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Volume 90 Issue 43 | p. 34 | Concentrates
Issue Date: October 22, 2012

Lunar Formation Revisited

Models and observations tackle giant-impact theory of moon’s origin
Department: Science & Technology
Keywords: moon origin, zinc isotopes
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Credit: NASA/JPL/USGS
09043-scicon-mooncxd
 
Credit: NASA/JPL/USGS

Three new studies flesh out, and possibly resolve complications in, the theory that the moon formed when a giant body collided with Earth (Nature, DOI: 10.1038/nature11507; Science, DOI: 10.1126/science.1225542 and 10.1126/science.1226073). Although the impact formation idea has been popular since the 1970s, more recently discovered chemical similarities between Earth and the moon are hard to reconcile with the idea that the moon was created from remnants of a separate body. But two new simulations paint plausible scenarios. One model, from Matija ćuk and Sarah T. Stewart of Harvard University, simulates a small planet about half the mass of Mars striking a rapidly spinning young Earth. Another, from Robin M. Canup of Southwest Research Institute, simulates a much larger impact by a body the size of Earth itself. Both models produced moons chemically similar to Earth. In another study, Frédéric Moynier of Washington University in St. Louis finds that heavy zinc isotopes are more abundant on the moon than on Earth, bolstering the notion that volatile substances were purged from a nascent moon after a violent impact. In addition, this discovery could temper the hope, spurred by recent discoveries of hydrated lunar minerals, that the moon’s interior could be rich in water.

In this computer model, a planet half the size of Mars crashes into a young, rapidly spinning Earth; the moon eventually forms from the ring of debris.
Credit: Sarah Stewart
 
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