Nickel reduces the undercooling needed for Earth's inner core nucleation.

Earth's solid inner core froze out of a molten outer core, but scientists face a puzzle: normal models require the liquid to cool down by an unrealistically huge amount before it can start freezing into a solid crystal. This freezing start is called nucleation. Researchers simulated the atoms of iron and nickel under inner-core conditions to see how the metals behave. They found that solid iron-nickel prefers to start forming a crystal structure called bcc rather than hcp. The bcc structure needs smaller starter crystals and less energy to form. Adding more nickel, plus natural patchy mixes in the liquid, lowers the barrier even more. Together, these effects let crystals freeze at about 250 K of undercooling, nearly matching real-world limits.

Why it matters

This work addresses a major paradox in Earth science: how the solid inner core could have formed without extreme undercooling. By showing that nickel and chemical fluctuations ease nucleation, it brings theoretical predictions closer to geophysical observations. This helps refine models of Earth's deep interior and its evolution.

Questions answered