Effects of initial conditions and impacts on the mantle dynamics and dynamo activity on early Mars

Mars has no dynamo-driven global magnetic field today, but strong crustal remanent fields [1] indicate that such a dynamo operated in the past. The disappearance of the magnetic field [2] at the end of a sequence of giant impact basins in the mid-Noachian [3] suggests a causal link between basin-forming impacts and the cessation of the dynamo. It has been proposed that shock heating of the mantle [4] due to basin-forming impacts could reduce heat flux at the core-mantle boundary (CMB) and suppress the core dynamo [5], or more effectively that direct shock heating of the core may lead to core stratification which cripples the core dynamo [6]. Here we suggest that the effects of impacts on Martian internal dynamics are also sensitive to pre-impact conditions in the mantle and to spacing of the impacts. Using 3D finite-element models of convection in a spherical shell [7], we modeled convection in the Martian mantle. After evolving for 500 My, we then simulated the effects of the Utopia impactor at three different points on Mars: over a convective upwelling, over a downwelling, and over a quiescent region. In each case, the CMB heat flux rises above the pre-impact level following the impact, reaches a peak, and subsequently evolves back down. However, the timing of the rise in CMB heat flux is strongly controlled by the impact location. The maximum heat flux at CMB occurs soonest for an impact over the upwelling, and latest over the downwelling. This peak in heat flux is likely due to global mantle convection induced by upwelling of the impact-heated upper mantle, which brings down cooler, near-surface material to the CMB. This overturn is more efficient when the impact heated region coincides with a pre-existing warm region. Our results suggest that the location of an impact with respect to the underlying convective pattern is an important control on subsequent mantle dynamics and core cooling. Heating at the CMB due to a single impact is restricted to a small region (within ~20° of the impact axis [6]). However, the Noachian basins are distributed about the globe, and closely spaced in time [2]. Thus, the individual impact-induced thermal anomalies may build up and blanket a wider area on the CMB. The ability of the mantle to remove impact heating depends on several factors including pre-impact temperature and viscosity of the mantle, and core temperature. These properties are also key in determining the ability of the planet to sustain a dynamo prior to putative perturbation by impacts. Preliminary results suggest that a superheated core (Tc = 2200 K) or a low-viscosity mantle (η0 < 1019 Pa s) is required to sustain dynamo activity in the early Noachian. [1] Acuña, M.H. et al. (2001), JGR 106, 23,403-23,418. [2] Frey, H.V. (2008), GRL 35, L13023. [3] Lillis, R.J. et al. (2008), GRL 35, L14203. [4] Watters, W.A. et al. (2009). JGR 114, E02001. [5] Roberts, J.H. et al. (2009) JGR 114, E04009. [6] Arkani-Hamed, J. and P. Olsen (2010), JGR 115, E07012. [7] Zhong, S. et al. (2010) JGR 105, 11,063-11,082.