Debris from a comet may have leveled an ancient village in Syria during a spate of several such explosions occurring around the world, according to new research.
The village of Abu Hureyra was a mound settlement in northern Syria around 13,000 years ago. The site was quickly excavated in 1972 and 1973, before the Euphrates River was dammed, flooding the site beneath Lake Assad. But the hurried excavations exposed charcoal-rich surfaces containing glass spheres formed from melting soil, melted iron- and sulfur-rich samples, and nanodiamonds. Such materials are all indicators of extremely high temperatures like those produced by a chunk of rock exploding in the air.
A team led by Andrew Moore, an archeologist at the Rochester Institute of Technology in New York who led the emergency excavations of the site in the '70s, recently reexamined some of the excavated material in greater detail. The scientists then developed experimental methods to replicate the materials they discovered at the village.
"These provided new insights into how the meltglass was formed and how plant and other materials became incorporated in it," Moore told Space.com by email.
Melting the minerals found in the soil requires temperatures over 3,630 degrees Fahrenheit (2,000 degrees Celsius), "hot enough to cause the quartz grains to boil," Moore said. That suggests something cataclysmic.
"It is impossible to explain these melted minerals on meltglass by any natural process other than a cosmic impact event," Moore said.
A deadly explosion
The first settlers of Abu Hureyra were hunter-gatherers who lived off the land. A terrible drought drove the people to begin cultivating grains that they had previously collected from the wild, turning them into the first known farmers, previous research has revealed.
Then, about 13,000 years ago, something very bad seems to have occurred, leaving a layer of carbon suggesting dramatic fires. But for much of the last decade, scientists inspecting the remnants of the village have debated what happened, unable to decide whether the carbon formed during an airburst or during more mundane fires among the thatched huts.
So Moore decided to reexamine the glass in more detail. His analysis of the glass composition matched a 2012 finding claiming an airburst had destroyed Abu Hureyra, suggesting that the villagers' bucolic lifestyle ended suddenly when one or more fragments from a passing comet exploded in the air nearby.
"People who were in or near the village of Abu Hureyra at the time the airburst exploded would have seen an immense flash in the sky, equivalent to a nuclear explosion," Moore said. "A few seconds later, they would have been incinerated by the blast emanating from the airburst. The heat wave destroyed the village and everything in it, leaving a layer of burned material across the surface."
Observers several tens of kilometers from the site would have seen the flash, heard the explosion, and felt the heatwave, but likely survived the detonation.
Moore and his colleagues heated fragments of the glass in a laboratory furnace until they had fully melted, which occurred at 2,400 F (1,300 C), establishing a lower limit for the temperature the spheroids had originally been exposed to. But it took higher temperatures for the quartz and other particles on the exterior to melt.
The researchers also compared the Abu Hureyra material with glass melted at other prehistoric impact sites on Earth and found many similarities. The wealth of meltglass dating to roughly the same timeframe suggests to researchers that thousands of pieces of debris shed from a comet slammed into Earth's atmosphere 12,800 years ago, impacting more than 40 sites across North America and Europe.
The new findings by Moore's team match a 2007 hypothesis that Earth experienced several multi-continental airbursts. Since an individual comet or asteroid large enough to cause such widespread destruction is unlikely, the researchers suspect the disparate impacts were possibly caused by cometary debris.
"The largest cometary debris clusters are proposed to be capable of causing thousands of airbursts within a span of minutes across one entire hemisphere of Earth," the authors wrote. "An encounter with such a million-km-wide debris cluster would be thousands of times more probable than a collision with a 100-km-wide comet or a 10-km-wide asteroid."
The research was published March 6 in the journal Scientific Reports.
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