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

Abstract

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.

Fig. 1. Effect of oxygen fugacity on the relative abundances of major gas species.

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).

Fig. 2. Relationship between iron oxidation state and oxygen fugacity in quenched peridotite liquid.

The log(XFe³⁺/XFe²⁺) ratios determined by Fe K-edge XANES in a peridotite liquid (1 bar, 2173 K) quenched to glass as a function of the imposed oxygen fugacity, expressed as the log unit deviation from the IW buffer (ΔIW). Error bars on points are the SD. The dashed black line is a linear regression through the data, and the gray curves represent the 95% confidence envelope on the regression. The shaded gray area pertains to the range of log(XFe³⁺/XFe²⁺) defined by the BSE, as inferred from Fe³⁺/Fe²⁺ ratios of whole-rock peridotites (table S6) (40, 41).

Fig. 3. Speciation during closed-system cooling of a magma ocean–generated atmosphere.

(A) A model atmosphere on the early Earth initially in equilibrium with a magma ocean of BSE composition with H = 0.01 weight % (wt %); C = 0.01 wt %, N = 0.0002 wt % (24, 53) before cooling to 300 K. Molar abundances of H, C, and N in the atmosphere are calculated with existing solubility laws (–56) yielding H/C = 0.22 and H/N = 5.8, while O is fixed to give an fO₂ of ΔIW + 0.5 at 2173 K. The resultant atmosphere of 140 bar is allowed to cool in a closed system. Vertical lines denote the temperatures of condensation of H₂O, graphite, and the 1-bar peridotite solidus. Speciation of the real gas is calculated at 50 K intervals using FactSage 7.3 (61) and is reported as fugacity (bar). (B) The present-day partial pressures of CO₂ and N₂ on Venus are shown for comparison at its surface temperature of 740 K.

Fig. 4. Effects of oxygen fugacity and H/C on atmospheric speciation.

Approximately 21,000 Gibbs free energy minimizations using FactSage 7.3 (61) were performed each at 100 bar and (A) 300 K and (B) 2173 K. Mole fractions of the gas species were linearly interpolated over a grid with variables of log(H/C) and logfO₂ expressed relative to the IW buffer. Colors shown are mixtures between end-members in (A) light green = CO₂; purple = N₂; gray = CH₄; white = NH₃; red = H₂, where contour lines correspond to the mole fractions of CO₂ below the N₂ band and CH₄ above; and in (B) red = H₂; purple = H₂O; green = CO₂; blue = CO. Contour lines show the mole fractions of each species listed in each corner. Overlain on these calculations are lines showing the atmospheric log(H/C) ratio (−0.66) and ΔIW (+0.5) calculated in equilibrium with the Earth’s magma ocean (MO) at 2173 K with an Fe³⁺/ΣFe = 0.037. Hydrogen degassing models (black curves) are shown for three different H₂/H₂O ratios whose value is listed in the white circles. Numbers next to dashed horizontal lines refer to the percentage of H degassed from the BSE composition of Hirschmann (2018) (H′18) (53). The BSE is a strict upper limit because it neglects the small solubility of H in nominally anhydrous minerals and/or inefficient degassing.