High-Pressure Chemistry Linking Dry and Wet Planets*

It has been believed that water-rich planets are formed outside the snow line. However, discovery of many close-in transiting sub-Neptune exoplanets, if at least some of them are water rich, challenges the notion. Therefore, sub-Neptunes have been believed to be large Earth-like planets with thick hydrogen rich atmosphere and hence dry. We have conducted a series of experiments under the pressure-temperature conditions expected at the boundary between hydrogen-rich atmosphere and rocky interior of a sub-Neptune at the GSECARS sector of Advanced Photon Source. Micro-second pulsed laser heating enables us to melt silicate in a hydrogen medium at high pressures. The reaction was monitored in situ using gated synchrotron X-ray diffraction. The recovered samples were analyzed with Raman spectroscopy and electron microscopy. We found reaction between hydrogen and magma which can unlock oxygen from silicate magma and enable oxygen to react with hydrogen to form water. The chemical reaction can convert a dry hydrogen-rich planet to a water-rich planet inside the snow line. H2/H2O ratio controls the degree of the conversion, explaining diverse mass-radius relations found among sub-Neptune exoplanets. We also found that high pressure promotes mixing between metal and hydrogen and mixing between oxide and water. Such mixing can result in a radial compositional gradient in the interiors of volatile-rich planets, which is different from well-differentiated layered interior structures of rocky planets. This dataset will advance our knowledge on atmosphere-interior interactions in large exoplanets, providing key data for understanding astrophysical measurements of their atmospheres.