  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
Earth's 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
Earth's oceans were already present when the late veneer came. The major
primary product of reaction between the late veneer's iron and Earth's 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
methane's gone is typically H2 and CO rich, with eventual oxidation to CO2
rate-limited by water photolysis and hydrogen escape.
