Primitive Crust Buried Below the Southern Highland of Mars

Geochemical studies of shergottites have predicted an enriched end member associated with the final residual liquid of the Martian magma ocean, which may result in the formation of a KREEP-like silica-rich primitive crust on early Mars. In this work, we investigate the evolution of the Martian crust using the 1D parameterized mantle convection model. By combining various observational and geophysical constraints, our model supports the KREEP-like silica-rich primitive crust that formed before 4.527 Ga, i.e., the time for the formation of the earliest basaltic secondary crust associated with the early mantle overturn, whose thickness would be at least 3–8 km. During the consequential evolution, the silica-rich primitive crust on the northern hemisphere would have been removed by the Borealis impact. However, the silica-rich primitive crust of the southern hemisphere would still survive and would be buried below the ejecta of the Borealis impact and local basaltic secondary crust, which may correspond to the low-density layer below the southern highland of Mars suggested by gravitational observations.