[U-08] CO環境の生命惑星化学

[U08-05] 初期火星大気COの生成消滅過程に関する同位体的洞察

*上野 雄一郎^1,2,3、Schmidt Johan⁴、Johnson Matthew⁴ (1.東京工業大学理学院地球惑星科学系、2.東京工業大学地球生命研究所、3.海洋研究開発機構、4.コペンハーゲン大学)

キーワード：生命起源、火星、安定同位体

In early atmosphere of Mars, CO may have played a significant role for prebiotic synthesis of organic compounds. Recent Mars exploration missions has revealed a presence of sedimentary organic matter in early Martian sediments (Ming et al., 2014; Eigenbrode et al., 2018), some of which exhibit a strong 13C depletion (d13C = -137‰ ~ -70‰: House et al., 2021). On the other hand, Martian atmospheric CO2 is rather rich in 13C compared to Earth (d13C = +46 ± 4‰: Webster et al., 2013). Therefore, an exceptionally large fractionation process of over 100‰ is required to account for these isotopic variations. We report a nobel laboratory experiments and theoretical calculations, which suggest that solar UV photo-dissociation of CO2 produces strongly 13C-depleted CO and thus remaining CO2 should be enriched in 13C. Considering irreversible hydrogen escape into space, the early Martian atmosphere should have been reducing than present-day Mars. In such a reducing atmosphere, CO is further transformed into soluble organic molecules such as aldehydes and organic acids (Zang et al., 2022). Therefore, the sedimentary organic matter in early Martian sediment may have been formed from CO in a reducing early atmosphere.

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Eigenbrode, J. L. et al. Organic matter preserved in 3-billion-year-old mudstones at Gale crater, Mars. Science 360, 1096-1101 (2018).
House, C. H. et al. Depleted carbon isotope compositions observed at Gale crater, Mars. Proc. Natl Acad. Sci. U. S. A. 119, e2115651119 (2022).
Webster, C. R. et al. Isotope ratios of H, C, and O in CO₂ and H₂O of the Martian atmosphere. Science 341, 260-263 (2013).
Zang, X., Ueno, Y. & Kitadai, N. Photochemical synthesis of ammonia and amino acids from nitrous oxide. Astrobiology 22, 387-398 (2022).