Posted on February 11, 2019
Glaciologist NASA has discovered a possible second impact crater buried under more than a mile of ice in northwest Greenland. This follows from a discovery in November 2018 of a 19-kilometer crater beneath the Hiawath glacier – the first crater with a meteorite impact that has ever appeared under the ice surface of the Earth. Although newly found impact sites in northwest Greenland are just 114 miles apart, they do not appear to have formed at the same time.
If the second crater, whose width exceeds 22 miles, is ultimately confirmed as a result of a meteorite impact, it will be the 22nd largest impact crater on Earth.
"We have explored the Earth in many different ways, from Earth, Air and Space – it is interesting that such discoveries are still possible," said Joe MacGregor, a glaciologist at the NASA Goddard Space Flight Center in Greenbelt, Maryland, who participated in both findings.
Before the discovery of the Hiawath Crater (see video below), scientists generally believed that most evidence of past impacts in Greenland and Antarctica would be destroyed by the relentless erosion of heavy ice.
After discovering the first crater, MacGregor checked the topographic maps of the rocks under the Greenland glacier for the marks of the other craters. By using images from the ice surface of Imaging Spectroradiometer on the Terra and Aqua NASA satellites, he soon noticed a circular pattern about 114 kilometers southeast of the Hiawatha glacier. The same circular pattern has also emerged in ArcticDEM, a high-resolution digital high-resolution model of the entire arctic area, obtained from commercial satellite imagery.
"I started to ask myself: Is it another impact crater? Supporting data support this idea?", Said MacGregor. "It helps to identify one large crater under the ice already very exciting, but now it seemed that it could be two."
To confirm his suspicion of the possible presence of a second impact crater, MacGregor studied uneven radar images that are used to map the topography of the rock bedrock under ice, including those collected by NASA IceBridge operations. What he saw beneath the ice was a few characteristic features of a complex crater: Flat, bowl depressions in the subsoil that were surrounded by a raised edge and center-positioned peaks that formed when the crater floor straightened after the impact. Although the structure is not as obvious as the circular Hiawatha crater, MacGregor estimated the average of the second crater at 22.7 miles. Measurement from the IceBridge operation also revealed a negative gravity anomaly over the area that is characteristic of impact craters.
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"The only circular structure that could approach this size would be a precipitated volcanic caldera," said MacGregor. "But areas of known volcanic activity in Greenland are several hundred miles away, and the volcano should have a clear positive magnetic anomaly and we can not even see it at all."
Although newly found craters in northwestern Greenland are only 114 miles apart, they do not seem to be created at the same time. From the same radar data and ice cores that were gathered nearby, MacGregor and his colleagues found that the ice in the area was at least 79,000 years old. Ice layers were smooth, suggesting that the ice was not severely disrupted at the time. This meant that either the impact hit more than 79,000 years ago, or – if it happened recently – any impact on the disturbed ice has long escaped from the area and replaced by ice from a distance.
Scientists then looked at the rate of erosion: they calculated that a crater of this size would initially be more than half a mile deep between the edge and the floor, which is a order larger than its current depth. Considering a number of likely erosion ratios, it was calculated that it would take from about 100,000 years to 100 million years when ice would erode the crater into its current shape – the faster the erosion decreases, the younger the crater would be within the acceptable range, and vice versa .
"The ice layers above this second crater are clearly older than those above Hiawatha, and the other crater is about twice damaged," said MacGregor. "If both of them were at the same time, probably the thicker ice over the second crater would be cracked much faster than the Hiawatha."
To calculate the statistical probability that two craters were generated by the adverse effects of events, MacGregor used recently published estimates that use the impact on the moon to better understand the impact of impacts on Earth. By using computer models that can monitor the production of large craters on Earth, they found that the number of craters that would naturally have to be created close together without the need for double impact was consistent with Earth's craters.
"This does not exclude the possibility that two new Greenland craters were made in a single event, such as the impact of a well-separated binary asteroid, but we can not even solve it," said William Bottke, a planetary scientist with the Southwest Boulder Research Institute in Colorado and co- MacGregor and a new study on the impact of lunar impacts.
In fact, two couples of unrelated but geographically close craters have appeared in Ukraine and Canada, but the age of craters in pairs is different from each other.
"The existence of the third pair of unconnected craters is modestly surprising, but we do not think it is unlikely," said MacGregor. "Overall, the evidence we have gathered suggests that this new structure is very likely to affect the crater, but at present it seems unlikely that it will be twins with Hiawatha."
Image credit: Science / AAAS
Day galaxy through the NASA / Goddard Space Flight Center