Approximately every 35 million years, a comet careens from outer space and lays waste to the Earth. For nearly two decades, researchers at Harvard have attempted to understand this “periodicity” of mass extinction. Now, in a paper recently submitted to Astrophysical Journal Letters, two Harvard graduate students used the 250 million-year-old crater record to show how a disk of dark matter suffusing the galaxy could spark mass extinctions.
Most scientists agree that the observable stuff in our galaxy, concentrated in a disk, is surrounded by a diffuse halo of dark matter. But some maintain that a portion of dark matter exists in a separate, condensed form—a “dark disk” located both on top of and within our own flat galaxy. It’s the relative movements of our galaxy and this disk that may trigger events that lead to mass die-offs.
“There’s a cycle in our galaxy; the solar system bobs up and down through the plane of the disk-shaped galaxy—we’re pulled towards the plane, we go through it, we overshoot, we go back,” said Michael Rampino, professor of biology at New York University. “The folks at Harvard are proposing that there is a thin disk of concentrated dark matter, and the solar system moves through it every 35 million years or so.”
Since this timeline matches the interval between comet impacts, the idea is that dark matter exudes powerful gravitational forces, disrupting the comet cloud around our solar system as we pass through the dark disk. Dislodged meteorites can then tumble towards Earth with dire repercussions.
The researchers’ analyses were unique in that they incorporated three important measurements related to our galaxy: specific estimations of its mass components, its rotating spiral arm pattern, and the radial bobbing motion of the solar system as it revolves around the galactic center. In fact, this was the first study to include either of the latter two features. Taking such factors into account, co-author Michael Rowan said the model accurately predicted the date 66 million years ago when the Chicxulub crater was created by the impact that annihilated the dinosaurs.
Harvard physics professor Lisa Randall, who served as an advisor for the paper, was largely responsible for revitalizing the dark disk theory in the early 2013 after its decline in the 1990s. “The dark disk is controversial because the data weren’t always correctly interpreted or understood,” Randall said. “The idea is re-emerging with a growing consensus that we don’t know the nature of dark matter well enough to rule out interesting candidates.”
Both Rampino and Manoj Kaplinghat, a professor of physics and astronomy at UC Irvine, are sympathetic to this view “If the normal stuff, which is just 20% of the matter in the universe by mass, is so complicated, then why should the rest of the 80%, which is dark, be simple?” Kaplinghat said. But, he added, as it currently stands, the theory requires additional data to back it up.
According to Kaplinghat, this model is novel because it includes a dark disk possibly thick enough to be stable, yet thin enough to match the periodicity in the crater record. He also said this was the first model to account for the motion of the solar system through the spiral arms of our galaxy. The gravitational forces exerted by the spiral arms impede the solar system as it bobs through the plane of the galaxy—which could explain why the periods between comet impacts are not always exactly 35 million years.
To test the effects of these adjustments, the researchers asked their model to predict the dates of multiple crater impacts already noted on the record. Rowan’s collaborator, Eric Kramer, said the Chicxulub prediction stuck out in particular. “Previously, it appeared the comet that killed the dinosaurs was a few million years off from the time the sun crossed the galactic plane,” he said. “But when you take spiral arm passages into account, the data line up quite well.”
Rowan said, in theory, the model could be used to anticipate future comet impacts. Kramer affirmed that we’re currently passing through the dark disk, which suggests a higher probability of comet impact. “Of course, these showers happen over timescales of millions of years, so hopefully we’re safe,” he said. “All it takes is one comet to have disastrous consequences.”