Penetrative Convection in Super-Earth Planets: Consequences of MgSiO_3 Postperovskite Dissociation Transition and Implications for Super-Earth GJ 876 d

摘要

Theoretical studies suggest that MgSiO_3 postperovskite dissociates into MgO and MgSi2O5 at 0.9 TPa, and MgSi_2O_5 dissociates into MgO and SiO_2 at 2.1 TPa, both with negative Clapeyron slopes. In addition, unlike a conventional view, the viscosity in super-Earth planets is proposed to decrease with pressure when pressure exceeds ~0.1 TPa. Employing 2-D-axisymmtric and 3-D-spherical control volume compressible models, we perform an investigation on the impact of deep mantle dissociation of postperovskite into oxides, in conjunction with variations of the mantle viscosity, on the degree of the deep mantle layering, and the consequences of this layering on cooling of the rocky planets similar to super-Earth GJ 876 d. Small-scale convection in a layer of thickness ~500 km from which penetrative plumes originate, develops above the core-mantle boundary (CMB) in the models for which the viscosity at the dissociation transition depth and below is less than ~10~(23) Pa·s. Due to the buffering effect of this deep mantle layering, while the mean mantle temperature above the layer decreases, resulting in a viscosity increase above the layer, it increases in the layered region above the CMB. This leads to a further reduction in viscosity at the bottom of the mantle. The effect is enhanced with increasing the CMB temperature and the contrast in thermal expansivity. The cooling rate of the planet decreases in the layered models due to the buffering effect of this deep mantle transition, as well as the influence of the viscosity increase above transition depth.