The origin of the Earth‐Moon system is very much related to the origin of the solar system as a whole. The ancient lunar surface has preserved a record of events over the last four billion years. Astronomers obtain relative crater ages from superimposition. For example, younger craters are found on top of older craters. Ejecta rays from younger craters also fall over older craters. Craters on lava flows (maria) similarly are younger than the lava. The purpose of the Apollo lunar missions was to obtain rock samples from different regions so that the relative age history of the lunar system could be translated into one with absolute ages. The planet Mercury, which is also heavily cratered with an apparently similar cratering history as the Moon, supplies additional evidence to theorize the Moon's history and origin. This, and other evidence, points to a process by which smaller objects ( planetesimals, or little planets) merged to form the surviving planetary objects of today's solar system.
Earth and the Moon are so similar they can be thought of as forming a binary planetary system. Study of their chemical makeup provides important information on how these two objects became permanently associated with each other. The Moon is relatively deficient in heavier elements (mean density 3.3 g/cm 3 compared to 5.5 g/cm 3 for Earth). More specific chemical analysis of Moon rocks shows that the chemistry of the two objects is otherwise very similar, but not identical. Traditionally, three theories explain the association of the two objects. The theory of coeval formation argues that the Moon and Earth coalesced together out of the same materials. The idea that their chemistry is not identical poses a severe problem for this theory. Fission theory suggests that a single, initially rapidly rotating object broke apart. But this theory would require nearly identical chemical composition for the surviving objects. Dynamical problems also hinder this idea. The capture hypothesis theorizes that the Moon formed elsewhere in the solar system and only later became bound to Earth. This model allows for differences in the chemical composition of the two objects; but the problem is that their chemistry is too similar. Also, dynamical problems exist involving a loss of orbital energy necessary to end up with the two objects orbiting each other.
The ability of modern high speed computers to numerically model planetary‐sized objects has led to a final theory that is likely correct—a grazing impact or collisional hypothesis. This theory provides that a Mars‐sized object (a proto‐moon about half the size of Earth) hit the proto‐earth nearly tangentially. The proto‐earth survived, but with significant crust/mantle material lost to a debris cloud surrounding the planet. The impactor was mostly disrupted into the debris cloud; its iron core survived more or less intact but was assimilated by Earth. Much of this debris (impactor mantle plus proto‐earth mantle) subsequently coalesced to form the present Moon. Debris also fell to Earth to become part of its mantle and crust, thus producing lunar/terrestrial chemistry that is very similar, but not identical. Detailed computer calculations have shown that this scenario is dynamically and energetically possible.