There are a number of ideas about where the Moon came from, but, based on orbital mechanics, the accepted theory is that about 150 million years after the Solar System formed some 4.6 billion years ago, the primordial Earth was struck by an object the size of Mars called Theia. Out of the debris of this massive impact, the Moon was formed.

This theory fit most of the facts, but not all of them. In particular, it didn't fit with the composition of isotopes found on the Earth and Moon. Isotopes are a very useful for finding where things come from because their ratios are pretty much set at an object's place of origin like a nuclear bar code. By proper measurement, scientists can tell everything from whether a meteor came from Mars to which farm a chicken was raised on. Unfortunately, the Moon turned out to be a bit of a poser.

If the Moon was created by Theia striking the Earth, then the the isotope ratios of the Earth and Moon should be very different, but they're not. They're similar – too similar to have a separate origin.

Scientists at the University of Maryland (UMD) have shed light on this mystery by analyzing the isotopic “fingerprints” of rock samples brought back by the Apollo 16 mission in 1972 – in particular, by measuring the isotopes of tungsten present and comparing them with those on Earth.

The team says that the impact was so severe that it formed a cloud of dust from the Earth and Theia that mixed thoroughly, then coalesced into the Moon as it cooled, giving it a composition very similar to the Earth's mantle. The result was the outer surfaces of the Earth and Moon were formed of a mixture of the same materials, resulting in the same isotopic levels.

There is more to solving the puzzle, however. After the Earth and Moon formed, more material continued to bombard them, but the Earth collected most of this debris. The clever bit is that this new material was light on the isotope tungsten-182. Because of this, the scientists expected to find comparatively less tungsten-182 on Earth than on the Moon, which turned out to be the case.

"The small, but significant, difference in the tungsten isotopic composition between Earth and the Moon perfectly corresponds to the different amounts of material gathered by Earth and the Moon post-impact," says Richard Walker, a professor of geology at UMD. "This means that, right after the Moon formed, it had exactly the same isotopic composition as Earth’s mantle."

The UMD findings were published this month in Nature.

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