Minerals in giant impact crater may be clues to moon's makeup, origin

Mineralogical discoveries in the moon's largest impact crater could yield clues to the evolution of our companion's crust and mantle, U.S. researchers say. The evidence is data from the Moon Mineralogy Mapper that flew aboard India's Chandrayaan-1 lunar orbiter showing a diverse mineralogy in the subsurface of the giant South Pole Aitken basin, scientists at Brown University reported Monday. The differing signatures detected in the crater -- at 1,500 miles wide the largest impact basin on the moon and perhaps the largest in the solar system -- could be of minerals dredged up at the time of the giant impact 4 billion years ago, they said. While the distinct minerals could have formed as the molten surface rock from the impact cooled, recent research at Brown and elsewhere suggests it's also possible the mineral differences reflect differences in rock types that were there before the giant impact. If the diversity reflects pre-existing material, the impact basin could hold important clues about the composition of the moon's lower crust and mantle, the Brown researchers said. "If you do the impact scaling from models, [the SPA impact] should have excavated into the mantle," Brown graduate student and study leader Dan Moriarty said. "We think the upper mantle is rich in a mineral called olivine, but we don't see much olivine in the basin. That's one of the big mysteries about the South Pole Aitken basin. So one of the things we're trying to figure out is how deep did the impact really excavate. If it melted and excavated any material from the mantle, why aren't we seeing it?" If the impact did excavate mantle material, and it doesn't contain olivine, that would have substantial implications for models of how the moon was formed, he said.