Primitive crust buried below southern highland of Mars

The geochemical studies for shergottites predicted an enriched end-member associated with the final residual liquid of Martian magma ocean, which may result in the formation of a KREEP-like silica-rich primitive crust on the early Mars. In this work, we investigate the evolution of Martian crust by the 1-D 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 earliest basaltic secondary crust associated with the early mantle overturn, whose thickness would be 3--8 km at least.} During consequential evolution, the silica-rich primitive crust on the northern hemisphere would be removed by Borealis impact. However, the silica-rich primitive crust of southern hemisphere would still survive and would be buried below the ejecta of Borealis impact and local basaltic secondary crust, which may correspond to the low-density layer below the southern highland of Mars suggested by the gravitational observations.