Redox state of the Earth’s magma ocean and its initial Venus-like atmosphere

Abstract

The exchange between an ocean of magma and steam produced the Earth’s first atmosphere. Its speciation depends on the fugacity of oxygen (fOR₂) set by Fe³⁺/ Fe²⁺ ratio of oceanic magma to its surface. Here, we establish the relationship between fOR₂ and Fe³⁺/ Fe²⁺ in hardened liquids of earth silica composition at 2173 K and 1 bar. Mantle-derived rocks have Fe³⁺/ (Fe³⁺+ Fe²⁺) = 0.037 ± 0.005, where the magma ocean defines a fOR₂ 0.5 log units above the ferro-wüstite buffer. To this fOR₂, the solubilities of HCNO species in the magma ocean produce a CO-rich atmosphere. Cooling and condensation of H.₂Or it would have led to a prebiotic earth atmosphere composed of CO₂-N₂, in proportions and pressures similar to those observed on Venus. Today’s differences between the Earth’s atmosphere and those of its planetary neighbors stem from the position and heliocentric mass of the Earth, which allowed geologically long-lived oceans, in turn facilitating CO₂ drawdown and, finally, the development of life.