Following the discovery of the 19.2-mile (31 km) wide Hiawatha impact crater beneath the northwest margin of the Greenland Ice Sheet, Dr. Joe MacGregor of NASA’s Goddard Space Flight Center and colleagues explored satellite and aerogeophysical data in search of additional such craters and found a possible second impact crater that is 22.7 miles (36.5 km) wide and 114 miles (183 km) southeast of the Hiawatha crater. The discovery is described in a paper in the journal Geophysical Research Letters.
Just 114 miles from the recently-discovered Hiawatha impact crater under the ice of northwest Greenland, lies a possible second impact crater. The 22.7-mile-wide feature would be the second crater found under an ice sheet, and if confirmed, would be the 22nd-largest crater on Earth. Image credit: NASA’s Goddard Space Flight Center / Jefferson Beck.
Before the discovery of the Hiawatha 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, Dr. MacGregor and co-authors checked topographic maps of the rock beneath Greenland’s ice for signs of other craters.
Using imagery of the ice surface from NASA’s Terra and Aqua satellites, they soon noticed a circular pattern some 114 miles 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.
To confirm their suspicion about the possible presence of a second impact crater, the researchers 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 they 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.
“The only other circular structure that might approach this size would be a collapsed volcanic caldera. 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,” Dr. MacGregor said.
Although the two impact craters in northwest Greenland are only 114 miles apart, they do not appear to have been formed at the same time.
From the same radar data and ice cores that had been collected nearby, the team determined that the ice in the area was at least 79,000 years old. The layers of ice were smooth, suggesting the ice hadn’t been strongly disturbed during that time. This meant that either the impact happened more than 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.
The scientists then looked at rates of erosion: they calculated that a crater of that size would have initially been more half a mile deep between its rim and floor, which is an order of magnitude greater than its present depth.
Taking into account a range of plausible erosion rates, they calculated that it would have taken anywhere between roughly a hundred thousand years and a hundred million years for the ice to erode the crater to its current shape — the faster the erosion rate, the younger the crater would be within the plausible range, and vice versa.
“The ice layers above this second crater are unambiguously older than those above Hiawatha, and the second crater is about twice as eroded. If the two did form at the same time, then likely thicker ice above the second crater would have equilibrated with the crater much faster than for Hiawatha,” Dr. MacGregor said.
To calculate the statistical likelihood that the two craters were created by unrelated impact events, the 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, the scientists 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.
“This does not rule out the possibility that the two new Greenland craters were made in a single event, such as the impact of a well separated binary asteroid, but we cannot make a case for it either,” said Dr. William Bottke, a planetary scientist at the Southwest Research Institute.
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’ve assembled indicates that this new structure is very likely an impact crater, but presently it looks unlikely to be a twin with Hiawatha,” Dr. MacGregor said.
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Joseph A. MacGregor et al. A Possible Second Large Subglacial Impact Crater in Northwest Greenland. Geophysical Research Letters, published online February 11, 2019; doi: 10.1029/2018GL078126
