The Effect of a Low Viscosity Layer on Convection in the Mantle

摘要

If thermal convection occurs in the Earth's mantle, it may also occur within the Moon and Mars. The dimensions of these planets are comparable to the thickness of the Earth's mantle. Presumably the amount of radioactive heat generated per unit mass is similar in all three bodies. However, the surface morphology of the Earth differs markedly from that of the Moon or Mars. The explanation for the difference is based on the effect produced on convection in the mantle by the presence of a low 'viscosity' or low creep strength layer. A low viscosity layer changes the amount of 'coupling' between the outer crust and mantle convection. The theory assumes that the viscosity or creep strength is essentially zero in the low viscosity layer. The analysis is similar to that developed earlier for the calculation of stresses within the mantle. The deeper the low viscosity layer lies within the mantle, the greater is the coupling of the outer crust of the mantle convection currents. If the low creep strength layer lies close to the surface the outer crust is decoupled from the interior. According to the literature, the depth of the low velocity layer is determined by the temperature and pressure profiles within the mantle. (Author)