Hydrogen May Be Flowing Through the Solid Iron of Earth’s Inner Core Like a Liquid Through a Sponge

Earth's core is mostly iron, but it cannot be only iron. Seismic measurements put the core's density several percent below that of pure iron at the relevant pressures, which means something lighter is mixed in. Hydrogen, the lightest and most abundant element in the universe, has been a leading suspect for decades — and a frustrating one, because different simulations have disagreed about how much of it the core can hold and where it would sit.

A study published in the Proceedings of the National Academy of Sciences by Zepeng Wu and colleagues attacks the problem with quantum-mechanical simulations of iron and hydrogen under core conditions, and comes back with a specific picture. Hydrogen is not spread evenly. At roughly 5,500 kelvin, its concentration runs about 16 atomic percent at the boundary between the solid inner core and the liquid outer core, and falls to about 9 percent at the very center of the planet.

That gradient is not an accident of how the core assembled. It is what thermodynamic equilibrium demands: hydrogen partitions between the solid and the liquid according to local conditions, and the equilibrium point shifts with depth. The controlling variable, the authors find, is not pressure on its own but temperature measured against iron's melting point at that depth — a distinction that explains why earlier calculations, each using different assumptions about the melting curve, produced results that could not be reconciled.

The stranger result concerns how the hydrogen behaves once it is there. In hexagonal close-packed iron, the crystal structure expected in the inner core, hydrogen can enter a superionic state: the iron atoms hold their lattice positions while the hydrogen atoms migrate through the gaps between them, diffusing like a fluid inside a solid. "Our calculations show that hydrogen can stabilize a superionic bcc phase at sufficiently high temperature and hydrogen content," the authors write, pointing to a second crystal structure that hydrogen itself helps hold together.

Superionic phases are not exotic in the abstract — they have been proposed for the water-ice mantles of Uranus and Neptune, where oxygen holds a lattice and protons flow through it. Finding the same physics in iron at the center of our own planet moves the idea from the outer solar system to the ground beneath everyone's feet.

The consequences reach into problems geophysicists care about for other reasons. A mobile species inside the inner core affects how the core conducts heat and electricity, which feeds directly into the dynamo that generates Earth's magnetic field, and a compositional gradient across the inner core offers a candidate explanation for the seismic anisotropy that has puzzled researchers for years. The paper also proposes a scaling relation that reconciles the earlier conflicting simulations with laboratory experiments, which may matter more in the long run than any single number it reports.