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-wüstite 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.

PAOLO A. SOSSI, ANTONY D. BURNHAM, JAMES BADRO, ANTONIO LANZIROTTI, MATT NEWVILLE, HUGH ST.C. O’NEILL, Redox state of Earth’s magma ocean and its Venus-like early atmosphere, Science Advances  (2020), Vol. 6, no. 48, eabd1387, DOI: 10.1126/sciadv.abd1387 abstract

Calculated H2/H2O (red) and CO/CO2 (blue) ratios of an ideal gas at 2173 K as a function of oxygen fugacity expressed relative to the ΔIW. These ratios are independent of molar H/C of the gas phase, as well as its total pressure (provided it remains ideal), and are controlled only by temperature (which determines the intercept, corresponding to the logarithm of the equilibrium constant of reaction) and (fO2)0.5 (the exponent being the slope of the line). Dashed gray lines denote the fO2 of a peridotite liquid with a BSE composition (24) and an Fe3+/ΣFe ratio 0.037 (fig. S8) (40, 41) calculated according to different model parameterizations of the relationship between fO2 and Fe3+/Fe2+ in silicate melts; K + C′91 = Kress and Carmichael (29); N′96 = Nikolaev et al. (36); B′18 = Borisov (37); J′04 = Jayasuriya et al. (38); O′N′18 = O’Neill et al. (39).