Melting in super-earths

Abstract

We examine the possible extent of melting in rockiron super-earths, focusing on those in the habitable zone.We consider the energetics of accretion and core formation, the timescale of cooling and its dependence on viscosity and partial melting, thermal regulation via the temperature dependence of viscosity, and the melting curves of rock and iron components at the ultra-high pressures characteristic of superearths. We find that the efficiency of kinetic energy deposition during accretion increases with planetary mass; considering the likely role of giant impacts and core formation, we find that super-earths probably complete their accretionary phase in an entirely molten state. Considerations of thermal regulation lead us to propose model temperature profiles of super-earths that are controlled by silicatemelting.We estimate melting curves of iron and rock components up to the extreme pressures characteristic of superearth interiors based on existing experimental and ab initio results and scaling laws. We construct superearth thermal models by solving the equations ofmass conservation and hydrostatic equilibrium, together with equations of state of rock and iron components. We set the potential temperature at the core-mantle boundary and at the surface to the local silicate melting temperature. We find that ancient (̃4Gyr) super-earths may be partially molten at the top and bottom of their mantles, and that mantle convection is sufficiently vigorous to sustain dynamo action over the whole range of super-earth masses. © 2014 The Author(s) Published by the Royal Society.