Everything we thought we knew about the moon's origin is wrong

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Dec 16, 2012

Think back to your last Earth Science course and you'll probably remember a little tale about how the moon came to be. Something about a big collision and a new planetary body emerging from the debris, right? Well, new research into minerals found in moon rocks suggests that could be all wrong, and it raises a whole new set of questions.

Until now, the prevailing theory of how the moon was created was what's known as the "giant impact hypothesis," which posits that sometime in the very distant past Earth collided with another planetary mass about the size of Mars, known as Theia. The collision produced a large amount of magma from both planetary bodies, which over time came together and cooled to become the moon. In order for this to be plausible, physicists have concluded that the moon must be made up of about 40 percent Theia, 60 percent Earth.

But new research by geochemists at the University of Chicago casts serious doubt on whether this is how it really happened. To understand why, we have to know a little chemistry.

One way of telling what planetary body a mineral sample came from is by testing its isotope composition. Most elements (oxygen, nitrogen, carbon, etc.) have several small variants called "isotopes." Each isotope has a slightly different atomic structure, but remains the same element. Oxygen has three such isotopes, and by measuring how many of each is found in a given sample, we can generally determine that sample's origin relative to another sample. Simply put, if you take two different samples of oxygen from two different parts of Earth and compare them, the isotope composition will be virtually identical, whereas if you take one sample from Earth and one from Mars or Mercury or Jupiter, the isotope composition will be noticeably different.

Puzzlingly, we've known for a while that the isotope composition of oxygen samples found in moon rocks is the same as the composition of oxygen samples from Earth. That doesn't make sense, since the moon is supposed to have gained a large percentage of its mass from another planetary body that presumably had different isotope compositions. There's a simple explanation for that, though, as oxygen is easily vaporized. It's possible that Earth exchanged oxygen gas with the magma formed shortly after the "giant impact," which would explain why the moon's oxygen composition is the same.

But data found when comparing other elements isn't so easy to explain away. The University of Chicago researchers recently tested the isotope composition of titanium samples recovered from the moon against titanium samples from Earth, and found the compositions to be nearly identical. Again, given that the moon is supposed to have at least partly come from somewhere else, this shouldn't be true, and researchers say the presence of identical titanium samples isn't as easy to account for as oxygen samples.

"The oxygen isotopic composition would be very easily homogenized because oxygen is much more volatile, but we would expect homogenizing titanium to be very difficult," said Junjun Zhang, who led the Chicago research team.

So, if this research is correct, it means the moon wasn't made the way we've always thought it was. So how did it get there, and how do we account for the fact that it seems to be all-Earth? One theory is that a different kind of collision caused it, a glancing impact from another body rather than a head-on one. This would have set the Earth spinning very, very fast until it threw some of itself out into space like an out-of-control pottery wheel. But there's a lot of leftover momentum in that theory that's unaccounted for.

Some researchers think this could mean that the "giant impact hypothesis" still has a ring of truth to it. It just might need a little tweaking.

"I think the general idea of having an impact forming a disk and this disk then forming a moon is probably right," said Matthias Meier, a planetologist at Lund University in Sweden, "but this paper shows us that we still don't understand exactly what the mechanism is, and there is a lot of work to be done in that field."

(via Wired)