HEAT-TRANSPORT EFFICIENCY FOR STAGNANT LID CONVECTION WITH DISLOCATION VISCOSITY - APPLICATION TO MARS AND VENUS

Mantle convection on Mars and Venus is likely to occur in the regime k nown as stagnant lid convection. We perform thermal boundary layer ana lyses as well as finite element simulations of stagnant lid convection with non-Newtonian viscosity (which is believed to be more appropriat e for the lithosphere and upper mantle) and discuss one particular app lication of the results, the efficiency of heat transport on the terre strial planets. As in the case of Newtonian viscosity, the efficiency of heat transfer in the stagnant lid regime is extremely low compared to plate tectonics: For example, in the absence of plate tectonics, th e mantle temperature on Earth, which is already dose to the solidus, w ould be about 700-1500 K higher for the present-day value of the surfa ce heat flux. For Venus, the critical heat flux which can be removed w ithout widespread melting is only 10 - 20 mW/m(2) For Mars, it is 15 - 30 mW/m(2). Therefore, there are no doubts that in the absence of mob ile plates, the mantle temperature would significantly exceed solidus during planetary evolution. It is hypothesized that this could cause o ne, or a combination, of two possible processes: (1) differentiation o f radiogenic isotopes into the crust during early planetary magmatism and (2) initiation of some kind of plate tectonics as a result of plat e weakening due to melting.