Scientists at the University of Maryland believe an Earth-Theia collision far more violent than previously thought could explain how the Earth’s Moon came into existence.
This is an artist’s depiction of a catastrophic collision between two celestial bodies; such an impact between the proto-Earth and Theia likely formed the Moon. Image credit: NASA / JPL-Caltech.
According to the giant impact hypothesis (sometimes called the Big Splash, or the Theia Impact), our Moon was created 4.5 billion years ago in a catastrophic collision between Earth and an impactor the size of Mars, known as Theia.
Over the past years planetary researchers have simulated this process and reproduced many of the properties of the Earth-Moon system. However, these simulations have also given rise to the so-called Lunar Paradox problem – the Moon appears to be made up of material that would not be expected if the collision theory is correct.
Analyses of the various simulations of the Earth-Theia collision predict that the Moon is mostly made up of material from Theia. However, elements found on the Moon show identical isotopic properties to those found on our planet.
Now, a new study published in the journal Nature has generated an isotopic ‘fingerprint’ of the Moon that could help researchers solve this problem.
The study suggests that the impact of Theia into early Earth was so violent, the resulting debris cloud mixed thoroughly before settling down and forming the Moon.
According to the authors, both Earth and the Moon gathered additional material after the main impact, and that Earth collected more of this debris and dust. This new material contained a lot of tungsten, but relatively little of this was of a lighter isotope of tungsten known as 182W. Taking these two observations together, one would expect that Earth would have less 182W than the Moon.
Sure enough, when comparing rocks from the Moon and Earth, the scientists found that the Moon has a slightly higher proportion of 182W. The key, however, is how much.
“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 Dr Richard Walker of the University of Maryland’s Department of Geology, the senior author on the study.
This finding supports the idea that the mass of material created by the Theia impact must have mixed together thoroughly before the Moon coalesced and cooled. This would explain both the overall similarities in isotopic fingerprints and the slight differences in 182W.
It also largely rules out the idea that the Mars-sized body was of similar composition, or that the Moon formed from material contained in the pre-impact Earth.
In both cases, it would be highly unlikely to see such a perfect correlation between 182W and the amounts of material gathered by the Moon and Earth post-impact.
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Mathieu Touboul et al. Tungsten isotopic evidence for disproportional late accretion to the Earth and Moon. Nature, published online April 08, 2015; doi: 10.1038/nature14355
