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Massive Crater Discovered Under Greenland Ice
An international team of researchers, including a NASA glaciologist, have discovered a large meteorite impact crater hiding beneath more than a half-mile of ice in northwest Greenland. The crater — the first of any size found under the Greenland ice sheet — is one of the 25 largest impact craters on Earth, measuring roughly 300 m deep and more than 31 km in diameter, an area slightly larger than that inside Washington's Capital Beltway, or corresponding to an area bigger than Paris. 1) 2)
The group, led by researchers from the University of Copenhagen's Center for GeoGenetics at the Natural History Museum of Denmark worked for the past three years to verify their discovery, which they initially made in 2015 using NASA data. Their finding is published in the 14 November issue of the journal Science Advances. 3)
"NASA makes the data it collects freely available to scientists and the public all around the world," said Joe MacGregor, a NASA glaciologist at Goddard Space Flight Center in Greenbelt, Maryland, who became involved in the investigation in its early stages. "That set the stage for our Danish colleagues' ‘Eureka' moment."
The researchers first spotted the crater in July 2015, while they were inspecting a new map of the topography beneath Greenland's ice sheet that used ice-penetrating radar data primarily from NASA's Operation IceBridge — a multi-year airborne mission to track changes in polar ice — and earlier NASA airborne missions in Greenland. The scientists noticed an enormous, previously unexamined circular depression under Hiawatha Glacier, sitting at the very edge of the ice sheet in northwestern Greenland.
Using satellite imagery from the Moderate Resolution Imaging Spectroradiometer instrument on NASA's Terra and Aqua satellites, MacGregor also examined the surface of the ice in the Hiawatha Glacier region and quickly found evidence of a circular pattern on the ice surface that matched the one observed in the bed topography map.
To confirm their suspicions, in May 2016 the team sent a research plane from Germany's Alfred Wegener Institute to fly over the Hiawatha Glacier and map the crater and the overlying ice with a state-of-the-art ice-penetrating radar provided by the University of Kansas. MacGregor, who is an expert in radar measurements of ice, helped design the airborne survey.
"Previous radar measurements of Hiawatha Glacier were part of a long-term NASA effort to map Greenland's changing ice cover," MacGregor said. "What we really needed to test our hypothesis was a dense and focused radar survey there. The survey exceeded all expectations and imaged the depression in stunning detail: a distinctly circular rim, central uplift, disturbed and undisturbed ice layering, and basal debris — it's all there."
The crater formed less than 3 million years ago, according to the study, when an iron meteorite more than half a mile wide smashed into northwest Greenland. The resulting depression was subsequently covered by ice.
"The crater is exceptionally well-preserved and that is surprising because glacier ice is an incredibly efficient erosive agent that would have quickly removed traces of the impact," said Kurt Kjær, a professor at the Center for GeoGenetics at the Natural History Museum of Denmark and lead author of the study.
Kjær said that the crater's condition indicates the impact might even have occurred toward the end of the last ice age, which would place the resulting crater among the youngest on the planet.
In the summers of 2016 and 2017, the research team returned to the Hiawatha Glacier to map tectonic structures in the rock near the foot of the glacier and collect samples of sediments washed out from the depression through a meltwater channel.
Possible Second Large Subglacial Impact Crater in Northwest Greenland
A NASA geologist has discovered a possible second impact crater buried under more than a mile of ice in northwest Greenland. This follows the finding, announced in November 2018, of a 19-mile-wide crater beneath Hiawatha Glacier - the first meteorite impact crater ever discovered under Earth's ice sheets. Though the newly found impact sites in northwest Greenland are only 114 miles apart, at present they do not appear to have formed at the same time. 4)
If the second crater, which has a width of over 22 miles, is ultimately confirmed as the result of a meteorite impact, it will be the 22nd largest impact crater found on Earth.
"We've surveyed the Earth 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 participated in both findings.
Before the discovery of the Hiawatha impact 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, 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 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.
"I began asking myself ‘Is this another impact crater? Do the underlying data support that idea?'," MacGregor said. "Helping identify one large impact crater beneath the ice was already very exciting, but now it looked like there could be two of them."
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 isn't as clearly 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 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."
Although the newly found 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, 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 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 researchers 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," MacGregor said. "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."
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.
"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 William Bottke, a planetary scientist with the Southwest Research Institute in Boulder, Colorado, and co-author of both MacGregor's paper and the new lunar impact record study. 5)
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," MacGregor said. "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."
1) "International Team, NASA Make Unexpected Discovery Under Greenland Ice," NASA Release 18-099, 14 November 2018, URL: https://www.nasa.gov/press-release/international-team-nasa-make-unexpected-discovery-under-greenland-ice
2) Faculty of Science - University of Copenhagen, "Massive impact crater from a kilometer-wide iron meteorite discovered in Greenland," Science Daily, 14 November 2018, URL: https://www.sciencedaily.com/releases/2018/11/181114160042.htm
3) Kurt H. Kjær, Nicolaj K. Larsen, Tobias Binder, Anders A. Bjørk, Olaf Eisen, Mark A. Fahnestock, Svend Funder, Adam A. Garde, Henning Haack, Veit Helm, Michael Houmark-Nielsen, Kristian K. Kjeldsen, Shfaqat A. Khan, Horst Machguth, Iain McDonald, Mathieu Morlighem, Jérémie Mouginot, John D. Paden, Tod E. Waight, Christian Weikusat, Eske Willerslev, Joseph A. MacGregor, "A large impact crater beneath Hiawatha Glacier in northwest Greenland," Science Advances, Vol 4, No. 11, 14 November 2018, eaar8173, DOI: 10.1126/sciadv.aar8173, URL: http://advances.sciencemag.org/content/advances/4/11/eaar8173.full.pdf
4) Maria-Jose Vinas, "NASA finds possible second impact crater under Greenland ice," NASA, 11 February 2019, URL: https://www.nasa.gov/feature/goddard/2019/nasa-finds-possible-second-impact-crater-under-greenland-ice
5) Joseph A. MacGregor, William F. Bottke Jr., Mark A. Fahnestock, Jeremy P. Harbeck, Kurt H. Kjær, John D. Paden David E. Stillman, Michael Studinger, "A Possible Second Large Subglacial Impact Crater in Northwest Greenland," " Geophysical Research Letters, First published: 11 February 2019, https://doi.org/10.1029/2018GL078126 URL: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL078126
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).