Oxygen Driving Hydrogen Into the Inner Core: Implications for the Earth's Core Composition

Earth's core should contain light elements to account for the density deficit relative to pure iron as inferred from seismic observations. Of particular interest is hydrogen, as planetary accretion models predict the delivery of water possibly sequestered in the core. In this study, we investigate the partitioning of hydrogen across the inner-core boundary using extensive atomistic simulations with machine learning potentials. While showing the tendency of hydrogen to dominantly remain in the liquid phase during inner core solidification, we find that the presence of oxygen would drive more hydrogen into the inner core, where 7 mol% oxygen even reverses the partitioning of hydrogen. By considering such mutual influences of partitioning among light elements in the core, we propose that the inner core can be an important reservoir of primordial hydrogen along with its growth. Plain Language Summary It is well known that Earth's core is composed of iron and nickel along with appreciable light elements such as silicon, oxygen, sulfur, carbon, and hydrogen. Among these candidates, hydrogen stands out given recent evidence for primordial H2O/H in Earth's core. In this study, by using extensive first principles simulations, we systematically investigate the thermodynamics of Fe-H-O systems. The partition coefficient of hydrogen across the inner-core boundary has been quantified for different amounts of oxygen. It is found that the presence of oxygen in the outer core tends to drive more hydrogen into the inner core and even reverse the partitioning of hydrogen. This study thus demonstrates the importance of accounting for the mutual influences among light elements for unraveling the still enigmatic composition of Earth's core as well as how inner core solidification over time may have caused compositional changes that affect the evolution of the core, its seismic properties, and its ability to power the magnetic field.