Investigating the Photochemical Properties of Cytosine and Thymine Under Mars-Like Conditions: Effect of UV Radiation and Calcium Perchlorate.

Mars missions actively search for organic matter as potential biosignatures. Yet harsh conditions at the surface, including unfiltered ultraviolet (UV) light above 190 nm and oxidizing agents such as perchlorates, challenge the preservation of compounds relevant to astrobiology, such as nucleobases. Since current instruments primarily analyze samples from the uppermost surface layer, understanding the stability of organic matter under Mars-like surface conditions is essential. Nucleobases have interesting photochemical properties, as they can dimerize under UV light. Previous work showed that UV exposure of uracil under Mars-like conditions rapidly leads to more photostable dimers, with an enhanced photodecomposition yield when perchlorates are present. Additional chemical groups, including alkyne (C≡C) and nitrile (C≡N), emerge in the presence of calcium perchlorate and indicate novel photoproducts. The present work investigates the evolution of two other pyrimidine nucleobases, cytosine and thymine, in simulated UV martian conditions with and without calcium perchlorate. Infrared spectroscopy monitoring of the sample throughout the duration of the experiment showed that cytosine and thymine both form photoproducts under UV light, likely dimers for thymine. Moreover, both molecules seem to interact spontaneously with calcium perchlorates prior to any UV exposure, to form either a complex with cytosine or a change in the crystalline phase with thymine.