LAUSR

We examine the fractional crystallization of intermediate depth (~160–260 km), impact‐induced magma oceans on Mars, and find that residual liquids become denser than normal Martian mantle. Fractional crystallization near 3 GPa establishes inverted density distributions that can generate melt descent within the mantle. Liquid compositions produced by ~45–80 wt% crystallization become dense enough to descend to the core‐mantle boundary and could form a stably stratified thermochemical boundary layer (TCBL). If this layer crystallizes, its mineralogy would be dominated by either garnet and ferropericlase or stishovite and ringwoodite. Although the size of Mars' core remains uncertain, the addition of such a thermal boundary layer would impede stabilization of (Mg, Fe)SiO3‐perovskite at the base of the mantle. A TCBL would both elevate the inferred temperature of the core and inhibit heat flow out of the core, with a potentially causal relation with the current lack of an internally generated Martian magnetic field.