Redox state of Earth's magma ocean and its Venus-like early atmosphere

Exchange between a magma ocean and vapor produced Earth's earliest atmosphere. Its speciation depends on the oxygen fugacity (fO(2)) set by the Fe3+/Fe2+ ratio of the magma ocean at its surface. Here, we establish the relationship between fO(2) and Fe3+/Fe2+ in quenched liquids of silicate Earth-like composition at 2173 K and 1 bar. Mantle-derived rocks have Fe3+/(Fe3++Fe2+) = 0.037 +/- 0.005, at which the magma ocean defines an fO(2) 0.5 log units above the iron-wiistite buffer. At this fO(2), the solubilities of H-C-N-O species in the magma ocean produce a CO-rich atmosphere. Cooling and condensation of H2O would have led to a prebiotic terrestrial atmosphere composed of CO2-N-2, in proportions and at pressures akin to those observed on Venus. Present-day differences between Earth's atmosphere and those of her planetary neighbors result from Earth's heliocentric location and mass, which allowed geologically long-lived oceans, in-turn facilitating CO2 drawdown and, eventually, the development of life.