An international team of researchers with participation of the University of Fribourg has discovered a 31-km wide meteorite impact crater buried beneath the ice-sheet in northern Greenland. This is the first time that a crater of any size has been found under one of Earth’s ice sheets. The researchers worked for the last three years to verify their discovery made in 2015. The research is described in a newly published study in the international journal Science Advances.

The crater measures 31 km in diameter, placing it among the largest known impact craters on Earth. «It is larger than the whole city of Paris», says Horst Machguth from University of Fribourg. «The crater formed when an iron meteorite of more than a kilometer in diameter smashed into northern Greenland, but has since been hidden under nearly a kilometre of ice.»

Giant circular depression
The crater was first discovered in July 2015 as Danish researchers inspected a new map of the topography beneath Greenland's ice-sheet. They noticed an enourmous, but previously undetected circular depression under the so called Hiawatha Glacier, sitting at the edge of the ice sheet in northern Greenland.

«We immediately knew this was something special but at the same time it became clear that it would be difficult to confirm the origin of the depression», says Kurt Kjær of the Natural History Museum of Denmark, lead author of the study. «We inferred that the depression could be a previously undescribed meterorite crater, but initially we lacked conclusive evidence.»

The crucial evidence
Their suspicion that the giant depression was a meteorite crater was reinforced when the team sent a German research plane from the Alfred Wegener Institute to fly over the Hiawatha Glacier and map the crater and the overlying ice with a new powerful ice radar. Joseph MacGregor, a glaciologist at NASA, who participated in the study adds: «To test our hypothesis we needed to look through the glacier ice and map the depression in detail. Our colleagues at the Alfred Wegener Institute and University of Kansas performed a dense and focused radar survey that exceeded all expectations and imaged the structure of the ice and the underlying rock surface in stunning detail. Features typically associated with an impact crater, such as a distinctly circular rim, a central uplift. It’s all there.»

But the fact that the feature looks like an impact crater is not conclusive proof of a meteorite impact. Hence, in the summers of 2016 and 2017, the research team visited the site to map tectonic structures in the rock near the foot of the glacier and collect samples of sediments.

«Some of the quartz sand we collected contains so called planar deformation features. These features form under extreme levels of pressure that are only reached for brief moments of time during a violent impact, and this is conclusive evidence that the depression beneath the Hiawatha Glacier is a meteorite crater», says Nicolaj K. Larsen from Aarhus University, Denmark. «However», adds Horst Machguth from the University of Fribourg, «we had to check whether these quartz sands were washed out from the actual depression and not from somewhere else. An extended study of satellite images allowed us to prove that the sand, containing the quartz grains, was deposited only a few years ago by a large meltwater river originating from underneath the Hiawatha glacier.»

The consequences of the impact on the Earth’s climate and life
Earlier studies have shown that large meteorite impacts can profoundly affect Earth’s climate, with major consequences for life on Earth at the time. It is therefore resonable to ask when and how the meteorite impact at the Hiawatha Glacier affected the planet.

«The crater is well-preserved, and that is surprising, because glacier ice erosion can efficiently remove the traces of the impact.» says Horst Machguth. «In our opinion, this indicates that the crater could be rather young from a geological perspective. Its condition suggests that it formed after ice began to cover Greenland 3 million years old, the onset of the last Ice Ages.»

«The next step in the investigation will be to confidently date the impact. This will be a challenge, because it will probably require recovering material that melted during the impact from the bottom of the structure, but this is crucial if we are to understand how the Hiawatha impact affected life on Earth», concludes Kurt Kjær.