Formation of the Moon

A related, but separate issue concerns the origin of the Moon. Some interesting facts about the Moon which need explanation are:

• the average density of the Moon is 3.3 g cm^-3, roughly the avearge density of an Earth rock but less than the average density of the Earth, 5.5 g cm^-3 ==> Moon is similar in composition to the crust of the Earth but is depleted in heavy elements when compared to the Earth.
• the Moon is also deficient in volatile elements compared to the Earth.
• The Earth and Moon share similar oxygen isotopic compostion but do show differences, the Moon shows slightly higher oxygen isotopes in its composition.

Although the Moon is roughly similar to the Earth's crust in composition, there are significant differences (heavy elements, volatile elements, isotope abundances). ===> the Moon probably formed in the same region of the Solar nebula as did the Earth (that is, it was not formed elsewhere and then captured), but it probably was not simply spun-off from the Earth or formed coevally with the Earth (as a mini-Solar System).

Currently, the party line which attempts to explain how the Moon formed is as:

Theia, a Mars-sized asteroid, hit the young Earth with a glancing blow forming a debris ring and knocking the Earth's rotation axis to its 23.5o tilt. Because the bulk of the material in the ring comes from the Earth's crust, the similarities to the Earth are naturally explained (in particular the deficiency of heavy elements, the lack of a large iron core for the Moon, and the same oxygen isotope composition for the Earth and Moon). However, since some of the asteroid material will also be included into the Moon, the different isotope abundances can be explained. Also, because the impact is likely to be violent, the more volatile elements will be vaporized and less likely to be re-captured into the Moon. These things are good. Further, if a theory for the origin of the Moon calls for an evolutionary process, it would have a hard time explaining why other planets do not have moons similar to ours. (Only Pluto has a moon that is an appreciable fraction of its own size.) The impact hypothesis has the advantage of invoking a catastrophic event that might happen only once or twice in our Solar System. See the Youtube video for a recent simulation of the formation of the Moon. Video made at NASA Ames Research Center.