Theories of Formation for the Moon

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.

The Metallic Core There is evidence that the Moon has a metallic core similar to that of Earth, but it is relatively smaller. This gives rise to the mean density of the Moon of 3.34 gcm-3 as compared to 4.05 gcm-3 for Earth. However, FeO is higher than that of Earth, but 13% lower than C1 chondrites. This is reflected in the Mg/(Mg+Fe) ratio of ) of 0.89 for Earth’s mantle as compared to 0.80 for the Moon, based on the composition of lunar basalts.

Trace Element Evidence The Moon is strongly depleted in volatile elements (e.g. Bi, Tl) and enriched in refractory elements (e.g. Ca, Al, Ti, REEs, U, Th). However, trace elements in general are depleted consistent with fractionation to a metallic core. Siderophile trace elements are dpleted in the Earth’s mantle and the Moon’s, but this can be explained by partitioning into the Earth’s core

Isotopic Evidence A major factor is seen in the oxygen isotopic composition of the Moon, which is very similar to that of Earth

Constraints from Recent Data A detailed comparison of the properties of Lunar and Earth rock samples has placed very strong constraints on the possible validity of these hypotheses. For example, if the Moon came from material that once made up the Earth, then Lunar and Terrestrial rocks should be much more similar in composition than if the Moon was formed somewhere else and only later was captured by the Earth. These analyses indicate that the abundances of elements in Lunar and Terrestrial material are sufficiently different 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 the accounted for.