Impact origin for the Martian crustal dichotomy: Half emptied or half filled?

One possible mechanism for Martian hemispherical dichotomy formation is excavation of the northern lowland basin in the existing crust by a single giant impact. Here we explore a related scenario in which the impact melt volume is sufficient to fill the excavated cavity, produce a thickened crust, and generate a topographic basin centered on the opposite side of the planet. We investigate this mechanism by formulating a model for viscous spreading of a partially molten layer over a spherical surface. We derive numerical solutions and calculate the final area of the spreading layer as a function of impactor size and shape of the layer boundary due to an initially asymmetric melt region. Results are compared with observations, including lowland basin size, shape of the dichotomy boundary, age difference between crustal provinces, and crustal thickness.