New study reveals evidence of aerial comet explosions dating back 12,800 years ago

Researchers continue to expand the case for the Younger Dryas impact hypothesis. The idea suggests that a fragmented comet crashed into Earth’s atmosphere 12,800 years ago, causing widespread climate change that, among other things, led to the abrupt reversal of Earth’s warming trend and to an anomalous quasi-glacial period called the Younger Dryas.

Now, UC Santa Barbara Professor Emeritus James Kennett and colleagues report the presence of proxies associated with the cosmic airburst spread across several distinct sites in the eastern United States (New Jersey, Maryland and South Carolina), materials indicative of the force and temperature involved in such an event, including platinum, microspheres, molten glass, and shock-fractured quartz. The study appears in the journal Air explosions and craters.

“What we found was that the pressures and temperatures were not characteristic of major cratering impacts, but were consistent with so-called ‘touchdown’ aerial explosions that do not form not a lot of craters,” Kennett said.

The Earth is bombarded every day by tons of celestial debris, in the form of tiny dust particles. At the other end of the scale are extremely rare and cataclysmic impacts like the Chicxulub event that caused the extinction of dinosaurs and other species 65 million years ago. Its 150-kilometer-wide impact crater is located in Mexico’s Yucatán Peninsula.

In between are impacts that do not leave craters on the Earth’s surface but are nonetheless destructive. The shock wave from the 1908 Tunguska event destroyed 2,150 square kilometers (830 square miles) of forest, as the asteroid about 40 meters (130 feet) in diameter collided with the atmosphere nearly 10 kilometers (6 miles) above the Siberian taiga.

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

Shocked quartz and amorphous silica

Researchers are particularly interested in the presence of shocked quartz, indicated by a pattern of lines, called lamellae, that exhibit stresses large enough to distort the crystal structure of quartz, a very hard material. This “cream of the crop” of cosmic impact evidence is present in impact craters, but linking shocked quartz to cosmic airborne explosions has proven to be more of a challenge.

“In the extreme form, like when an asteroid hits the Earth’s surface, all the fractures are very parallel,” Kennett explained. In the realm of cosmic air explosions, different variables are present. “When you think about it, the pressures and temperatures that produce these fractures vary depending on density, entry angle, impact altitude and impactor size.

“What we found – and this is characteristic of the impact layer, called the Younger Dryas boundary – is that while we occasionally see examples of ‘traditional’ shocked quartz with parallel fractures in quartz grains, we mostly see grains that are not parallel,” he said. These fractures appear as an irregular, web-like pattern of sinuous lines and surface and subsurface cracks, contrasting with the parallel, planar deformations of the shocked quartz associated with impact found in craters. These subparallel and subplanar deformations are due in large part to the relatively lower pressures caused by explosions that occur above ground, the researchers say, as opposed to impacts that make contact with Earth.

What these sediments share with the shocked quartz of crater sites is the presence of amorphous silica (molten glass) in these fractures. And that, the researchers say, is evidence of the combination of high pressure and temperatures (above 2,000 degrees Celsius) that could have come from a low-altitude fireball aerial explosion. Similarly, fractured quartz grains and molten glass have been found in more recent samples from surface explosions, such as at the Trinity atomic bomb test site in New Mexico. The approximately 20-kiloton bomb exploded atop a 30.5-meter (100-foot) tower.

These low-pressure shocked quartz grains join a growing suite of impact indicators that together argue for a fragmented comet that not only caused widespread fires, but also abrupt climate change that led to the extinction of 35 genera of megafauna in North America, such as mammoths and giant sloths, and led to the collapse of a thriving human culture called Clovis, according to the researchers.

“There are a whole range of different shocked quartzes, so we need to demonstrate in a well-documented way that they are indeed important for interpreting cosmic impact, even if they don’t reflect a traditional major crater-forming event,” he said. Kennett said. “These are very low altitude aerial explosions, almost certainly associated with a cometary impact.”