Evolution of an Early Reduced Earth’s Atmosphere Driven by Photochemistry and Hydrodynamic Escape

Earth is expected to have acquired a reduced proto-atmosphere enriched in H2 and CH4 through the accretion of building blocks that contain metallic Fe and/or the gravitational trapping of surrounding nebula gas. Such an early reduced atmosphere that covers a proto-ocean would then ultimately evolve toward oxidized chemical compositions through photochemical processes that involve reactions with H2O-derived oxidant radicals and the selective escape of hydrogen to space. However, the photochemistry and hydrodynamic escape inducing the atmospheric evolution along with the organic synthesis have not been fully investigated. In this study, we developed a hydrodynamic escape model [1-3] and a photochemical model [4] to clarify the evolution of a reduced Earth’s atmosphere mainly composed of H2 and CH4.

Our calculations for the hydrodynamic escape found that the molecular radiative cooling by CH4, CO, CO2, H2O, and their photochemical products can significantly suppress the hydrodynamic escape. Even when the photochemically unstable molecules such as CH4, CO2, and H2O are dissociated, their photochemical products like CH3, CO, and OH serve as effective coolants. These radiative cooling processes can extend the lifetime of H2-rich atmospheres by about one order of magnitude compared to the case of pure hydrogen atmospheres on early Earth, which also results in negligible escape of heavier carbon- and nitrogen-bearing molecules and noble gases. Details of the hydrodynamic escape modeling are provided in references [1-3].

Our photochemical calculations show that UV absorptions by gaseous hydrocarbons such as C2H2 and C3H4 significantly suppress H2O photolysis and subsequent CH4 oxidation during the photochemical evolution of a reduced atmosphere enriched in H2 and CH4. As a result, nearly half of the initial CH4 converted to heavier organics along with the deposition of prebiotically essential molecules such as HCN and H2CO on the surface of a primordial ocean for a geological timescale order of 10–100 Myr. Our results suggest that the accumulation of organics and prebiotically important molecules in the proto-ocean could produce a soup enriched in various organics, which might have eventually led to the emergence of living organisms. Further details of the photochemical modeling are presented in reference [4].

References:

[1] Yoshida, T., & Kuramoto, K. (2021). Hydrodynamic escape of an impact-generated reduced proto-atmosphere on Earth. Monthly Notices of the Royal Astronomical Society, 505(2), 2941.

[2] Yoshida, T., Terada, N., Ikoma, M., & Kuramoto, K. (2022). Less effective hydrodynamic escape of H2-H2O atmospheres on terrestrial planets orbiting pre-main-sequence M dwarfs. The Astrophysical Journal, 934(2), 137.

[3] Yoshida, T., Terada, N., & Kuramoto, K. (2024). Suppression of hydrodynamic escape of an H2-rich early Earth atmosphere by radiative cooling of carbon oxides. Progress in Earth and Planetary Science, 11(1), 59.

[4] Yoshida, T., Koyama, S., Nakamura, Y., Terada, N., & Kuramoto, K. (2024). Self-Shielding Enhanced Organics Synthesis in an Early Reduced Earth’s Atmosphere. Astrobiology, 24(11), 1074.