Tuesday , January 26 2021

In Greenland under the glacier ice

Asteroid strikes hit Earth for its 4.5 billion-year history, and some of them are responsible for global extinctions, such as the death of dinosaurs about 66 million years ago. But the planet bears the scars of only a few hundred of such blows due to weathering processes, such as erosion.

Now, using radar measurements related to the breakthrough of ice, the researchers found a huge impact crater, which was well preserved on almost a kilometer of glacial ice in the northwestern part of Greenland. By analyzing the size of craters and precipitation from the crater, they calculated that the asteroid that crashed into Earth was more than a kilometer in diameter, consisting mainly of iron. This discovery suggests that the Earth may have more signs of its turbulent history under its glaciers and ice sheets.

Strange depression

Marc Fahnestok, a glaciologist from the University of Alaska Fairbanks, and his colleagues did not intend to find an asteroid effect. They used the archival measurements of the Greenland ice sheet to study its ice flows and underlying bedrock. But when a group of Danish explorers noticed a circular depression in a rock on the northwestern edge of the ice sheet beneath the Hyavath Glacier, their curiosity was aroused.

“There are only a few ways to get a closed depression,” said Fakhnastok. "The river cannot do this." The asteroids were probably guilty, and it so happened that some of the Danish researchers worked at the Natural History Museum of Denmark at the University of Copenhagen, which houses a collection of iron meteorites from Greenland. The hunt continued.

The data clearly showed a circular depression about 31 kilometers in diameter with a raised rim and a raised area near its center.

By May 2016, researchers received support from a private foundation to fund a series of flights over the mysterious depression in the Hyavath Glacier. Flying at an altitude of only 350 meters in the converted DC-3 plane, the researchers used ice-protection radar to accurately display layers of ice and rocks beneath the Hyavath glacier. The data clearly showed a circular depression about 31 km in diameter with a raised rim and a raised area near its center, a geometry characteristic of a impact crater.

Shocked grains and gold

In order to better limit whether this depression was really caused by an asteroid strike, Fahnastok and his colleagues collected sandy sediments that flowed out from under the Hyavath glacier. Even in the laboratory, scientists found that some of the quartz grains in the sediments showed that a high-pressure shock wave passed through the rocks.

“This is an exposure diagnosis,” said Fahnestk. Along with affected quartz grains, scientists also found elevated levels of elements such as nickel, cobalt, chromium and gold, an elemental signature indicating an iron meteorite.

This crater, if confirmed, will become the 25th largest crater on Earth according to the Earth Impact Database, the team writes in its article that was published last week in Scientific achievements, According to researchers, the asteroid that created this crater had a diameter of about 1.5 kilometers. For comparison, it was estimated that the asteroid that cut down the dinosaurs and created the crater Chicxulub on the Yucatan Peninsula was about 10 kilometers in diameter.

No debris?

One lasting secret is the absence of any leaks associated with this alleged cataclysm that would scatter debris around the perimeter. Four deep ice cores were drilled in central and northern Greenland, but none of them recovered emissions.

“Thorough work done on deep ice cores would not have missed the blanket,” said Fahnestok.

One theory to explain this discrepancy is that an asteroid hit Greenland, traveling north, which would have mostly dispersed debris in the north and created an asymmetric ejection layer. It is not known whether any of the iron meteorites located at the University of Copenhagen came from this event.

The exact age associated with this crater is difficult, Fahnestk said, but it is possible to establish younger and older boundaries. The crater cannot be younger than 11,000 years old, because the ice deposited during the Holocene is not broken; if the asteroid had fallen during the Holocene, these layers would have been destroyed.

And the discovery of shocked grains that are still being carried away from the lower side of the glacier is a hint that the crater is probably no older than several million years, said Fahnestok. "If a [the crater] were frozen again and again, this sediment will disappear. ”

Chris Lowry, a paleogeographer from the Institute of Geophysics at the University of Texas at Austin, who studied the Chicxulub crater, suggests turning to the ocean to reach the age of this crater. “I hope someone plans to take the oceanic sediment cores nearby to place the crater ejection into the exact context of the climate records that are stored in the marine sediments,” said Lowry, who did not participate in the study.

Environmental effects

Regardless of the exact age of the crater, the impact probably had a significant impact on the environment. The tremendous amount of heat generated during the impact was supposed to melt near the ice. The researchers suggest that the subsequent flow of melt-water-fresh water into the ocean may affect global patterns of ocean circulation.

"This impact most likely had significant environmental consequences in the Northern Hemisphere and, possibly, on a global scale."

Since fresh water is more buoyant than salt water, the influx of glacial melt can slow down or stop the heating of cold, dense water in the North Atlantic. This dive is a critical component of ocean circulation and weather patterns of the planet. "This impact most likely had significant environmental consequences in the Northern Hemisphere and, possibly, on a global scale," the team wrote in its article.

Fakhnastok and his colleagues hope to return to Greenland. Their previous fieldwork there was brief — only a few days — and they want to nail down more details about the crater. “If you want to know the age and geochemical signatures of the impact,” said Fahnestk, “you probably want to drill ice and restore the stone.

-Katherine Corney (email: [email protected]; @katherinekornei) Freelance Science Journalist

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Citation: () ,, Eos, ,
DOI: 10,1029 /.
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