The fission theory states that the moon broke from a rapidly spinning "proto-earth." If this happened after the Earth differentiated (dense materials sunk inward, less dense floated to top) it would explain why the moon is less dense than Earth. In fact, the moon has a very low iron content and this fits in perfectly with this theory. What killed this theory was analysis of the lunar rocks. They differ chemically from the crustal rocks of Earth. Another blow to this theory was that if the proto-earth was spinning fast enough to "spit" out the moon, there would be more angular momentum in the Earth-Moon system than is observed today.
The next theory to be offered up was the condensation theory which suggested that the Earth and Moon condensed from the same cloud of material. This theory died quickly because the Earth and Moon have quite different densities and compositions. The Moon is poor in volatile materials such as water, which is abundant on the Earth.
The capture theory suggests that the Moon formed elsewhere and was later captured by the Earth. This would explain the different densities and compositions, but leads to some tough questions with no satisfactory answers. There would have to be a complex set of circumstances for Earth to be able to capture a body as large as the Moon. It is also believed that it would take such strong gravitational forces by the Earth to capture the Moon, that it would likely tear the Moon apart.
The most widely supported theory as to the origin of the Moon is the large impact theory. If a very large body (such as Mars size) struck the Earth, a huge amount of eject would be thrown up and into orbit around the Earth. It is possible the Moon could have formed from this debris. This material would be a combination of debris from both Earth and the impacting body. This theory explains the differences in density and composition.
Copyright © 1997 Kathy A. Miles and Charles F. Peters II