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Atmospheric comet explosion may have set Earth on fire 12,800 years ago

Earth’s geological history is written in layers of dirt and stone – evidence of climatic shifts, ancient cultures, and celestial impacts. The Younger Dryas period is one of the most widely studied examples of extreme environmental change.

The significant climatic shift abruptly ended a warming trend and ushered in a near-glacial period. The Younger Dryas Impact Hypothesis suggests this cooling event was triggered by a cosmic airburst, a fragmented comet crashing into Earth’s atmosphere approximately 12,800 years ago.

Evidence of a Younger Dryas airburst

Recently, scientists have unveiled more evidence supporting this hypothesis. Traces of cosmic airbursts were found scattered across several sites in the eastern United States (New Jersey, Maryland, and South Carolina).

The new evidence includes materials indicative of the force and temperature involved in such an event – like platinum, microspherules, meltglass, and shock-fractured quartz.

“What we’ve found is that the pressures and temperatures were not characteristic of major crater-forming impacts but were consistent with so-called ‘touchdown’ airbursts that don’t form much in the way of craters,” said UC Santa Barbara Professor James Kennett, who led the study.

Understanding the Younger Dryas period

The Younger Dryas holds great importance for climate researchers. It offers valuable insights into the sensitivity and potential for rapid changes in Earth’s climate system. This period perfectly illustrates the interconnectedness of our oceanic, atmospheric, and terrestrial systems.

As we move towards the conclusion of our journey through the Younger Dryas, it is worthwhile noting the significance of understanding this past climatic event.

The Younger Dryas signifies a sudden return to icy conditions, disrupting the gentle warming of the Earth’s climate following the last Ice Age. This chilly phase spanned roughly from 12,900 to 11,700 years ago.

Potential causes

Scientists have suggested various reasons that could have led to the Younger Dryas. For instance, a large pool of freshwater entering the North Atlantic could have messed with the customary flow of ocean currents.

Shifting solar activity might have had an effect on the global climate. Even a comet or asteroid’s impact could have sparked a cooling effect all over the world.

The Younger Dryas ushered in a swift drop in temperature, down by 10°C in some places. Ice caps and glaciers grew in size, while many regions saw their moisture levels drop. Wind routes and ocean currents underwent major shifts.

Abrupt ending

This abrupt transformation also influenced the global vegetation layouts. It led to the extinction of several large animal species and nudged human societies to tweak their hunting and gathering maneuvers.

As suddenly as it had started, the Younger Dryas period ended, with temperatures in Greenland shooting up by 10°C in just ten years. This swift rise in temperature marked the advent of the Holocene epoch, the geological period we find ourselves in today.

Recognizing and learning from the dramatic changes that occurred during this relatively short period can offer us valuable insights into the workings of our planet. It underscores the fact that our climate system is sensitive and can change rapidly.

Cosmic impacts on Earth

Earth is showered daily by tons of celestial debris, from minuscule dust particles to cataclysmic impacts such as the Chicxulub event that caused the extinction of dinosaurs about 65 million years ago.

While the former hardly leaves a trace on our planet, the latter leaves massive craters, like the 150-kilometer-wide impact crater on the Yucatan Peninsula in Mexico.

Somewhere in between are impacts that, while not forming craters, still create significant destruction.

An example is the 1908 Tunguska event, which flattened a hefty 2,150 square-kilometer swath of Siberian forest despite the asteroid being only 40 meters diameter and disintegrating almost 10 kilometers above ground.

Tracing the Younger Dryas culprit

The comet that supposedly triggered the Younger Dryas cooling episode was significantly larger. It is estimated to have been 100 kilometers wide before fragmenting into thousands of pieces.

It scattered a sediment layer that stretches across much of the northern hemisphere and even some locations south of the equator.

This layer contains high levels of rare materials associated with cosmic impacts, such as iridium and platinum, and materials formed under high temperatures and pressures, like microspherules, meltglass, and nanodiamonds.

Shocked quartz: A clue from the past

One type of evidence that is particularly interesting to the researchers is shocked quartz. This form of quartz contains a pattern of lines that represent exposure to intense pressure – great enough to deform the crystalline structure of the quartz.

Shocked quartz grains are commonly found in impact craters but associating them with cosmic airbursts has been challenging.

The researchers noted that these quartz grains did not always present the “traditional” shocked quartz with parallel fractures.

Instead, they discovered grains with an irregular, web-like pattern of intersecting, meandering lines and surface and subsurface fissures.

The research suggests that these deformations are due to the relatively lower pressures caused by explosions that take place above the ground, as opposed to impacts that make contact with the Earth.

Cataclysmic event with far-reaching effects

These insights, coupled with other findings, strengthen the case for a fragmented comet that not only caused widespread burning but also triggered climatic change resulting in the extinction of 35 genera of megafauna in North America and the collapse of the flourishing Clovis human culture.

As scientists continue to unearth new clues about our planet’s geological history, we are given a more in-depth understanding of the celestial events that shaped our world and the life that inhabits it.

The study is published in the journal Airbursts and Cratering Impacts.


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