Mass-dependent dynamics of terrestrial exoplanets using ab initio mineral properties

We present new modelling results for the internal structure and convective dynamics of large terrestrial (rocky) exoplanets. By going out to 20 Earth masses our results show that pressure and temperature can reach several Terapascal (TPa) and 10000 K respectively in the silicate mantle of these planets. Ab initio calculations have predicted that the main constituent magnesium-silicate mineral of the Earth's mantle can fully dissociate, in a stepwise fashion, into the oxides SiO₂ and MgO under these P,T conditions (Umemoto et al., 2017). Based on calculated properties of the new phases and phase transitions by Umemoto et al. (2017) we have modelled the internal structure of large terrestrial planets with an Earth-like core mass fraction of 0.3 and one to twenty times the Earth's mass. We found that full dissociation into oxides occurs inside planets that are more massive than thirteen Earth masses when pressure at the core mantle boundary exceeds ∼ 2.4 TPa. Our modelling results of Rayleigh–Benard mantle convection for exo-planets with mass values within the range 1–20 Earth masses show strong differences in the internal structure and the convection dynamics between the different cases. First, due to the increasing internal pressure the number of phase transitions increases from zero in the smallest case to four transitions, with five layers of different mineral associations, for the larger planet cases > 13M_⊕. The bottom layer of the latter case corresponds to the layer of oxides. Furthermore, from the modelling results we also observe three regimes of convective dynamics, with: (1) smaller planets (< ∼ 4M_⊕), showing vigorous convection, (2) intermediate cases (< ∼ 12M_⊕), with sluggish penetrative convection, concentred in a single shallow mantle zone of higher flow velocity, and (3) large planets, (> ∼ 12M_⊕), showing vigorous convection in two zones near the top and bottom, separated by a high viscosity mid-mantle region mantle with sluggish convection. These convective regimes are directly related to the pressure effect on mantle viscosity, first increasing followed by a decrease due to pressure weakening. Here the mass of the planet is the control variable because it sets the mantle pressure range. For the larger planet cases including a bottom layer of oxides, weakened by the reduced viscosity, we observe vigorous convection and small scale structure in the deepest part of the mantle that interacts with the dissociation phase boundary. The third of the above convective regimes shows the important role played by penetrative convection in the interior of super-earths, as there are two actively convecting layers at the top and bottom, driving a slowly moving interior layer. This impacts the heat-flux from the core and the viability of core dynamo processes.