Originally published 2018-12-22.
Artist's impression of an iron asteroid striking northwest Greenland. When this picture was made, scientists thought the impact might be geologically recent. Image: NASA.
Under roughly a kilometre of ice at the edge of northwest Greenland lies a bowl-shaped impact crater about 31 kilometres (19 miles) across. It sits beneath Hiawatha Glacier and was announced in November 2018 by an international team led by Kurt Kjær of the Natural History Museum of Denmark. When it was first described, some researchers suggested it might be very young, possibly about 13,000 years old, and might even have helped trigger a sudden cold spell at the end of the last ice age.
That idea did not survive. In March 2022 a follow-up study dated the impact to about 58 million years ago, long before any ice sheet existed and millions of years before modern humans. So the crater is real and remarkable, but it has nothing to do with the end of the ice age or with climate change today. Here is how it was found, why scientists once linked it to the Younger Dryas cold snap, and how the dating settled the question.
How a hidden crater was spotted
The first clue came from a map, not a field trip. In 2015 Kjær and a colleague were studying a new model of Greenland's bedrock, built from ice-thickness measurements and the way the ice surface flows. Under Hiawatha Glacier it showed a curious circular depression. The glacier's front also bulges outward in a neat semicircle, as if ice were spilling out of a basin.
The team then turned to radar, which can see through ice and bounce off the rock below. Joseph MacGregor, a glaciologist at NASA's Goddard Space Flight Center, dug through archived data from NASA's airborne surveys of the Arctic. Those flights had sometimes passed over the glacier with instruments switched on, and the old echoes outlined what looked like a crater rim.
To see more, Germany's Alfred Wegener Institute flew a dedicated survey in May 2016 using a new ultra-wideband ice-penetrating radar mounted on its polar research aircraft. Science magazine's report on the discovery said the three flights added about 1,600 kilometres of fresh lines across the ice and were financed in part by Denmark's Carlsberg Foundation. The images showed a raised rim, a jagged floor and a cluster of bumps in the middle: a central uplift, the rebound feature that forms when rock briefly behaves like a fluid during a large impact.
The view from a NASA survey aircraft over northwest Greenland. Airborne radar revealed the crater's rim and central peak through the ice. Photo: John Sonntag/NASA.
The proof: shocked quartz and melted rock
Many things in nature make circles, including volcanic calderas and collapsed basins, so a round shape alone does not prove an impact. In summer 2016 Kjær flew by helicopter to the glacier front and spent a short window collecting sand and pebbles washed out by the river that drains from under the ice. That sediment comes from the crater floor.
Back in Copenhagen, Adam Garde of the Geological Survey of Denmark and Greenland (GEUS) found what the team was hoping for. There were grains of quartz with microscopic planar features, a type of damage that forms only under the extreme shock pressures of a hypervelocity impact or a nuclear explosion. There was also glassy material melted at very high temperatures. Chemical traces in the sediment pointed to an iron-rich impactor. The 2018 paper in Science Advances argued that the impactor was an iron asteroid about 1.5 kilometres across. If confirmed as an impact, the structure ranks among the 25 largest known impact craters on Earth.
A quartz grain seen under polarised light. Its banded internal pattern is a hallmark of shock from an extraterrestrial impact. Photo: Adam Garde, GEUS.
Kurt Kjær sampling glacial outwash sand at the edge of Hiawatha Glacier in 2016, where shocked grains from the crater floor had been carried out by meltwater. Photo: Svend Funder.
Why the age mattered so much
In 2018 the crater's age was the open question. The radar showed a surprisingly sharp, little-eroded crater, and some of the ice layers above it looked disturbed. Using volcanic ash horizons that can be matched to dated ice cores, the team found neatly layered ice back to about 11,700 years ago, with jumbled older ice below. To some on the team, that hinted at an impact during the last ice age, perhaps even around 12,800 years ago.
