Martian Dichotomy: Impact-induced Crustal Production in Mantle Convection Models

The Martian crustal dichotomy features a ~5.5 km contrast in topography and ~25 km difference in crustal thickness between the northern and southern hemispheres, and is thought to have formed within the first 400-500 Myr of the planets history. Among the different theories which invoke an endogenic and/or impact origin for the dichotomy, Golabek (2011) [1] and Reese & Solomatov (2011) [2] suggested that a giant impact induces melt and hence crustal production, which creates the highlands in the southern hemisphere. On the other hand, it is shown that the Martian dynamo was active at 4.5 and 3.7 Ga [3]. While it has been suggested that early tectonics could facilitate the presence of a dynamo [4], it is not clear whether the dichotomy-associated crustal processes have such effect on the core. In this study, we explore the potential linkage between the crustal dichotomy formation and dynamo activity. We use the mantle convection code StagYY to simulate the long-term thermochemical evolution subsequent to a giant impact on Mars in 2D spherical annulus geometry. The impact is parametrized as a thermal anomaly at the start of simulation, and the resultant magma pond is treated with an enhanced thermal conductivity to account for the heat loss through turbulent flow. We systematically vary parameters that affect the interaction between the magma pond and surrounding solid mantle, including the impactor size, reference viscosity and magma thermal conductivity. Specifically, we look at the CMB heat flux as a criterion for a dynamo. Our preliminary results demonstrate that a giant impact induces crust production on the impacted hemisphere, and delamination of this crust can increase the heat flux at core mantle boundary on the same hemisphere, favoring a hemispherical dynamo. [1] Golabek et al. (2011) [2] Reese & Solomatov (2011) [3] Mittelholz et al. (2020) [4] Nimmo & Stevenson (2000)