COMPRESSIBLE CONVECTION IN A VISCOUS VENUSIAN MANTLE

Finite element simulations of axisymmetric spherical shell compressible convection were carried out to investigate the effect of various surface boundary conditions in a Venusian mantle. We employed a thermal expansivity-alpha which decreased with depth, a uniform viscosity an order of magnitude greater than the Earth's, and zero and chondritic quantities of internal heating. As long as hot plumes from the core-mantle boundary were strong, the convection pattern was typical of that for variable alpha-flow; that is, it was characterized by steady upflowing regions, unsteady collections of downflowing plumes, and large aspect ratio cells. Increases in the internal heating or the temperature T0 at the top of the convecting layer weakened the hot plumes and therefore decreased the width of the cells. A rigid surface increased the internal temperature and also decreased the width of convection cells. Extensive regions of subadiabaticity were found in the mantle. We compare our results with those for fully three-dimensional convection under similar conditions (Schubert et al., 1990).