The formation of the Earth set the initial temperature and composition for its later evolution. The onset of plate tectonics and the generation of Earth’s magnetic field both depend on these initial conditions. Based on isotopic and geochemical observations, we know that the metallic core separated from the mantle silicates during Earth formation, when high-energy planetary collisions were building up its mass. However, we do not know how mixed metal and silicates were during, and after, each collision. Their chemical equilibration, and the resulting composition of the core and the mantle, depend on this mixing.

We investigate the fluid dynamics of planetary collisions in laboratory experiments. From our findings, we infer the efficiency of metal-silicate mixing within the forming Earth. Our results also indicate that the stratification in the present-day Earth’s core could be a vestige of the giant impact that formed the Moon.