Convection in the Moon: Effect of variable viscosity

Previous tests of the lunar interior for stability to thermal convection have been limited to consideration of constant viscosity models. Since diffusion creep viscosity is an exponential function of temperature and can vary within the moon by many orders of magnitude, the effects of variable viscosity must be considered. This work determines stability criteria for the case of variable viscosity. The conditions necessary for solid convection to play an important role in the moon's evolution are determined by application of the calculated stability criteria to a variety of thermal history models. Most thermal history temperature profiles would be unstable to solid convection at the present time if currently used estimates of the viscosity function are applicable. A possible explanation of lunar seismic data (Nakamura et al., 1973) is that temperatures near the melting point of silicates exist at depths greater than 800 km. If this is the case, either the lunar interior is stable, or solid convection is occurring at a rate insufficient to cool the lunar interior substantially. If the former is true, the viscosity of lunar material is higher than that usually assumed.