  It has long been assumed the Earth's solid inner core started to grow when
molten iron cooled to its melting point. However, the nucleation mechanism,
which is a necessary step of crystallization, has not been well understood.
Recent studies found it requires an unrealistic degree of undercooling to
nucleate the stable hexagonal close-packed (hcp) phase of iron, which can never
be reached under the actual Earth's core conditions. This contradiction leads
to the inner core nucleation paradox [1]. Here, using a persistent-embryo
method and molecular dynamics simulations, we demonstrate that the metastable
body-centered cubic (bcc) phase of iron has a much higher nucleation rate than
the hcp phase under inner-core conditions. Thus, the bcc nucleation is likely
to be the first step of inner core formation instead of direct nucleation of
the hcp phase. This mechanism reduces the required undercooling of iron
nucleation, which provides a key factor to solve the inner-core nucleation
paradox. The two-step nucleation scenario of the inner core also opens a new
avenue for understanding the structure and anisotropy of the present inner
core.
