Redox state of Earth's magma ocean and its Venus-like early atmosphere.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 33239296.
- Also identified by DOI 10.1126/sciadv.abd1387 and PMC identifier 7688334.
- Licence recorded as CC BY-NC.
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Abstract
Exchange between a magma ocean and vapor produced Earth's earliest atmosphere. Its speciation depends on the oxygen fugacity (<i>f</i>O<sub>2</sub>) set by the Fe<sup>3+</sup>/Fe<sup>2+</sup> ratio of the magma ocean at its surface. Here, we establish the relationship between <i>f</i>O<sub>2</sub> and Fe<sup>3+</sup>/Fe<sup>2+</sup> in quenched liquids of silicate Earth-like composition at 2173 K and 1 bar. Mantle-derived rocks have Fe<sup>3+</sup>/(Fe<sup>3+</sup>+Fe<sup>2+</sup>) = 0.037 ± 0.005, at which the magma ocean defines an <i>f</i>O<sub>2</sub> 0.5 log units above the iron-wüstite buffer. At this <i>f</i>O<sub>2</sub>, the solubilities of H-C-N-O species in the magma ocean produce a CO-rich atmosphere. Cooling and condensation of H<sub>2</sub>O would have led to a prebiotic terrestrial atmosphere composed of CO<sub>2</sub>-N<sub>2</sub>, 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 CO<sub>2</sub> drawdown and, eventually, the development of life.