The origin of life on Earth seems to demand a highly reduced early atmosphere, rich in CH4, H2, and NH3, but geological evidence suggests that Earths mantle has always been relatively oxidized and its emissions dominated by CO2 H2O, and N2. The paradox can be resolved by exploiting the reducing power inherent in the late veneer, i.e., material accreted by Earth after the Moon-forming impact. Isotopic evidence indicates that the late veneer consisted of extremely dry, highly reduced inner solar system materials, suggesting that Earths oceans were already present when the late veneer came. The major primary product of reaction between the late veneers iron and Earths water was H2. Ocean vaporizing impacts generate high pressures and long cooling times that favor CH4 and NH3. Impacts too small to vaporize the oceans are much less productive of CH4 and NH3, unless (i) catalysts were available to speed their formation, or (ii) additional reducing power was extracted from pre-existing crustal or mantle materials. The transient H2-CH4 atmospheres evolve photochemically to generate nitrogenated hydrocarbons at rates determined by solar radiation and hydrogen escape, on timescales ranging up to tens of millions of years and with cumulative organic production ranging up to half a kilometer. Roughly one ocean of hydrogen escapes. The atmosphere after the methanes gone is typically H2 and CO rich, with eventual oxidation to CO2 rate-limited by water photolysis and hydrogen escape.
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