Large-scale hydrogen reactions with magma oceans change the atmosphere of exoplanets

Exoplanet super-Earths and sub-Neptunes are likely more prevalent than stars, yet their nature remains uncertain. Notably, sub-Neptunes orbit in close proximity to their host stars, often enveloped in substantial and hot hydrogen-rich atmospheres. These conditions, characterized by intense irradiation and the presence of greenhouse gases, accelerate the melting of the underlying silicate surface and facilitate the exchange of volatiles with the atmosphere. In this study, we conduct extensive large-scale ab initio simulations to investigate the reaction between hydrogen gas and molten multi-component silicates. Our findings reveal that magma oceans possess the capability to dissolve up to 2 wt% hydrogen at typical conditions pertinent to the interface between the atmosphere and the condensed surface of such planets. The influx of hydrogen into the molten silicate results in a reduction in magma density, leading to the formation of a substantial buffer layer between the condensed interior and the outer atmosphere. This layer effectively suppresses convection and restricts chemical exchanges within these planets.

Furthermore, the incorporation of hydrogen into the molten silicate alters the redox state of the magma ocean, causing it to become reduced. Consequently, a substantial outflux of oxygen is generated, which combines with atmospheric hydrogen to produce substantial amounts of water vapor. Consequently, the release of oxygen will profoundly alter the atmospheric chemistry and manifest spectral signatures detectable from space telescopes. These atmospheric composition modifications constitute a testable signature of concealed magma oceans on exoplanets. The uptake of volatiles by the magma implies that sub-Neptunes exhibit a higher volatility compared to previously assumed. Additionally, our atomistic simulations suggest that the dissolution and evaporation process is chemically intricate, resulting in the formation of hundreds of distinct molecular species within the atmosphere. These magma-buffered atmospheric recipes will necessitate consideration in future studies focused on the origins of exoplanet atmospheres.