Moon formation

Theories have been developed regarding Moon formation, the formation of the Moon, as well as other moons, which likely result from various processes. The giant-impact hypothesis is currently the standard model of the formation of the Moon, that sometime in the very early history of the solar system, a Mars-sized planet (termed Theia) struck the Earth sending debris from both into space which eventually coalesced into a circumplanetary disk. This explains why lunar material is strikingly similar to Earth material in isotope abundances (isotope signature) and radioactivity characteristics (though the Moon has a lower level of volatiles, which also requires explanation). A current issue with the model is that lunar material would be expected to show some signs of the different material from an impacting planet and it does not. Variations on the hypotheses to explain this:

• fast-spinning Earth model - that the Earth had a very short day prior to the impact, causing more Earth material to be sent space-ward, resulting in a greater similarity.
• half-Earth model - that the impact was between two planets of roughly equal size (smaller than the present Earth) and both the present Earth and the Moon harbor a blend of their material.

Less well-established variations:

• multiple-impact model - that a succession of impacts created the Moon.
• synestia model - that the impact vaporized the two planets, resulting in a synestia (a rapidly spinning torus-shaped body of vaporized rock) which considerably mixed the material that ultimately formed the two current bodies.

Each of these models has its problems.

Other solar system moons may have formed through one or more of these processes, with an additional possibilities of some other type of formation of a circumplanetary disk or of capture of an asteroid. Suitable capture methods (subsequent to the solar nebula's dissipation) require some coincidence, e.g., the simultaneous nearby presence of a third body, but it is seriously considered, for example, for Neptune's moon, Triton, due to its a retrograde orbit. Capture seems unlikely for the Moon because of its Earth-like isotope signature.