Phase transitions and the three‐dimensional planform of thermal convection in the Martian mantle

Abstract

Previous convection models for Earth have shown that the endothermic phase transition from spinel to perovskite plus magnesiowüstite at 660 km depth has a strong effect on mantle convection. For Mars, the depth of the main phase transitions is estimated using a set of 32,000 density models with randomly perturbed input parameters. It is found that, on average, the depth of the spinel to perovskite transformation is 1910 km. Thus a perovskite layer exists in the Martian mantle only if the core radius is less than 0.45 of the planetary radius. Recent results on the density of FeS at pressure and temperature conditions relevant to the Martiam core indicate that such a small core size is consistent with the cosmochemical estimate of around 14% sulphur content of the core. Numerical simulations of three‐dimensional spherical convection demonstrate that the presence of the endothermic phase transition close to the core‐mantle boundary is the crucial factor determining the style of convection. In addition to the endothermic phase boundary, the simulations assume an isoviscous mantle, which is heated both from below and within, a rigid upper boundary, and Rayleigh numbers of the order of 106. Under these conditions, it is shown that the natural planform of mantle convection exhibits only one or two strong mantle plumes. The number of plumes decreases with time during planetary evolution. For high Rayleigh numbers, the plumes start to oscillate about a mean position and pulsate in time. Strongly localized mantle upwellings are obtained even when the proportion of basal heating is only 10%. These results are robust in the sense that they do not critically depend on the parameters of the phase transition, the Rayleigh number, or the rate of basal heating. This planform of mantle convection is consistent with the volcanic history of Mars, where early volcanism was widespread, but later concentrated in two provinces, Tharsis and Elysium, and finally in Tharsis alone.