A NASA scientist has discovered a possible second impact crater, with a width of over 36 kilometres (km), buried under ice in northwest Greenland, the US space agency said Tuesday.
This follows the discovery of a over 30-km-wide crater beneath Hiawatha Glacier -- the first meteorite impact crater ever discovered under Earth's ice sheets -- announced in November last year.
Though the newly found impact sites in northwest Greenland are only 183 km apart, at present they do not appear to have formed at the same time, NASA said in a statement.
If the second crater is ultimately confirmed as the result of a meteorite impact, it will be the 22nd largest impact crater found on Earth, according to the findings published in the journal Geophysical Research Letters.
"We have surveyed the Earth in many different ways, from land, air and space -- it is exciting that discoveries like these are still possible," said Joe MacGregor, a glaciologist with NASA's Goddard Space Flight Center in the US, who participated in both findings.
Before the discovery of the Hiawatha impact crater, scientists generally assumed that most evidence of past impacts in Greenland and Antarctica would have been wiped away by unrelenting erosion by the overlying ice.
Following the finding of that first crater, MacGregor checked topographic maps of the rock beneath Greenland's ice for signs of other craters.
Using imagery of the ice surface from the Moderate Resolution Imaging Spectroradiometer instruments aboard NASA's Terra and Aqua satellites, he soon noticed a circular pattern some 183 km to the southeast of Hiawatha Glacier.
The same circular pattern also showed up in ArcticDEM, a high-resolution digital elevation model of the entire Arctic derived from commercial satellite imagery, NASA said.
To confirm his suspicion about the possible presence of a second impact crater, MacGregor studied the raw radar images that are used to map the topography of the bedrock beneath the ice, including those collected by NASA's Operation IceBridge.
What he saw under the ice were several distinctive features of a complex impact crater: a flat, bowl-shaped depression in the bedrock that was surrounded by an elevated rim and centrally located peaks, which form when the crater floor equilibrates post-impact.
Though the structure is not as clearly circular as the Hiawatha crater, MacGregor estimated the second crater's diameter at 36.5 km.
Measurements from Operation IceBridge also revealed a negative gravity anomaly over the area, which is characteristic of impact craters.
"The only other circular structure that might approach this size would be a collapsed volcanic caldera," MacGregor said.
"But the areas of known volcanic activity in Greenland are several hundred miles away. Also, a volcano should have a clear positive magnetic anomaly, and we don't see that at all," he said.
From the same radar data and ice cores that had been collected nearby, MacGregor and his colleagues determined that the ice in the area was at least 79,000 years old.
The layers of ice were smooth, suggesting the ice had not been strongly disturbed during that time.
This meant that either the impact happened over 79,000 years ago or -- if it took place more recently -- any impact-disturbed ice had long ago flowed out of the area and been replaced by ice from farther inland.
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