Germination

Boundary layer

Magnetic field

Physics

Geology

Core–mantle boundary

Stratification (seeds)

Earth's magnetic field

Dormancy

Astrophysics

Projectile

Planet

Mechanics

Botany

Quantum mechanics

Mantle (geology)

Inner core

Geophysics

Biology

Seed dormancy

Outer core

Nature Geoscience

The Earth’s outermost core is thought to be stratified. Turbulent mixing experiments suggest that merging between the cores of projectile and planet following the Moon-forming giant impact could have produced the stratification. A stratified layer below the core–mantle boundary has long been suspected on the basis of geomagnetic and seismic observations1,2,3. It has been suggested that the outermost core has a stratified layer about 100 km thick3,4,5,6 that could be due to the diffusion of light elements7,8. Recent seismological evidence, however, supports a layer exceeding 300 km in thickness of enigmatic origin9,10,11. 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-Earth12,13 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 core9,10 can be explained as a vestige of the Moon-forming giant impact during the late stages of planetary accretion.

Core merging and stratification following giant impact