A NASA glaciologist has discovered a possible second impact crater buried under more than a mile of ice in northwest Greenland.
Follow the discovery, announced in November 2018, of a 19-mile-wide crater beneath the Hiawatha Glacier, the first meteorite crater discovered beneath Earth's ice sheets. Although the newly found impact sites in northwest Greenland are separated by only 114 miles, at present they do not appear to have formed at the same time.
If the second crater, which has a width of more than 22 miles, is ultimately confirmed as a result of a meteorite impact, it will be the 22nd largest impact crater on Earth.
"We have inspected the Earth in different ways, from land, air and space, it's exciting that discoveries like these are still possible," said Joe MacGregor, a glaciologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who participated in both findings.
Prior to the discovery of the Hiawatha impact crater, scientists generally assumed that most of the evidence of past impacts in Greenland and Antarctica would have been erased by the relentless erosion caused by the ice. After the discovery of that first crater, MacGregor reviewed the topographic maps of the rock under the Greenland ice in search of signs of other craters. Using images from the surface of the ice from the moderate-resolution spectroradiometer instruments aboard NASA's Terra and Aqua satellites, he soon noticed a circular pattern about 114 miles southeast of the Hiawatha Glacier. The same circular pattern also appeared in ArcticDEM, a high resolution digital elevation model of the entire Arctic derived from commercial satellite images.
"I started asking myself," Is this another impact crater? Does the underlying data support that idea? ", Said MacGregor. "Helping to identify a large impact crater under the ice was already very exciting, but now it looked like there could be two of them."
MacGregor reported on the discovery of this second possible crater in Geophysical research letters on February 11.
To confirm his suspicion about the possible presence of a second impact crater, MacGregor studied the raw radar images used to map the topography of the bedrock under the ice, including those compiled by Operation IceBridge of the POT. What he saw under the ice were several distinctive features of a complex impact crater: a flat basin-shaped depression in the base rock that was surrounded by a raised rim and peaks located in the center, which form when the ground of the Crater is balanced after impact. Although the structure is not as clearly circular as the Hiawatha crater, MacGregor estimated the diameter of the second crater at 22.7 miles. Measurements from Operation IceBridge also revealed an anomaly of negative gravity over the area, which is characteristic of impact craters.
"The only other circular structure that could approach this size would be a collapsed volcanic caldera," said MacGregor. "But the areas of known volcanic activity in Greenland are several hundred miles away, in addition, a volcano should have a clear positive magnetic anomaly, and we do not see that at all."
While the newly found impact craters in northwest Greenland are separated by only 114 miles, they do not appear to have formed at the same time. 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 ice layers were smooth, suggesting that the ice had not been disturbed strongly during that time. This meant that the impact occurred more than 79,000 years ago or, if more recently, all the ice disturbed by the impact had left the area a long time ago and was replaced by ice from the interior.
Then, the researchers observed the erosion rates: they calculated that a crater of that size would initially have a depth of half a mile between its edge and the floor, which is an order of magnitude greater than its current depth. Considering a range of plausible erosion rates, they calculated that it would have taken between about one hundred thousand years and one hundred million years for ice to erode the crater to its current form: the faster the erosion rate, the younger the crater. The crater would be within the plausible range, and vice versa.
"The ice sheets on this second crater are unequivocally older than those on Hiawatha, and the second is about twice as eroded," said MacGregor. "If the two formed at the same time, then, probably, a thicker ice on the second crater would have balanced with the crater much faster than for Hiawatha."
To calculate the statistical probability that the two craters were created by unrelated impact events, the MacGregor team used recently published estimates that take advantage of lunar impact rates to better understand the hardest-to-detect impact record on Earth. Using computational models that can track the production of large craters on Earth, they found that the abundance of such craters that should naturally form close to each other, without the need for double impact, was consistent with Earth's crater history. .
"This does not rule out the possibility that the two new Greenland craters were created in a single event, such as the impact of a well separated binary asteroid, but we can not defend it either," said William Bottke, a planetary scientist. with the Southwest Research Institute in Boulder, Colorado, and co-author of the MacGregor article and the new lunar impact record study.
In fact, 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 each other.
"The existence of a third pair of unrelated craters is modestly surprising, but we do not consider it unlikely," said MacGregor. "In general, the evidence we have gathered indicates that this new structure is very likely an impact crater, but at present it seems unlikely that it is a twin with Hiawatha."
Huge crater discovered in Greenland by the impact that shook the northern hemisphere
Joseph A. MacGregor et al. A possible second great subglacial impact crater in northwestern Greenland, Geophysical research letters (2019). DOI: 10.1029 / 2018GL078126