Hidden bowl A crater was discovered at the edge of Hiawatha Glacier in northwest Greenland. Airborne radar data revealed a round depression (bottom image) buried beneath almost a kilometer of ice. Researchers also found deformed quartz minerals and other signatures of an ancient impact within sediments collected just outside the edge of the ice (black circle). K.H. Kjær et al/Science Advances 2018

Members of the research team first spotted a curiously rounded shape at the edge of Hiawatha Glacier in northwest Greenland in 2015, during a scan of the region by NASA’s Operation IceBridge. The mission uses airborne radar to map the thickness of ice at Earth’s poles. The researchers immediately suspected that the rounded shape represented the edge of a crater, Kjær says.

For a more detailed look, the team hired an aircraft from Germany’s Alfred Wegener Institute that was equipped with ultra-wideband radar, which can send pulses of energy toward the ice at a large number of frequencies. Using data collected from 1997 to 2014 from Operation Icebridge and NASA’s Program for Arctic Regional Climate Assessment, as well as 1,600 kilometers’ worth of data collected in 2016 using the ultra-wideband radar, the team mapped out the inner and outer contours of their target.

The object is almost certainly an impact crater, the researchers say. “It became clear that our idea had been right from the beginning,” Kjær says. What’s more, it is not only the first crater found in Greenland, but also one of the 25 or so largest craters yet spotted on Earth. And it has held its shape beautifully, from its elevated rim to its bowl-shaped depression.

“It’s so conspicuous in the satellite imagery now,” says John Paden, an electrical engineer at the University of Kansas in Lawrence and a member of the team. “There’s not another good explanation.”

On the ground, the team hunted for geochemical and geologic signatures of an asteroid impact within nearby sediments. Sampling from within the crater itself was impossible, as it remains covered by ice. But just beyond the edge of the ice, meltwater from the base of the glacier had, over the years, deposited sediment. The scientists collected a sediment sample from within that glacial outwash and several from just outside of it.

The outwash sample contained several telltale signs of an impact: “shocked” quartz grains with deformed crystal lattices and glassy grains that may represent flash-melted rock. The sample also contained elevated concentrations of certain elements, including nickel, cobalt, platinum and gold, relative to what’s normally found in Earth’s crust. That elemental profile points not only to an asteroid impact, the researchers say, but also suggests that the impactor was a relatively rare iron meteorite.

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HT/David Hagen, ResourceGuy, Fred Nicol and maybe some I missed.