A massive impact crater the size of the city of Paris has been discovered under the ice sheet in northern Greenland. Partially hidden under the Hiawatha Glacier, this 31-kilometer (19.3-mile) crater is estimated to be no older than three million years, but the researchers believe that it was formed much more recently, possibly as late as 12,000 years ago, making it the largest impact crater of its kind on Earth.

The object that formed the crater was an iron meteorite that is estimated to have been 1.5 kilometers (0.93 miles) across, that would have vaporized or melted 20 cubic kilometers (4.8 cubic miles) of the Earth’s crust when it struck. The impact would also have melted a massive amount of ice that would have been released into the North Atlantic, disrupting the North Atlantic current and triggering climate disruptions around the globe.

Initially spotted by a group of Danish geologists in 2015, the last three years have been spent studying both radar-sounding data from NASA’s Program for Arctic Regional Climate Assessment and Operation IceBridge, and visiting the site to look for signs of a meteorite impact.

"We immediately knew this was something special but at the same time it became clear that it would be difficult to confirm the origin of the depression," geologist Kurt H. Kjær of the Natural History Museum of Denmark explains. After spotting the crater in the radar data, the team set out to conduct direct surveys of the site.

"You can see the rounded structure at the edge of the ice sheet, especially when flying high enough," according to John Paden, an electrical engineer with the University of Kansas.

"For the most part the crater isn’t visible out the airplane window. It’s funny that until now nobody thought, ‘Hey, what’s that semicircular feature there?’ From the airplane it is subtle and hard to see unless you already know it’s there.

"Using satellite imagery taken at a low sun angle that accentuates hills and valleys in the ice sheet’s terrain – you can really see the circle of the whole crater in these images."

Surveys of the site included detailed radar soundings that revealed what is known as a complex crater: above a certain size, the rim of an impact crater collapses downward, while a mound forms at the center, where the liquefied rock has solidified after "splashing" upward in response to the initial downward force imparted by the impacting object.

The ground survey also conducted a geochemical analysis of the area that revealed the presence of iron, indicating that the meteor that formed the crater was largely made of iron. Shocked quartz, crystals that have a different microscopic structure from regular quartz crystals that are formed under intense pressure, were found, along with natural glass, formed from silica from the bedrock that had been melted by the intense heat of the impact.

At this point, the age of the crater is difficult to determine, with the current estimate being anywhere between 3 million years and 12,000 years, due to it being under a kilometer (nearly two-thirds of a mile) of ice. The research team is leaning toward a younger age, however, as the crater’s features are still quite fresh for being under an ice sheet, as ice is notorious for its rapid erosion of geological features. The team is considering drilling through the ice to take samples of the bedrock to narrow down the date of the impact.

Regardless of where the Hiawatha crater’s age falls within the 3-million to 12,000 year range, that makes it the largest impact crater of such a young age that has ever been discovered. The only crater larger than this one formed within the last 10 million years is the 52 kilometer (32.3 mile) Karakul crater in Tajikistan, and that is estimated to be around 5 million years old. The Earth does bear much more massive scars from far more violent impacts, but these occurred much earlier in Earth’s history. Amongst all known impact craters of all ages, the Hiawatha Crater now stands as the 25th largest in the world.

If the 12,000-year-old date does prove to be accurate, this adds to a growing body of evidence that a major impact event occurred over the Laurentide Ice Sheet at the end of the last ice age, causing a cataclysmic melt event that left its mark in not only the geological and climatological records, but also on the traditions of our ancient cultures

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