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Earth meets its match in this artist's conception depicting the moon's birth.

Earth meets its match in this artist's conception depicting the moon's birth.

Hagai Perets

How did the moon really form?

Early in its life, a Mars-sized object slammed into Earth and gave birth to the moon. That's been the prevailing idea for decades, though it’s taken some hits recently. Now, new simulations suggest that the original idea may indeed be correct.

Planetary scientists proposed the collision hypothesis in the 1970s. It seemed to explain a basic fact about the moon: Its composition is similar to Earth's mantle. This similarity arose, the thinking went, because the impact blasted material from the mantle into space, where it gathered together to become the moon. Later, however, computer simulations showed that most lunar material actually came from the impactor instead, which means the impactor's composition must have resembled Earth's mantle. No one knows where this impactor came from, but had it not blasted itself to smithereens by bashing Earth, it might have survived as another planet of our solar system.

Recent studies have questioned the original impactor scenario. That’s because most objects hitting modern Earth—like meteorites—differ from our world, having different proportions of oxygen isotopes. A 2007 computer simulation, for example, suggested that only about 1% of the large objects hitting early Earth matched its isotopic composition. Thus, few impactors would have produced a moon with a composition so similar to that of Earth. This has caused scientists to propose alternative models. One idea was that Earth spun very fast when the impactor hit, so that centrifugal force lifted a large amount of mantle material into the moon, thereby explaining the similarity in composition.

Now, new work finds that a much greater percentage of these impactors actually matched Earth's composition, thereby bolstering the original collision hypothesis. Planetary scientists Alessandra Mastrobuono-Battisti and Hagai Perets of the Technion-Israel Institute of Technology in Haifa and Sean Raymond of the Laboratory of Astrophysics of Bordeaux in France have analyzed simulations of how the inner solar system may have formed through the collision and merger of 1000 to 2000 protoplanets into three or four final planets. Although the compositions of the final planets were very different, as expected, when the scientists looked at the composition of the final large impactor that struck each planet it was often quite similar in makeup to the planet it hit. "What we found is that these two bodies [planet and impactor] can be very similar," Mastrobuono-Battisti says. As the scientists report online today in Nature, 20% to 40% of the planet-impactor pairs had similar enough compositions to produce the sort of Earth-moon similarity we see today.

"This is a very important piece of the puzzle," says Robin Canup, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. She had grown pessimistic that the standard model was the right answer. "At face this makes it look like [that] scenario is the most probable," she says.