Evidence points to 'smashup' moon origin

U.S. astronomers say they've confirmed a long-held theory that the moon broke off from Earth as a result of a cosmic body the size of Mars smashing into Earth. Researchers at Washington University in St. Louis say an analysis of moon rocks collected during NASA Apollo missions provided evidence for the conclusion -- a tiny but significant variation in the amount of the element zinc that would have condensed out of the roiling cloud of vaporized rock created by such a catastrophic collision. The moon rocks, geochemists discovered, while otherwise chemically similar to Earth rocks, were woefully short on so-called volatiles, easily evaporated elements. A giant impact could explain this depletion, whereas alternative theories for the moon's origin do not, the researchers said. The rocks could be depleted of volatiles because they had been vaporized in a huge impact, they said. The Giant Impact Theory, proposed in 1975, suggests the moon was created in an apocalyptic collision between a planetary body called Theia (in Greek mythology the mother of the moon Selene) and the early Earth. The collision released so much energy it melted and vaporized Theia and much of the proto-Earth's mantle, the theory holds, and the moon then condensed out of the cloud of rock vapor.