Ever since the Apollo program, the predominant theory of Lunar formation is that, shortly after the solar system formed, a planet which we call Theia collided with the Earth, and the debris ejected into orbit formed the Moon.
The problem is that the Moon and Earth are isotopically very similar - it was thought exactly similar. Until now. A type of primitive space rock from the very early days of the solar system, called an enstatite chondrite, has an isotopic signature very similar to the Earth, but not exactly. And moon rocks brought back from the Apollo mission bear their telltale signature, suggesting that about 50% of the Moon's material is from Theia (the rest being from the nearly destroyed Earth.)
According to the lead researcher, Dr Daniel Herwartz, from the University of Goettingen, no one has found definitive evidence for the collision theory, until now.
"It was getting to the stage where some people were suggesting that the collision had not taken place," he told BBC News.
"But we have now discovered small differences between the Earth and the Moon. This confirms the giant impact hypothesis."
The collision theory was given its first serious boost when the Apollo astronauts brought back rocks that were about 100 million years younger than the oldest asteroid materials found on Earth. So if the Moon was younger, what held back its formation? Well the fact that it isn't an original solar system object would be one reason.
If you transport yourself back about 4.4 billion years ago, the collision wouldn't have been the picture that Armageddon has probably planted in your head - it was a slow glancing collision that destroyed the impactor, and sent the material of Earth spinning such that, when the Earth re-solidified, a day would have lasted an entire four hours. (The conservation of angular momentum in the Earth Moon system has slowly slowed the Earth's day down, and moved the Moon into a larger orbit.)
it isn't entirely clear whether the initial Lunar configuration was a single moon - there is some evidence that a smaller moon also formed, which later splattered on the Moon's far side, leaving the much different surface that the other side of the moon has.
Here is the original report on the data by Dr. Herwartz and his collaborators, Andreas Pack, Bjarne Friedrichs, and Addi Bischoff. (Subscription required)
http://www.sciencemag.org/...
Abstract: the Moon was probably formed by a catastrophic collision of the proto-Earth with a planetesimal named Theia. Most numerical models of this collision imply a higher portion of Theia in the Moon than in Earth. Because of the isotope heterogeneity among solar system bodies, the isotopic composition of Earth and the Moon should thus be distinct. So far, however, all attempts to identify the isotopic component of Theia in lunar rocks have failed. Our triple oxygen isotope data reveal a 12 ± 3 parts per million difference in Δ17O between Earth and the Moon, which supports the giant impact hypothesis of Moon formation. We also show that enstatite chondrites and Earth have different Δ17O values, and we speculate on an enstatite chondrite–like composition of Theia. The observed small compositional difference could alternatively be explained by a carbonaceous chondrite–dominated late veneer.