City-size impact crater found under Greenland ice

The first large crater ever found under ice, the discovery could possibly be linked to a controversial extinction theory. Wednesday, 14 November 2018

By Brian Clark Howard

Today, an international team of scientists describes what they say is a huge new impact crater that lies under northwestern Greenland’s Hiawatha Glacier. If confirmed, it would be the first impact crater on Earth discovered under ice, the team reports in the journal Science Advances. At an estimated 19 miles wide, it would rank among the top 25 known craters in the world.

“Until 2015, no one had paid much attention to this part of the planet,” says study co-author Joseph MacGregor, a glaciologist with the NASA Goddard Space Flight Centre in Maryland. But that year, scientists began flying over the area with highly sensitive scanning instruments, such as lasers and radar, through NASA’s Operation IceBridge.

Like all IceBridge data, the scans were made public, and a group of Danish glaciologists noticed something interesting when they reviewed the material: a large, bowl-shaped depression was clearly visible in the bedrock under the ice.

“Could that be an impact crater? they asked,” MacGregor says. “They all laughed. But then they said, Maybe it is.”

As they looked closer, someone on the team also pointed out that a large meteorite in the collection at the Natural History Museum of Denmark—near where they parked their bicycles every day—had come from that same region of Greenland.

“We asked ourselves, could the two be linked?” says lead author Kurt Kjær, a glacial geologist and curator at the Natural History Museum of Denmark and the University of Copenhagen.

Uncovering a crater

To find out more, the Danish scientists reached out to MacGregor, who is the chief scientist for the IceBridge project. To get more high-resolution scans of the Hiawatha Glacier, the team also enlisted the Alfred Wegener Institute in Germany, which provided them with additional surveying flights in May 2016 carrying newer, more sensitive instruments. They also sent a ground team in July 2016 to map surrounding structures on the surface and collect samples of sediments that had drained out from under the glacier.

With the precise radar data, the team was able to more completely work out the shape of the proposed crater. The walls of the circular rim are roughly 320 metres (1,050 feet) above the floor of the crater, they found. The team also identified an uplifted area 50 metres to 70 metres high in the centre of the crater, which Kjær says is an expected feature and is the result of the force of the strike.

In the sediment samples, the researchers found grains of what's known as shocked quartz—a rare form of the ubiquitous mineral that has been deformed in a characteristic way by very high-energy events, such as in a large impact. Some of the grains also showed a brown color known as toasting, again a sign of intense energy release. Other minerals showed signs of shock metamorphism, to the point of turning into glass.

Based on the size of the crater, the team estimates that the asteroid would have been around 0.75 miles across and would have weighed 11 to 12 tonnes as it entered the atmosphere. And based on their mineral analysis, they believe it was an iron-rich space rock—the same type of rock as the meteorite fragment in the museum, although more tests would need to be done to establish a firm link, Kjær notes.

Now that he knows the circular depression is there, Kjær adds that he can even see its outline on the surface of the ice.

MacGregor agrees: “I have a coffee mug with a three-inch map of Greenland on it. I can see the Hiawatha Glacier on there. So this really was hiding in plain sight.”

Icy impact

But impact crater expert Ludovic Ferriere of the Natural History Museum in Vienna, who was not associated with the study, is sceptical of these conclusions.

“I can say what they are presenting as shock quartz is definitely shock quartz,” says Ferriere, who is also a National Geographic Explorer. But he cautions that he would like to see a larger sample size of sediments tested, as he isn’t sure that the quartz taken from under the glacier necessarily came from the presumed impact crater.

Kjær counters that they have many more samples still to sort through, grain by grain. And based on their radar mapping of the drainage system under the glacier, “where could the material come from if it didn’t come from inside the glacier?”

Ferriere also says that the uplift the team reports for such a large impact crater is too small for what should be expected. Either it isn’t what they suspect, or the uplift feature has been heavily eroded. Kjær responded that the impact through the ice sheet can explain the more muted central uplift, as well as the debris and sloping sediments they observed with radar.

Both Kjær and Ferriere agree that the next steps likely include additional analysis of the existing samples, including the possibility of radioisotope dating, as well as collecting more material from the site. Ideally, the scientists say it would be best to drill through the glacier—which is nearly 1km thick over the crater—and into the rock below. A heavy drilling operation in such a remote area would be challenging and expensive, but not without precedent.

“I think they have something here, but they make strong conclusions based on very preliminary data and a lot of gaps,” Ferriere says. “With drilling, they might find something very different.” As it stands, Ferriere argues, their discovery of the shock quartz “is like if you arrive on the scene of a murder and you find one poor guy there; he’s not necessarily the murderer.”

World-changing event?

The scientists also don’t currently have enough information to assign an age to the proposed impact crater, but based on their analysis, they have suggested bookends for the date of the event. Given the structure of the rock and ice that can be 'read' with radar, the team believes that the glacier was in place at the time of the strike, and that the impact punched a hole in the ice and resulted in a significant amount of melting and refreezing. That would suggest that the impact happened sometime before the end of the Pleistocene epoch around 11,700 years ago.

“It’s likely quite young, geologically speaking,” MacGregor says. “It’s likely less than three million years old and possibly as young as 12,000 to 15,000 years old.”

If the discovery holds, the Hiawatha Crater could therefore be a tantalising new piece of evidence for a very controversial idea. Called the Younger Dryas impact hypothesis, the notion is that some kind of large impact occurred in northern North America about 10,900 to 12,900 years ago, during the Younger Dryas Ice Age. This impact, the idea goes, caused massive wildfires across much of the continent that in turn led to the extinction of many of the large Ice Age mammals, like mammoths and mastodons, as well as the human Clovis culture.

One big problem with this hypothesis has long been the lack of a suitably large impact crater. If it's real and the dates match up, the Hiawatha Crater could be a plausible explanation, MacGregor says: “It’s a very speculative idea, but if this does turn out to be [the link], it would have had an outsize impact on human history.”

“We do not discuss it in the paper, but I think it is a possibility,” Kjær adds. “This may generate a lot of discussion, and we need to find out. We won’t know until we have a proper date.”