It’s hard to know something is right in front of you if you’re not looking for it. Sometimes that’s true with love, other times that’s true with opportunity. And sometimes that thing that’s hidden in plain sight is a giant impact crater, wider than Paris, that sits beneath a half-mile-thick ice sheet. On Wednesday, a collaborative team of Danish and American researchers announced it had discovered just that — the aftermath of a meteorite’s landing sometime between 12,000 and 3 million years ago.

Buried beneath Hiawatha Glacier in northwest Greenland, the impact crater described in Science Advances is now categorized as one of the 25 largest impact craters on Earth and is the first crater of any size to be found under one of the planet’s continental ice sheets. For the last three years, scientists have worked to verify their discovery — combining cutting-edge radar technology, aerial and land surveys, and geological analysis to prove that many, many years ago an iron meteorite nearly a mile wide smashed into Greenland.

Study lead author and project leader Kurt Kjær, Ph.D., tells Inverse that if this event were to happen today it would “be an unpleasant experience.” Unpleasant, here, is a bit of an understatement.

“Imagine 12 billion tons of iron coming down,” explains the University of Copenhagen professor. “Just the energy released on impact would be equal the energy from 45 Hiroshima atom bombs, creating strong earthquakes 100 kilometers [62 miles] away from the impact site and covering large areas with hot ejecta material. It would instantly kill life in the large surrounding area.”

Map of the bedrock topography beneath the ice sheet and the ice-free land surrounding the Hiawatha impact crater. Natural History Museum of Denmark, NASA Goddard Space Flight Center

John Paden, Ph.D., a University of Kansas associate scientist and one of the study’s co-authors, adds that because this particular crater is on the edge of the Greenland ice sheet — the body of ice that covers about 80 percent of present-day Greenland — when the ancient meteorite landed it likely caused a temporary change in the flow of the ocean in the Nares Strait between Greenland and Canada. He tells Inverse that because of the massive injection of dust into the atmosphere post-crash, sunlight was likely blocked, and “there would be substantial cooling similar to the effect we have seen with volcanic eruptions.”

In a way, the discovery of the crater began in 1993 when the University of Kansas and the NASA Program for Arctic Regional Climate Assessment began to work together to conduct radar depth sounding measurements over the Greenland ice sheet. Data has been collected annually since — the purpose of which, Paden explains, is “to improve our understanding of the ice sheet because the ice sheet is connected to sea level rise and the Earth’s climate.”

Radar data revealed both the topography of the impact crater beneath the ice and the layering of the ice itself. Natural History Museum of Denmark, NASA Goddard Space Flight Center

Scientists consider Greenland a canary in a coal mine for climate change. Because polar regions are more sensitive to a warming Earth than other parts of the planet, what happens in Greenland alerts scientists to what’s in store for other regions. It’s also integral to know what is happening to its ice — Greenland has more land ice than any other spot on Earth, apart from Antarctica, and if all of that ice melted, Earth’s sea levels would rise by 23 feet.

Over the last two decades, this process of ice-penetrating radar analysis has been pieced together by glaciologists to produce maps of what Greenland is like underneath its ice. Danish scientists, led by Kjær, were using one of these topographic maps when they observed from land an enormous, undetected circular pattern upon the Hiawatha glacier that aligned with the map’s telling of a craterlike depression under the ice sheet.

“My research group and most of the other co-authors have been involved in research in Greenland for decades, which involves the entire ice sheet and marginal areas,” Kjær explains. “But it was when we started to inspect the newest map of the topography beneath the ice sheet in 2015 that we could see a large, circular feature near the ice sheet margin in Inglefield Land, northwest Greenland.”

While a circular depression doesn’t automatically mean it was caused by an intergalactic visitor, something tugged at the Kjær’s mind: In the courtyard at the Geological Museum in Copenhagen sits a 20-tonne iron meteorite that was found in North Greenland, not too far from the Hiawatha Glacier.

“An idea came to mind that this feature might be an impact crater,” Kjær explains, “partly because we pass a large meteorite each day in the courtyard of our museum.”

Kurt Kjær collects sand samples at the front of the Hiawatha Glacier. Svend Funder

That’s when the hunt to prove it was a meteorite crash site began: In the summers of 2016 and 2017, the scientists conducted aerial surveys and went directly to the site to map tectonic structures in the rock and collect sediment samples deposited by a river that drains out of the glacier. They found angular quartz grains that revealed signs of shock from an impact, as well as other bits of carbonaceous materials and glass — evidence that they emerged from melting bedrock. In the river, they found further scientific treasure: Sediments with elevated concentrations of nickel, cobalt, chromium, and gold; their presence indicates a rare, iron meteorite.

This discovery, says Kjær, is a career highlight. Now, what needs to be done is further analysis to confirm the precise dating of the crash. Evidence so far indicates it occurred during the Pleistocene, but that time period is very broad — ranging from 2.59 million to 11,700 years ago — and the theory is still uncertain. To date the impact, they’ll have to recover material at the bottom of the ice sheet — a challenge, but one that Kjaer says is a crucial one. Understanding exactly when this happened won’t just solve a mystery, it’ll add to the understanding of how Earth’s climate has historically been shaped. It’s a collaborative mission, one that incorporates multiple factions of science.

It’s also not a task to take lightly. As Paden explains “this is probably a once in a lifetime opportunity for most of us.”

Abstract:

We report the discovery of a large impact crater beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, we identify a 31-kilometer-wide, circular bedrock depression beneath up to a kilometer of ice. This depression has an elevated rim that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be actively eroding. From ground investigations of the deglaciated foreland, we identify overprinted structures within Precambrian bedrock along the ice margin that strike tangent to the subglacial rim. Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris rich or heavily disturbed. The age of this impact crater is presently unknown, but from our geological and geophysical evidence, we conclude that it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet.