In the world of impact craters, Chicxulub is a celebrity: The 180-kilometer-diameter maw, in the Gulf of Mexico, was created by a cataclysmic asteroid impact at the end of the Cretaceous that spelled the demise of most dinosaurs. But researchers have now uncovered another crater off the coast of Guinea that might well be Chicxulub’s cousin. The newly discovered feature, albeit much smaller, is also about 66 million years old. That’s a curious coincidence, and scientists are now wondering whether the two impact structures might be linked. Perhaps Chicxulub and the newly discovered feature—dubbed Nadir crater—formed from the breakup of a parent asteroid or as part of an impact cluster, the team suggested. These results were published in Science Advances.
Rocks of Concern
Every day, tons of cosmic dust rain down on our planet. That microscopic debris poses no danger to life on Earth, but its larger brethren are very much cause for concern: A space rock measuring hundreds of meters in size is apt to cause regional destruction, and the arrival of something measuring kilometers in size could spell global havoc. That’s what happened 66 million years ago when a roughly 12-kilometer-wide asteroid slammed into a shallow reef in the Gulf of Mexico. That event, now known as Chicxulub after the small town that’s grown up nearby in Mexico, launched shock waves, powerful tsunamis, and blasts of superheated air that decimated life in the vicinity. And airborne particles—bits of dust, soot, and sulfate aerosols born from the sulfur-rich rocks that existed at the Chicxulub impact site—choked the atmosphere and plunged the entire planet into a sunlight-starved “impact winter” that lasted for years. When the air finally cleared, over 75% of all species had gone extinct.
The newly discovered Nadir crater appears to have formed right around the same time as that cataclysm. Uisdean Nicholson, a sedimentary geologist at Heriot-Watt University in Edinburgh, Scotland, and his colleagues discovered the candidate crater while they were poring over observations of seafloor sediments originally collected for oil and gas exploration. The team spotted the roughly 8-kilometer-wide structure in seismic reflection imaging data obtained off the coast of West Africa. “It was pure serendipity,” said Nicholson.
Signs of an Impact
The putative crater is buried under roughly 300 meters of sediments topped by 900 meters of water, and its appearance strongly suggests it was created by a hypervelocity impact, said Nicholson. For starters, it’s circular in shape, with a pronounced rim. Second, it contains a small central peak, a feature that often arises in large impact craters. And perhaps most important, there’s clear evidence of deformed sediments—caused by faulting and folding—persisting hundreds of meters below what would be the crater floor. “There’s a lot of things that suggest it’s an impact,” said Gavin Kenny, a geochemist at the Swedish Museum of Natural History in Stockholm who was not involved in the research.
Numerical simulations run by team member Veronica Bray, a planetary scientist at the University of Arizona, have suggested that the impactor was about 400 meters in diameter. The arrival of such an object moving at roughly 20 kilometers per second would have produced tsunami waves more than a kilometer in height and ground shaking equivalent to that of a magnitude 7 earthquake, Bray estimated. But the mayhem that ensued, intense as it was, was mostly limited to a regional scale, said Bray. “This wasn’t a global killer.”
On the basis of assemblages of microfossils unearthed close to Nadir crater, Nicholson and his colleagues estimated that this feature formed at or near the end of the Cretaceous period. But it’s too simplistic to assume that a pair of gravitationally bound asteroids—a binary asteroid—formed Chicxulub and Nadir crater in a one-two punch, the authors suggested. That’s because of the extreme distance between the two sites 66 million years ago: roughly 5,500 kilometers. (They’re even farther apart now—about 8,000 kilometers—because of spreading of the Atlantic seafloor.) Binary asteroids tend to hit much closer to one another: The one example on Earth of a so-called “impact doublet” formed by a binary asteroid is characterized by craters just a little over 10 kilometers apart. “So Chicxulub and Nadir couldn’t have formed from a direct hit of a binary asteroid,” said Nicholson.
Looking to Jupiter
A more likely scenario, Nicholson and his collaborators suggested, is something akin to what happened to comet Shoemaker-Levy 9. In 1992, the roughly 2-kilometer-diameter comet had fragmented into more than 20 pieces after passing very close to Jupiter. Two years later, those fragments slammed into the gas giant over the course of several days, creating a series of dark scars that stretched across a wide swath of the planet.
Perhaps a similar breakup of a common parent asteroid occurred on Earth 66 million years ago, Nicholson and his colleagues proposed. An asteroid—there’s good evidence that the Chicxulub impact was due to an asteroid rather than a comet—orbiting Earth could have been torn apart by our planet’s gravity. Those fragments could have then dispersed sufficiently in space such that they smashed into Earth within days of one another yet in very disparate locations, the researchers suggested.
Another possibility is that one or more asteroids collided somewhere in deep space—most likely in the asteroid belt between Mars and Jupiter—and an ensemble of cosmic shrapnel traveled en masse to Earth. The result would have been an uptick in cratering that persisted not over days, as in the case of the breakup of a common parent asteroid, but over a million or so years. Scientists are aware of only one such event—known as an impact cluster—in Earth’s history, and it occurred roughly 460 million years ago. “We think an asteroid parent body broke up somewhere in the solar system and sent material flying towards Earth,” said Kenny.
The impact cluster scenario might be more likely, Nicholson and his colleagues suggested. That’s because a third large crater—the 24-kilometer-diameter Boltysh crater in central Ukraine—also dates to around 66 million years ago. Research published last year suggested that Boltysh formed just 650,000 years after the Chicxulub impact.
There’s also the possibility that Nadir crater was simply created by an unrelated impact, Nicholson and his colleagues acknowledged. Perhaps a stroke of bad cosmic luck led to Earth being pummeled in relatively close succession.
It’s clearly key to more precisely constrain the age of Nadir crater, Nicholson and his collaborators maintain. Right now, the uncertainty in the structure’s age is about a million years, and that’s too large to discriminate between the breakup of a common parent asteroid and impact cluster scenarios. Drilling sediment cores from the crater would allow scientists to look for stratigraphic signatures like the iridium layer from Chicxulub that could yield a much more precise date. Nicholson and his colleagues recently submitted a drilling proposal to the International Ocean Discovery Program to do just that.
—Katherine Kornei (@KatherineKornei), Science Writer