Most people are already aware of the awe-inspiring scale of comets. These icy celestial bodies have been flinging themselves about all around the Solar System since its infancy, and have long been a considerable interest for study to astronomers and other scientists for what secrets they may hold within them.
One undeniable fact about these comets, though, is their constant presence in our cosmic neighborhood. These things may range in sizes from the small to the truly colossal, like the 150-km-wide Bernardinelli-Bernstein Comet that’s hurtling through the Solar System right now. It’s unavoidable to think of the possibility that these things might one day just graze past us or hit our planet—but what’s absolutely certain is the fact that comets have already hit our planet before.
Some of these impacts we already know all too well—perhaps the most famous one being the impact that bookended the Cretaceous Period, and the Mesozoic Era along with it, some 65 million years ago. Countless more have either hit or grazed our planet, but have left traces that were either buried under the waves or erased by erosion and time.
Sometimes, though, they leave traces that don’t seem that obvious at first. It really just takes a team with some time on their hands to figure out that features that we thought were just supposed to be there weren’t from Earthly origins at all. One such instance, recorded as a study published in the journal Geology, showcases just that: a feature in South America’s Atacama Desert that turned out to be a signature left behind by our icy wanderers from beyond the orbit of Neptune.
You see, east of a plateau within the desert named Pampa del Tamarugal are patches of dark material scattered across the dry soils. A team of scientists from Brown University took interest in these patches, as they appeared to be dark silicate glass.
The team may have recognized immediately that there’s a story to be found within these rocks. You see, silicate glass requires really high temperatures to form, much like how special furnaces are required to both make and mold glass articles like bowls and pitchers. Clearly something happened in this area that caused its temperature to go so high as to melt the surrounding rocks into silicate glass. From there, it’s just a matter of finding the identity of the culprit.
The team of scientists, led by Brown University Department of Earth, Environmental and Planetary Sciences professor emeritus Pete Schultz, then proceeded to date the dark silicate glass, as well as check its composition for possible signs of its origins. Dating found the glass to be around 12,000 years ago; nevertheless, it’s the characterization data that revealed its true nature.
Previous studies on these desert glass seemed to point to the culprit being ancient grass fires; after all, the Atacama Desert wasn’t always a desert. The morphology of the resulting glass, however, prompted Schultz and team to take a second look into the odd feature’s true origins.
Turns out, the silicate glass contained within itself crystals called zircon, which in this case “decomposed” into a mineral called baddeleyite. Baddeleyite can only form from zircon decomposition in temperatures north of 1,600 °C—a fact which, to the team, proves that whatever it was that caused the formation of these glasses, it surely must have been a high-temperature event.
The silicate glass’ internal structure also showed signs that they were “twisted, folded, rolled, and even thrown” while still molten, according to a press release by Brown University. The team regarded this as a sign that the high-temperature event must have been accompanied by an airburst explosion. To Schultz and team, the identity of their culprit was slowly coming into view.
The final piece of the puzzle that they needed came in the form of other crystals embedded in the desert glass. Within the samples they obtained were minerals like cubanite and troilite. While at first they seem innocuous, these minerals were also spotted in abundance from NASA’s previous Stardust mission, which collected samples from comets.
Said co-author and Fernbank Science Center planetary geologist Scott Harris: “Those minerals are what tell us that this object has all the markings of a comet. 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 [an] […] airburst.”
Combining these facts, one thing emerged as the true cause of these dark silicate glass swaths across the Atacama Desert; an object flying above the skies of the desert which exploded in midair, instantly melting and distorting the rocks below it into silicate glass. The presence of the odd crystals point to the culprit being a celestial object like a comet which exploded above the Atacama Desert some 12,000 years ago.
“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 Schultz 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 fireballs streaking across the sky, but those are tiny blips compared to this.”
A similar event is thought to have caused the end of a particular ancient city called Tall el-Hammam; it, however, arrived some 8,000 years after the Atacama Desert visitor barged into our atmosphere and created what must have been quite the show, had we been around to see the lights.
Schultz and team recognize that much work is to be done if we are to truly understand the events surrounding this odd desert feature. For one, a more detailed dating study is required to truly ascertain when this comet or asteroid arrived and caused havoc to an ancient Atacama Desert.
There was a chance, however, that the American continent’s earliest inhabitants may have played witness to this ancient fiery explosion. As Schultz put it: “It would have been quite a show.”
References
- Brown University. (2021a, November 2). Vast patches of glassy rock in Chilean desert likely created by ancient exploding comet. Brown University. https://www.brown.edu/news/2021-11-02/comet
- Brown University. (2021b, November 2). Vast patches of glassy rock in Chilean desert likely created by ancient exploding comet. ScienceDaily. https://www.sciencedaily.com/releases/2021/11/211102180530.htm
- Saplakoglu, Y. (2021, November 4). Mysterious glass in the Atacama Desert may be from an ancient exploding comet. LiveScience. https://www.livescience.com/glassy-rock-chile-atacama-desert-ancient-comet
- Schultz, P. H., Harris, R. S., Perroud, S., Blanco, N., & Tomlinson, A. J. (2021). Widespread glasses generated by cometary fireballs during the late Pleistocene in the Atacama Desert, Chile. Geology. https://doi.org/10.1130/G49426.1
- Strickland, A. (2021, November 3). An ancient fireball turned miles of this desert to glass. CNN. https://www.cnn.com/2021/11/03/world/atacama-desert-glass-comet-scn/index.html
