Earth is subjected to a constant cosmic rain of material. The vast majority of it is tiny micrometeorites that burn up in the atmosphere, up to 100 tons per day by some estimates.
But sometimes, much larger objects strike Earth. The most notable is probably the Chicxulub impactor that wiped out the dinosaurs and left a massive crater, now buried.
There are many other large potential impactors that explode above the surface, called touchdown airbursts, and their effect on Earth is much harder to quantify.
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New research suggests that a swarm of debris from an exploding comet left its mark by triggering the Younger Dryas, a period of abrupt cooling around 12,000 years ago. The authors say that the touchdown airburst and the resulting Younger Dryas led to the extinction of megafauna, and the disappearance of the Clovis culture.
Their findings support the Younger Dryas Impact Hypothesis (YDIH), which states that the impact of a disintegrating asteroid or comet is responsible for abruptly cooling the Earth.
The YDIH isn't widely accepted in the science community. Critics tout the lack of an impact crater as evidence against the YDIH. They also say that other evidence supporting it can best be explained by other causes.
New research found evidence of comet debris impacts at sites of the Clovis culture, which came to an end at the same time as the Younger Dryas. Will this lead to wider acceptance of the YDIH?
The study appears in PLOS One. It's titled "Shocked quartz at the Younger Dryas onset (12.8 ka) supports cosmic airbursts/impacts contributing to North American megafaunal extinctions and collapse of the Clovis technocomplex," and the lead author is James Kennett, the UC Santa Barbara Emeritus Professor of Earth Science.
The research is based on the discovery of shocked quartz at three well-known Clovis sites: Murray Springs in Arizona, Blackwater Draw in New Mexico and Arlington Canyon in California's Channel Islands.
"These three sites were classic sites in the discovery and the documentation of the megafaunal extinctions in North America and the disappearance of the Clovis culture," lead author Kennett said in a press release.
Shocked quartz consists of sand grains deformed by extreme pressure and heat. It was first discovered after underground nuclear weapons were tested. It's also found inside impact craters, and lightning is known to create it.
"When cosmic airbursts detonate with enough energy and at sufficiently low altitude, the resultant relatively small, high-velocity fragments may strike Earth's surface with high enough pressures to generate thermal and mechanical shock that can fracture quartz grains and introduce molten silica into the fractures," the authors write.
"Here, we report the discovery of shocked quartz grains in a layer dating to the Younger Dryas (YD) onset (12.8 ka) in three classic archaeological sequences in the Southwestern United States."
The researchers used 10 different analytical techniques, including electron microscopy, and found grains with glass-filled fractures that are very similar to the type created by nuclear explosions and found in 27 different impact craters. They were also produced in 11 laboratory shock experiments.
"All research, including this study, has found that non-shocked quartz fractures without glass filling are very common in non-impact layers, but quartz fractures filled with melted silica have only been reported in impact layers," the researchers write.
"These shocked grains co-occur with previously reported peak concentrations in platinum, meltglass, soot, and nanodiamonds, along with microspherules, similar to those found in ~28 microspherule layers that are accepted as evidence for cosmic impact events, even in the absence of a known crater," the researchers explain.
The YDIH states that the exploding comet also created widespread fires and choked the sky with ash, leading to the abrupt cooling that defines the Younger Dryas. In these harsh conditions, the Clovis culture collapsed and megafauna like woolly mammoths went extinct.
"In other words, all hell broke loose," Kennett said.
The YDIH has many proponents, and over the course of the last couple of decades, they've unearthed evidence in support of it. One piece of evidence is the "black mat" layer found in sediments at different locations, predominantly in the northern hemisphere.
YDIH proponents say this indicates the mass burning triggered by the airburst. Other evidence includes microspherules, nanodiamonds, and platinum.
"The YDB layer at the three sites was previously interpreted as resulting from multiple airbursts/impacts from large comet fragments based on peak abundances of inferred airburst/impact-related proxies," the authors write.
Impact craters are slam-dunk evidence of deadly impacts. But in their absence, according to the researchers, shocked quartz with glass-filled fractures is the next best thing. When combined with other evidence, their presence adds to the weight of the YDIH.
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The YDIH has faced and continues to face strong headwinds. Other researchers say there are other explanations for the evidence supporting the hypothesis. They also point out that there have been many episodes like the Younger Dryas and that no exploding comet is needed to explain any of them.
But the authors say that their new findings "provide strong support for the hypothesis," while also acknowledging that "this interpretation has faced challenges."
"By connecting the physical evidence of an impact event with well-established archaeological and paleontological records, our findings contribute to a more comprehensive understanding of this critical period in Earth's recent history," the researchers write.
"This research sheds light on past events and provides insights into the potential global effects of cosmic impacts on climate, ecosystems, and human societies," they conclude.
This article was originally published by Universe Today. Read the original article.