Constraining Earth's core composition from inner core nucleation.

Growth of Earth's solid inner core drives the geodynamo, generating a global magnetic field that protects the atmosphere and surface from harmful solar radiation.Yet despite its importance, the origins of the inner core remain enigmatic.Traditional models of inner core evolution ignore the physical requirement that liquids must be supercooled below their melting temperature before freezing.Recent estimates of the supercooling required to homogeneously nucleate the inner core from a variety of iron alloys are unrealistically large and incompatible with the current size and thermal structure of the inner core.Here we show, using molecular dynamics simulations, that nucleation of solids from an Fe 1-x Cx liquid with x = 0.1-0.15reduces the required supercooling to 250-400 K, which is consistent with present-day thermal profiles.Though our required compositions are not a complete description of core chemistry, which requires at least ternary systems, they are consistent with a number of constraints derived from seismology, mineral physics, and geochemistry.Crucially, our demonstration that specific compositions are able to account for the formation of the inner core shows that the nucleation process can provide a new and strong constraint on core composition.The estimated supercooling has implications for the thermal evolution of the core, paleomagnetic signatures of inner core formation, and the seismically observed structure of the inner core.