  A recent study by Ramirez et al. (2014) demonstrated that an atmosphere with
1.3-4 bar of CO2 and H2O, in addition to 5-20% H2, could have raised the mean
annual and global surface temperature of early Mars above the freezing point of
water. Such warm temperatures appear necessary to generate the rainfall (or
snowfall) amounts required to carve the ancient martian valleys. Here, we use
our best estimates for early martian outgassing rates, along with a 1-D
photochemical model, to assess the conversion efficiency of CO, CH4, and H2S to
CO2, SO2, and H2. Our outgassing estimates assume that Mars was actively
recycling volatiles between its crust and interior, as Earth does today. H2
production from serpentinization and deposition of banded iron-formations is
also considered. Under these assumptions, maintaining an H2 concentration of
~1-2% by volume is achievable, but reaching 5% H2 requires additional H2
sources or a slowing of the hydrogen escape rate below the diffusion limit. If
the early martian atmosphere was indeed H2-rich, we might be able to see
evidence of this in the rock record. The hypothesis proposed here is consistent
with new data from the Curiosity Rover, which show evidence for a long-lived
lake in Gale Crater near Mt. Sharp. It is also consistent with measured oxygen
fugacities of martian meteorites, which show evidence for progressive mantle
oxidation over time.
