[P-PS05] Mars and martian moons

[PPS05-P08] Lifetime of ancient Martian prebiotic compounds on the surface of Phobos modeled by an electron irradiation experiment of rock minerals mixed with amino acids

*Tomoki Takeda^1,2, Tomoki Kimura¹, Hideaki Miyamoto², Tomoki Nakamura⁴, Naoki Terada³ (1.Tokyo University of Science, 2.Tokyo University, 3.Graduate school of Science, Tohoku University, 4.Department of Earth and Planetary Materials Sciences, Faculty of Science, Tohoku University)

Keywords:Solar wind, Amino acids, Electrons, Phobos, Surface layer, Simulant

On the surface of Mars, numerous pieces of topographic evidence have been found, which suggest the existence of liquid water on early Mars (Ehlmann et al., 2011). It was also suggested that early Mars had prebiotic compounds like the amino acids on the surface as possible life precursors. Numerical simulations have suggested that the Martian satellite Phobos was formed by a giant impact on early Mars (Rosenblatt, P.et al., 2016), possibly preserving the prebiotic compounds. Phobos, which is an airless body, is subject to the space weathering processes by Solar Energetic Particles (SEP) (Pieters et al., 2016). It is expected to shorten the lifetime of prebiotic compounds in the satellite surface layer. Laboratory experiments for modeling the SEP irradiation have been conducted for the prebiotic compounds under various environmental conditions, and their space-weathering processes have been discussed. In these experiments, amino acids were dissociated and bound by electron irradiation at keV particle energies, leading to the creation of amines, cyanates, and dipeptides (Maté et al., 2014; Maté et al., 2015; Corr and Silveira., 2022., etc). However, the process of amino acid space-weathering in the Phobos surface environment remains unresolved. In this study, we attempted to elucidate the amino acid space-weathering by SEP in the Phobos surface layer based on electron irradiation of a Phobos simulant (UTPS-TB, Miyamoto et al., 2021) mixed with protein-synthesized amino acids. At room temperature (300K), powder samples of a mixture of 50wt% UTPS-TB and 50wt% and amino acids (alanine or cysteine) were irradiated with 5keV electrons at a fluence of 4e+16particles/cm2. The infrared reflectance measurements of the irradiated sample confirmed the degradation of alanine and cysteine in the sample surface layer. Quantitative analysis of amino acids (glycine, alanine, and cysteine) by high-performance liquid chromatography (HPLC) showed that the column density of alanine in the Phobos-alanine sample decreased from 2.42-2.58e+20 particles/cm2 to 2.15-2.37e+20 particles/cm2. The Phobos-cysteine sample also showed a decrease in column density of cysteine from 2.04-2.10e+20 particles/cm2 to 5.44-5.70e+19 particles/cm2. Other amino acids were not newly created in either sample. Based on these results, alanine and cysteine on the surface of Phobos are estimated to be depleted in 0.158-1.28e+4 [year] and 4.68-4.81e+2[year], respectively. The higher degradation rate of cysteine than that of alanine may be attributed to the greater degradation rate of the carbon-sulfur bond by electron irradiation compared to other functional groups. It was inferred from this study that the main products of irradiation are hydrocarbon compounds with relatively lower molecular weights dissociated from amino acids with larger molecular weights. In the future, we will make a more realistic estimation of the amino acid degradation process in the surface layer of Phobos to elucidate the degradation and formation process of prebiotic compounds by changing the ratio of amino acids to a Phobos simulant and irradiation fluence.