Early magnetic field and magmatic activity on Mars from magma ocean cumulate overturn

Significant and perhaps complete melting of the young terrestrial planets is expected from their heat of accretion and core formation. The process of subsequent magma ocean fractional solidification creates a cumulate mantle unstable to gravitational overturn. Overturn should be fast (≤1 to 10 Ma) and result in increasing mantle density with depth. This stable stratification inhibits later thermal convection, preserving geochemical heterogeneities. Overturn places cold cumulates against the core-mantle boundary, which creates sufficient heat flux to drive a core dynamo, producing a brief, strong magnetic field. During overturn, hot cumulates rise from depth and melt adiabatically, creating an early crust to record this field and leaving behind mantle reservoirs with isotopic fractionations dating from the early evolution of the planet.