Five serious theories have been proposed for the formation of the Moon (not counting the one involving green cheese):

1.    The Fission Theory: The Moon was once part of the Earth and somehow separated from the Earth early in the history of the Solar System. The present Pacific Ocean basin is the most popular site for the part of the Earth from which the Moon came.

2.    The Capture Theory: The Moon was formed somewhere else, and was later captured by the gravitational field of the Earth.

3.    The Condensation Theory: The Moon and the Earth condensed together from the original nebula that formed the Solar System.

4.    The Colliding Planetesimals Theory: The interaction of earth-orbiting and Sun-orbiting planetesimals (very large chunks of rocks like asteroids) early in the history of the Solar System led to their breakup. The Moon condensed from this debris.

5.    The Ejected Ring Theory: A planetesimal the size of Mars struck the earth, ejecting large volumes of matter. A disk of orbiting material was formed, and this matter eventually condensed to form the Moon in orbit around the Earth.

Comparison of the properties of Lunar samples brought back by the Apollo Missions and Earth rock samples has shown evidence that strongly disagrees with hypotheses.

 

For example,

 

If the Moon came from material that once made up the Earth, then Lunar and Earth rocks should be much more similar in composition than if the Moon was formed somewhere else and only later was captured by the Earth.

 

Study of the amounts and types of elements in Lunar and Earth material are different enough to make it unlikely that the Moon formed directly from the Earth.

 

Generally, work over the last 10 years has essentially ruled out the first two explanations and made the third one rather unlikely.

 

At present the fifth hypothesis, that the Moon was formed from a ring of matter ejected by collision of a large object with the Earth, is the favored hypothesis; however, the question is not completely settled and many details remain to be accounted for.

 

This section from Astronomy 161 by Dr. Stephen Daunt University of Tennessee

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