Core merging and stratification following giant impact

A stratified layer below the core-mantle boundary has long been suspected on the basis of geomagnetic and seismic observations(1-3). It has been suggested that the outermost core has a stratified layer about 100 km thick(3-6) that could be due to the diffusion of light elements(7,8). Recent seismological evidence, however, supports a layer exceeding 300 km in thickness of enigmatic origin(9-11). Here we show from turbulent mixing experiments that merging between projectile and planetary core following a giant impact can lead to a stratified layer at the top of the core. Scaling relationships between post-impact core structure and projectile properties suggest that merging between Earth's protocore and a projectile core that is enriched in light elements and 20 times less massive can produce the thick stratification inferred from seismic data. Our experiments favour Moon-forming impact scenarios involving a projectile smaller than the proto-Earth(12,13) and suggest that entrainment of mantle silicates into the protocore led to metal-silicate equilibration under extreme pressure-temperature conditions. We conclude that the thick stratified layer detected at the top of Earth's core(9,10) can be explained as a vestige of the Moon-forming giant impact during the late stages of planetary accretion.