Deep beneath our feet, at the very center of Earth, lies a solid iron core. But it's not pure iron — there are lighter elements too, such as hydrogen. Exactly how hydrogen is distributed within the core has long been a mystery.
Scientists built a computer model of hydrogen behavior in iron under the gigantic pressures and temperatures found in the real core. They found that if you take the core temperature and divide it by the melting temperature of pure iron, this ratio controls how much hydrogen ends up in the solid part versus the liquid part. Pressure has little effect. This simple rule reconciled previous calculations that gave different numbers.
It turns out that closer to the center of the core, there should be more hydrogen, and less toward its edge. This is not a coincidence but a consequence of equilibrium: at those temperatures and pressures, it is energetically favorable for hydrogen to gather in the depths. The same mechanism likely works for other light elements in the core.
If all the iron from Earth's core were spread on the surface, it would cover the planet with a layer about 2 meters thick.