Posted on February 11, 2019
A NASA glaciologist has discovered a potential second impact crater buried in more than a mile of ice in northwest Greenland. This follows the November 2018 report of a 19-mile wide crater beneath the Hiawatha Glacier – the first meteorite impact crater ever discovered beneath the earth's ice sheets. Although the newly found impact locations in northwest Greenland are only 114 miles apart, they do not seem to have formed at the same time.
If the second crater has a breadth of over 22 miles, it is ultimately confirmed as the result of a meteor impact, it will be the 22nd largest impact crater on Earth.
"We've surveyed the Earth in various ways from land, air and space – it's exciting discoveries like these are still possible," said Joe MacGregor, a glaciologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Prior to the discovery of the Hiawatha impact crater (see video below), scientists generally conclude that most of the evidence of past influences in Greenland and the Antarctic is due to inexorable erosion from the overlying Ice had been destroyed.
After finding this first crater, MacGregor examined topographic maps of the underlying rock of Greenland ice for signs of other craters, using images of the ice surface from the Moderate Resolution Imaging Spectroradiometer instruments aboard the NASA satellites Terra and Aqua he soon noticed a circular pattern about 114 Miles southeast of Hiawatha Glacier. The same circular pattern was also seen in ArcticDEM, a high-resolution digital elevation model of the entire Arctic, derived from commercial satellite imagery.
"I began to wonder, is this another impact crater? Do the underlying data support this idea? "Said MacGregor. "It was exciting to identify a big impact crater under the ice, but now it looks like there might be two of them."
To confirm his suspicion about the possible presence of a second impact crater, MacGregor examined the raw radar Images used to map the topography of the rock beneath the ice, including those collected by NASA's IceBridge operation. What he saw under the ice were several characteristic features of a complex impact crater: a shallow, bowl-shaped depression in rock surrounded by a raised rim and centrally located peaks that form when the crater floor balances after impact. Although the structure is not as circular as the Hiawatha crater, MacGregor estimated the diameter of the second crater to be 22.7 miles. Measurements of Operation IceBridge also showed a negative gravity anomaly over the area characteristic of impact craters.
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"The only circular structure that could approach this size would be a collapsed volcanic caldera," MacGregor said. "However, the areas of known volcanism in Greenland are several hundred miles away. Also, a volcano should have a distinct positive magnetic anomaly, and we do not see that at all.
Although the newly found impact craters in northwestern Greenland are only 114 miles apart, they do not appear to have formed the same time. From the same radar data and ice cores gathered nearby, MacGregor and his colleagues found that the ice in the area was at least 79,000 years old. The ice sheets were smooth, indicating that the ice was not severely disturbed during this time. As a result, either the impact occurred more than 79,000 years ago or, if more recently, an impacted ice had long since flowed out of the area and been replaced by more distant ice.
The researchers then looked at erosion rates: they calculated that a crater of this size would initially have been more than half a mile deep between the rim and the ground, which is an order of magnitude above the current depth. Considering a number of plausible erosion rates, they calculated that it would have taken between about a hundred thousand years and a hundred million years for the ice to bring 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 sheets over this second crater are clearly older than those above Hiawatha, and the second crater is about twice as strong," MacGregor said. "If the two formed at the same time, the probably thicker ice over the second crater would have been compared much faster to the crater than to Hiawatha."
To calculate the statistical probability that the two craters were generated by unrelated impact events The MacGregor team used recently published estimates that use the influence rates of the moon to record the Earth's impacts, which are harder to detect , to understand better. By using computer models that can track the production of large craters on Earth, they discovered that the abundance of these craters, which naturally should be close together without requiring a double strike, is in line with Earth's crater record.
"This does not exclude the fact that the two new Greenland craters were made in a single event, such as the effects of a well-separated binary asteroid, but we can not speak for it," planetary scientist William Bottke said with the Southwest Research Institute of Boulder, Colorado, and co-author of both MacGregor's article and the new study on the effects of lunar months.
Two pairs of unrelated but geographically close craters have already been found in Ukraine and Canada. The age of the craters in the pairs is different.
"The existence of a third pair of unrelated craters is not surprising, but we do not consider it unlikely," MacGregor said. "Overall, the evidence we've collected indicates that this new structure is likely to be an impact crater, but at the moment it seems unlikely that it will be a twin with Hiawatha."
Credit: Science / AAAS
The Daily Galaxy on NASA / Goddard Space Flight Center