A NASA scientist has discovered a possible second impact crater, with a width of over 36 km, buried under ice in northwest Greenland, the US space agency said. This follows the discovery of a 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 are only 183 km apart, at present they do not appear to have formed at the same time, NASA said. 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 - and 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 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 at 183 km southeast of Hiawatha Glacier. The same pattern also showed up in ArcticDEM, a high-resolution digital elevation model of the entire Arctic derived from commercial satellite imagery.

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.”

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.

To calculate the statistical likelihood that the two craters were created by unrelated impact events, MacGregor’s team used recently published estimates that leverage lunar impact rates to better understand Earth’s harder-to-detect impact record. By employing computer models that can track the production of large craters on Earth, they found that the abundance of said craters that should naturally form close to one another, without the need for a twin impact, was consistent with Earth’s cratering record. Indeed, two pairs of unrelated but geographically close craters have already been found in Ukraine and Canada, but the ages of the craters in the pairs are different from one another.

“The existence of a third pair of unrelated craters is modestly surprising but we don’t consider it unlikely. On the whole, the evidence we have assembled indicates that this new structure is very likely an impact crater, but presently it looks unlikely to be a twin with Hiawatha,” MacGregor said.