What killed the dinosaurs? It maybe have been not one, but two asteroid strikes that caused the climate change that killed the dinosaurs. It may even have been a cluster of asteroids.

The discovery of a new impact crater off the west coast of Africa suggests that the huge asteroid that crashed into the Gulf of Mexico 66 million years ago had a smaller cosmic companion.

Scientists already know that an asteroid—or perhaps a comet—struck Earth off Mexico’s Yucatán Peninsula. The resulting 110 miles/80 kilometers wide Chicxulub crater is thought to have caused a decades-long “impact winter” that killed the dinosaurs.

Now scientists have used seismic measurements to find evidence of an asteroid impact crater—named “Nadir”—below the North Atlantic Ocean that measures five miles/9 kilometers wide. It’s 1,300 feet below the seabed about 250 miles off the coast of Guinea, West Africa.

Detailed in a study published in Science Advances yesterday, the asteroid that caused the Nadir Crater it is estimated to have also been about 1,300 foot-wide.

That’s about the same size as asteroid Bennu, which NASA asteroid sample return mission OSIRIS-REx visited from 2018 through 2020. Bennu is considered to be the most hazardous object in the near-Earth asteroid catalog, with a 1-in-1750 chance of collision in the next few hundred years.

Crucially, the Nadir Crater is dated to around 66 million years ago—the same as the Chicxulub crater.

Could the discovery of a second, albeit smaller asteroid impact site mean a rethink about how the dinosaurs died-off? Were there, in fact, more than two asteroids—or even a cluster that all struck Earth and whose craters have not yet been discovered?

“It would have generated a tsunami over 3,000 feet high, as well as an earthquake of more than magnitude 6.5,” said Veronica Bray, a research scientist in the University of Arizona Lunar and Planetary Laboratory, and second author on the paper. “Although it is a lot smaller than the global cataclysm of the Chicxulub impact, Nadir will have contributed significantly to the local devastation.”

The impact may also have released substantial quantities of greenhouse gases from shallow buried black shale deposits at the impact site, according to the scientists.

Despite asteroids striking Earth for its entire existence only 200 impact craters have so far been have been discovered, fewer than 20 of them on or under the seabed. At Nadir Crater the researchers discovered an elevated rim above a terraced crater floor, a pronounced central uplift and extensive subsurface deformation—all tell-tale signs of san impact crater.

As to where the asteroid that caused the Nadir Crater may have come from, the authors present four hypothesis:

• It’s not related to the Chicxulub asteroid.
• It broke-off the main Chicxulub asteroid.
• The Chicxulub asteroid was a binary asteroid.
• The Chicxulub asteroid and the “Nadir asteroids” were part of a cluster of asteroids that struck Earth.

If the latter then there could be more Chicxulub “siblings” to discover.

“The Nadir Crater is an incredibly exciting discovery of a second impact close in time to the Cretaceous–Paleogene extinction,” said study co-author Sean Gulick, an impact expert at the University of Texas at Austin. “While much smaller than the extinction causing Chicxulub impactor, its very existence requires us to investigate the possibility of an impact cluster in the latest Cretaceous.”

The researchers now want to drill into the seabed to both confirm that Nadir Crater is an asteroid impact crater and to test its precise age.

Despite the new possibility of two impacts in a relatively short period of time in the Cretaceous Period the chances of another similar impact in the modern era is unlikely. Scientists rate the frequency of an an impact by an asteroid over six miles/10 kilometers in size are rare and happen only about once every 250 to 500 million years.

Wishing you clear skies and wide eyes.