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Mysterious glass beads in shells suggest an ancient meteorite strike near Florida



A pair of microtectites found in old seashells.
Image: Kristen Grace, Florida Museum

Since their discovery 13 years ago, scientists have been thinking about the origin of tiny glass beads in antique seashells. New evidence suggests that these are microtectites – a byproduct of meteorite impacts – which are the first time that these celestial remains have been found in ancient shell shells, according to new research published in Meteoritics and Planetary Science. The research is still incomplete, but the discovery points to a previously unknown meteorite impact (or meteor strikes) off the coast of Florida about 2 to 3 million years ago.

"This is the first report on microtectites in Florida and one of the few found objects of space debris in the state," said Mike Meyer, lead author of the new study and researcher at Harrisburg University of Science and Technology Gizmodo. "These bullets can also help us date the bowl beds they were found in, since we do not have a precise age for them."

These tiny, spherical glass beads are created during the explosive impact of a meteorite falling into the earth strikes.

"When a sufficiently large impact occurs, the impactor evaporates – a meteorite or comet or something – mostly, but also the rock and the ground it hits," Meyer said. "Most rocks are minerals that contain silica. Therefore, much of it has to be melted. These molten debris fly away from the impact site and cool as they travel through the air, usually giving it a kind of aerodynamic shape. "

Four microtectites.
Image: Meyer et al., 2019

Finally, these materials cool in the atmosphere and rain back on the surface. Large pieces called tektites often form teardrops, and small droplets called microtectites produce bullets. Over time, these materials, like any other sediment, are transported by water and wound down to the sea, Meyer said, adding that this extraterrestrial material can then collect in mussel shells. Finally, these shells are covered with sediments and retain their contents for a long time.

This survey dates back to 2006, when Meyer was a student at the University of South Florida. He and his classmates searched Florida's Pinecrests beds looking for shells produced by single-celled organisms known as benthic foraminifera. Instead, they found dozens of glass beads in fossilized shells, most of them in fossils of Mercenaria campechiensis, also known as southern quahogs. These particles were very small and had a diameter of about 200 microns. In total, 83 of these mysterious bullets were found. At that time nobody knew what to do with these transparent spheres, which is why the pearls languished for years at the university.

Having had the time and inclination to re-examine the problem, Meyer decided to look at the old pearls with new eyes. Together with colleagues Roger Portell and Peter Harries, he analyzed the structures using scanning electron microscopy, backscatter imaging and X-ray spectroscopy, allowing the researchers to determine their composition. In addition, the researchers performed comparative analyzes with other microtectites, cosmic spheres (also called micrometeorites) and volcanic rocks.

Microtectite under the microscope.
Image: Meyer et al., 2019

"Volcanoes were excluded by the shape – our were mostly too round – and the composition," Meyer told Gizmodo. "There is the slightest possibility that it might be reflective paint or coal ash, but the shapes of our material were too uniform, and we found them in those enclosed shell fossils that were closed off and cut off from their environment for millions of years It seemed unlikely that any kind of human contamination would have had time to get in there, "he said.

Microtectites, according to Meyer, remained the only plausible answer. These materials consisted primarily of silicic acid and came from "the debris created by an impact event".

Interestingly, the microtectites were found in four different sediment depths, due either to multiple impact events or to the spread of materials from a single tectite bed. Looking to the future, Meyer wants to use the argon-argon radiometric dating method to date the beads. He would also like to find more bullets because his team has only a few dozen to work with. To this end, researchers plan to recruit citizen scientists looking for more material in southwest Florida.

In the press release, microtectites were referred to as "cosmic pearls," which is cute but not exactly accurate.

"It was a catchy name," Meyer admitted, "although I generally refer to them as" the bullets "as they are spherical" mollusks like shells or oysters.


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