Thermophysical States of MgSiO₃ Liquid up to Terapascal Pressures: Implications for Magma Oceans in Super-Earths and Sub-Neptunes

Thermophysical properties of silicate liquids under extreme conditions are critical for understanding the accretion and evolution of super-Earths and sub-Neptunes. The thermal equation of state and viscosity of silicate liquids determine the adiabatic profiles and dynamics of magma oceans. However, these properties are challenging to constrain at elevated pressures in experiments. Here, we perform ab initio molecular dynamics simulations of MgSiO₃ liquid across a wide range of pressures (0–1,200 GPa) and temperatures (2200–14000 K) and analyze its structure, the Grüneisen parameter, and viscosity. Our results reveal the clear temperature and pressure dependence of the Grüneisen parameter, which varies synchronously with the O-O coordination number. The Grüneisen parameter shifts from positive to negative temperature dependence between ∼20 and 70 GPa, corresponding to a peak in the O-O coordination number and SiO₅ abundance. Initially, the Grüneisen parameter increases with pressure and then decreases, showing limited temperature dependence above ∼300 GPa, where its behavior resembles that of solids. Furthermore, we determine the adiabat and viscosity profiles of magma oceans in super-Earths and sub-Neptunes. The results suggest that the mantles of super-Earths and sub-Neptunes may solidify either from the bottom up or at pressures of ∼120–150 GPa, depending on the curvature of the mantle melting line. The low viscosity of magma oceans likely enhances convective currents and facilitate efficient differentiation. These thermophysical properties, now quantified up to terapascal pressures, enable updates to the mass-radius relation of magma ocean exoplanets, showing notable differences compared to their solid counterparts.