The influences of surface temperature on upwellings in planetary convection with phase transitions

The importance of surface temperature for mantle convection appears with the presence of adiabatic heating and cooling and the release and consumption of latent heat in the presence of phase transitions. For some planetary bodies these effects cannot be neglected. The dimensionless surface temperature T₀, which is the ratio between the temperature at the top of the convective region and the temperature drop across the mantle, is close to one for Mars and Venus. For the Earth, T₀ lies between 0.2 and 0.5. The dynamical influence of T₀ is especially poignant for internally heated convection with temperature-dependent viscosity. There is a tight coupling between the magnitude of the temperature field and the viscosity itself. We have studied temperature-dependent viscosity convection for both low-T₀ (0.2) and high-T₀ (1.2) situations and with internal heating in mantle convection with two upper-mantle phase transitions. Our results show that within this range of T₀ there exist two regimes for the evolution of upwellings in the mantle. In transient situations plume-plume collisions lead to the formation of megaplumes for high-T₀ regimes but are less likely to do so for low T₀. In the long-term regime, plumes with low T₀ are prone to develop from the transition zone with a supply of hot material coming from the shallow lower mantle. In systems with high T₀, however, long-lived plumes tend to have deeper mantle origins. In quasi-layered situations high T₀ may act as a positive feed-back mechanism in inducing powerful hot upwellings into the upper mantle.