Impact models
Moon-forming collisions are studied through simulations. Because the energy caused by the impact of the colliding planets is high enough to melt or even partially vaporize them, pressure forces and phase changes are incorporated into the models. Gravitational interactions and torques are also included because the collision distorts the planets and ejects debris into a disk. Mantle and core materials need to be tracked.
In the canonical giant-impact model, developed since the late 1970s, the Moon is explained as the product of a slow, glancing blow from a Mars-sized body — about 10–15% of Earth's mass — on the early Earth6. The collision left Earth spinning rapidly, once every five hours, with the Moon orbiting close to Earth. Gravitational interactions and torques then caused the Moon's orbit to expand and Earth's rotation to slow to our current 24-hour day. This model is consistent with the Moon's mass, its lack of iron and the angular momentum of the Earth–Moon system.
More detailed chemical properties are harder to explain. The giant-impact model has the Moon condensing from material in a disk, which in canonical impacts is derived primarily from the impactor's mantle. But it is improbable that the impactor had the same composition as the early Earth. The oxygen isotope composition of Mars, for example, differs from that of the Earth by more than a factor of 50 (ref. 1). If the impactor was as different from Earth as Mars is, its signature would still be detectable in the Moon, even after a giant collision.