Large-scale atmosphere dissolution in magma oceans in exoplanets

Exoplanet sub-Neptunes are nearly as common as stars, but their nature remains uncertain. Sub-Neptunes orbit close to their host stars, and many are apparently shrouded in large and hot hydrogen-dominated atmospheres. Harsh irradiation conditions and the presence of greenhouse gases enhance the melting of the underlying silicate surface and promote the exchange of volatiles between the atmosphere and a magma ocean. Here we show that magma oceans can dissolve ≥2 wt% hydrogen at typical conditions relevant to the interface between the atmosphere and the condensed surface in such planets. The influx of hydrogen into the molten silicate lowers magma density, which alters the sub-Neptune radius-mass relationship (equation of state). Moreover, the hydrogen influx changes the redox state of the magma ocean (which becomes reduced), and generates a massive outflux of oxygen, which reacts with atmospheric hydrogen to form large amounts of water vapor. Thus, the release of oxygen will profoundly change the atmospheric chemistry and bear spectral signatures observable from space telescopes. These atmospheric-composition changes are a testable signature of hidden magma oceans in exoplanets. The uptake of volatiles by the magma implies that the interiors of sub-Neptunes are more volatile-rich than once thought. Moreover, our atomistic simulations suggest that the dissolution and evaporation process is chemically complex, forming hundreds of molecular species in the atmosphere. These magma-buffered atmospheric recipes will need to be accounted for in future studies addressing models of exoplanet atmosphere origins.