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Buried under half a mile of snow and ice in Greenland, the scientists discovered an impact crater large enough to engulf the District of Columbia.
The discovery suggests that a giant iron asteroid crashed on what is today a glacier during the last Ice Age, an era known as the Pleistocene Era. which started 2.6 million years ago. When it ended only 11,700 years ago, a mega-fauna, like saber-toothed cats, went extinct while humanity had inherited the Earth.
The discovery could lead to a better understanding of the climate of glaciation and the effects of the eruption of debris on it resulting from such a cataclysmic collision.
"This is the first impact crater found under one of the ice floes on our planet," Kurt Kjær, a geologist at the Geogenetics Center of the Natural History Museum of Denmark and lead author of the study, said on Wednesday. in the journal Science Advances.
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In 2015, Dr. Kjær and a colleague were analyzing a map of NASA's Greenland when they noticed a huge circular depression on the Hiawatha Glacier at the northwestern tip of Greenland.
"A hidden landscape was beginning to emerge," said Dr. Kjær. "We looked at it and said:" What is it? "
At that moment, Mr. Kjær thought of the meteorite the size of a car exposed in the courtyard near his office in Copenhagen, which had been found by chance in northwestern Greenland. He and his colleague joked that the circular structure was perhaps a crater left by an asteroid.
After the laughter subsided, they realized that their suggestion might not have been exaggerated. "There are only many ways to create a circular function under a layer of ice," said Dr. Kjær.
In May 2016, for three days, his team flew over the crater in a German plane with a radar penetrating the ice, drawing imaginary grid lines on the surface.
The aerial survey confirmed the presence of a huge pit with a raised circular rim and elevating structures in the center, all obvious signs of an impact crater. The team's analysis showed that the Hiawatha crater was nearly 300 meters deep and 20 miles in diameter, making it one of the 25 largest impact craters on earth. although much smaller than that of 90 miles left by the impact of Chicxulub.
"As soon as you start looking for structures under the ice that look like an impact crater, Hiawatha stands out like a thumb sore," said Joseph MacGregor, a glaciologist at the NASA Goddard Space Flight Center in Maryland and co-author. author of the study.
The crater bowl rests well against the edge of the glacier, giving the wandering ice cap a semi-circular appearance visible from above. From this semicircle stands a tongue of white ice, a large river containing sediments from the bottom of the ice cap.
Dr. Kjær ventured into the floodplain by helicopter and collected sediment. He found that what he said were very shocked pieces of quartz, indicating that a violent impact had occurred at some point in the history of the region. Sediments in the area also had high concentrations of nickel, cobalt, chromium, gold and platinum, indicating that the meteorite was iron.
As the team has not yet searched for material ejected from the impact in ice cores, they can not establish a specific date for impact, unless to say that this it happened during the Pleistocene. The next step is to determine if the asteroid has crashed into a glacier or an area that was then covered with ice, and to assess the climatic effects of the impact.
Dr. Kjær still wonders if the meteorite in the courtyard in front of his office, which was found about 200 miles from the Hiawatha crater, might contain clues to better understand the ancient impact.
"Even though we have looked so much at the surface of the planet, with all types of equipment," said Dr. Kjær, "the age of discovery is not over yet."
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