Sunday, 31 March 2019

Possible second large impact crater beneath the Greenland Ice Sheet.

The discovery of large impact craters is now a rare event in even the most remote locations on Earth, and it is highly unlikely that any such structures larger than about 6 km in diameter are left to be discovered exposed on the Earth's surface. However such structures are not always exposed at the surface, and large impact craters are still sometimes discovered beneath desert sands, shallow seas or polar ice sheets. One such discovery was announced in a paper published in the journal Science Advances on 14 November 2018, by a team of scientists led by Kurt Kjær of the Centre for GeoGenetics at the Natural History Museum of Denmark at the University of Copenhagen, who described a 31 km diameter impact crater beneath the Hiawatha Glacier in northwest Greenland.

In a paper published in the journal Geophysical Research Letters on 11 February 2019, Joseph MacGregor of the Cryospheric Sciences Laboratory at NASA's Goddard Space Flight Center, William Bottke of the Department of Space Studies at the Southwest Research Institute, Mark Fahnestock of the Geophysical Institute at the University of Alaska Fairbanks, Jeremy Harbeck, also of the Cryospheric Sciences Laboratory at NASA's Goddard Space Flight Center, and of ADNET Systems, Inc., Kurt Kjær of the Centre for GeoGenetics at the Natural History Museum of Denmark at the University of Copenhagen, John Paden of the Center for Remote Sensing of Ice Sheets at the University of Kansas, and Michael Studinger, again of the Cryospheric Sciences Laboratory at NASA's Goddard Space Flight Center, describe a possible second large impact crater beneath the Greenland Ice Sheet.

MacGregor et al. examined commercial satellite data from the ArcticDEM Polar Geospatial Center, as well as aerogeophysical surveys and satellite data produced by NASA over the past 25 years. Using this data they identify a structure roughly 183 to the southeast of the Hiawatha Crater, which they provisionally name the Peterson Crater, in honour of the eminent Scottish glaciologist Stan Paterson, who helped to survey the area during the 1953–1954 British North Greenland Expedition. The structure is approximately 36.5 km in diameter and is covered by up to 2180 m of ice. It has a maximum depth of 160 m, from the highest point on the rim to the lowest point in its basin, with an inner ring of peaks 23 km in diameter (inner rings of peaks are common in larger impact structures).

(a) Hillshaded ArcticDEM surface elevation across northwestern Greenland, showing both the Hiawatha impact crater along the ice margin and the presently identified structure farther inland to the southeast. Horizontal lines across the panel are mosaicking artifacts. Magenta arrows indicate location of both structures. Locations of 1953–1954 British North Greenland Expedition (BNGE) traverse stations, 1959–1967 Camp Century station, and 1995 Humboldt Glacier shallow ice core sites. (b) Map of Greenland with black box showing location of panels (a) and (c). (c) Gridded subglacial topography across northwestern Greenland. MacGregor et al. (2019).

Based upon ice cores measurements in the region, which enable the dating of reflective surfaces within the ice, MacGregor et al. conclude that the Paterson Crater is at least 79 000 years old. They also note that, given the crater's size, it must have had an original depth in excess of 1km, and that for this to have eroded down to the current profile would have taken at least 100 000 years. This in turn suggests that the structure is somewhat older than the Hiawatha Crater, which has a minimum age of about 11 700 years (though it is probably older than this), and which is much less eroded, ruling out the possibility of the two craters having originated from a double impact (where a single large object broke up as it entered the atmosphere and hit the ground in two pieces).

Finding two large impact craters within 183 km with separate origins at first seems somewhat unlikely, indeed MacGregor et al. calculate that the odds against  a crater-forming object hitting the ground within 183 km of any given point within 1.5 million years is 2.1 in 10 000. However, whilst this seems somewhat unlikely, MacGregor et al. also calculate that given a random distribution of impacts on Earth, and the amount of available stable terrain in which craters can be preserved, the Earth's surface should contain 1-2 craters of this size that lie within 183 km of one another by random chance, making the discovery significant, but less statistically implausible.

See also...

http://sciencythoughts.blogspot.com/2019/03/discovery-of-large-impact-crater.htmlhttps://sciencythoughts.blogspot.com/2019/03/investigating-meteoroid-impact-on-moon.html
https://sciencythoughts.blogspot.com/2019/01/could-microbes-from-earth-have-reached.htmlhttps://sciencythoughts.blogspot.com/2018/11/glacial-flour-creates-dust-storm-in.html
https://sciencythoughts.blogspot.com/2018/09/understanding-formation-of-coesite-in.htmlhttps://sciencythoughts.blogspot.com/2017/09/understanding-deposition-of-suevites-in.html
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