Planetary controls on magma ocean crystallisation, re-melting and overturn

In their hot initial phase, rocky planetary bodies undergo a magma ocean (MO) stage. Crystallisation of this magma ocean sets the initial structure of planetary mantles, and thus determines the early stages, and long term evolution, of solid-state mantle convection, thus regulating the litrhospheric tectonic, core convection and associated magnetic field. This major planetary differentiation process also controls the outgassing of the primary atmosphere, and therefore the long-term surface evolution and habitability. While several studies have addressed this crystallisation process from a mass-balance or a dynamical point of view, few have studied remelting of the convecting solid mantle while a magma ocean was still present. We here present spherical annulus numerical calculations of mantle convection and melting under a magma ocean to address the role of heterogeneity and dynamic recrystallisation on remelting and differentiation. Results indicate that the parameters that typically impact mantle convection (viscosity, density anomaly, etc) also impact the differentiation of the magma ocean. In particular, dynamic topography has a great influence on the composition of the magma ocean and its differentiation, as it conditions both, excess melting above upwellings (e.g. Figure 1) and excess crystallisation above downwellings. These topography effects are greater the closest the system is to a magma ocean overturn. Our findings can help to understand the differences between solar system bodies, such as the presence or absence of basal magma oceans in terrestrial bodies, or to predict the convective evolution of rocky exoplanets.

Figure 1: Effects of different MO density on mantle upwellings, the greater topography due to higher density of the MO  causes increased excess melting.