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In a study published in the journal Geology, researchers show that samples of the desert glass contain tiny fragments with minerals often found in rocks of extraterrestrial origin. Those minerals closely match the composition of material returned to Earth by NASA's Stardust mission, which sampled the particles from a comet called Wild 2. The team concludes that those mineral assemblages are likely the remains of an extraterrestrial object -- most likely a comet with a composition similar to Wild 2 -- that streamed down after the explosion that melted the sandy surface below.
"This is the first time we have clear evidence of glasses on Earth that were created by the thermal radiation and winds from a fireball exploding just above the surface," said Pete Schultz, a professor emeritus in Brown University's Department of Earth, Environmental and Planetary Sciences. "To have such a dramatic effect on such a large area, this was a truly massive explosion. Lots of us have seen bolide fireballs streaking across the sky, but those are tiny blips compared to this."
The glasses are concentrated in patches across the Atacama Desert east of Pampa del Tamarugal, a plateau in northern Chile nestled between the Andes Mountains to the east and the Chilean Coastal Range to the west. Fields of dark green or black glass occur within a corridor stretching about 75 kilometers. There's no evidence that the glasses could have been created by volcanic activity, Schultz says, so their origin has been a mystery.
Some researchers have posited that the glass resulted from ancient grass fires, as the region wasn't always desert. During the Pleistocene epoch, there were oases with trees and grassy wetlands created by rivers extending from mountains to the east, and it's been suggested that widespread fires may have burned hot enough to melt the sandy soil into large glassy slabs.
But the amount of glass present along with several key physical characteristics make simple fires an impossible formation mechanism, the new research found. The glasses show evidence of having been twisted, folded, rolled and even thrown while still in molten form. That's consistent with a large incoming meteor and airburst explosion, which would have been accompanied by tornado-force winds. The mineralogy of the glass casts further serious doubt on the grassfire idea, Schultz says. Along with researchers from the Fernbank Science Center in Georgia, Chile's Universidad Santo Tomás and the Chilean Geology and Mining Service, Schultz and colleagues performed a detailed chemical analysis of dozens of samples taken from glass deposits across the region.
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