Posted on February 11, 2019
A NASA glaciologist has discovered a possible other battleground buried under more than a mile of ice in northwest Greenland. This follows the result announced in November 2018 by a 19-km-wide crater under Hiawatha Glacier – the first meteorite war crater ever discovered under the Earth's ice sheet. Although the newly found sites in northwest Greenland are only 114 kilometers apart, they do not seem to have formed at the same time.
If the second crater, which has a width of over 22 miles, is ultimately confirmed as a result of a meteorite impact, it will be the 22nd largest crash crater found on Earth.
"We've explored the land in many different ways, from land, air and space – it's exciting that discoveries like these are still possible," said Joe MacGregor, a glaciologist with NASA's Goddard Space Flight Center in Greenbelt, Maryland, who attended both results.
In general, before the discovery of the Hiawatha impact crater (see video below), scientists assumed that most evidence of past impacts in Greenland and Antarctica would have been eradicated by unsustainable erosion of the overlying ice.
Following the finding of the first crater, MacGregor checked topographic rock maps under greenland ice for signs of other craters. Using images of the ice surface from the instrument's moderate resolution imaging spectral radio instruments onboard NASA's Terra and Aqua satellites, he soon saw a circular pattern about 114 miles southeast of the Hiawatha glacier. The same circular pattern also appeared in ArcticDEM, a high-resolution digital altitude model of the entire Arctic derived from commercial satellite images.
"I started asking myself" Is this another battleground? Do the underlying data support this idea? "MacGregor said." Helping to identify a large crater under the ice was already very exciting, but now it looked like there might be two of them. "
To confirm its suspicion of the possible presence of another battleground, MacGregor studied the radar radar images used to map topography of the bedrock below the ice, including those collected by NASA's Operation IceBridge. What he saw under the ice was several characteristic features of a complex battleground: a flat, bowl-shaped depression in the bedrock, surrounded by an elevated edge and centrally located peaks that form when the bottom of the crater balances the post-impact. Although the structure is not as clear circular as the Hiawatha crater, MacGregor estimated the second crater's diameter at 22.7 miles. Measurements from Operation IceBridge also revealed a negative gravity anomaly over the area, which is characteristic of crash craters.
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"The only other circular structure that can 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. A volcano must also have a clear positive magnetic anomaly, and we do not see it at all."
Although the newly found craters in northwest Greenland are only 114 kilometers apart, they do not seem to be formed at the same time. From the same radar data and ice cores that had been gathered nearby, MacGregor and his colleagues stated that the ice in the area was at least 79,000 years old. The impacts were smooth, suggesting that the ice was not severely disturbed during that time. This meant that either the impact occurred more than 79,000 years ago or – if it happened recently – someone had disturbed ice for a long time since the flight out of the area and was replaced by ice from longer inland.
The researchers looked at the degree of erosion: they calculated that a crater of this size would initially be more than half a mile deep between the rim and the floor, which is an order of magnitude greater than its current depth. Considering a number of probable erosion rates, they calculated that it would have taken anywhere between about a hundred thousand years and a hundred million years for the ice to destroy the crater to its present form – the sooner the erosion rate, the younger the crater would be within the plausible area, and the other way around.
"The strokes over this second crater are unequivocally older than those over Hiawatha, and the other crater is about twice as hollowed out," MacGregor said. "If the two are formed at the same time, thicker ice over the other crater would probably have equated with the crater much faster than for Hiawatha."
To calculate the statistical probability that the two craters were created by unrelated impact events, MacGregor's team used recently published estimates that exploit the lunar impact rates to better understand the Earth's harder-to-record power record. Using computer models that can track the production of large craters on Earth, they found that abundance of the said craters, which would naturally form close to each other without the need for a twin impact, was consistent with Earth's crater record.
"This does not exclude the possibility that the two new Greenlandic craters were made in a single event, such as the effect of a well-separated binary asteroid, but neither can we do anything for it either," says William Bottke, a planet scientist with Southwest Research Institute in Boulder, Colorado, and co-author of both MacGregor's paper and the new moon-stimulating record survey.
In fact, two pairs of independent but geographically close craters have already been found in Ukraine and Canada, but the crater's age in the pairs is different.
"The existence of a third pair of independent craters is modest, but we do not consider it unlikely," MacGregor said. "On the whole, they show evidence that we have gathered that this new structure is very likely a hit crater, but at the moment it seems unlikely to be twin with Hiawatha."
Image Credit: Science and AAAS
Daily Galaxy via NASA / Goddard Space Flight Center