A second asteroid may have struck during the dinosaurs' demise

In a catastrophic instant some 66 million years ago, the course of life on Earth was forever changed. A six-mile-wide space rock slammed into the coast of Mexico’s Yucatán Peninsula—and it kicked off a global cataclysm. The towering tsunamis that followed crashed on shores for thousands of miles. Wildfires raced across vast swaths of land. And the vaporization of rock along the seafloor unleashed gasses that sent the climate into wild swings—all of which led to the extinction of some 75 percent of all species, including all non-avian dinosaurs.

But this might not be the whole story. Buried under layers of sand on the coast of West Africa are hints that the giant space rock might not have been alone.

Researchers discovered a possible crater that spans some 5.3 miles wide, revealed in seismic surveys of the seafloor, according to a new study published in Science Advances. The crater, dubbed Nadir after a submarine volcano nearby, appears to have been carved by the impact of a space rock at least a quarter of a mile wide—and it may have formed around the same time as the Chicxulub crater, the expansive scar in Earth’s surface from the dinosaur-killing asteroid.

“A lot of people have questioned: How could the Chicxulub impact—albeit a huge one—be so globally destructive?” says study author Veronica Bray, a planetary scientist from the University of Arizona. “It might be that it had help.”

The object that created Nadir would have been considerably smaller than the Chicxulub impactor, so its effects were likely regional. But if confirmed, the second meteorite strike in quick succession could have delivered a one-two punch in the global catastrophe at the end of the Cretaceous period, according to the study. In one scenario, the pair of asteroids could have come from a single parent body that fractured in two before colliding with Earth’s atmosphere and punching the ground more than 3,400 miles apart.

While additional analysis is needed to confirm the suspected crater’s age and identity, and whether it’s linked with Chicxulub, scientists are cautiously excited about the potential for a newfound impact site.

Earth’s record of ancient impacts is woefully incomplete thanks to the planet’s active geologic churn. Swaths of the surface are recycled into the planet’s mantle, while other areas are repaved with fresh volcanic rock, and still others are ground away by shifting glaciers. Only about 200 impact craters have been confirmed on the planet, which prevents scientists from fully understanding how these strikes affected the ancient Earth—and what role they might play in our planet’s future.

“Earth does a great job of destroying impact craters,” says Jennifer Anderson, an experimental geologist who studies impact cratering at Winona State University but was not part of the study team. Because of the planet's active geology, she says, “any discovery of a new impact crater on Earth is always important.”

A seismic surprise

Like many discoveries, the possible new crater was found by accident. Geologist Uisdean Nicholson of Heriot-Watt University in Edinburgh was interested in reconstructing how South America broke apart from Africa roughly 100 million years ago.

For clues, Nicholson examined the features beneath the seafloor between the two continents, collaborating with the commercial companies WesternGico and TGS for seismic data. The analysis tracked how seismic waves ricocheted underground to illuminate subterranean features. Almost immediately, Nicholson spotted something odd.

An expert in seismic surveys, Nicholson has seen the data for many features that create lumps and dips in the layers underground, such as salt domes rising through denser surrounding rock. But the wiggles of the data before him hinted at something more cataclysmic. “I’ve never seen anything like it,” he says.

Nicholson reached out to other scientists, including Bray, to ask if they thought this could be an impact crater, and they all agreed: The feature consists of a depression surrounded by a rim with a prominent peak at its center, which is common among such craters.

By analyzing the structure’s shape and size, the team modeled how it might have formed. The results suggest the crater came from the impact of a space rock roughly a quarter of a mile wide that screamed through the atmosphere, hitting the sea surface at nearly 45,000 miles per hour. As it plunged into the ocean, Bray says, it “would move through the water like it wasn’t even there.”

The collision would have unleashed the energy of 5,000 megatons of TNT, the team estimates, almost instantly vaporizing the water and layers of the seafloor below. Then a shockwave would have raced across the surface, causing once solid rock to flow like liquid. Within minutes, the seabed would have rebounded upward in a central peak and then collapsed back on itself. The result would be a mound within a bowl-like depression in the middle—exactly what the scientists think they’ve discovered buried off Africa’s west coast.

By correlating the sediment layers in this area with dated samples at other sites, researchers estimate that the feature formed roughly 66 million years ago—tantalizingly similar to Chicxulub.

A planetary one-two punch?

Studying the environmental consequences of the Nadir event could help us better understand what future impacts might do to our planet. The theoretical Nadir impactor would have been comparable in size to the asteroid Bennu, which has a 1-in-1,750 chance of colliding with Earth over the next three centuries, making it one of the most likely asteroids to hit our planet. Such an event would be far from insignificant, stirring tsunamis for hundreds of miles. Or as Bray puts it: “It’s big enough to wipe out a city or two.”

But what the discovery means for our understanding of the events immediately following the Chicxulub impact and the end of the dinosaurs' reign remains uncertain. The energy released by the Nadir impact and its environmental consequences would have been dwarfed by the six-mile-wide Chicxulub asteroid’s collision with Earth and the global cataclysm that followed.

“That is simply a different league,” says Martin Schmieder, an expert in large impact structures at Neu-Ulm University of Applied Sciences in Germany, who reviewed the study ahead of its publication.

But the Nadir impact could have “added insult to injury” in an already devastated ecosystem, Bray says. There’s also the question of whether there were other impacts around this same period. The study authors note that at 65.4 million years, the Boltysh impact crater in Ukraine is slightly younger than Chicxulub.

Clusters of strikes from the fragments of comets or asteroids have previously been documented on Earth and other worlds. For example, near where Anderson lives in the upper U.S. Midwest, a trio of craters dates back roughly 460 million years. They are part of a spike in impacts during the Ordovician period, which scientists have tied to a possible collision in the asteroid belt that sent a parade of meteorites hurtling toward our planet over millions of years.

However, identifying these clusters in Earth’s spotty record of ancient strikes presents a challenge. An impact the size of Nadir is estimated to occur slightly less than every 100,000 years, Schmieder says. “So this could basically happen anytime.”

And for Nadir itself, more study is needed to determine how it formed at all.

“This is an exciting discovery,” Gareth Collins, a planetary scientist who specializes in impact cratering at Imperial College London, says via email, though he cautions not much can be concluded yet about the find. Direct samples are needed to confirm the feature’s origin as well as more precise dates for the possible impact that formed it.

The study authors have already applied for emergency funds to drill into the Nadir formation and collect samples of the possibly shocked, melted, and jumbled crater rock, as well as the sediment layers above. The thickening bed of sand and mud atop the buried structure may have not only preserved the features of the crater, but it could help reveal the state of ocean life in the years after the impact—providing a trove of new data about what happens to our planet when an asteroid strikes.

“But of course,” Bray says, “we'll only know for sure when we drill into it.”