The composition of the Earth’s earliest atmosphere

A new experimental study shows that ferric iron (Fe³⁺) is more efficiently formed from ferrous iron (Fe²⁺) in magma under high-pressure conditions than previously thought. The estimated oxidation state of such a Fe³⁺-rich magma is quantitatively consistent with highly oxidized magmas inferred from geological records from 4.0-4.4 billion years ago (Hadean Eon). The Earth's atmosphere at that time was mainly composed of CO₂ and SO₂, suggesting the formation of an uninhabitable surface environment for primitive life.

Elucidating the atmosphere and surface environment of the early Earth, especially before the origin of life, is a key to 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.
Ehime University has 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. 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 URL: https://www.nature.com/articles/s41561-023-01169-4

Bibliographic Information

Hadean mantle oxidation inferred from melting of peridotite under lower-mantle conditions, Hideharu Kuwahara, Ryoichi Nakada, Shintaro Kadoya, Takashi Yoshino, Tetsuo Irifune, Nature Geoscience, 16, 461-465, doi:10.1038/s41561-023-01169-4, 2023 (May 4).

Fundings

• Japan Society for the Promotion of Science(JSPS) KAKENHI Grant Number 20H01994 and 21K18655

Media

• 

  Quenched texture of the recovered sample

  The bright area at the center of the image shows quenched metallic melt and the surrounding grey area indicates quenched silicate melt. The sample was encapsulated into a graphite capsule, which is transformed into diamond during heating experiments.

  credit : Geodynamics Research Center, Ehime University
  Usage Restriction : Please get copyright permission