Hydrodynamic escape of an impact-generated reduced proto-atmosphere on Earth

Recent cosmochemical studies have shown that most of Earth's building blocks were close to enstatite meteorites in isotopic compositions. This implies the formation of an impact-induced proto-atmosphere enriched in H-2 and CH4 on accreting Earth. Such a reduced proto-atmosphere would have been largely lost by hydrodynamic escape, but its flux and time-scale for hydrogen depletion remain highly uncertain. Here we carry out 1D hydrodynamic escape simulations for such an H-2-CH4 proto-atmosphere by incorporating expanded chemical networks and radiative cooling processes for estimation of the duration of the H-2-rich surface environment on early Earth. In the escape outflow, CH4 is dissociated effectively by direct photolysis and chemical reactions with photochemically produced ion species. On the other hand, radiative cooling by photochemical products such as H, CH, and CH3 significantly suppresses atmospheric escape. Even though CH4 and their concentrations are small, the heating efficiency decreases to when CH4/H-2 = 0.007 in the lower atmosphere and CH4 would suffer negligible escape when CH4/H-2 greater than or similar to 0.01. The time-scale for H-2 escape consistent with the constraints of the isotopic compositions and the amount of C and N on the present Earth is possibly more than several hundred million years. Our results suggest that a long-lived hydrogen-rich reduced environment played important roles in climate warming and the generation of organic matters linked to the emergence of living organisms during the first several hundred million years of Earth.