Degassing of CO₂ triggers large-scale loss of helium from magma oceans

The moon-forming impact caused widespread melting and vaporization of the proto-Earth, leading to the formation of a protolunar disk. Volatile elements trapped in the inner parts of the disk condensed together with heavier elements into a global magma ocean. By subsequent cooling and degassing, volatiles contributed to the formation of the Earth's secondary atmosphere. The thermodynamic conditions at the magma ocean-atmosphere interface profoundly impacted the degree of degassing that occurred. Today, our understanding of these conditions remains unclear. Here, we simulate the degassing of carbon and helium from a magma ocean with a bulk silicate Earth composition at presumed typical conditions for the early Earth. We employ first-principles molecular dynamics calculations. We find that volatile loss is determined by pressure, temperature, and melt composition. Carbon and helium devolatilize more easily when both elements are present in the melt, leading to pronounced loss from the magma ocean. Our findings suggest that the early Earth atmosphere was carbon-rich, with a high fraction of helium and other noble gases, and thicker and hotter than previously thought. We evaluate the implications for Earth's composition and thermal evolution, as well as other planets that may undergo similar processes with different chemistries.

Anne H. Davis, Razvan Caracas