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

Abstract

Numerical simulations of mantle convection in Mars, using an axisymmetric spherical-shell model, show partial layering caused by the two exothermic olivine-spinel (α-β, β-γ) phase transitions. An extended Boussinesq approximation has been used in which viscous dissipation, adiabatic heating and cooling, and latent heat are included. The Rayleigh number (Ra) has been varied between 5 x 105 and 108. The partial layering with the vertical velocity at the exothermic phase transitions varying strongly in space and time is the result of two opposing effects: the enhanced buoyancy of the phase boundaries by thermal anomalies and the impeding influences from the latent heat release (or consumption). The effect of the latent heat is stronger in Mars than the Earth because of the comparatively low pressure gradient in the Martian mantle and the smaller excess temperature of upwellings and downwellings. The time-series of the mean vertical mass transport across the phase transitions show oscillations between blocking and acceleration of the flow. The amplitude and the oscillations in the time-series increase with increasing Ra. Because of the partial layering, the planet will cool more slowly and less uniformly than suggested by thermal evolution models with parameterized convection. In addition, the number of strong mantle plumes is reduced to only a few upwellings. Such a pattern is suggested for Mars by the existence of two pronounced volcanic centers, Tharsis and Elysium. This could also cause a strong time dependence in the Martian volcanic activity. The latent heat release 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-mantle boundary. A layer thicker than about 300 km would convect separately, and induce leaking to the mantle above at a significantly smaller rate compared to the layers induced by the olivine-spinel phase boundaries. For a perovskite layer smaller than about 300 km, the convective vigor near the core-mantle boundary decreases with the layer thickness.