The Moon has been Earth's companion for far longer than man has been looking to it, but since we have been looking at it, we have been wondering how it came to be. Several theories have been proposed over the theories and finally there is evidence to support one. Researchers at Washington University in St. Louis and University of California, San Diego have analyzed moon rock samples and found that the Moon was likely formed according to the Big Impact Theory.
According to this theory, billions of years ago there was a collision between the early Earth and a planetary body called Theia. Though Theia was likely the size of Mars, the Earth absorbed it and spat out enough rock and debris for the Moon to form, as the material coalesced. The evidence the researchers found is a significant isotope fractionation of zinc. What that means is they found more heavy zinc isotopes than light ones. Such a difference is only explained by the fact that lighter isotopes more easily evaporate away then heavy isotopes. As this was found in samples taken from multiple areas of the Moon, this was not caused by some local event, which indicates the whole moon was at one point hot enough to boil away the lighter zinc.
Understanding how the Moon formed is very important for understanding how the Earth formed and came to be what it is today. After all, if that collision with Theia formed the Moon, how else might it have affected the Earth?