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Title: How did the inner core nucleate without implausible supercooling?
Plain: 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: 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.
Settled by: Molecular dynamics or free-energy calculations of nucleation rates for realistic Fe alloys at inner-core-boundary pressure, reproducing an allowed supercooling.
Kind: well-posed
Answer type: mechanism
References: (empty)
Literature status: open
Status note: 2021 to 2026 simulations propose two-step bcc nucleation and alloying effects (e.g. arXiv:2502.19763, arXiv:2608.27139); no accepted resolution.
Posed since: 2018