Super-Earth Internal Structures and Initial Thermal States

Abstract

The presence of a planetary magnetic field is an important ingredient for habitability. The coexistence of a solid and a liquid core can facilitate the maintenance of a compositionally driven dynamo; however, the likelihood of such a configuration in super-Earths is unknown. Recently, shock experiments and ab initio calculations have constrained the stability, equations of state, and melting properties of ultrahigh pressure core and mantle phases. Here, we investigate the internal structure of super-Earth exoplanets with a range of total masses and core mass fractions ranging from that of Mars (0.2) to Mercury (0.68), including an Earth-like bulk composition. We examine the effect of the initial core-mantle boundary temperature (TCMB) on their internal structure and identify regimes with coexisting solid and liquid cores, and deep mantle melting. We find that the range of TCMB for which an inner core is growing increases with the total planet mass and even more with the core mass fraction. Therefore, our results suggest that super-Earths should have a crystallizing core over a large temperature range. We also find that the presence of a growing inner core is likely to be accompanied by a partially liquid lower mantle, which will likely influence planetary thermal evolution. We estimate the initial CMB temperature after super-Earth accretion by assuming an accretional heat retention efficiency similar to Earth. We find that massive super-Earths are expected to have an initial internal temperature consistent with a partially liquid core, allowing for the possibility of thermal and compositional dynamo action.