Constraining the Composition of Earth’s Core: Insights from Nucleation in FeC Liquids

The growth of Earth's solid inner core powers the geodynamo in the liquid outer core, creating a global magnetic field that helps to shield the planet from harmful solar radiation. However, the origins of the inner core are still not fully understood. Traditional models of core evolution overlook the necessity for liquids to be supercooled below their melting point before freezing. Recent estimates of the required supercooling for the inner core's homogeneous nucleation are unrealistically high and conflict with the expected current thermal structure of the core. Through molecular dynamics simulations, we show that nucleation from an Fe_1-xC_x liquid, with x=0.1-0.15, reduces the supercooling requirement to 250-400 K, broadly compatible with expected current thermal profiles of the core. Though these 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 results demonstrate that whilst some potential compositions of the core cannot explain the presence of the inner core, others can. The nucleation process of the inner core can therefore provide a new and strong constraint on core composition.