When the Earth was very young, about 150 million years old, it collided with another object roughly the size of Mars. The violent collision ejected massive amounts of material from the Earth and that material formed the moon.

For the last 30 years, that has been the collective and fairly undisputed theory of where the moon came from. However, there has always been one serious problem with the whole idea.

The isotopic composition of a galactic body is sort-of it’s geological DNA and the isotopic “fingerprint” of the Earth and the moon are nearly identical. On the surface, this means that the Earth and the object that crashed into it were nearly identical.

If Theia, the name given to this object, came from elsewhere in the solar system it would have had a much different isotopic composition than the Earth.

Several theories have been proposed to explain the similarities. One states that the impact created a cloud that mixed so thoroughly the two becme indistinguishable by the time the moon formed. One theory holds that Theia and Earth just happened to be isotopically similar to begin with. A third holds that the moon formed entirely from material from the Earth with the Earth absorbing Theia’s material.

Now a new paper published in the journal Nature by scientists at the University of Maryland hopes to lay the controversy to rest. The researchers created a new isotopic fingerprint that zeroed in on the isotope of Tungsten in the moon and the Earth.

The results suggest that that the Earth-Theia impact was so violent that the resulting cloud mixed thoroughly before re-coalescing into the Earth and Moon.

“The problem is that Earth and the moon are very similar with respect to their isotopic fingerprints, suggesting that they are both ultimately formed from the same material that gathered early in the solar system’s history. This is surprising, because the Mars-sized body that created the moon is expected to have been very different. So the conundrum is that Earth and the moon shouldn’t be as similar as they are,” said Richard Walker, a professor of geology at UMD and co-author of the study in a statement.

The evidence, according to the researchers, suggests that the Earth and the moon both gathered additional material after the impact event. The Earth, with its larger gravitational pull, would have gathered much more material but relatively little of the light isotope Tungsten-182. The researchers expected that the moon would have larger amounts of Tungsten-182 than the Earth.

Their analysis confirmed this prediction, with a slightly higher proportion of Tungsten-182 on the moon.

“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. This means that, right after the moon formed, it had exactly the same isotopic composition as Earth’s mantle,” said Walker.

The paper supports the idea of a thorough mixing of the material from the impact prior to the formation of the moon and explains the similarities in isotopic fingerprints as well as the slight difference in Tungsten-182.

It also rules out the idea that Theia oringally had the same composition as the Earth.

“This result brings us one step closer to understanding the close familial relationship between Earth and the moon. We still need to work out the details, but it’s clear that our early solar system was a very violent place,” said Walker.