A new study reveals evidence of a comet explosion 12,800 years ago

Researchers continue to expand the case for the Younger Dryas Impact hypothesis. The idea posits that a fragmented comet crashed into Earth’s atmosphere 12,800 years ago, causing a large-scale climate shift that led to, among other things, an abrupt reversal of Earth’s warming trend and an anomalous period of nearby glaciation called the Younger Dryas.

Now, UC Santa Barbara professor emeritus James Kennett and his colleagues report the presence of spaceflight-related proxies distributed at several separate locations in the eastern United States (New Jersey, Maryland, and South Carolina), materials indicative of the power and temperature of such an event, including platinum, microspheres, melts and impact fractured quartz. The study will appear in the journal Airbursts and craters.

“We found that the pressures and temperatures were not characteristic of large cratering impacts, but were consistent with so-called ‘landing’ airbursts, which do not form as much as craters,” Kennett said.

Earth is bombarded every day with tons of celestial debris in the form of tiny dust particles. At the other end of the scale are extremely rare and cataclysmic impacts, such as the Chicxulub event, which caused the extinction of dinosaurs and other species 65 million years ago. Its 150 kilometer wide (93 mi) impact crater can be found on the Yucatán Peninsula in Mexico.

Somewhere in between are impacts that don’t leave craters on the Earth’s surface, but are still destructive. The shock wave from the 1908 Tunguska event felled 2,150 square kilometers (830 sq mi) of forest when an asteroid roughly 40 meters (130 ft) in diameter collided with the atmosphere nearly 10 kilometers (6 mi) above the Siberian taiga.

The comet believed to be responsible for the Younger Dryas cooling episode is estimated to have been 100 kilometers (62 miles) across – much larger than the Tunguska object and fragmented into thousands of pieces. The layer of sediment associated with the airburst extends over much of the Northern Hemisphere, but can also be found in places south of the equator. This layer contains unusually high amounts of rare materials associated with cosmic impacts, such as iridium and platinum, and materials formed at high pressures and temperatures, such as magnetic microspheres (cooled metal droplets), molten glass, and nanodiamonds.

Shocked quartz and amorphous silica

Scientists are particularly interested in the presence of shocked quartz, indicated by a pattern of lines, called lamellae, that exhibits enough stress to deform the crystal structure of quartz, a very hard material. This “crème de la crème” of cosmic impact evidence is present in impact craters, however associating shocked quartz with cosmic impacts has proven more challenging.

“In an extreme form, like when an asteroid hits the Earth’s surface, all the fractures are very parallel,” Kennett explained. There are various variables present in the space raid realm. “When you think about it, the pressures and temperatures that cause these fractures will vary depending on the density, the entry angle, the height of the impact and the size of the impactor.

“What we found – and this is what is characteristic of an impact layer called the Younger Dryas Boundary – is that although we occasionally see examples of ‘traditional’ shocked quartz with parallel fractures in the quartz grains, mostly we see grains that are not parallel ,” he said. These faults are seen in an irregular, web-like pattern of intersecting meander lines and surface and subsurface cracks, in contrast to the parallel and planar deformations of the impact-related impact quartz found in the craters. These sub-parallel and sub-planar deformations are largely due to the relatively lower pressures caused by explosions that occur above the ground, the researchers say, as opposed to impacts that touch the ground.

What these sediments share with the shocked quartz at crater sites is the presence of amorphous silica—molten glass—in these fractures. And this, according to scientists, is evidence of a combination of pressure and high temperatures (higher than 2000 degrees Celsius) that could have come from the explosion of the bolide at low altitude. Similarly, broken quartz grains and molten glass have been found in present-day samples of above-ground explosions, such as at the Trinity atomic bomb test site in New Mexico. A roughly 20 kiloton bomb was detonated at the top of the 30.5 meter (100 ft) tower.

These low-pressure shocked quartz grains join a growing set of impact proxies that together make the case for a fragmented comet that not only caused widespread burning, but also a sudden climate change that resulted in the extinction of 35 genera of megafauna in North America, such as mammoths and giant ground sloths and, according to researchers, led to the collapse of a flourishing human culture called Clovis.

“There are a number of different shocked quartzes, so we have to make a well-documented case that they are indeed significant for cosmic impact interpretation, even if they don’t reflect a traditional large crater-forming event,” Kennett said. “They are from very low altitude ‘touchdown’ takeoffs almost certainly associated with a comet impact.”