[U-08] Biogeochemistry of CO world

[U08-05] Isotopic insights into source and sink of CO in early Martian atmosphere

*Yuichiro Ueno^1,2,3, Johan Schmidt⁴, Matthew S Johnson⁴ (1.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2.Earth-Life Science Institute, Tokyo Institute of Technology, 3.Japan Agency for Marine-Earth Science and Technology, 4.University of Copenhagen)

Keywords:Origin of Life, Mars, Stable isotope

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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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).