A scar beneath the ice
Hidden beneath nearly a kilometer of ice at the margin of the Hiawatha Glacier lies a massive impact crater, first identified in 2015 from airborne radar surveys. Data from NASA's Operation IceBridge and Germany's Alfred Wegener Institute revealed a circular bedrock depression 31 kilometers (19 miles) wide and about 320 meters (1,050 feet) deep. It is one of the 25 largest impact craters found on Earth. The structure is remarkably well-preserved, with a distinct elevated rim and a subtle central uplift, features of large complex craters.
The discovery was confirmed by analyzing sediments washed out from under the glacier. Field teams from the Natural History Museum of Denmark collected samples from the river draining the crater, which contained proof of a violent impact. These samples included shocked quartz grains, whose crystal structures are deformed in a specific way that only occurs under the immense pressure of an asteroid strike. Geochemical analysis of the sediment also identified the projectile: a fractionated iron asteroid, estimated to have been at least 1.5 kilometers (nearly a mile) wide. The energy released by this impact would have been millions of times greater than an atomic bomb.
Solving the age mystery
The crater's excellent preservation initially suggested it might be very young, geologically speaking. This led to a hypothesis that the impact occurred just 12,800 years ago. Such a recent event could have triggered the Younger Dryas, a sudden and intense period of global cooling that coincides with the extinction of North American megafauna. Accessing the crater directly for dating is impossible due to the 930 meters of ice covering it.
Scientists found a way to bypass obstacle by dating the impact-altered materials in the outwash sediments. Two independent methods were used on rocks and sand grains that were superheated by the impact and later carried out by meltwater. The first method, uranium-lead dating of shocked zircon crystals, yielded an age of 57.99 ± 0.54 million years. The second, argon-argon dating of impact-melted glass, confirmed this result. The impact occurred during the late Paleocene epoch, about 8 million years after the asteroid that wiped out the dinosaurs. At that time, Greenland was not covered by ice; it was a temperate landscape with rainforests. The new date definitively rules out any connection to the Younger Dryas event.