  The giant impact hypothesis for Moon formation successfully explains the
dynamic properties of the Earth-Moon system but remains challenged by the
similarity of isotopic fingerprints of the terrestrial and lunar mantles.
Moreover, recent geochemical evidence suggests that the Earth's mantle
preserves ancient (or "primordial") heterogeneity that predates the
Moon-forming giant impact. Using a new hydrodynamical method, we here show that
Moon-forming giant impacts lead to a stratified starting condition for the
evolution of the terrestrial mantle. The upper layer of the Earth is
compositionally similar to the disk, out of which the Moon evolves, whereas the
lower layer preserves proto-Earth characteristics. As long as this predicted
compositional stratification can at least partially be preserved over the
subsequent billions of years of Earth mantle convection, the compositional
similarity between the Moon and the accessible Earth's mantle is a natural
outcome of realistic and high-probability Moon-forming impact scenarios. The
preservation of primordial heterogeneity in the modern Earth not only
reconciles geochemical constraints but is also consistent with recent
geophysical observations. Furthermore, for significant preservation of a
proto-Earth reservoir, the bulk composition of the Earth-Moon system may be
systematically shifted towards chondritic values.
