Massive impact crater from a kilometre-wide iron meteorite discovered in Greenland

An international team lead by researchers from the Centre for GeoGenetics at the Natural History Museum of Denmark, University of Copenhagen have discovered a 31-km wide meteorite impact crater buried beneath the ice-sheet in the northern Greenland. This is the first time that a crater of any size has been found under one of Earth's continental ice sheets. The researchers worked for last three years to verify their discovery, initially made in the 2015. The research is described in a new study just published in the internationally recognized journal Science Advances.

Close-up of the northwestern ice-sheet margin in Inglefield Land. The Hiawatha impact crater was discovered beneath the 
semi-circular ice margin. The structure is also imprinted on the shape of the ice surface, even though it lies nearly 1,000 
meters below the ice surface. Hiawatha is named after outlet glacier at the edge of the ice sheet. The name was given by 
Lauge Koch in 1922 during an expedition around northern Greenland, while thinking of the pre-colonial native American
 leader and co-founder of the Iroquois Confederacy [Credit: The Natural History Museum of Denmark]

The crater measures more than 31 km in diameter, corresponding to an area bigger than Paris, and placing it among the 25 largest impact craters on Earth. The crater formed when a kilometre-wide iron meteorite smashed into northern Greenland, but has since been hidden under nearly a kilometre of 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. But that means the crater must be rather young from a geological perspective.

Geomorphological and glaciological setting of Hiawatha Glacier, northwest Greenland. (A) Regional view of northwest 
Greenland. Inset map shows location relative to whole of Greenland. Magenta box identifies location of panels B-D. 
(B) 5-m ArcticDEM mosaic over eastern Inglefield Land. Colors are ice surface velocity. Blue line illustrates an 
active basal drainage path inferred from radargrams. (C) Hillshade surface relief based on the ArcticDEM mosaic
 which illustrates characteristics such as surface undulations. Dashed red lines are the outlines of the two subglacial
 paleo-channels. Blue lines are catchment outlines, i.e., solid blue line is subglacial and hatched is supraglacial. 
(D) Bed topography based on airborne radar sounding from 1997-2014 NASA data and 2016 AWI data. Black 
triangles represent elevated rim picks from the radargrams and the dark purple circles represent peaks in the 
central uplift. Hatched red lines are field measurements of the strike of ice-marginal bedrock structures. 
Black circles show location of the three glaciofluvial sediment 
[Credit: University of Kansas]

So far, it has not been possible to date the crater directly, but its condition strongly suggests that it formed after ice began to cover Greenland, so younger than 3 million years old and possibly as recently as 12,000 years ago - toward the end of the last ice age" says Professor Kurt H. Kjær from the Center for GeoGenetics at the Natural History Museum of Denmark.

Giant circular depression

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

An artist’s depiction of a possible impact into the Greenland Ice Sheet. The iron meteorite penetrated 7 km into the Earth’s
 crust, creating a crater that was initially 20 km wide and collapsed within minutes into the final 31 km crater 
we see today [Credit: Carl Christian Tofte]

"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 Professor Kjær.

In the courtyard at the Geological Museum in Copenhagen just outside the windows of the Center for GeoGenetics sits a 20-tonne iron meteorite found in North Greenland not far from the Hiawatha Glacier.

Map of the bedrock topography beneath the ice sheet and the ice-free land surrounding the Hiawatha impact crater. The 
structure is 31 km wide, with a prominent rim surrounding the structure. In the central part of the impact structure, 
an area with elevated terrain is seen, which is typical for larger impact craters. Calculations shows that in order 
to generate an impact crater of this size, the earth was struck by a meteorite more than 1 km wide 
[Credit: The Natural History Museum of Denmark]

"It was therefore not such a leap to infer that the depression could be a previously undescribed meterorite crater, but initially we lacked the evidence," reflects Associate Professor Nicolaj K. Larsen from Aarhus University.

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 and is an expert in ice radar measurements adds:

Quartz grains with planar deformation features. These features are diagnostic of the quartz having experienced 
the shock of a massive impact event [Credit: The Natural History Museum of Denmark]

"Previous radar measurements of Hiawatha Glacier were part of a long-term NASA effort to map Greenland's changing ice cover. What we really needed to test our hypothesis was a dense and focused radar survey there. Our colleagues at the Alfred Wegener Institute and University of Kansas did exactly that with a next-generation radar system that 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."

In the summers of 2016 and 2017, the research team returned to the site 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.

Kurt Kjær collecting sand samples at the front of Hiawatha Glacier. This sand was transported by the glacier from 
the bottom of the impact crater to the ice margin, and it has yielded a wealth of information on the impact 
[Credit: Svend Funder]

"Some of the quartz sand washed from the crater had planar deformation features indicative of a violent impact, and this is conclusive evidence that the depression beneath the Hiawatha Glacier is a meteorite crater, " says Professor Larsen.

The consequences of the impact on the Earth's climate and life

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

"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 Professor Kjær.

Source: Faculty of Science - University of Copenhagen [November 14, 2018]