That date is loaded. It matches the start of the Younger Dryas, a cold snap that lasted from about 12,900 to 11,700 years ago. As the world was warming out of the last glaciation, much of the Northern Hemisphere plunged back towards ice-age conditions within decades, and in Greenland the shift may have taken only a few years. The period is named after Dryas octopetala, a small white tundra flower whose fossils are common in sediments from that time.
Since 2007 a group of researchers led by physicist Richard Firestone had argued that a comet or asteroid strike caused the Younger Dryas, set off wildfires and helped wipe out mammoths and the Clovis culture of North America. That idea, the Younger Dryas impact hypothesis, had one obvious gap: no crater. Hiawatha seemed, briefly, like the missing piece.
A 2018 graphic from Science comparing Hiawatha with the Chicxulub and Chesapeake Bay craters. Its suggested age range, 100,000 to 12,800 years ago, was later shown to be far too young.
The skeptics in 2018
Many scientists were cautious from the start. The impact physicist Jay Melosh pointed out that objects big enough to dig a 31-kilometre crater hit Earth only every few million years, so a very recent strike was statistically unlikely. Glaciologist Richard Alley of Penn State suggested the odd ice textures could have other causes and that the crater could be far older. Greenland's deep ice cores, which cover about the last 100,000 years, showed no layer of impact debris of the kind such a strike should leave. Ocean researchers also noted that the freshwater pulse thought to have weakened Atlantic circulation at the start of the Younger Dryas appears to have drained through the Mackenzie River into the Arctic, not from Greenland.
More broadly, the Younger Dryas impact hypothesis has struggled. Claimed nanodiamonds and spherules have not been reliably replicated, the "black mat" layers occur at many different ages, and large reviews in 2011 and 2023 concluded the evidence does not support it. Most researchers favour meltwater from retreating ice sheets disrupting ocean circulation as the cause of the cold snap.
The 2022 answer: about 58 million years old
The question was settled by dating rock that was melted or shocked in the impact itself. In a study published in Science Advances in March 2022, Gavin Kenny of the Swedish Museum of Natural History and colleagues used two independent methods. One lab measured argon released by laser-heating tiny grains of impact-melted sand. Another measured uranium and lead in shocked zircon crystals from pebbles carried out by the meltwater river. Both pointed to the same answer: 57.99 ± 0.54 million years.
That places the impact in the late Paleocene, roughly eight million years after the Chicxulub asteroid ended the age of the dinosaurs. Greenland then had no ice sheet at all. According to the University of Copenhagen, the region was covered by temperate rainforest. Kenny said the result "surprised us all." Whatever the impact did to the Paleocene world, it cannot have caused the Younger Dryas, and it played no role in the ice age.
What the crater still tells us
Hiawatha is still one of the most interesting craters found in decades. It shows that large impact structures can stay hidden beneath ice, and that radar surveys made for climate and sea-level research can turn up unexpected geology. The glacier above it has preserved an unusually fresh-looking crater simply because ice has protected it in recent geological time.
It is also a useful lesson in how science corrects itself. The first paper was careful to call the age uncertain. Others tested the boldest interpretation, and better dating methods produced a clear answer. Some people also wondered about the Cape York meteorites, the huge iron masses the Inuit of northwest Greenland used for tools for centuries. One fragment, the 20-tonne Agpalilik, stands outside the Copenhagen geology museum. According to Science's 2018 report, Kjær said it was one of the things that first made him curious about Hiawatha. No link between those irons and the crater has been established.
Sources: University of Copenhagen: Hiawatha crater dated to about 58 million years (2022); Wikipedia: Hiawatha Glacier (Kjær et al. 2018; Kenny et al. 2022, Science Advances); Wikipedia: Younger Dryas; Wikipedia: Younger Dryas impact hypothesis; Wikipedia: Cape York meteorite; original reporting by Science magazine (2018). Images: NASA, John Sonntag/NASA, Adam Garde/GEUS, Svend Funder, Science.
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