Question

### Models of the Martian Crustal Dichotomy

Mars's topography is divided into two distinct regions: the flat northern lowlands, which have a thin crust, and the cratered southern highlands, which have a thick crust. Scientists have proposed two models to explain the origin of this dichotomy.

Model 1
The dichotomy was formed by internal geological processes. Early in Mars's history, a single massive upwelling mantle plume developed beneath the southern hemisphere, accompanied by a corresponding downwelling in the northern hemisphere. This convection pattern caused crustal thinning in the north due to partial melting and lithospheric recycling, while volcanic material accumulated in the south, thickening the southern crust. This internal redistribution of heat and mass occurred gradually over approximately $100$ to $300$ million years.

Model 2
The dichotomy was created by an external event. An oblique impact by a large planetary body (approximately $1,600$ to $2,700$ kilometers in diameter) struck Mars’s northern hemisphere during the late stage of Mars's accretion. The energy from this giant impact melted and excavated the existing northern crust, forming the Borealis Basin. The southern highlands represent the pre-impact crust that remained largely unaffected. The initial excavation of the basin and removal of the northern crust occurred within a few hours of the impact.

Based on the models, Model 1 and Model 2 differ on which of the following aspects of the formation of the Martian crustal dichotomy?