It is now generally accepted that 66 million years ago a large asteroid smacked into the Earth, causing the large Chicxulub crater off the coast of Mexico. This was a catastrophic event, affecting the entire globe. Fire rained down causing forest fires across much of the globe, while ash and debris blocked out the sun. A tsunami washed over North America – one site in North Dakota contains fossils from the day the asteroid hit, including fish with embedded asteroid debris. About 75% of species went extinct as a result, including all non-avian dinosaurs.

For a time there has been an alternate theory that intense vulcanism at the Deccan Traps near modern-day India is what did-in the dinosaurs, or at least set them up for the final coup de grace of the asteroid. I think the evidence strongly favors the asteroid hypothesis, and this is the way scientific opinion has been moving. Although the debate is by no means over, a majority of scientists now accept the asteroid hypothesis.

But there is also a wrinkle to the impact theory – perhaps there was more than one asteroid impact. I wrote in 2010 about this question, mentioning several other candidate craters that seem to date to around the same time. Now we have a new candidate for a second KT impact – the Nadir crater off the coast of West Africa.

Geologists first published about the Nadir crater in 2022, discussing it as a candidate crater. They wrote at the time:

“Our stratigraphic framework suggests that the crater formed at or near the Cretaceous-Paleogene boundary (~66 million years ago), approximately the same age as the Chicxulub impact crater. We hypothesize that this formed as part of a closely timed impact cluster or by breakup of a common parent asteroid.”

Now they have published a follow up study, having been given access to private seismic data that allows for a detailed 3D analysis of the crater site. This is important because of how scientists identify impact craters. The gold standard is to identify physical evidence of impact, such as shock crystals. There are telltale minerals that can only be formed by the intense sudden power of an impact, or that form when debris is thrown into the high atmosphere while molten and then rains back down. These are conclusive signs of an impact. But there are many somewhat circular structures in the world, and often they may be prematurely declared a crater without solid evidence. So geologists are cautious and skeptical.

For the Nadir crater, which is on the sea floor, we do not have physical evidence. The initial study showed that it has a candidate circular structure, but this was not enough evidence to convince the scientific community. The detailed new analysis, however, is more compelling. First the scientists find that it does have a complete circular structure consistent with an impact basin. Even more significant, however, is that they have documented a “central uplift”, which is a characteristic sign of an impact crater. When asteroids hit they cause a depression and liquify the underlying rock. This causes a shockwave which rebounds, causing the molten rock to uplift in the center of the crater, leaving behind an uplift. This means that a circular basin with an uplift in the exact middle is a signature of an impact. This is solid and convincing evidence, even without the physical evidence of impact crystals. They write:

“Our new study published in Communications Earth & Environment ³ presents this new, state-of-the-art 3D data, revealing the architecture of the crater in exceptional detail and confirms (beyond reasonable doubt!) an impact origin for the crater. This is the first time that an impact structure has ever been imaged fully with high-resolution seismic data like this and it is a real treasure trove of information to help us to reconstruct how this crater formed and evolved.”

Sounds pretty convincing. But this leaves the questions they raised in their study two years ago – did this asteroid hit at the exact same time as the Chicxulub asteroid? If not, how far apart were they? If they did hit at the same time, were they originally part of the same asteroid? Given that there are other candidate craters that date to the same period of time, perhaps the asteroid broke up into multiple pieces that all struck the Earth at the same time.

If these asteroids were not originally part of the same asteroid, then what are the other possibilities? It is possible, although statistically unlikely, that there were simply different independent major impacts within a short time of each other. There is nothing to keep this from happening, and given the history of life on Earth perhaps it’s not that surprising, but it would be a statistical fluke.

The other possibility is that, even if they were different asteroids, perhaps there was some astronomical event that caused multiple chunks of rock and ice to swarm into the inner solar system. This would have caused a temporary period of relatively high bombardment. Perhaps a rogue planet swung by our solar system, scattering material from the Kuiper belt, some of which found its way to Earth.

The authors propose to drill into this structure, to get that physical evidence that would be so helpful. Not only would this confirm its impact status, but we may be able to tell if the chunk of rock that caused the Nadir crater has the same mineral signature as the Chicxulub asteroid. I suspect we could also tell their relative timing. Perhaps we could see the iridium layer from the Chicxulub impact, and see how that relates to the Nadir impact.

We may be able to answer – were the dinosaurs just really unlucky, or was the Chicxulub impact event more devastating than we even realized? Either way I look forward to more scientific investigation of the Nadir crater.