Earth's inner core nucleation paradox

The conventional view of Earth's inner core is that it began to crystallize at Earth's center when the temperature dropped below the melting point of the iron alloy, and has grown steadily since that time as the core continued to cool. However, this model neglects the nucleation energy barrier, which is typically represented in terms of the critical supercooling required to stabilize a crystal nucleus. We show that the critical supercooling in a homogeneous liquid iron alloy is of order 1000 K, which is far too large to be a plausible mechanism for the origin of Earth's inner core. Instead, growth may have been facilitated by a solid substrate that lowered the energy barrier to nucleation. Internal mechanisms for producing such a substrate do not appear to be feasible, and hence we suggest that it dropped from the mantle, most likely as a nugget of solid iron-nickel alloy. Its delivery to Earth's center, either before or after this region had cooled to the saturation point, would profoundly alter the thermal and dynamic evolution of the core.