The Moon
There are many theories covering the formation of the moon. The first is that the moon is an external body captured in Earth's orbit, but this theory is negated by isotopic evidence stating that both formed in the same location.
Another theory is that the moon and earth simply formed side by side. This theory is also negated by the fact that the moon contains far less iron than the Earth, as shown by its lower density.
Finally, some scientists theorised that the moon was formed by material "spinning off the Earth". This violates physical considerations such as angular momentum.
One theory that allows for this evidence is the collision theory. [1]
Collision Theory
The idea that the moon was formed when a giant protoplanet collided with a possibly molten Earth some 50 thousand years after the formation of the solar system is widely supported by scientists. This theory, in which a giant protoplanet roughly the size of Mars collided with the Earth, ejecting matter in to Earth's orbit, is supported by supercomputer simulations and evidence determined from the moon's composistion.
When the two bodies collided, matter from the Earth's crust was ejected. The heat from the collision could have vaporised any volatile compounds, explaining their abscence on the moon.
This theory is not without its flaws, however. There is no evidence to suggest that Earth ever had the oceans of magma implied by this theory. The presence of water trapped in lunar basalts is also har to explain. Finally, oxygen isotopic ratios, which are unique to each solar body, are identical between the moon and the Earth. If the moon had been formed when a seperate protoplanet collided with Earth, these isotopes would be different.
Bibliography.
- [1]: What Evidence Supports the Collision Theory?, Astronomy.com. Accessed 24 April 2010.