Effects of adiabatic compression on thermal convection in super-Earths of various sizes

We present two-dimensional numerical models of thermal convection of a compressible fluid in the mantles of super-Earths calculated under the truncated anelastic liquid approximation to discuss how adiabatic compression affects the thermal convection, depending on planetary mass. The convection is driven by basal heating, the viscosity depends on temperature, and the thermal expansivity and the reference density depend on the depth. We varied all of the magnitude of adiabatic heating, the Rayleigh number, the depth profile of the thermal expansivity, and that of the reference density in accordance with the planetary mass. The effects on thermal convection become substantial, when the planetary mass normalized by the Earth’s mass M p exceeds a threshold M c , about 4. Hot plumes ascending from the core–mantle boundary become thinner with increasing M p ; they become almost invisible except around the core–mantle boundary, when M p > M c . The lithosphere that develops along the surface boundary due to the temperature dependence of viscosity becomes thicker with increasing M p and is about twice as thick as that at M p = 1 when M p = 9.4. The convective velocity is almost independent of M p . These results are in a striking contrast with earlier predictions that are made based on the models where the effects of adiabatic compression are neglected; it is important to take account of the effects of adiabatic compression properly in the exploration of mantle dynamics such as plate tectonics and hot spot volcanisms in massive super-Earths. Further researches are necessary to clarify the dependence of M c on the surface temperature and the material properties of the convecting mantle.