PHASE-TRANSITIONS IN THE MARTIAN MANTLE - IMPLICATIONS FOR PARTIALLY LAYERED CONVECTION

Numerical simulations of mantle convection in Mars, using an axisymmet ric spherical-shell model, show partial layering caused by the two exo thermic olivine-spinel (alpha-beta, beta-gamma) phase transitions. An extended Boussinesq approximation has been used in which viscous dissi pation, adiabatic heating and cooling, and latent heat are included, T he Rayleigh number (Ra) has been varied between 5 X 10(5) and 10(8). T he partial layering with the vertical velocity at the exothermic phase transitions varying strongly in space and time is the result of two o pposing effects: the enhanced buoyancy of the phase boundaries by ther mal anomalies and the impeding influences from the latent heat release (or consumption). The effect of the latent heat is stronger in Mars t han the Earth because of the comparatively low pressure gradient in th e Martian mantle and the smaller excess temperature of upwellings and downwellings. The time-series of the mean vertical mass transport acro ss the phase transitions show oscillations between blocking and accele ration of the flow. The amplitude and the oscillations in the time-ser ies increase with increasing Ra. Because of the partial layering, the planet will cool more slowly and less uniformly than suggested by ther mal evolution models with parameterized convection, In addition, the n umber of strong mantle plumes is reduced to only a few upwellings. Suc h a pattern is suggested for Mars by the existence of two pronounced v olcanic centers, Tharsis and Elysium. This could also cause a strong t ime dependence in the Martian volcanic activity. The latent heat relea se causes the mantle temperature to increase across each transition by about 50 K and produces a hot lower mantle and a liquid core, We have tested the case of a 85-350 km thick perovskite layer at the core-man tle boundary. A layer thicker than about 300 km would convect separate ly, and induce leaking to the mantle above at a significantly smaller rate compared to the layers induced by the olivine-spinel phase bounda ries. For a perovskite layer smaller than about 300 km, the convective vigor near the core-mantle boundary decreases with the layer thicknes s.