Asteroid impacts have pummeled Earth over its 4.5-billion-year history, and some have been responsible for global extinctions like the demise of the dinosaurs roughly 66 million years ago. But the planet bears the scars of only a few hundred of these impacts because of weathering processes like erosion.
Now, using ice-penetrating radar measurements, researchers have detected an enormous impact crater that’s well preserved under nearly a kilometer of glacial ice in northwestern Greenland. Through analysis of crater size and sediments from the crater, they estimate that the asteroid that slammed into Earth was more than a kilometer in diameter, composed mainly of iron. This discovery suggests that Earth may harbor more signs of its tumultuous history beneath its glaciers and ice sheets.
A Weird Depression
Mark Fahnestock, a glaciologist at the University of Alaska Fairbanks, and his colleagues didn’t set out to find an asteroid impact. They were using archival measurements of the Greenland Ice Sheet to study its ice flows and underlying bedrock. But when a group of Danish researchers noticed a circular depression in the bedrock at the northwest edge of the ice sheet, under Hiawatha Glacier, their curiosity was piqued.
“There are only a few ways you can get a closed depression,” said Fahnestock. “A river can’t do it.” One likely culprit was an asteroid impact, and it just so happened that some of the Danish researchers worked at the Natural History Museum of Denmark of the University of Copenhagen, which houses a collection of iron meteorites from Greenland. The hunt was on.
By May 2016, the researchers had secured support from a private foundation to fund a series of flights over the mysterious depression in Hiawatha Glacier. Flying at an altitude of just 350 meters in a converted DC-3 plane, the researchers used ice-penetrating radar to accurately map the layers of ice and rock beneath Hiawatha Glacier. The data clearly revealed a circular depression roughly 31 kilometers in diameter with an elevated rim and an uplifted area near its center, geometry characteristic of an impact crater.
Shocked Grains and Gold
To better constrain whether this depression was truly caused by an asteroid impact, Fahnestock and his colleagues collected sandy sediments that had drained out from underneath Hiawatha Glacier. Back in the laboratory, the scientists found that some of the quartz grains in the sediments showed evidence that a high-pressure shock wave had passed through the rocks.
“It’s diagnostic of an impact,” said Fahnestock. Along with the shocked quartz grains, the scientists also found elevated levels of elements like nickel, cobalt, chromium, and gold, an elemental signature indicative of an iron meteorite.
This crater, if confirmed, will be the 25th largest crater on Earth according to the Earth Impact Database, the team wrote in their paper, which was published last week in Science Advances. On the basis of its size, the researchers estimated that the asteroid that created this crater was about 1.5 kilometers in diameter. For comparison, it’s been calculated that the asteroid that felled the dinosaurs and created the Chicxulub crater in the Yucatán Peninsula was roughly 10 kilometers in diameter.
One enduring mystery is the lack of any rock ejecta associated with this supposed cataclysm, which would have scattered debris far and wide. Four deep ice cores have been drilled in central and northern Greenland, but none have recovered ejecta.
“The careful work that’s done on deep ice cores would not have missed an ejecta blanket,” said Fahnestock.
One theory to explain this discrepancy is that an asteroid struck Greenland while traveling north, which would have predominantly scattered debris to the north and produced an asymmetric ejecta layer. It’s not known whether any of the iron meteorites housed at the University of Copenhagen came from this event.
Precisely age dating this crater is difficult, said Fahnestock, but it’s possible to place younger and older bounds. The crater can’t be any younger than about 11,000 years because the ice deposited during the Holocene period is not disturbed; if the asteroid had hit during the Holocene, those layers would have been destroyed.
And finding shocked grains still being carried away from the underside of the glacier is a hint that the crater is probably no older than a few million years, said Fahnestock. “If [the crater] had been glaciated over and over again, that kind of sediment would be gone.”
Chris Lowery, a paleoceanographer at the Institute for Geophysics at the University of Texas at Austin who has studied the Chicxulub crater, suggests turning to the ocean to age date this crater. “I hope that someone is planning to take cores of the ocean sediments nearby to place the crater ejecta into the precise context of the climate records that are preserved in marine sediments,” said Lowery, who was not involved in the research.
Regardless of the crater’s exact age, the impact likely had a significant effect on the environment. The enormous quantity of heat generated in the impact would have melted nearby ice. The ensuing flow of meltwater—fresh water—to the ocean might have then affected global patterns of ocean circulation, the researchers propose.
Because fresh water is more buoyant than salt water, an influx of glacial melt might have slowed or halted the sinking of cold, dense water in the North Atlantic. This sinking is a critical component of the ocean’s circulation and, by extension, the planet’s weather patterns. “This impact very likely had significant environmental consequences in the Northern Hemisphere and possibly globally,” the team wrote in their paper.
Fahnestock and his colleagues are hoping to return to Greenland. Their previous fieldwork stints there were brief—only a few days—and they’re eager to nail down more details about the crater. “If you want to know the age and geochemical signatures of the impact,” said Fahnestock, “you probably want to drill through the ice and recover rock.”