Scientists Find ‘Chemical Fingerprint’ of Comet Airburst That May Have Ignited the Decline of Hopewell Culture

About 1,500 years ago, a comet may have exploded in Earth's atmosphere over North America.

"What's fascinating is that many different tribes have similar stories of the event," says University of Cincinnati anthropologist Kenneth Tankersley, who is a member of the Piqua Tribe of Alabama. "The Miami tell of a horned serpent that flew across the sky and dropped rocks onto the land before plummeting into the river. When you see a comet going through the air, it would look like a large snake. The Shawnee refer to a 'sky panther' that had the power to tear down a forest. The Ottawa talk of a day when the sun fell from the sky. And when a comet hits the thermosphere, it would have exploded like a nuclear bomb."

In corroboration with Indigenous oral histories, scientists have found micrometeorites and other “chemical fingerprints” of a cosmic event at 11 archeological sites associated with the Hopewell culture, a network of Indigenous nations connected by trade routes between 200 B.C.E. to 500 C.E. They describe their findings in a study published in the journal Scientific Reports. Researchers suspect the event could have been the beginning of the ancient civilization’s decline, reports David Nield for Science Alert.

The Hopewell culture was centered in southern Ohio, with connected groups spread across North America as far north as the Canadian Great Lakes and as far south as the Gulf Coast. Hopewell villages grew corn, beans and squash, and they collected nuts, seeds and berries. They handcrafted elaborate thin pottery as well as copper jewelry and other metal objects to trade at elaborate ceremonies. They also built massive earthworks, or geometrically-designed mounds of dirt used for burial or ritual purposes, per the National Park Service.

By 400 C.E., notable features of Hopewell culture gradually disappear from the archeological record. According to Archaeology magazine, the number and quality of mounds diminished along with the importance of ritual, art, and trade.

The fiery blast of a comet tearing through the atmosphere would have devastated the landscape below, clearing forests, damaging agriculture and possibly wiping out villages. These losses would have greatly disrupted crop harvest, limited access to resources and likely interrupted trade activity, reports Vishwam Sankaran for the Independent.

"It looks like this event was very injurious to agriculture. People didn't have good ways to store corn for a long period of time. Losing a crop or two would have caused widespread suffering," says study author David Lentz, a biologist at the University of Cincinnati, in a statement.

This type of cosmic event is called an airburst because a comet or meteorite explodes high in the Earth's atmosphere, instead of striking the surface. Airbursts create immense heat, release shockwaves and shower micrometeorites onto the ground. The debris has a “chemical fingerprint,” Tankersley, who co-authored the study, says in a statement.

"Cosmic events like asteroids and comet airbursts leave behind high quantities of a rare element known as platinum," Tankersley says. "The problem is platinum also occurs in volcanic eruptions. So we also look for another rare element found in non-terrestrial events such as meteorite impact craters: iridium. And we found a spike in both iridium and platinum."

Using radiocarbon and typological dating, researchers estimate the cosmic event occurred between 252 and 383 C.E. Researchers also found a charcoal layer within sediment, which suggests the area was exposed to extreme heat. They estimate the explosion in the atmosphere could have scorched 9,200 square miles, an area the size of New Jersey.

The researchers also note that the find coincides with 69 near-Earth comets that Chinese astronomers documented during this timeframe, per the Independent.

Archeologists have also found that the Ohio Hopewell peoples collected the micrometeorites and forged jewelry and pan flutes out of them. They also constructed a comet-shaped mound called Milford Earthworks that the Hopewell people may have constructed near the airburst's epicenter, per a statement.

Currently, the earthwork is nearly gone and sits beneath a cemetery, per the Washington Post. The remaining images and evidence of the earthwork were illustrated in 1823 by United States Army Corps of Engineers and included in a book called Ancient Monuments of the Mississippi Valley published by the Smithsonian Institution in 1848.

Comet airbursts have wreaked havoc on Earth before. One of the most well-known meteorite airburst events was the Tunguska event, when a meteorite over the Podkamennaya Tunguska River in Russia exploded in 1908. The force flattened about 830 square miles, destroyed 80 million trees, and shattered windows hundreds of miles away.

Past research has also suggested war or climate change may have caused the dissipation of Hopewell society, though the civilization may have eventually collapsed due to several causes, including a cosmic airburst. Researchers are planning more studies to understand further what occurred 1,500 years ago.

"It's hard to know exactly what happened," Lentz said in a statement. "We only have a few points of light in the darkness. But we have this area of high heat that would have been catastrophic for people in that area and beyond."

Elizabeth Gamillo | | READ MORE

Elizabeth Gamillo is a daily correspondent for Smithsonian and a science journalist based in Milwaukee, Wisconsin. She has written for Science magazine as their 2018 AAAS Diverse Voices in Science Journalism Intern.