A 1.5-kilometer asteroid, intact or in pieces, may have smashed into an ice sheet just 13,000 years ago. NASA
On a bright July day in 2016, Kurt Kjær was in a helicopter flying over northwest Greenland. Soon, his target came into view: it was the Hiawatha Glacier, a slow-moving sheet of ice over a kilometer thick. It advanced on the Arctic Ocean in a conspicuous semicircle, as though spilling out of a basin. Kjær, who is a geologist at the Natural History Museum of Denmark in Copenhagen, thought that the glacier was hiding an explosive secret. His helicopter landed close to the surging river which drains the glacier, sweeping out rocks from beneath it. Kjær had 18 hours to discover the mineral crystals which would confirm his suspicions.
What he brought back home clinched the case for a great discovery. He found a 31-kilometer-wide impact crater hidden beneath Hiawatha. It was left when an iron asteroid 1.5 kilometers across slammed into Earth, probably within the past 100,000 years.
Although it wasn't as cataclysmic as the dinosaur-killing Chicxulub impact, that carved out a 200-kilometer-wide crater in Mexico around 66 million years ago, the Hiawatha impactor, too, might have left an imprint on Earth's history. The timing is still up for debate, though some researchers on the discovery team think that the asteroid struck at a crucial moment: almost 13,000 years ago, as the world was thawing from the last ice age. This would mean that it crashed into Earth when mammoths and other megafauna were in decline, and humans were spreading across North America.
The impact would've been a spectacle for anybody within five hundred kilometers. A white fireball three times brighter and four times larger than the sun would've streaked across the skies. If this object struck an ice sheet, it would've tunneled through to the bedrock, vaporizing water and stone alike in a flash. The resulting explosion packed the energy of a 700 megaton nuclear bomb; even an observer hundreds of kilometers away would've experienced a buffeting shock wave, a monstrous thunder-clap, and hurricane-force winds. Then, rock debris might've rained down on Europe and North America, and the released steam, a greenhouse gas, could've locally warmed Greenland, melting even more ice.
The news of the impact discovery has reawakened an old debate among researchers that study ancient climate. A massive impact on the ice sheet would've sent meltwater pouring into the Atlantic Ocean—possibly disrupting the conveyor belt of ocean currents and causing temperatures to plunge, particularly in the Northern Hemisphere.
A small group of researchers proposed a similar scenario a decade ago, as they were trying to explain a cooling event, over 1000 years long, called the Younger Dryas, which started 12,800 years ago, as the last ice age was ending. The controversial solution was to invoke an extraterrestrial agent: the impact of one or more comets. The researchers suggested that besides changing the plumbing of the North Atlantic, the effect ignited wildfires across two continents too which led to the extinction of large mammals and the disappearance of the mammoth-hunting Clovis people of North America. The research group marshaled suggestive yet inconclusive evidence, and few other scientists were convinced. However, the idea caught the public's imagination despite an obvious limitation: Nobody could find an impact crater.
It began with a hole. In 2015, Kjær and a colleague of his were studying a new map of the hidden contours under Greenland's ice. Based on variations in the ice's depth and surface flow patterns, the map offered a coarse suggestion of the bedrock topography—including a hint of a hole under Hiawatha.
Kjær recalled a giant iron meteorite in his museum's courtyard, close to where he parks his bicycle. Called Agpalilik, Inuit for "the Man," the 20-ton rock is a fragment of an even more massive meteorite, the Cape York, discovered in pieces on northwest Greenland by Western explorers yet long used by Inuit people as a source of iron for harpoon tips and tools. Kjær wondered whether the meteorite was a remnant of an impactor which dug the circular feature under Hiawatha. However, he still wasn't confident it was an impact crater. He needed to see it more clearly with radar, that can penetrate ice and reflect off bedrock.
Kjær's team started to work with Joseph MacGregor, a glaciologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, that dug up archival radar data. MacGregor discovered that NASA aircraft often flew over the site on their way to survey Arctic sea ice, and sometimes the instruments were turned on, in test mode, on the way out.
NASA and German aircraft used radar to see the contours of an impact crater beneath the ice of Hiawatha Glacier. JOHN SONNTAG/NASA
The radar photos more clearly showed what looked like the rim of a crater, but they were still too fuzzy in the middle. Many features on Earth's surface, like volcanic calderas, can masquerade as circles. However, only impact craters contain central peaks and peak rings, which form at the center of a new crater when molten rock rebounds just after a strike. To look for these features, the researchers needed a dedicated radar mission.
The Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, had coincidentally purchased a next-generation ice-penetrating radar to mount across the wings and body of their Basler aircraft, a twin-propeller retrofitted DC-3 that is a workhorse of Arctic science. However, they also needed financing and a base close to Hiawatha.
Kjær took care of the money. He thought that traditional funding agencies would be too slow, or prone to leaking their idea. He, therefore, petitioned Copenhagen's Carlsberg Foundation, that uses profits from its global beer sales to finance science. MacGregor, for his part, enlisted NASA colleagues to persuade the U.S. military to let them work out of Thule Air Base, a Cold War outpost on northern Greenland, where German members of the team had been trying to get permission to work for twenty years.
Three flights, in May 2016, added 1600 kilometers of fresh data from dozens of transits across the ice. The radar revealed five prominent bumps in the crater's center, indicating that a central peak was rising about 50 meters high. The crater bottom was exceptionally jagged. If the asteroid had struck earlier than 100,000 years ago when the area was ice-free, erosion from melting ice farther inland would've scoured the crater smooth, MacGregor says. Furthermore, the radar signals showed that the thick layers of ice were jumbled up—another sign of a recent impact.
Banded patterns in the mineral quartz are diagnostic of shock waves from an extraterrestrial impact. ADAM GARDE, GEUS
However, the team needed direct evidence to overcome the skepticism they knew would greet a claim for a massive young crater, one which seemed to defy the odds of how often large impacts happen. That is why on July 2016 Kjær found himself frenetically sampling rocks all along the crescent of terrain encircling Hiawatha's face. His most crucial stop was in the middle of the semicircle, close to the river, where he collected sediments which appeared to have come from the glacier's interior.
In that outwash, Kjær's team closed its case. Sifting through the sand, Adam Garde, a geologist at the Geological Survey of Denmark and Greenland in Copenhagen, discovered glass grains forged at temperatures higher than a volcanic eruption could generate. He also discovered crystals of quartz, which contained a distinctive banded pattern which can be formed only in the intense pressures of extraterrestrial impacts or nuclear weapons.
The Younger Dryas, named after a small white and yellow arctic flower which flourished during the cold snap, had long fascinated scientists. Until human-driven global warming set in, that period reigned as one of the sharpest recent swings in temperature on the planet. As the last ice age waned, approximately 12,800 years ago, temperatures in parts of the Northern Hemisphere plunged by as much as 8°C, all the way back to ice age readings. They stayed that way for over a thousand years, turning advancing forest back into the tundra.
The trigger could've been a disruption in the conveyor belt of ocean currents, such as the Gulf Stream that carries heat northward from the tropics. In a 1989 paper in Nature, Kennett, with Wallace Broecker, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory, and others, laid out how meltwater from retreating ice sheets could've shut down the conveyor. As warm water from the tropics travels north at the surface, it cools while evaporation makes it saltier. Both factors boost the water's density until it sinks into the abyss, helping to drive the conveyor. Adding a pulse of less-dense freshwater could hit the brakes. Paleoclimate researchers have primarily endorsed the idea, although evidence for such a flood has lacked until recently.
In 2016, Kurt Kjær looked for evidence of an impact in sand washed out from underneath Hiawatha Glacier. He would find glassy beads and shocked crystals of quartz. SVEND FUNDER
In 2007, Kennett suggested a new trigger. He teamed up with scientists led by Richard Firestone, a physicist at Lawrence Berkeley National Laboratory in California, that proposed a comet strike at the critical moment. Exploding over the ice sheet covering North America, the comet or comets would've tossed light-blocking dust into the sky, cooling the region. Farther south, fiery projectiles would've set forests alight, producing soot that deepened the gloom and the cooling. The impact also could've destabilized ice and unleashed meltwater that would've disrupted the Atlantic circulation.
The climate chaos could explain why the Clovis settlements emptied, and the megafauna vanished soon afterward. However, the evidence was scanty. Firestone and his colleagues flagged thin sediment layers at dozens of archaeological sites in North America. These sediments seemed to contain geochemical traces of an extraterrestrial impact, like a peak in iridium, the exotic element which helped cement the case for a Chicxulub impact.
