It was the worst day in the history of life on Earth. One moment, the Age of Dinosaurs lumbered on as it had for millions and millions of years. The next, a roughly six-mile-wide chunk of space rock slammed into the Earth, kicking off a mass extinction that would wipe out the non-avian dinosaurs and many other forms of life. And now, more than 66 million years later, researchers have begun to pinpoint where that cataclysm-sparking piece of rock came from.
The fact that a huge piece of extraterrestrial rock struck what is now the Yucatan Peninsula 66 million years ago is not controversial. And, year by year, scientists working in different disciplines keep amassing more evidence that this unprecedented event caused our planet’s fifth mass extinction. The incredible heat of impact debris returning to the atmosphere, global wildfires and a dust cloud that blocked the sun for years all played a role. In the end, almost three quarters of known species went extinct during the cataclysm.
So far, however, most of what we know about the event has come from earthbound evidence. No one really knew where the dino-destroying rock came from or how it came to intersect our planet’s orbit.
Published in Scientific Reports today, the new study by astronomers Amir Siraj and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics, propose that a series of break-ups and chance events sent the huge chunk of space rock our way.
The new hypothesis was discovered by looking outward, then looking inward. “My work on the asteroid impact rates for Earth-like exoplanets prompted me to investigate the properties of cometary impact rates on such systems,” Siraj says. Naturally, what better way to understand Earth-like planets than studying our own solar system? By looking at our astronomical neck of the woods, Siraj noticed that some comets came very close to Earth after having close brushes with the sun.
The story likely started in the Oort Cloud. This is a cloud-like field of debris around the sun. But the debris don’t always stay there. The gravitational pull of the sun and Jupiter can pull comets and asteroids out of the cloud and inadvertently set them on a course for other parts of the solar system.
Some of the Oort Cloud comets are often big, between 10 and 37 miles across. And, Siraj noticed, when such large chunks of rock pass close enough to the sun, its massive gravitational forces can tear the rocks into smaller chunks. Those chunks might not be small in an absolute sense. Some can still be miles across, just like the one that struck the Earth 66 million years ago.
Both the sun and the planet Jupiter are so large that their gravity alters the orbits of comets that pass towards the middle of our solar system. Jupiter’s massive gravitational field sometimes disrupts their orbit and sends them closer to the sun. The overall effect, Siraj says, is “like a pinball machine.”
Some of these rocks passing close to the sun, Siraj says, “produce fields of cometary shrapnel.” The breakup of the comet Shoemaker-Levy 9 is an example of just this sort of interstellar interaction. This comet was pulled apart by Jupiter’s gravity in 1992 before the pieces crashed into the planet in 1994.
Not that all experts agree with this new model. Asteroid expert Bill Bottke of the Southwest Research Institute says that events like Shoemaker-Levy 9 may have more stringent requirements than the new model suggests. Further, Bottke says, the model predicts that other planets in the solar system should show signs of these large impacts through time.
Siraj responds that time is a critical factor, with the new model focusing on objects that don’t immediately crash into Jupiter but make their way further into the solar system. And, Siraj says, “It is certainly possible that Mercury, Venus, or Mars had similar impact events,” but this was outside the scope of the present study.
The implications of the hypothesis go beyond the fate of Tyrannosaurus and Triceratops. Other impacts marked Earth both before and after the end-Cretaceous collision. These impacts didn’t trigger mass extinctions, but they still created massive craters like the Zhamanshin crater in Kazakhstan and the Vredefort crater in South Africa
If all of this has you a little nervous looking at the night sky, though, don’t worry. The new model predicts that a comet or asteroid the size of the one that struck at the end of the Cretaceous is only going to strike Earth every 250 to 730 million years or so. What happened 66 million years ago was a truly exceptional and rare event, underscored by the fact that it is the only mass extinction in the history of life on Earth to be caused by an impact rather than Earth-bound causes like intense volcanic activity.
Most of the daughter rocks created by the Jupiter “pinball machine” just go sailing right on by. In fact, Siraj notes: “The major, short-term risks to the Earth still come from near-Earth asteroids, which are the focus of most planetary defense efforts.” That’s small comfort in a big universe.