The Moon is a very unusual object, since, compared to the other moons and their planets, it is very large relative to the size of its host planet. At least partly because of this, scientists have exerted considerable effort in explaining how it came to be. They have come up with 4 main theories about why the Earth has the Moon it does: The Moon developed at the same time as the Earth, from the same cloud of debris. The Moon tore out of the Earth, due to the Earth’s early high rotation. The Moon was a foreign object that was captured by the Earth as it wandered by. The Moon was created when a Mars sized object slammed into it about 50 million years after Earth’s formation.
Data from the Apollo missions showed that the Moon has virtually no iron core as the Earth does, which suggests that it wasn’t formed from the same cloud of debris. This theory also can’t account for the large angular momentum of the current Earth-Moon system. For the fission theory to be valid, the Earth’s angular momentum would have had to be far higher than current models indicate. The foreign capture theory requires a very unlikely near perfect coming together of the two objects, and doesn’t explain why the two objects have very similar oxygen-isotope profiles.
So, currently the most likely theory seems to be that the Moon was formed about 4.5 billion years ago in a collision between a Mars sized object and the Earth. This object would have formed in the same neighborhood as the Earth around the Sun, which explains the similar oxygen-isotope profile of the pair. The iron cores of the Earth and object merged, and much of the lighter materials that were blown off from the pair made up the Moon (the rest fell back to Earth). This theory also helps explain why the moon has very few volatile materials, since it was formed through a process that would have baked off the non-refractory elements. Also, this impact might be the reason the Earth is tilted on its axis, and therefore why we have seasons today.
Life might not have been possible without the Moon, because it acts as a gyroscope and stabilizes the Earth, and so prevents it from wobbling and causing climate extremes. One theory holds that the Moon might have been responsible for life in another way. The idea is that in the beginning it was much closer than today, and caused huge tides. These huge tides could possibly have put DNA through temperature cycles, which would have caused it to duplicate in a process similar to the modern PCR duplication process used in labs today. (The Origin of the Moon, NOVA – Origins, Origin of the Moon, Where did the Moon come from?, If We Had no Moon, No Moon, no life on Earth, suggests theory) (See also: Scientific American’s article by G. Jeffrey Taylor in the July 1994 issue (pp 40-47))
Tags: Moon