Uracil evolution under Mars surface-like UV radiation conditions with the MOMIE (Mars Organic Matter Irradiation and Evolution) experiment

Abstract

International audienceThe detection and identification of organic molecules at Mars are of primary importance for astrobiology, as some of these molecules are life precursors and components. While in situ exploration missions are searching for them at the surface of the planet, it is essential to understand how organic molecules evolve and are preserved at the surface of Mars. Indeed the harsh conditions of the environment of Mars, such as ultraviolet (UV) radiation or oxidative processes, due for example to perchlorate ions, could explain the low abundance and diversity of organic molecules detected so far [1][2]. The MOMIE (Mars Organic Matter Irradiation and Evolution) experiment has been set up to study the evolution of organic matter under simulated martian radiation within the laboratory [3]. Organic samples are exposed under a wide range UV lamp (200-400 nm) as thin homogenous solid films (a fraction of μm thick), by a mean temperature of -55°C and a mean pressure of 6 mbar, close to Mars surface conditions. Most of the organic molecules studied so far were quickly destroyed under Mars surface-like UV radiation conditions [4]. Uracil is a one-ringed heterocyclic compound, a nucleobase found in RNA, and has been detected in meteorites thus is likely to be found at Mars [5]. Pure uracil samples were irradiated for several weeks in the MOMIE experiment, as well as uracil mixed with perchlorate salts. To characterize the evolution of uracil samples, analyses by infrared spectroscopy (FTIR) were performed all along the experiment. These analyses allowed determining whether uracil is preserved or photodegraded, and if so, its photolysis rate. The FTIR successive analysis showed new absorption band appearance and increase along the UV irradiation. High-Resolution Mass Spectrometry (HRMS) analysis of irradiated uracil samples has shown that uracil seems to form polymers under martian-like UV radiation. These polymers are far more resistant to UV radiation. The effect of perchlorates salts on uracil evolution and preservation has been investigated as well. The MOMIE experiment allows to understand which organic molecules in situ missions should target, as some of them are quickly destroyed in Mars surface-like conditions while others are likely to form new compounds. Uracil polymers, formed by uracil molecules under Mars surface-like radiation conditions, would be very relevant compounds to search for at the surface of Mars