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Many sub-Neptune exoplanets have been believed to be composed of a thickhydrogen-dominated atmosphere and a high-temperature heavier-element-dominantcore. From an assumption that there is no chemical reaction between hydrogenand silicates/metals at the atmosphere-interior boundary, the cores ofsub-Neptunes have been modeled with molten silicates and metals (magma) inprevious studies. In large sub-Neptunes, pressure at the atmosphere-magmaboundary can reach tens of gigapascals where hydrogen is a dense liquid. Arecent experiment showed that hydrogen can induce the reduction of Fe$^{2+}$ in(Mg,Fe)O to Fe$^0$ metal at the pressure-temperature conditions relevant to theatmosphere-interior boundary. However, it is unclear if Mg, one of the abundantheavy elements in the planetary interiors, remains oxidized or can be reducedby H. Our experiments in the laser-heated diamond-anvil cell found that heatingof MgO + Fe to 3500-4900 K (close to or above their melting temperatures) in aH medium leads to the formation of Mg$_2$FeH$_6$ and H$_2$O at 8-13 GPa. At26-29 GPa, the behavior of the system changes, and Mg-H in an H fluid andH$_2$O were detected with separate FeH$_x$. The observations indicate thedissociation of the Mg-O bond by H and subsequent production of hydride andwater. Therefore, the atmosphere-magma interaction can lead to a fundamentallydifferent mineralogy for sub-Neptune exoplanets compared with rocky planets.The change in the chemical reaction at the higher pressures can also affect thesize demographics (i.e., "radius cliff") and the atmosphere chemistry ofsub-Neptune exoplanets.

Stability of Hydrides in Sub-Neptune Exoplanets with Thick Hydrogen-Rich Atmospheres