for National Geographic News
The moon was formed from fragments of Earth after a collision with a giant asteroid relatively late in our planet's formation, new tests of moon rocks show.
The finding upends many of the prior theories for how the moon came to be, researchers say.
Scientists have long believed that the moon was formed by a collision between our planet and a Mars-size object.
Computer models have shown that in this scenario 80 percent of the moon's material should have come from the asteroid, with only 20 percent from Earth.
But the new study of moon rocks collected three decades ago by Apollo astronauts, however, found that Earth and the rocks were too similar for that to be the case.
The most likely explanation is that the moon was formed primarily of Earthly material, the authors say.
Lead author Mathieu Touboul of the Swiss Federal Institute of Technology in Zurich said there is another theory that may explain its formation.
"Alternatively, the material from which the moon eventually formed was a magma disk, connected to the Earth by a common atmosphere," he said in a statement.
Material from Earth and the nascent moon could then be exchanged via a shared metal-vapor atmosphere.
By the time the two worlds had settled back down and begun drifting apart, their compositions would have been virtually identical.
"New simulations of such a process have recently shown that such exchange is possible," Touboul said.
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