The large temperature difference between the core and mantle of Mars at the end of planetary accretion creates a hot, internally convecting thermal boundary layer at the base of the mantle, whose viscosity is several orders of magnitude lower than the viscosity of the mantle above it. Theoretical analysis and numerical simulations of the instability of this thermal boundary layer show that it is likely that only one large plume forms. This superplume may play a role in the formation of crustal dichotomy and generation of the magnetic field in the early history of Mars.