A titanic impact could have stripped away a portion of its rocky mantle, leaving behind a metallic core whose density is out of proportion with the original ratio of rock to metal.
Computer simulations of this impact show that both of the objects would melt in the impact and the dense core of the impactor would fall as molten rock into the liquefied iron core of Earth.
The explanation may be in Earth's mantle, the layer of rock underlying the tectonic plates and extending down over 1,800 miles to the outer edge of Earth's iron core.