Do Larger Rocky Exoplanets Outgas More Oxidized Atmospheres?

Telescopes with higher resolution are enabling humanity to explore and characterize rocky worlds beyond the Solar System. These low-mass exoplanets tend to lose their primordial H₂-He atmospheres, and derive secondary atmospheres from outgassing supplied by silicate mantles. The oxidation state of an outgassed atmosphere has broad implications for habitability, and it hinges on the redox state of degassing rocks or melts. Over the past decades, both experimental and modeling studies have pointed out that high pressures stabilize ferric (Fe³⁺) over ferrous (Fe²⁺) iron in a magma ocean. This implies that a larger planet with a deeper magma ocean would have a higher Fe³⁺/Fe²⁺ ratio, which would lead to a higher mantle oxygen fugacity and thus a more oxidized atmosphere. We synthesize previous experimental and modeling findings into an improved computational tool to explore how a rocky (exo)planet’s interior redox state depends on its size, density, and bulk silicate composition. Our model predictions are testable through future observations of rocky exoplanets.