Self-Shielding Enhanced Organics Synthesis in an Early Reduced Earth's Atmosphere.

Earth is expected to have acquired a reduced proto-atmosphere enriched in H and CH through the accretion of building blocks that contain metallic Fe and/or the gravitational trapping of surrounding nebula gas. Such an early, wet, reduced atmosphere that covers a proto-ocean would then ultimately evolve toward oxidized chemical compositions through photochemical processes that involve reactions with HO-derived oxidant radicals and the selective escape of hydrogen to space. During this time, atmospheric CH could be photochemically reprocessed to generate not only C-bearing oxides but also organics. However, the branching ratio between organic matter formation and oxidation remains unknown despite its significance on the abiotic chemical evolution of early Earth. Here, we show via numerical analyses that UV absorptions by gaseous hydrocarbons such as CH and CH significantly suppress HO photolysis and subsequent CH oxidation during the photochemical evolution of a wet proto-atmosphere enriched in H and CH. As a result, nearly half of the initial CH converted to heavier organics along with the deposition of prebiotically essential molecules such as HCN and HCO 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.