Sunday, November 07, 2021

Massive comet exploding over Chile 12,000 years ago may have created strange glassy rocks


By Meghan Bartels 3 days ago

Dark silicate glass is seen against the Atacama Desert in northern Chile.
(Image credit: P.H. Schultz/Brown University)

Be grateful you weren't in what is now Chile's Atacama Desert 12,000 years ago.

Today, a 47-mile (75 kilometers) swath of the desert is strewn with strange dark, glassy rocks that have long puzzled scientists. New research finds that those rocks are quite similar to comet particles collected by a NASA mission. The scientists behind the work now think that all those years ago, one or more huge comets exploded together in the skies over the region, causing tornado-force winds, scorching a wet, grassy landscape, and scattering the area with warped and twisted glass that still contains minerals generally found only in meteorites.

"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," Pete Schultz, a geologist at Brown University in Rhode Island, said in a statement.

"To have such a dramatic effect on such a large area, this was a truly massive explosion," he said. "Lots of us have seen bolide fireballs streaking across the sky, but those are tiny blips compared to this."

Schultz and his colleagues wanted to compare the strange Chilean glass, which ranges from dark green to black, with the work of a NASA mission called Stardust, which launched in 1999. Stardust visited a comet called Wild-2 to trap particles of its dust, which the spacecraft delivered to Earth in 2006. The samples were scientists' first-ever pristine samples of cometary dust from out beyond the orbit of the moon, according to NASA.

For the new research, Schultz and his colleagues collected 300 pieces of the strange glassy rock to analyze from two patches of the region. In addition, the team cut thin, polished slices out of 20 of those samples to study under a microscope. In all of those slices, the researchers found dozens of grains and fragments that didn't match the region.

Those anomalies included a mineral called troilite, which is typically found in meteorites, and decomposed zircons that suggest the samples reached temperatures hotter than 3,000 degrees Fahrenheit (1,670 degrees Celsius). Other traits the researchers noted aren't exclusive to meteorites but are quite common in an extraterrestrial context and match what scientists have seen in meteorites or the Wild-2 samples.

"Those minerals are what tell us that this object has all the markings of a comet," Scott Harris, a planetary geologist at the Fernbank Science Center in Atlanta and a co-author on the new study, said in the same 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."

In addition, the researchers noted signs of a dramatic past on the glass, which show "sliding, shearing, twisting, rolling, and folding (in some cases, more than twice) before being fully quenched," the scientists wrote in the paper.

Scientists have considered other scenarios to explain the strange glass over the years. At the time, the region featured oases and grassy wetlands, so some have suggested that vast grass fires could have melted the sandy soil to form the glassy rocks.

"There may be lots of these blast scars out there, but until now we haven't had enough evidence to make us believe they were truly related to airburst events," Schultz said. "I think this site provides a template to help refine our impact models and will help to identify similar sites elsewhere."

Scientists can't yet precisely date the glassy rocks, although other research on the phenomenon has suggested the strange rocks formed between 12,300 and 11,500 years ago.

Intriguingly, that's close to the time when the remains of large mammals like horses and ground sloths stop showing up in the fossil record, and around when some scientists think humans started building settlements in northern Chile — although there's no way to know right now whether any of the three events are connected.

"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."

The research is described in a paper published Tuesday (Nov. 2) in the journal Geology.

An ancient fireball turned kilometres of the world's driest desert into glass

Ashley Strickland
CNNDigital
Thursday, November 4, 2021 


Researchers believe the Atacama Desert in Chile was the site of an ancient comet explosion intense enough to create giant slabs of silicate glass, according to a new study
. (P.H. Schultz/Brown University/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 75 kilometres, 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 or 1648 degrees Celsius, 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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