Metal fragmentation in magma oceans following large planetary collisions

Abstract:

The disruption of impactor cores following large planetary collisions controls the equilibration of metal and silicates during the differentiation of terrestrial planets. Accretion models and impact simulations indicate that the Earth formed by high-energy collisions that melted the impacting embryos. Following each impact, the metallic core of the impactor fell in a molten silicate magma ocean. Previous fluid dynamics experiments showed that, in this ocean, the impactor core fragmented into drops at depths 2-4 times the impactor radius. However, what controls the size of these metal drops remains unclear.

To answer this question, we use laboratory experiments where a piston injects a volume of aqueous solution (representing the impactor core) into a pool of silicone oil (representing the magma ocean). We vary the Weber number, which measures the importance of inertia to interfacial forces, reaching values nearly two orders of magnitude larger than previous studies. We observe that the released liquid entrains ambient fluid into a ring, inside which it fragments into drops. We obtain scaling laws for the drop size and the entrainment rate as a function of the size, density and speed of the released liquid. Applied to planet formation, our scalings on the drop size suggest that the impactor core fully equilibrates with the silicates that it entrains.