The Photochemistry on Space Station (PSS) Experiment: Organic Matter under Mars-like Surface UV Radiation Conditions in Low Earth Orbit

Abstract : The search for organic molecules at the surface of Mars is a top priority of the Mars Science Laboratory (NASA) and ExoMars 2020 (ESA) space missions. Their main goal is to search for past and/or present molecular compounds related to a potential prebiotic chemistry and/or a biological activity on the Red Planet. A key step to interpret their data is to characterize the preservation or the evolution of organic matter in the martian environmental conditions. Several laboratory experiments have been developed especially concerning the influence of ultraviolet (UV) radiation. However, the experimental UV sources do not perfectly reproduce the solar UV radiation reaching the surface of Mars. For this reason, the International Space Station (ISS) can be advantageously used to expose the same samples studied in the laboratory to UV radiation representative of martian conditions. Those laboratory simulations can be completed by experiments in low Earth orbit (LEO) outside the ISS. Our study was part of the Photochemistry on the Space Station experiment on board the EXPOSE-R2 facility that was kept outside the ISS from October 2014 to February 2016. Chrysene, adenine, and glycine, pure or deposited on an iron-rich amorphous mineral phase, were exposed to solar UV. The total duration of exposure to UV radiation is estimated to be in the 1250–1420 h range. Each sample was characterized prior to and after the flight by Fourier transform infrared (FTIR) spectroscopy. These measurements showed that all exposed samples were partially degraded. Their quantum efficiencies of photodecomposition were calculated in the 200–250 nm wavelength range. They range from 10⁻⁴ to 10⁻⁶ molecules·photon⁻¹ for pure organic samples and from 10⁻² to 10⁻⁵ molecules·photon⁻¹ for organic samples shielded by the mineral phase. These results highlight that none of the tested organics are stable under LEO solar UV radiation conditions. The presence of an iron-rich mineral phase increases their degradation.