Mercury and other unusually metal-rich objects in the Solar System may be relics left behind by collisions in the early Solar System that built the other planets, according to a study published online in Nature Geoscience.

Erik Asphaug and Andreas Reufer conducted numerical simulations of impacts between a proto-Mercury and another planetary embryo in the early Solar System. They found that if the speed of the impact is sufficiently high and the other planetary embryo sufficiently large, proto-Mercury will escape the orbit of the other planet, but will have its non-metallic mantle stripped by the larger body. Thus, the researchers propose one or two of these hit-and-run collisions can explain Mercury’s massive metallic core and very thin rocky mantle in comparison to other rocky planets.

Similarly, metal-rich asteroids such as Psyche may also be survivors of the violent early Solar System, saved by hit-and-run collisions that only stripped their rocky mantles.