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 (fO2) set by the Fe3+/Fe2+ ratio of the magma ocean at its surface. Here, we establish the relationship between fO2 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 fO2 0.5 log units above the iron-wuestite buffer. At this fO2, 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-N2, 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.P.A.S. acknowledges support from SNSF Ambizione Fellowship grant no. 180025 and H.S.O. on IPGP visiting professorship in September, 2017. A.D.B. and H.S.O. acknowledge support from ARC Laureate Fellowship no. FL130100066. Parts of this work were supported by the UnivEarthS Labex program (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02) and IPGP multidisciplinary program PARI (Paris–IdF region SESAME Grant no. 12015908). The authors acknowledge the facilities and the scientific and technical assistance of Microscopy Australia at the Centre for Advanced Microscopy, Australian National University, a facility that is funded by the University and the Federal Government. Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation–Earth Sciences (EAR: 1634415) and Department of Energy- GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357