New study reveals comet airburst evidence from 12,800 years ago

By Jurassic JennJun 28, 2024 10:05 AMScience
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New study reveals comet airburst evidence from 12,800 years ago
New study reveals comet airburst evidence from 12,800 years ago. Source: astrobiology.com

New evidence supporting the Younger Dryas Impact hypothesis has emerged from a recent study led by UC Santa Barbara emeritus professor James Kennett and his team. The hypothesis suggests that approximately 12,800 years ago, a fragmented comet collided with Earth's atmosphere, triggering a transformative climatic shift known as the Younger Dryas. Kennett's study, published in the journal Airbursts and Cratering, examines the distribution of proxies associated with the cosmic airburst across multiple sites in the eastern United States—specifically, New Jersey, Maryland, and South Carolina. This was reported by SSPDaily.

The materials discovered within these sites, including platinum, microspherules, meltglass, and shock-fractured quartz, provide valuable indicators of the force and temperature involved in such an event. However, it is noteworthy that these proxies align with the characteristics of "touchdown" airbursts rather than major crater-forming impacts, as emphasized by Kennett. Correspondingly, the pressures and temperatures present in the findings are consistent with the proposed mechanism of a fragmented comet airburst but deviate from those typically associated with significant crater formation.

Understanding the impact of celestial bodies on Earth involves considering a wide spectrum of events. While humankind experiences daily bombardment of minuscule dust particles, catastrophic events like the Chicxulub impact—responsible for the extinction of dinosaurs and numerous species—are exceedingly rare. The aftermath of more moderate impacts, which lack significant craters, can still result in destructive consequences. For instance, the Tunguska event in 1908 devastated 2,150 square kilometers (830 square miles) of the Siberian taiga when an asteroid roughly 40 meters (130 ft) in diameter collided with the atmosphere above the region.

In the case of the Younger Dryas, the suspected comet responsible for the cooling episode is estimated to have had a width of 100 kilometers (62 miles), making it substantially larger than the object involved in the Tunguska event. Additionally, the comet fragmented, resulting in thousands of smaller pieces. Although the sediment layer stemming from the airburst covers vast areas of the Northern Hemisphere, its traces extend to locations south of the equator. This layer exhibits elevated levels of rare materials associated with cosmic impacts such as iridium and platinum, as well as evidence of high-pressure and high-temperature environments, including magnetic microspherules, meltglass, and nanodiamonds.

Of particular interest to researchers like Kennett is the presence of shocked quartz. The characteristic lamellae pattern resulting from stress significant enough to deform the crystal structure of quartz can be observed in impact craters. However, definitively linking shocked quartz to cosmic airbursts has presented challenges. Major crater-forming impacts exhibit uniformly parallel fractures, while the realm of cosmic airbursts encompasses variables such as density, entry angle, impact altitude, and object size.

Surprisingly, the Younger Dryas Boundary, representing the impact layer, showcases quartz grains with fractures that mostly deviate from parallel patterns. Instead, they adopt an irregular, web-like formation of intersecting and meandering lines, along with surface and subsurface fissures. This contrast to the expected parallel fractures signifies lower pressures resulting from explosions above-ground—a characteristic feature of airburst events rather than impacts making direct contact with the Earth's surface.

Another similarity between these sediments and shocked quartz at crater sites lies in the presence of amorphous silica—melted glass—within the fractures. This glass material is clear evidence of the combination of pressure and high temperatures exceeding 2000 degrees Celsius, which is likely associated with a low-altitude bolide airburst. Comparative fractures in quartz grains and meltglass have been discovered in present-day samples from above-ground explosions, such as those seen at the Trinity atomic bomb test site in New Mexico, where a 20-kiloton bomb was detonated atop a 30.5 meter (100 foot) tower.

By including these lower-pressure shocked quartz grains in the range of impact proxies, researchers can build a stronger case for a fragmented comet's role in not only triggering widespread burning but also inducing abrupt climatic changes. The results of such changes led to the extinction of 35 genera of megafauna in North America, including species like mammoths and giant ground sloths, and accelerated the collapse of the Clovis culture—an established human civilization at the time.

James Kennett emphasizes the need to establish comprehensive evidence of the significance of these different forms of shocked quartz to interpret cosmic impacts, even if they do not indicate traditional major crater-forming events. Ultimately, the findings highlight the occurrence of "touchdown" airbursts from very low altitudes that are highly likely to be associated with cometary impacts.

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