Hydrogen in the Earth's Outer Core From Density Measurements of Liquid Fe‐H up to 102 GPa and 4,100 K

Abstract Among several candidate light elements in the Earth's core, hydrogen may be of particular importance due to its abundance in the universe and siderophile nature at high pressure and temperature ( P‐T ) conditions. However, the hydrogen content in the core remains unclear due to the lack of reliable experimental constraints on the thermoelastic properties of Fe‐H alloys. In this study, we report the density of liquid Fe‐H alloys with various hydrogen contents up to 102 GPa and 4,100 K based on the measured X‐ray diffraction diffuse signals using a diamond‐anvil cell. Combined with previous measurements of the density of liquid pure Fe, we quantitatively evaluated the effect of hydrogen on the density of liquid Fe‐H alloys at high P‐T by using a thermoelastic model. Our results show that hydrogen could greatly decrease the density of liquid Fe‐H alloys but weakly affects its isothermal bulk modulus and the Grüneisen parameter. Models show that liquid Fe with 0.83–0.90 wt% H in the outer core (the maximum value) along an expected geotherm could have densities and bulk sound velocities comparable with seismic observations after considering experimental uncertainties. Although these results indicate that hydrogen could be a major light element in the Earth's outer core, other light elements (Si, O, S, and C) might be further considered in future studies to better interpret geophysical, geochemical, and cosmochemical observations of our planet.