TanDEM-X image of Hiawatha Glacier
Glaciers abound on Greenland's coastline; fed by the Greenland ice sheet, they flow towards the Arctic Ocean. In the northwest, Hiawatha Glacier is located at 78.8 degrees north, 67 degrees west. It emerges from a semi-circular lobe at the ice sheet margin and forms a narrow tongue with a length of 10 kilometres extending onto the ice-free Inglefield Land. Hiawatha Glacier’s northern neighbour, the large Humboldt Glacier, is much more widely known. The front of the Humboldt Glacier is over 100 kilometres wide where it flows into the Nares Strait. The TanDEM-X image shows the region around Hiawatha Glacier.
Recently, however, Hiawatha Glacier has received worldwide attention. Some years ago, radar measurements performed as part of NASA’s Operation IceBridge, a campaign to monitor changes in the polar ice caps, revealed a circular depression in the ground underneath the ice where Hiawatha Glacier emerges from the ice sheet. Subsequent surveying by an international research team using a more advanced airborne radar system on board the Polar 6 aircraft operated by the Alfred Wegener Institute (AWI) yielded a more detailed view of that bowl-shaped feature. With a diameter of 31 kilometres and a depth of more than 300 metres, it resembles impact craters on Earth or the solid surface of other celestial bodies.##markend##
Remote sensing data alone cannot determine whether or not a circular terrestrial structure is the result of an asteroid impact. Other geological phenomena such as volcanic craters and calderas or intrusions display similar morphologies and are often misleading. Unambiguous identification of an impact crater has to rely on the confirmation of shock effects in the rock affected by the impact. This includes shatter cones and planar deformation features or planar fractures, with the latter only being observable in crystals at the microscopic level. These rock metamorphoses are unmistakable signs of an impact. Only when an extraterrestrial projectile impacts at a speed in excess of 11 kilometres per second (hypervelocity) does the resulting shock wave cause the extremely high pressures and temperatures required for producing such metamorphic effects. In Earth’s crust, no geological processes occur that could create comparable conditions.
Detecting changes in rocks that have been caused by shock metamorphosis, especially at the microscopic level, is often a difficult task. In the case of the structure beneath Hiawatha Glacier, it is not only the very remote location that makes accurate investigations difficult, but continental ice several hundred metres thick above the structure makes it impossible to directly extract rock samples. For this reason, the science team collected sediments in the melt flow channel at the western edge of the structure in order to search for shock characteristics. And indeed, planar deformation features were found in quartz grains, which could be interpreted as shock effects.
How old is the Hiawatha impact site? Direct age determination using radiometric techniques was impossible because of the lack of suitable rock samples. Therefore, the age was estimated based on the structure’s young-looking morphology and certain properties of the overlying ice sheet. Impact during the Pleistocene epoch is considered to be most likely. This dates the event to the past 2.6 million years with a lower limit of only 12,500 years. In the latter case, the impact would have occurred in prehistoric times. It would have been comparable with the impact that produced the Ries Crater 15 million years ago. This crater is located approximately 110 kilometres northwest of Munich and is one of the world’s most extensively studied impact sites.
The identification of the structure found beneath Hiawatha Glacier as an impact site has not, however, received unanimous approval. Renowned impact geologists doubt the evidence that has been presented. They argue that the origin of the quartz samples showing shock effects needs further analysis, particularly proof that they come from the suggested impact. Even more challenging is explaining its morphology and its very young age, which implies that the structure must be very well preserved. It should thus possess, in addition to a rim, a central uplift or an inner ring. These are features that occur when the impacting asteroid exceeds a given size and produces a crater larger than four kilometres across, resulting in a complex crater. The impact of a smaller projectile generates a simple crater, a bowl-shaped depression with a raised rim. The morphology of the structure under Hiawatha Glacier lacks a pronounced central uplift, so it more closely resembles an oversized simple crater. An impact of these dimensions should also have caused thick ejecta layers in an area covering up to 10 times the crater diameter. These have not been found.
For the time being, Hiawatha Glacier structure remains a controversial topic, making the otherwise rather inconspicuous glacier a very interesting object in Greenland’s far northwest. Perhaps, after further scientific investigations, there will be an unambiguous confirmation of an impact. If not, it will be another example showing how difficult it is to identify the scars of cosmic impacts on Earth.