We will consider the consequences of a Venus mantle enriched in water relative to the Earth. If the Moon-forming impact hypothesis is correct, Earth may have lost substantial volatiles. Venus, however, experienced no such impact and is therefore likely to be relatively more water-rich. We demonstrate that a higher abundance of water in the Venus mantle has the effect of inhibiting the rise of crust-forming melts and volatiles to the surface, which results in a thin stable crust, a dry, stiff upper mantle, and restricted magmatism. Conduction through a shallow thermal lithosphere dominates the removal of heat from the interior, and flow in the mantle is strongly coupled to the surface deformation. Current observations of Venus are consistent with this model. A review of the consequences of this model allows us to make predictions for the Magellan radar images. These include: 1) No evidence for plate tectonic processes; 2) younger apparent surface ages and enhanced crater relaxation for highland areas; 3) limited quantity of hot-spot volcanism, and 4) occasional evidence of localized magmatism (diatremes) associated with a volatile-rich mantle source. The model has implications to the Earth as well: in the absence of the Moon-forming impact, crustal formation in the water-rich Earth mantle may have been arrested, and plate recycling would have ceased early in the planet's history. ¿ American Geophysical Union 1990