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Competing ideas abound for how Earth got its moon

Earth's satellite may have formed from one giant impact or from about 20 small ones

By
6:00am, April 4, 2017
the moon

CRASH TEST   There is some debate about just what kind of collision formed the moon. 

The moon’s origin story does not add up. Most scientists think that the moon formed in the earliest days of the solar system, around 4.5 billion years ago, when a Mars-sized protoplanet called Theia whacked into the young Earth. The collision sent debris from both worlds hurling into orbit, where the rubble eventually mingled and combined to form the moon.

If that happened, scientists expect that Theia’s contribution would give the moon a different composition from Earth’s. Yet studies of lunar rocks show that Earth and its moon are compositionally identical. That fact throws a wrench into the planet-on-planet impact narrative.

Researchers have been exploring other scenarios. Maybe the Theia impact never happened (there’s no direct evidence that the budding planet ever existed). Instead of a single colossal collision, scientists have proposed that a string of impacts created miniature moons largely from terrestrial material. Those mini moons

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