Greenland impact crater
© Kjær, et alThe Greenland impact crater, superimposed over Paris.
Scientists have discovered a crater beneath Greenland's Hiawatha Glacier that they say could be one of the 25 largest impact structures on Earth.

It's a 31-kilometre-wide circular bedrock depression up to a kilometre below the ice and was likely caused by a fractionated iron asteroid about a kilometre wide.

Its impact would have had substantial environmental consequences in the Northern Hemisphere and perhaps even more widely, say the researchers, led by led by Kurt Kjær from the University of Copenhagen, Denmark.

The crater is the only one of its size that retains a significant portion of its original surface topography.

The researchers are unsure of its exact age, but suggest it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet, more than two million years ago.

Using dating techniques, they inferred that the young ice covering the crater is "well behaved" but that deeper and older ice was debris-rich and heavily disturbed.

The findings are reported in a paper published in the journal Science.

The crater is the first of its kind to be discovered in northwest Greenland, and adds another important piece to the jigsaw that is the long-hidden landscape lying underneath its giant ice sheet.

"While airborne radar sounding of the Greenland Ice Sheet began in the 1970s, comprehensive surveying of the ice sheet has only become possible over the past two decades," the authors write.

After making their initial discovery, Kjær and colleagues retrieved three sediment samples deposited by a river draining out of the glacier. In one sample, angular quartz grains with small fluid inclusions were present and showed signs of being shocked by an impact.

Several of these grains consist of carbonaceous materials and glass that are likely derived from impact melting of mineral grains in the bedrock.

Further testing of subsamples found the sediment contained elevated concentrations of nickel, cobalt, chromium and gold, indicative of a relatively rare iron meteorite.