How did the inner core nucleate without implausible supercooling?
In plain words
For the first crystal of solid iron to form at Earth's center, the liquid must cool well below its freezing point. The cooling required appears far larger than the core ever experienced.
Precise statement
Classical homogeneous nucleation of hcp Fe at the center requires supercooling of order 1000 K, whereas thermal histories permit at most a few hundred K. Identify a pathway (metastable bcc two-step nucleation, Ni, C, Si or O effects on the liquid, heterogeneous seeds) that yields at least one critical nucleus in the core volume within about 1 Gyr at supercooling consistent with thermal models. An answer is a computed nucleation rate versus supercooling for a realistic core composition.
What would settle it
Molecular dynamics or free-energy calculations of nucleation rates for realistic Fe alloys at inner-core-boundary pressure, reproducing an allowed supercooling.
Status in the literature
Unverified note
2021 to 2026 simulations propose two-step bcc nucleation and alloying effects (e.g. arXiv:2502.19763, arXiv:2608.27139); no accepted resolution.