Two-phase flow in the Earth's core

The solid inner core of the Earth is growing as the Earth cools. Fluid outer core material comprises liquid iron and lighter components, and as the iron crystallizes on the inner core, light material is released and floats upwards stirring the inner core. It is this convective stirring which powers the geodynamo, the source of the Earth’s magnetic field. It has been known for fifty years that this compositional convection is the main energy source for core dynamics, but recent seismological observations indicate that a layer 150 km thick near the inner core boundary (the F layer) is stably stratified.

It is likely that this layer has variable composition and is on the liquidus dividing solid and liquid at the relevant temperature and pressure. The way in which the solid material builds up the inner core and the light material escapes to the bulk of the outer core to drive the geodynamo is not yet understood, though a number of suggestions have been made. The aim would be to build a simple model of this process which is dynamically consistent. Recent advances in high pressure physics mean that the physical properties of core fluid material are much more reliably known, which will help the investigation, and make it timely.