Fluid dynamics of local martian magma oceans

The evolution of a melt region produced by a large impact during Mars formation is addressed. While some impact induced melt is redistributed during crater excavation, sufficiently large impacts (much larger than basin forming impacts) generate an intact melt region which is retained beneath the excavation zone, i.e., a local magma ocean. Local magma ocean evolution depends on the effective rheology controlling large scale deformation of the solid part of the planet, mechanism of crystallization, and melt region size. Within the uncertainties of various parameters, two scenarios are possible. For sufficiently weak rheology or large melt region size, evolution is characterized by rapid extrusion and formation of a global magma ocean. For sufficiently strong rheology or small melt region size, in situ crystallization to a partially molten solid state occurs prior to isostatic adjustment. Subsequent to in situ crystallization, local magma ocean evolution depends on melt region size and efficiency of lateral redistribution compared to bulk conductive cooling. For large melt regions, lateral spreading occurs via plastic deformation and results in an asymmetric, global, partial melt layer. For small melt region size, viscous spreading viscous can result in bulk cooling below the solidus prior to formation of a global layer. A hypothesis for the origin of the hemispherical crustal dichotomy and Tharsis rise is suggested. The dichotomy is associated with a global partial melt layer produced by evolution of a large, local magma ocean. After dichotomy formation, evolution of a second, smaller, local magma ocean is related to Tharsis development.