The Mars-sized object that hit baby Earth some 4.5 billion years ago, creating plumes of debris that eventually coalesced into the moon, was a head-on strike, new research shows.
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An analysis of seven moon rocks collected by Apollo astronauts, one lunar meteorite and six volcanic rocks from Earth's mantle shows that the moon and Earth have nearly identical oxygen isotopes, an indication that Earth and second body, known as Theia ("mother of the moon"), ended up quite thoroughly mixed.
The finding, reported in this week's Science, runs contrary to previous research showing less mixing of the bodies.
"We don't see any difference between the Earth's and the moon's oxygen isotopes. They're indistinguishable," geochemist Edward Young, with the University of California Los Angeles, said in a press release.
If Earth and Theia collided in a glancing side blow, as previously hypothesized, most of the moon would be comprised of Theia and it would have a different lineup of oxygen isotopes than Earth.
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A head-on crash would cause more mixing, giving Earth and the moon a more similar list of building materials.
"Theia was thoroughly mixed into both the Earth and the moon and (was) evenly dispersed between them," Young said. "This explains why we don't see a different signature of Theia in the moon versus the Earth."