The Chilling Regularity of Mass Extinctions

Scientists say new evidence supports a 26-million-year cycle linking comet showers and global die-offs.

An artist's rendering of an asteroid or comet striking Earth. (Andrea Danti / Shutterstock)

One thing we know for sure is that conditions on Earth were, shall we say, unpleasant for the dinosaurs at the moment of their demise. Alternate and overlapping theories suggest the great beasts were pelted with monster comets, drowned by mega-tsunamis, scorched with lava, starved by a landscape stripped of vegetation, blasted with the radiation of a dying supernova, cloaked in decades of darkness, and frozen in an ice age.

Now, a pair of researchers have new evidence to support a link between cyclical comet showers and mass extinctions, including the one that they believe wiped out the dinosaurs 66 million years ago. Michael Rampino, a geologist at New York University, and Ken Caldeira, an atmospheric scientist at the Carnegie Institution for Science, traced 260 million years of mass extinctions and found a familiar pattern: Every 26 million years, there were huge impacts and major die-offs. Their work was accepted by the Monthly Notices of the Royal Astronomical Society in September.

In recent decades, researchers using other methods have found evidence for a 26-million-year cycle of extinction on Earth, but the idea has remained controversial and unexplained. “I believe that our study, using revised dating of extinctions and craters, and a new method of spectral analysis, is strong evidence for the cycles,” Rampino told me.

Other scientists who have researched mass extinctions are more measured about the latest findings. “I’m sort of agnostic [about the larger theory],” said Paul Renne, the director of Berkeley Geochronology Center. “But I was really disappointed to see they used an age-database for the craters which is full of outdated information.”

Renne is the author of another new study that focuses on the Chicxulub crater, the massive divot beneath the Yucatán Peninsula that was created by the same impact blamed for the extinction of the dinosaurs. Renne and his colleagues believe that the comet or asteroid that blasted into Earth and made Chicxulub also set off a global chain-reaction of volcanic eruptions that accelerated the end of the dinosaurs. Volcanoes were, they believe, erupting continuously for millions of years. Long enough to make Hawaii’s Kilauea, which has been flowing since 1983, seem laughable. (“Kilauea is nothing,” Renne told me. “Kilauea is a flea.”)

And while Renne is interested in the possibility that volcanism is tied to intervals of mass extinction, that possible connection doesn’t explain what kind of cycles might trigger the awakening of Earth’s most powerful magma systems on a global scale. That’s where theories about galactic periodicity come back into play.

“One of the earliest proponents of a periodic record [of mass extinction] was by a guy named Rich Muller,” Renne said. “He proposed a kind of phenomenological periodicity in which they didn’t really have a mechanism.” In other words, Muller found the 26-million-year pattern of mass extinctions on Earth, but didn’t immediately know what drove the cycle.

The latest findings from Rampino and Caldeira build on the idea that regular comet showers cause intervals of mass extinctions. The showers, the theory goes, are triggered by the movement of the sun and planets through the crowded mid-plane of our galaxy. As the sun crosses that region, it disrupts great clouds of space dust. Those clouds, in turn, throw off the orbit of comets, sending them careening toward Earth.

In another theory, planetary scientists suggested that one region of the solar system in particular, known as the Oort comet cloud, plays a key role in mass extinctions. The Oort cloud is a sprawling region at the border of our solar system that contains trillions of icy bodies. Muller put forth a popular hypothesis in the 1980s that said our sun has a sort of evil twin in the Oort cloud. This hypothetical star, he suggested, has an orbital cycle such that it would perturb its neighboring objects, and send 1 billion of them hurtling toward Earth every 26 million years. The star, a binary to the sun, was nicknamed Nemesis, and playfully referred to as the death star. “The binary star, or Nemesis theory, was an alternate to the Galactic-plane story,” Rampino told me. “But the star was looked for, but never found, so Nemesis theory is out of favor now.”

“[Muller] doesn’t even believe that anymore,” Renne told me.

If Rampino and Caldeira are correct, the next mass extinction may not be far off—in geologic terms, anyway. Our little corner of the solar system crossed the plane about 2 million years ago, and we are now moving up and through it. “In the Galactic theory, we are near the Galactic plane, and we have been in the danger zone for a couple of million years,” Rampino said. “We are still close to the plane, maybe 30 light years above the plane, [and] a light year is 6 trillion miles … We won’t come back across the plane for about another 30 million years.”

And while scientists can’t be sure when the next major comet or asteroid impact on Earth will be, the one that is believed to have killed the dinosaurs still stands out as extraordinary, even by mass-extinction standards. The city-sized asteroid that created Chicxulub, for instance, released more energy than 1 billion nuclear bombs when it hit the Earth.

“There hasn’t been an impact large enough to cause a major mass extinction since the impact 66 million years ago,” Rampino said. “That was a 10-kilometer [six-mile] diameter asteroid or comet. The largest impactor in the last 66 million years was only 5 kilometers [3 miles] in diameter, which only has one-tenth the energy, so it probably wouldn’t have taken out the dinosaurs. In fact, if a 10-kilometer-sized object had hit in the last 66 million years, we wouldn’t be here. Our ancestors probably would have been knocked out.”

The obvious next question, of course, is how do we prevent the terrible fate the dinosaurs suffered? “These events are so rare in geologic time that the odds of even our great-great-great-great-great-great-grandchildren witnessing them are really low,” Renne said. “The ultimate proof, which is observation, is not going to be available to us, unfortunately.” (Or fortunately, depending on your priorities.)

In the meantime, scientists are actively scouring the skies, and calculating the orbits of monstrous comets and asteroids. “So far, none are on a collision course, but the work has just begun in earnest,” Rampino said. “Once we know one is coming, then there are several options to divert the object. (You don’t want to blow it up, that will just increase the numbers of impactors.) One possibility is to have a nuclear explosion off to one side of the comet or asteroid, pushing it just slightly off course, or possibly just hitting the object with a rapidly moving space-craft would provide enough of a nudge.”

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