Mantle convection — the cycling of heat and material in the silicate interiors of planets — dictates many planetary characteristics, from planetary cooling and magnetism to atmospheric composition and habitability. Characterizing the relationship between mantle convection and planetary mass is a key challenge for understanding rocky exoplanets.

Credit: Reproduced with permission from van den Berg et al. (2019), Elsevier

So far, equation-of-state models based on extrapolation from conditions in Earth’s mantle have ignored mineral phase transitions at pressures expected in super-Earths. To overcome this limitation, Arie van den Berg and colleagues developed a new material model based on ab initio calculations of mineral properties. They simulated convection in planets up to 20 Earth masses and identified three convective regimes: below 4 Earth masses, vigorous convection occurs throughout the mantle; from 4 to 12 Earth masses, convection is slow and concentrated in a shallow, mobile zone; and above 12 Earth masses, a mid-mantle layer of high viscosity separates two vigorous-convection zones. In the lower zone, perovskite (MgSiO3) breaks down to Mg and Si oxides, leading to a low viscosity layer above the core–mantle boundary, characterized by small-scale convective structures (see the image).

van den Berg et al. suggest that further research into high-pressure phase changes and lower mantle convective regimes could reveal whether super-Earths have core dynamos capable of generating magnetic fields.