Around 12,000 years ago, something burned a vast swath of the Atacama Desert in northern Chile. The heat was so intense that it turned the sandy soil into large slabs of silicate glass. Now, an investigative team has come to a conclusion about what caused that hell. As published in the magazine
‘Geology’, it was the impact of a comet.
The researchers found that desert glass samples contain small fragments with minerals that are often found in rocks of extraterrestrial origin. Those minerals match the composition of the material returned to Earth by NASA’s Stardust mission, which sampled particles from a comet called Wild 2. The team concludes that these mineral assemblages are likely the remains of an alien object, very probably a comet with a composition. similar to Wild 2, the explosion of which melted the sandy surface below.
“This is the first time that we have clear evidence of crystals on Earth that were created by thermal radiation and winds from a fireball that exploded just above the surface,” says Pete Schultz, a professor at Brown University. “To have such a drastic effect on such a large area, this was a truly massive explosion. Many of us have seen bolide fireballs streaking through the sky, but those are small dots compared to this. “
The crystals are concentrated in patches throughout the Atacama Desert to the east of Pampa del Tamarugal, a plateau in northern Chile located between the Andes Mountains to the east and the Cordillera de la Costa de Chile to the west. The fields of dark green or black glass lie within a corridor that stretches for about 75 kilometers. There is no evidence that the crystals could have been created by volcanic activity, says Schultz, so their origin has been a mystery until now.
Tornado force winds
Some researchers have postulated that the glass was the result of ancient pasture fires, as the region was not always a desert. During the Pleistocene epoch, there was an oasis with trees and grassy wetlands created by rivers that ran from the mountains to the east. It has been suggested that the widespread fires could burn 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 crystals show evidence of having been twisted, bent, rolled, and even thrown while still in molten form. That is consistent with a large explosion from an incoming meteorite, which would have been accompanied by tornado-force winds.
Together with researchers from the Fernbank Science Center in Georgia, the Universidad Santo Tomás in Chile and the Geology and Mining Service of Chile, Schultz and his colleagues conducted a detailed chemical analysis of dozens of samples taken from glass deposits throughout the region.
The analysis found minerals called zircons that had thermally decomposed to form baddeleyite. This mineral transition generally occurs at temperatures above about 1,650ºC, much hotter than what could be generated by pasture fires.
The analysis also revealed sets of exotic minerals found only in meteorites and other extraterrestrial rocks. Specific minerals such as cubanite, troilite, and inclusions rich in calcium and aluminum matched mineral signatures from comet samples recovered from NASA’s Stardust mission.
“Those minerals are what tell us that this object has all the markings of a comet,” says Scott Harris, a planetary geologist at the Fernbank Science Center and a co-author of the study. “Having the same mineralogy that we saw in the Stardust samples in these crystals is really powerful evidence that what we are seeing is the result of a cometary outburst.”
Although the researchers acknowledge that more work is needed to establish the exact age of these crystals, Schultz believes that the impact coincides in time with the disappearance of large mammals from the region.
“It is too early to say whether there was a causal connection or not, but what we can say is that this event occurred around the same time as when we thought the megafauna disappeared, which is intriguing,” he says. “There is also the possibility that this was witnessed by the first inhabitants who had just arrived in the region. It would have been quite a show, “he adds.
The team also hopes to collect more data to find out the size of the impactor. Additionally, he believes this study can help identify similar blast sites elsewhere.