The team met immediate criticism. The decline of mammoths, giant sloths, and other species had begun well before the Younger Dryas. Besides, no sign existed of a human die-off in North America, according to archaeologists. The nomadic Clovis people would not have stayed long in any site. The distinctive spear points which marked their presence probably vanished not because the people died out, but rather because those weapons were no longer useful once the mammoths waned, according to Vance Holliday, an archaeologist at The University of Arizona in Tucson.
The geochemical evidence also started to erode. Outside scientists couldn't detect the iridium spike in the group's samples. The beads were real, yet they were abundant across many geological times, and soot and charcoal didn't seem to spike at the time of the Younger Dryas.
The impact hypothesis never quite died though. Its proponents kept on studying the putative debris layer at other sites in Europe and the Middle East. They also reported discovering microscopic diamonds at different locations that, they say, could have been formed only by an impact.
The impact would've melted 1500 gigatons of ice, according to the team's estimations; that's approximately as much ice as Antarctica has lost due to global warming in the past decade. The local greenhouse effect from the released steam and the residual heat in the crater rock would've added more melt. Much of that freshwater could've ended up in the nearby Labrador Sea, a primary site pumping the Atlantic Ocean's overturning circulation.
The evidence begins with the ice. In the radar images, grit from distant volcanic eruptions makes some of the boundaries between seasonal layers stand out as bright reflections. The bright layers can be matched to the same layers of grit in cataloged, dated ice cores from other parts of Greenland. Using that technique, Kjær's team discovered that most ice in Hiawatha is perfectly layered through the past 11,700 years. However, in the older, disturbed ice below, the bright reflections disappear. Tracing the thick layers, the team matched the jumble with debris-rich surface ice on Hiawatha's edge which was previously dated to 12,800 years ago.
Other lines of evidence also suggest Hiawatha could be the Younger Dryas impact. In 2013, Jacobsen examined an ice core from the center of Greenland, 1000 kilometers away. He expected to put the Younger Dryas impact theory to rest by demonstrating that, 12,800 years ago, levels of metals that asteroid impacts tend to spread didn't spike. He instead discovered a peak in platinum, similar to ones measured in samples from the crater site.
Nobody can be sure of the timing though. The disturbed layers could reflect nothing else than normal stresses deep in the ice sheet. Richard Alley, a glaciologist at Pennsylvania State University in University Park, thinks that the impact is much older than 100,000 years and a subglacial lake can explain the odd textures near the base of the ice.
A recent impact should have also left its mark in the half-dozen deep ice cores drilled at other sites on Greenland, that document the 100,000 years of the current ice sheet's history. However, none exhibits the thin layer of rubble that a Hiawatha-size strike should have kicked up.
Brandon Johnson, a planetary scientist at Brown University, is not so sure. After reading a draft of the study, Johnson, who models impacts on icy moons like Europa and Enceladus, used his code to recreate an asteroid impact on a thick ice sheet. An impact digs a crater with a central peak similar to the one seen at Hiawatha, he discovered, but the ice suppresses the spread of rocky debris.
Even if the asteroid struck at the right moment, it might not have unleashed all the disasters envisioned by proponents of the Younger Dryas impact. It may not even have triggered the Younger Dryas. Ocean sediment cores show no trace of a surge of freshwater into the Labrador Sea from Greenland, according to Lloyd Keigwin, a paleoclimatologist at the Woods Hole Oceanographic Institution in Massachusetts. Keigwin adds that the best recent evidence suggests a flood into the Arctic Ocean through western Canada instead.
An external trigger might be unnecessary in any case, according to Alley. During the last ice age, the North Atlantic saw 25 other cooling spells, potentially triggered by disruptions to the Atlantic's overturning circulation. None of these spells, known as Dansgaard-Oeschger (D-O) events, was as serious as the Younger Dryas; yet their frequency suggests an internal cycle played a role in the Younger Dryas, too.
Hiawatha's full story will still come down to its age. Even an exposed impact crater can be a challenge for dating, that requires capturing the moment when the impact altered existing rocks. The researchers are inspecting a blue crystal of the mineral apatite for lines left by the decay of uranium, but it is a long shot. The team also discovered traces of carbon in other samples, which may someday yield a date, Kjær says. However, the ultimate answer might require drilling through the ice to the crater floor, to rock which melted in the impact, resetting its radioactive clock. With big enough samples, researchers should be able to pin down Hiawatha's age.
Given the remote location, a drilling expedition to the hole at the top of the world would be costly. However, an understanding of recent climate history—and the giant impact it could do to our planet—is at stake.
Reference: Sciencemag
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