Peppered across the Atacama Desert in Chile are mysterious shards of dark, gnarled glass. A total of 47-miles of the desert landscape is strewn with patches of dark silicate glass, and scientists have long wondered how the glass came to be. The opaque glass is similar to minerals collected from comets during NASA's Stardust mission.
Now, researchers suspect the glass may have formed from an exploding comet that entered Earth's atmosphere 12,000 years ago, reports Vishwam Sankaran for the Independent. The study was published this month in the journal Geology.
"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 the study's first author Pete Schultz, a Brown University geologist, in a statement.
Schultz and his team collected 300 pieces of the rock from the desert and cut 20 of those samples into thin slices to view under the microscope, reports Meghan Bartels for Space.com. When observing the pieces under the microscope, the researchers saw particles and grains that did not match those found in the gritty region.
Some researchers suspected the glass resulted from ancient grass fires because the area was not always a vast desert, according to a Brown University statement. During the Pleistocene era, the region had trees, grassy wetlands and rivers extending from the mountains to the east. Large grass fires may have rolled through, hot enough to char the sandy soil into sleek glass. However, the amount of glass present in the desert today and some of the shards' physical characteristics make this hypothesis nearly impossible.
When observing the glass, researchers found it showed evidence of being gnarled, twisted, and rolled while still molten liquid. This evidence is consistent with the glass forming from an enormous incoming meteor, with an airburst explosion that would have created tornado-force winds, the university statement explains.
Further examination of the glass found minerals called zircons that were charred enough to form baddeleyite. The transition from zircon to baddeleyite occurs at over 3,000 degrees Fahrenheit, which is way hotter than temperatures a grass fire can produce. Grass fires can reach, in extreme cases, 1,472 degrees Fahrenheit. Chemical analysis of the glass identified minerals only found in extraterrestrial rocks and minerals, such as cubanite, troilite, pyrrhotite lath, or calcium-aluminum-rich inclusions. These minerals were also found in dust collected from the Wild-2 comet in 2004 by NASA during the Stardust mission, reports Isaac Schultz for Gizmodo. Stardust, launched in 1999, was the first mission to return a comet sample and other extraterrestrial material to Earth from outside the moon's orbit.
"Those minerals are what tell us that this object has all the markings of a comet," study co-author Scott Harris, a planetary geologist at the Fernbank Science Center, explained 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."
When the comet burst in the Earth's atmosphere is still up for debate, and the research team is working on pinning down the exact ages of the glass to determine for sure when the event took place.
"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 in a statement. "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."