Dissolution of gases in magma oceans on Sub-Neptunes

Sub-Neptunes are a category of exoplanets, with radius between 1.75 and 3 Earth radius, sufficiently large to accumulate a thick atmosphere composed mostly by H₂ and He on top of a rocky support. Most of the observed Sub-Neptunes orbit around their star in less than 100 days. Based on the relative position to their star and the size of the atmospheres, the temperatures and pressures at the atmosphere/mantle interface could go up to 4000K and 10GPa^[1]. At such conditions, their condensed surface should be completely molten creating magma oceans. This magma ocean or magma ponds react with the atmosphere in a way that it can affect the mass and composition of the planets ^[2]. Such interactions forge the mass-radius relation.

Here, we use ab-initio molecular dynamics to study the chemical exchanges between magma ocean and atmosphere. These exchanges consist of outgassing, dissolution of volatiles into the magma, and redox reactions ^[3]. We focus on the redox reactions mediated by the presence of Fe. We work on two extremes systems: Fe + H₂O and FeO + H₂. We monitor the chemical reactions between the different phases present in our systems, the oxidation state of iron and finally the catalytic role of Fe. Our simulations show that Fe is a catalyst for H₂O dissociation of the Fe + H₂O systems, and a H₂O generator in FeO + H₂ systems. The immiscibility gap is closed at 4000K for chemical systems at all pressures.