ON THE THERMAL EVOLUTION OF VENUS

Several models are calculated in order to assess the effects of differ ent physical parameters on the thermal evolution of Venus. The models are based on three-dimensional thermal convection calculations in an i ncompressible mantle of infinite Prandtl number using a modified Bouss inesq approximation. The mantle is assumed to have a temperature- and pressure-dependent viscosity, temperature-dependent thermal conductivi ty, depth-dependent thermal expansion coefficient, and time-dependent internal heat generation rate. The physical parameters considered are the initial temperature distribution, a possible D''-like layer at the base of the mantle, the temperature at the core/mantle boundary, the core solidification, the decrease of thermal expansion coefficient wit h depth, the rate of internal heat generation, the radially dependent viscosity, and the velocity boundary condition at the surface. A const ant temperature at the core/mantle boundary develops a strong thermal boundary layer at the base of the mantle, resulting in highly oscillat ory mantle convection. Allowing the core to cool suppresses the bounda ry layer and reduces the amplitude of the oscillations substantially. The initial temperature distribution, the core solidification, and a D ''-like layer have minor effects on the overall cooling of the mantle, although the enhanced heat of fusion of the core hampers the cooling of the core. The decrease of the thermal expansion coefficient with de pth lowers the slope of the adiabatic temperature gradient in the mant le and reduces the temperature in the lower part of the mantle and the core appreciably. The heat generation rate has a significant effect o n the present thermal state of the mantle; a higher rate of heat gener ation enhances the mantle temperature. Similarly, a higher mantle visc osity decreases the convection velocities and hampers the heat loss fr om the mantle. However, the most important parameter that controls the thermal evolution of the planet is the velocity boundary condition at the surface. A stress-free (we examined semifree) surface allows mant le material to approach the surface and coal efficiently, whereas a ri gid (we examine semirigid) surface hampers heat loss from the planet, resulting in a hot planet even when the internal heat sources are redu ced by about an order of magnitude.