand the possible preservation mechanisms. Results indicate that L-histidine intercalates into the mineral interlayer at acidic pH, and undergoes minor degradation after UV exposure compared to the pure molecule. At basic pH, polymolecular layers are formed and no degradation is observed. These results show that nontronite acts as a photoprotective mineral for L-histidine both at acidic and basic pH, making it a suitable mineral target for organic detection on Mars.