Earth's natural satellite: Formation of the Moon

The Moon is Earth's only natural satellite, and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary, having a quarter the diameter of Earth and 1⁄81 its mass. Charon is larger relative to Pluto, the Pluto, at slightly more than 1/9 (11.6%) of Pluto's mass, but Pluto has been re-classified as dwarf planet.

However, the Earth and Moon are still considered a planet–satellite system, rather than a double-planet system, as their barycentre, the common centre of mass, is located 1,700 km (about a quarter of the Earth's radius) beneath the surface of the Earth.

Stats

Diameter: 3,474 km

Semi-major axis: 384,399 km

Orbital Period: 27.32 days

Formation

Several mechanisms have been proposed for the Moon's formation 4.527 ± 0.010 billion years ago, some 30–50 million years after the origin of the Solar System. These include:

1. Fission of the Moon from the Earth's crust through centrifugal forces.
-- but which would require too great an initial spin of the Earth.

2. Gravitational capture of a pre-formed Moon.
-- but which would require an unfeasibly extended atmosphere of the Earth to dissipate the energy of the passing Moon.

3. Co-formation of the Earth and Moon together.
-- but which does not explain the depletion of metallic iron in the Moon.

All these hypotheses also cannot account for the high angular momentum of the Earth–Moon system.

Giant impact hypothesis

The giant impact hypothesis proposes that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. The colliding body is sometimes called Theia (or Euryphaessa) for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.

The giant impact hypothesis is the current favoured scientific hypothesis for the formation of the Moon. Supporting evidence includes Moon samples which indicate the surface of the Moon was once molten, the Moon's apparently relatively small iron core and a lower density than the Earth, and evidence of similar collisions in other star systems (which result in debris disks). Further, giant collisions are consistent with the leading computer models of the formation of the solar system.

Still, there remain several unanswered questions concerning the best current models of the giant impact hypothesis. The energy of such a giant impact is predicted to heat Earth to produce a global 'ocean' of magma; yet there is no evidence of the resultant planetary differentiation of the heavier material sinking into Earth's mantle. Further, lunar samples do not have expected ratios of oxygen isotopic ratios, volatile elements, iron oxide, or siderophilic elements compared to those of Earth.