Buoyant Impact Partial Melts on Ancient Mars

Most of the Martian crust formed prior to similar to 4 Ga, but the magmatic processes responsible for finalizing the structure and composition of the ancient crust remain enigmatic. Impacts can produce large volumes of melt under a wide range of melting pressures, temperatures, and degrees of melting. Hellas, Argyre, Isidis, and Utopia basins date to around 4 Ga, demonstrating that basin-scale impacts helped to place the finishing touches on an already established crust. In this work, we focus on the ascent and intrusion of impact partial melts generated at mantle depths and the consequences for the petrology and structure of the Martian crust. Specifically, we show that the majority of impact partial melts are buoyant, favoring ascent to the surface or to neutral buoyancy levels in the crust, where magmas solidify as intrusive rocks. The composition of these polybaric melts overlaps with some ancient Martian igneous materials. We propose that the process of ascent of deep-seated impact partial melts and intrusion at shallower levels may have contributed to the observed crustal stratification and ancient petrologic diversity on Mars. When large asteroids collide with planets, they can cause melting of that planet's crust and mantle. This process of impact melting was likely more common when the solar system was young. In this work, we focus on partial melting during impacts on early Mars. We combine results from the modeling of impacts and magmas. Because we find that melts are less dense than their surroundings, they are likely to rise toward the surface of Mars. Based on their density, we predict that a large fraction of these magmas will freeze at the base of the Martian crust or within the crust, forming layers with distinct compositions that can be compared with rocks examined by Mars rovers and with seismic data from the InSight lander. Impact partial melts are a volumetrically substantial counterpart to high-degree meltsDeep-seated impact partial melts are buoyant relative to surrounding mantle and crustal rocksAscent of ancient impact melts to neutral buoyancy levels in the Martian crust contributed to crustal layering and compositional diversity