Impact Theory Gets Whacked

Dan is a deputy news editor for Science.

Where did the moon come from? For 3 decades, planetary scientists have agreed that it happened something like this: While the solar system was still forming, a body the size of Mars struck Earth a glancing blow that reduced both to rubble. The cloud of debris reformed itself into the modern Earth and moon. This "giant impact" theory neatly explained why the rocks Apollo astronauts brought back from the moon closely resembled rocks on Earth—or so it seemed at first. Actually, recent computer models show, such a collision wouldn't have scrambled the two bodies together enough to explain the similarity. Meeting last month in London to discuss the problem, scientists agreed that the origin of the moon must have been messier and more complicated than anyone had assumed.

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Posted in Space