  Much of the Earth was built by high-energy impacts of planetesimals and
embryos, many of these impactors already differentiated, with metallic cores of
their own. Geochemical data provide critical information on the timing of
accretion and the prevailing physical conditions, but their interpretation
depends critically on the degree of metal-silicate chemical equilibration
during core-mantle differentiation, which is poorly constrained. Efficient
equilibration requires that the large volumes of iron derived from impactor
cores mix with molten silicates down to scales small enough to allow fast
metal-silicate mass transfer. Here we use fluid dynamics experiments to show
that large metal blobs falling in a magma ocean mix with the molten silicate
through turbulent entrainment, with fragmentation into droplets eventually
resulting from the entrainment process. In our experiments, fragmentation of
the dense fluid occurs after falling a distance equal to 3-4 times its initial
diameter, at which point a sizable volume of ambient fluid has already been
entrained and mixed with the dense falling fluid. Contrary to what has usually
been assumed, we demonstrate that fragmentation of the metallic phase into
droplets may not be required for efficient equilibration: turbulent mixing, by
drastically increasing the metal-silicate interfacial area, may result in fast
equilibration even before fragmentation. Efficient re-equilibration is
predicted for impactors of size small compared to the magma ocean depth. In
contrast, a much smaller re-equilibration degree is predicted in the case of
large impacts for which the impactor core diameter approaches the magma ocean
thickness.
