Coupled Core Cooling and Mantle Dynamics on Mars

Several giant impact basins of mid-Noachian age have been identified on Mars, and the global magnetic field appears to have vanished at about the same time. The impacts that formed these basins delivered a large amount of heat to the planetary interior. Such impact heating may have modified the pattern of mantle convection, and suppressed core cooling, contributing to the cessation of dynamo activity. Here we investigate the thermal evolution of Mars in response to the largest basin-forming impacts, using coupled models of mantle convection and parametrized core cooling. The chief difficulty in full coupling of 3D mantle convection models to core dynamo models is that relevant timescales are quite different. Most studies therefore are of either the core or the mantle, and treat the other layer as a boundary condition. Here, we consider the heating by large impacts, which instantaneously change the temperature structure in the core and mantle, and the thermal coupling of the core and mantle while both are simultaneously cooling. We model convection in the mantle using the finite element code Citcom in 2D axisymmetric geometry, appropriate for a single vertical impact scenario. At the time of an impact we introduce a temperature perturbation resulting from shock heating into the core and mantle layers. Because lateral mixing and stratification of the core occurs very quickly compared to mantle dynamics, we assume the core becomes stratified and its temperature varies only radially. At a given timestep, we fix the mantle temperature and solve the 1D enthalpy equation in the core and lower thermal boundary layer of the mantle over a time corresponding to a mantle timestep. We then update the temperature at the core-mantle boundary (CMB) and mantle boundary layer, and let the mantle convection progress for another timestep. We continue this iteration until the core temperature becomes almost adiabatic and the entire core is convecting. Mantle convection then proceeds as before. Preliminary models have been performed in 2D to speed computation times. We have imposed the heating due to a 1000 km diameter rocky projectile impacting Mars at 10 km/s. In the mantle, the impact heating generates a strong hemispheric upwelling, which quickly spreads into a warm layer beneath the stagnant lid. In the stratified core, the outermost layers are strongly heated. This acts as a thermal "blanket" that prevents cooling of the interior, and shuts down core convection, although the CMB heat flux actually rises as this layer cools into the mantle. While the thermal blanket in the outermost core disappears relatively quickly, the core does not return to a fully convective state for ~0.5 Gy following the impact. Our results suggest that a pre-existing core dynamo would have been crippled for at least a similar time-scale. Theoretically, the dynamo could restart once core temperature became adiabatic. If the pre-impact dynamo is subcritical or weakly supercritical, however, the CMB heat flow will be insufficient for such a restart, resulting in the permanent disappearance of the global magnetic field.