Ultraviolet Radiation-Induced Degradation Kinetics and Transfer Mechanisms in Biomolecular Thin Films on Martian Iron Oxide Analogs

High Resolution Image Download MS PowerPoint Slide The photochemical stability of organic thin films in contact with iron oxides was investigated under simulated Martian near-surface conditions to elucidate radiation-induced degradation and transformation processes by in situ ultraviolet–visible (UV–Vis) spectroscopy. Highly uniform 20–30 nm films of quercetin, iron tetraphenyl porphyrinato chloride (FeTPPCl), and magnesium tetraphenyl porphyrin (MgTPP) were deposited by thermal evaporation onto UV-transparent magnesium fluoride (MgF 2 ) substrates, either bare or coated with ultrathin iron oxide layers. Irradiation was performed in a CO 2 -dominated atmosphere (≈10 mbar) at −20 °C with a UV flux comparable to present-day Martian surface conditions. The experimental concept and thin-film preparation protocols were motivated by the hardware and sample design of the OREOcube space experiment on the International Space Station (ISS), enabling future comparison between laboratory Mars simulations and space-exposure studies of radiation-driven chemical evolution in organic thin films. UV–Vis measurements reveal molecule-dependent kinetic regimes. Quercetin exhibits apparent first-order decay on all substrate types, with iron oxide coatings altering degradation rates. In contrast, metal porphyrin degradation is more accurately described by kinetic approaches incorporating film-internal optical attenuation effects, indicating that changes in effective light penetration contribute to the observed behavior. Overall, quercetin degradation is largely governed by intrinsic photochemical reactivity, whereas porphyrin kinetics are more sensitive to electronic structure and attenuation effects. Iron oxide interfaces modulate, rather than dominate, degradation pathways, suggesting that mineral association may influence the persistence and spectroscopic detectability of organic compounds on the present-day Martian surface.