Redox Controls on Sulfur Degassing in the Magma Ocean

Degassing of the magma ocean shaped the Earth’s early atmosphere and volatile budget. Despite its fundamental importance, the oxidation conditions of the magma ocean and the associated degassing processes remain poorly constrained. Sulfur, an abundant volatile element with multiple valence states, provides a sensitive tracer of redox-dependent degassing, making it an ideal probe for these processes.

Here, we present the first systematic investigation of sulfur degassing under realistic magma ocean conditions typical of the beginning of the Haden, using ab initio molecular dynamics simulations. Our results reveal that sulfur volatility and its speciation in the gas phase are strongly controlled by redox conditions: oxidizing conditions make sulfur highly volatile as sulfur oxides, reducing conditions keep it bound to the silicate melt. In view of our results, the observations of sulfur depletion in the Earth today, can be explained if degassing of the early magmas from planetesimals during accretion occurred under relatively reducing conditions. Sulfur degassing at the magma ocean stage of the early Earth brought reducing species to the early atmosphere, with the sulfur vapor phases being favorable for the prebiotic synthesis of amino acids. Our sulfur degassing results establish a direct link between the depletion of volatile elements, the redox state of the magma ocean, and the composition of the early atmosphere, providing new insights into the evolution of early Earth.