Hydrogen in the Earth core inferred from neutron imaging and diffraction

The density of the Earth’s core is lower than that of pure iron; this is considered to be caused by the presence of light elements in the core. Hydrogen is one of the most important light elements in the Earth’s core because of its high cosmochemical abundance and its nature as a siderophile element under high pressure. Thus, the hydrogen content in liquid iron under high pressure is required to constrain the chemical composition of the Earth’s core. However, this value has been estimated based on the observation of quench products, and the true hydrogen content in liquid iron remains unclear. Here, we performed high-pressure and high-temperature neutron diffraction and imaging experiments in situ to determine the hydrogen content in liquid iron. We observed that liquid iron contains 0.17(3) wt.% H at 3.4 GPa and 1400 K, indicating that liquid iron is hydrogenated in the magma ocean during core formation. For the hydrogen content in the liquid iron at the base of the magma ocean, we estimated that the outer and inner cores contain 0.60–0.72 and 0.30–0.44 wt.% H, corresponding to 70–85 and 1.9–2.7 times the mass of hydrogen in the ocean, respectively. This suggests that hydrogen can contribute more than half of the density deficit in the outer core. For the magma ocean equilibrating with the hydrogen-rich primary atmosphere, the study findings show that liquid iron plays a crucial role in transporting a large amount of hydrogen into the core.