New research suggests early Earth had a highly oxidized magma ocean, leading to a CO2 and SO2-rich atmosphere. The subsequent accretion of reducing materials was critical for creating a habitable environment.
Elucidating the atmosphere and surface environment of the early Earth, especially before the origin of life, is crucial for understanding the habitability of the Earth. The atmosphere of terrestrial planets has been thought to be formed by degassing volatiles from the interior and its composition is mainly controlled by the oxidation state of the mantle. To understand the mantle oxidation state, the abundance of ferrous (Fe2+) and ferric (Fe3+) iron in the mantle are key because the mantle oxidation state varies with the relative abundance of these two iron oxides.
Mantle Oxidation State and Research Findings
Ehime University in Japan led an experimental study showing that the formation efficiency of Fe3+ via redox disproportionation of Fe2+ in metal-saturated magma under high pressures corresponding to the depth of the lower mantle is higher than previously thought. In this reaction, Fe3+ and metallic iron (Fe0) are formed from 2Fe2+, and the segregation of Fe0 into the core increases the content of Fe3+ in the residual magma and its oxidation state. The experimental results indicate that the Fe3+ content of the Earth’s magma ocean during core formation was about one order of magnitude higher than the present upper mantle.
Implications for Early Earth’s Magma Ocean
This suggests that the magma ocean was much more oxidizing than the present Earth’s mantle after the core formation, and the atmosphere formed by the degassing of volatiles from such a highly oxidizing magma would have been rich in CO2 and SO2.
In addition, the authors found that the estimated oxidation state of the Earth’s magma ocean can explain that of Hadean magmas of more than 4 billion years ago by inference from geological records. Because the formation efficiency of biomolecules in a CO2-rich atmosphere is quite low, the authors speculated that the late accretion of reducing materials after the formation of the Earth played an important role in supplying biologically available organic molecules and the formation of a habitable environment.
Reference: “Hadean mantle oxidation inferred from melting of peridotite under lower-mantle conditions” by Hideharu Kuwahara, Ryoichi Nakada, Shintaro Kadoya, Takashi Yoshino and Tetsuo Irifune, 4 May 2023, Nature Geoscience.
Funding: Japan Society for the Promotion of Science