Photostability of Quercetin under Oxidative Conditions Relevant to Mars

Understanding the photochemical stability of organic compounds under Martian surface conditions is essential for assessing their detectability, as such compounds would be exposed to intense ultraviolet (UV) radiation and diverse oxidizing environments. This study investigates the photodegradation of quercetin, a polyphenolic compound discussed as a potential indicator of past organic processes and a target molecule for the European Space Agency (ESA) missions OREOcube and ExocubeChem. Oxidative influences were systematically isolated under experimentally simulated Martian conditions by controlled variation of atmospheric composition and the inclusion of the clay mineral montmorillonite (Mont) as a matrix. Quercetin thin films were exposed to simulated Mars-surface UV radiation and temperatures under two distinct conditions: (i) CO₂-dominated Mars atmosphere (CO₂, 95%; Ar, 3%; and N₂, 2%) and (ii) the same atmosphere with adding 5% relative humidity (RH). Additional experiments were conducted with quercetin intercalated into Mont mineral. Degradation kinetics were monitored by ultraviolet–visible (UV–vis) and infrared (IR) transmission spectroscopy, while vacuum ultraviolet–UV (VUV–UV) spectroscopy enabled detection of gaseous photoproducts. Complementary irradiation under oxygen-rich conditions at room temperature (RT) provided additional kinetic constraints. In a CO₂-dominated atmosphere, quercetin undergoes conformational rearrangement without fragmentation. Humidity accelerates degradation, consistent with hydroxyl radical attack on the conjugated system. Oxygen-rich conditions induce the fastest decay, accompanied by C═O formation and CO release, indicative of superoxide-mediated oxidation. In contrast, intercalation into Mont prolongs quercetin lifetimes, attributed to UV shielding and geometric confinement. These results demonstrate that reactive oxygen species (ROS) govern quercetin photochemistry under Martian-relevant conditions, whereas Mont association can enhance molecular persistence.