An ancient fireball turned miles of the world’s driest desert into glass

By Ashley Strickland, CNN

The Atacama Desert in Chile has been used as a way to simulate alien environments, like Mars on Earth. Now, researchers believe it was the site of an ancient comet explosion intense enough to create giant slabs of silicate glass, according to a new study.

The research published Tuesday in the journal Geology.

About 12,000 years ago, intense heat turned Atacama’s sandy soil into vast areas of glass stretching for 46.6 miles (75 kilometers), but researchers weren’t sure what caused such a drastic change.

The Atacama Desert is the driest desert region on Earth, with incredibly little moisture or precipitation. The fragmented desert glass contains tiny mineral fragments that are often found in meteorites that land on Earth.

The minerals found in this glass matched up with particles collected by NASA’s Stardust mission, which sampled a comet known as Wild 2. The researchers are confident that the minerals found in the Chilean desert are what’s left after a comet similar to Wild 2 exploded over the sands and melted them.

“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, study author and a professor emeritus of geological science at Brown University and research professor at Brown’s department of earth, environmental and planetary sciences, in a statement. “To have such a dramatic effect on such a large area, this was a truly massive explosion. Lots of us have seen bolide (bright meteor) fireballs streaking across the sky, but those are tiny blips compared to this.”

The startling fields of glass, which appear dark green or black, stretch across an area east of the Pampa del Tamarugal plateau, located between the Andes Mountains and the Chilean Coastal Range. While volcanic activity can create this kind of glass, there was no evidence to support that the Atacama glass was formed that way.

Previously, researchers have suggested that ancient fires were the cause. The area once hosted grassy wetlands derived from rivers. If those ancient grasses burned in widespread wildfires, some believe it may have created the glass.

However, the glass itself is more complicated. Up close, it appears the glass pieces had been twisted, folded, rolled and thrown while they were still molten. This, the researchers say, would only be possible with an airburst explosion that can unleash winds rivaling those of tornadoes.

A chemical analysis of the glass revealed zircons, or minerals that thermally decomposed to form baddeleyite crystals. This change can only happen when temperature spike above 3,000 degrees Fahrenheit, which would definitely exceed the heat generated by grass fires.

The analysis also showed minerals like cubanite and troilite, both found in the Wild 2 comet and meteorites.

“Those minerals are what tell us that this object has all the markings of a comet,” said Scott Harris, study coauthor and a planetary geologist at the Fernbank Science Center in Georgia, in a statement. “To have the same mineralogy we saw in the Stardust samples entrained in these glasses is really powerful evidence that what we’re seeing is the result of a cometary airburst.”

The researchers want to focus on dating the glass to determine its exact age, as well as the potential size of the comet, but their current expectation that the impact occurred 12,000 years ago aligns with when large mammals disappeared from the area.

“It’s too soon to say if there was a causal connection or not, but what we can say is that this event did happen around the same time as when we think the megafauna disappeared, which is intriguing,” Schultz said. “There’s also a chance that this was actually witnessed by early inhabitants, who had just arrived in the region. It would have been quite a show.”

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