The day, some 4.5 billion years ago, was just five hours long, but it was a momentous one for Earth. A Mars-size object roaring in at 25,000 m.p.h. struck the young planet, already largely formed but devoid of life. The glancing blow hurled molten and vaporized debris into space, where it cooled, began circling Earth and eventually coalesced to form the moon.
This scenario, reported in the journal Nature last week, is drawn from a new computer simulation that goes far toward resolving puzzling inconsistencies in earlier studies of the moon's formation. That event was, of course, of overwhelming importance in...
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