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 silicate melting. 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 of mass 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 (∼4 Gyr) 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.

Spin crossover in liquid (Mg,Fe)O at extreme conditions

“…This is followed by another 3 ps of simulation, over which all physical time averages are computed. These temperatures and pressures encompass typical conditions in newly accreted terrestial planets as well as prevailing conditions inside super-Earths [2].…”

“…The role of (Mg,Fe)O and other liquid oxides as a source of dynamo action in Earth is likely historical, and may be preserved in the geological record, providing a test of the existence of a magma ocean. Magma oceans may persist inside super-Earths for much longer times [2], granting a plausible and generic mechanism for magnetic-field generation in these large, extra-solar bodies.…”

“…Planets with an Earth-like, rock-dominated composition tend to be partially or entirely molten at the early stages of their evolution [1,2]. This is due to the large amounts of energy deposited into the early planet during the accretion process through which the body is formed.…”

A Simple Analytical Model for Rocky Planet Interiors

“…Due to a feedback mechanism of silicate melting, there is good reason to set T mp ≈ 1600 K for Earth and super-Earths for a first approximation (Stixrude 2014). Then the temperature profiles inside these planets can be estimated using formulae in Stixrude (2014).…”

Characterizing exoplanets

“…So Lars Stixrude [22] has carried out studies, based on scaling from known silicate and iron melting curves, combined with ab initio calculations and experimental data, to determine the effects of accretion on these bodies in their early stages of evolution. This leads to the conclusion that super-Earths will be at least partially molten at the top and bottom of their mantles, and that dynamo actions are likely for all of these planets found so far.…”