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.

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 follows:

• approach
• impact
• accretion

The above scenario is a promising explanation for the origin of the Moon. Because the bulk of the material comes from the Earth's crust, the similarities to the Earth are naturally explained (in particular the deficiency of heavy elements). However, since some of the asteroid material will also be included into the Moon, the different isotope abundances can be explained. Finally, 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.