Iron-snow in Mercury's core

Convection within Earth's fluid core is likely powered (in part) by crystallization of iron-rich material at the inner core boundary, leading to buoyant upwellings of iron-depleted fluid and thermal buoyancy from the release of latent heat. However, depending on a planet's size and core composition, crystallization may occur instead in the volume of its fluid core, in the form of iron-rich snow. This is the case if the gradient of the melting temperature of the metallic alloy of the core with depth is less steep than the adiabatic temperature gradient (Figure 1).

In a series of papers, we have recently investigated which of these crystallization regimes most likely applies in Mercury's core at present day. This involved building interior models of Mercury to track the variation in density, pressure and temperature as a function of depth, as well as the melting temperature of the iron alloy in its liquid core. We then constrained these models to match the geodetic observations on the moments of inertia of Mercury.

However, many dynamical questions associated with this regime remain to be investigated. Iron-snow formation should be accompanied by the development of a stably stratified compositional gradient. Convection can still occur in the form of double-diffusive convection (or semi-convection), though whether a magnetic field can be sustained in such a regime (and what morphology it would take) are questions that we are currently investigating.

Figure 1: Schematics for 2 different crystallization regimes. (a) Earth-like regime. Cooling leads to first crossing of the liquidus at the center. An inner core grows as cooling proceeds. (b) Snow regime. Cooling leads to first crossing of the liquidus near the bottom of the thermally stratified region. Fe-rich snow forms, sinks under gravity; buoyant light elements (S) float upward. Cooling leads to an extension of the snow formation to greater depth, accompanied by the formation of a stable compositional gradient. From Dumberry and Rivoldini, Icarus, 2015.

Paper on this topic