Photostability of polycyclic aromatic hydrocarbons in hydrated magnesium sulfate under Martian ultraviolet irradiation to assist organics detection on Mars

The Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument, which is mounted on the Mars 2020 Perseverance rover, detected Raman signals in spectral regions relevant to organics, plausibly polycyclic aromatic hydrocarbons (PAHs), co-located with sulfate minerals, in the Quartier abrasion of the Issole outcrop in the Jezero crater floor on Mars. In order to ascertain the plausibility of the organic origin of these signals, it is essential to determine whether organics can withstand the effects of ambient Martian ultraviolet (UV) radiation after they are exposed by the Perseverance’s abrasion tool and prior to the analysis by SHERLOC. In this work, the stability under Martian-like UV irradiation of PAHs like 2,6-dihydroxynaphthalene and benzo[a]pyrene in hydrated magnesium sulfate, one of the main sulfate phases present in Quartier, is investigated. Our findings indicate that the spectroscopic features of 2,6-dihydroxynaphthalene and benzo[a]pyrene in hydrated magnesium sulfate remain unaltered when exposed to UV radiation comparable to that experienced at Jezero crater over a period of dozens of Martian days (or sols). Consequently, due to the photoprotective properties of this mineral, after the abrasion exposes them to the radiation, these compounds can still be detectable by the SHERLOC measurement and also by SuperCam because some organic bands fall in its infrared spectral range. In addition, photoproducts due to the UV exposure for both PAHs were detected. These results corroborate the hypothesis that the Raman signals detected by SHERLOC co-located with sulfates may arise from organic compounds.