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The impacts of asteroids have hammered the Earth during its 4.5 billion years of history, and some have been responsible for global extinctions such as the disappearance of dinosaurs there is about 66 million years old. But the planet only bears the scars of a few hundred of these impacts because of climatic processes such as erosion.
Now, using radar measurements penetrating the ice, the researchers have detected a huge well-preserved impact crater under nearly a kilometer of ice in northwestern Greenland. By analyzing the crater size and crater sediments, they estimate that the asteroid that struck the Earth was more than one kilometer in diameter, composed mainly of iron. This discovery suggests that the Earth could harbor more traces of its tumultuous history under glaciers and ice sheets.
A strange depression
Mark Fahnestock, a glaciologist at the University of Alaska Fairbanks, and his colleagues did not look for an asteroidal impact. They used archival measurements of the Greenland Ice Sheet to study its ice flows and bedrock. But when a group of Danish researchers noticed a circular depression in the bedrock north-west of the pack ice, under the Hiawatha glacier, their curiosity was piqued.
"There are only a few ways you can get a closed depression," Fahnestock said. "A river can not do it." One of the potential leaders was an asteroid impact. Some Danish researchers have worked at the Natural History Museum of Denmark's University of Copenhagen, which houses a collection of ferrous meteorites. from Greenland. The hunt was on.
The data clearly revealed a circular depression of about 31 kilometers in diameter with a raised ledge and elevated area near its center.
In May 2016, researchers had secured the support of a private foundation to fund a series of flights over the mysterious depression of the Hiawatha Glacier. Flying at an altitude of only 350 meters in a converted DC-3 aircraft, the researchers used radar penetrating the ice to accurately map layers of ice and rock under the Hiawatha Glacier. The data clearly revealed a circular depression about 31 kilometers in diameter with a raised rim and an elevated area near its center, characteristic geometry of an impact crater.
Shocked grains and gold
In order to better determine if this depression was actually caused by an asteroid impact, Fahnestock and his colleagues collected sandy sediments that had flowed under the Hiawatha Glacier. Back in the laboratory, scientists discovered that some of the quartz grains in the sediments showed evidence that a high-pressure shock wave had passed through the rocks.
"It's an impact diagnosis," Fahnestock said. In addition to the shocked quartz grains, scientists have also discovered high levels of elements such as nickel, cobalt, chromium and gold, elemental signature indicative of a ferrous meteorite.
This crater, if confirmed, will be the 25th largest crater in the world according to the Earth Impact database, writes the team in its article published last week in Progress of science. The researchers estimated that the asteroid at the origin of this crater had a diameter of about 1.5 km. For comparison, it was calculated that the asteroid that shot down the dinosaurs and created the Chicxulub Crater in the Yucatan Peninsula was about 10 kilometers in diameter.
No debris?
A persistent mystery is the absence of rock ejections associated with this so-called cataclysm, which would have scattered debris from afar. Four deep ice cores were drilled in central and northern Greenland, but none recovered from ejecta.
"The careful work done on the deep ice cores would not have missed a layer of ejecta," Fahnestock said.
One theory to explain this discrepancy is that an asteroid struck Greenland as it headed north, which would have mainly resulted in scattered debris to the north and created an asymmetrical ejecta layer. It is not known if one of the iron meteorites hosted at the University of Copenhagen came from this event.
It's hard to date this crater by age, says Fahnestock, but it's possible to place younger and older kiosks. The crater can not be less than 11,000 years old, because the ice deposited during the Holocene period is not disturbed; if the asteroid had struck during the Holocene, these layers would have been destroyed.
And finding shocked grains that are still washed away from the bottom of the glacier is a clue that the crater is probably not older than a few million years ago, said Fahnestock. "Yes [the crater] had been glaciated over and over again, this kind of sediment would have disappeared. "
Chris Lowery, paleoceanographer of the Institute of Geophysics at the University of Texas at Austin, who studied the crater of Chicxulub, suggests turning to the ocean to date this crater. "I hope somebody is considering recovering nearby ocean sediment cores to relocate the crater ejecta in the precise context of the weather records kept in marine sediments," Lowery said. did not participate in the research.
Environmental consequences
Regardless of the exact age of the crater, the impact likely had a significant impact on the environment. The enormous amount of heat generated during the impact would have melted the ice nearby. The researchers suggest that the resulting meltwater flow – freshwater – has subsequently affected global patterns of ocean circulation.
"This impact is very likely to have significant environmental consequences in the northern hemisphere and perhaps even the world."
Freshwater being more buoyant than salt water, an influx of glacial melt could have slowed or halted the sinking of a dense and cold water in the North Atlantic. This sinking is an essential element of ocean circulation and, by extension, the weather conditions of the planet. "This impact is very likely to have significant environmental consequences in the northern hemisphere and perhaps in the world," the team wrote in its article.
Fahnestock and his colleagues hope to return to Greenland. Their previous fieldwork was brief – only a few days – and they look forward to more details on the crater. "If you want to know the age and the geochemical signatures of the impact," Fahnestock said, "you probably want to drill through the ice and grab rocks."
-Katherine Kornei (email: [email protected]; @katherinekornei), Independent Scientific Journalist
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