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New clues on the perplexing origin of the moon

This artist's rendering shows the collision of two planetary bodies. A collision like this is believed to have formed the moon within the first 150 million years after our solar system formed. (NASA/JPL-Caltech)
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Scientists are pretty sure that the moon formed when a smaller planet -- probably about the same size as Mars -- collided with a young Earth, with most of the moon's material coming from this unknown planet (sometimes called Theia). But the Earth and the moon are surprisingly similar at the chemical level, so we're still trying to figure out how a foreign planet from another part of space could have created it.

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In a pair of new studies published Wednesday in Nature, researchers present evidence that this chemical similarity is due to a violent mixing of material that occurred when Theia hit Earth. Another study presents evidence that an alternate explanation -- that against all odds, the far-flung Theia happened to be made of similar stuff to our Earth -- may be more plausible than previously assumed.

The authors of the first two studies looked at tungsten in Earth and the moon, tracing how much of a particular isotope has formed in each body.

"The problem with the giant impact model is that the Earth and moon are really incredibly similar in terms of their genetic makeup, by which I mean the isotopic composition of their elements," study author Richard Walker of the University of Maryland said, "and these genetic indicators, if you will, tell us where the materials that built the planet came from."

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So it would be unlikely, he explained, for a random interloper like Theia, presumably born long before and in another part of the solar system, to have the same "genetic" makeup.

But Tungsten-182, he said, doesn't come from these basic building blocks. It's created by another element as it decays. So by comparing the ratio of the parent element to the daughter element (in this case tungsten) the researchers were able to work backward and establish that the moon and the Earth had the same isotopic compositions when they formed. Similar findings were produced by a separate group of researchers whose work was also published Wednesday in Nature.

While a third study found that planetary impacts are more likely to feature similarly composed planets than previously thought, the odds are still placed at just 20-40 percent. And Walker thinks the tungsten findings make the coincidence theory even less likely. Even if Theia happened to be a long-lost sibling of our planet, Walker said, it would have to be older, having been formed in another region of space before coming in on its crash course with Earth. It's not impossible that this older planet would be at precisely the same Tungsten-182 ratio as Earth when the impact occurred, but it's unlikely.

But the case is far from closed. Walker explained that the physics involved in this supposed collision -- the churning of gas and dust debris with a molten planet, leaving a blended bunch of materials for the Earth and the moon to build with -- is poorly understood.

"The origin of the moon is still hotly debated, and these studies don't solve that problem," he said. "But they might be driving us in the right direction."

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