Some 66 million years ago, a catastrophic event occurred that wiped out three-quarters of all plant and animal species on Earth, most notably taking down the dinosaurs. An errant asteroid from the asteroid belt has been deemed the most likely culprit. However, in a new paper published in Scientific Reports, Harvard astronomers offer an alternative: a special kind of comet—originating from a field of debris at the edge of our solar system known as the Oort cloud—that was thrown off course by Jupiter’s gravity toward the Sun. The Sun’s powerful tidal forces then ripped pieces off the comet, and one of the larger fragments of this “cometary shrapnel” eventually collided with Earth.
The most widely accepted explanation for what triggered that catastrophic mass extinction is known as the “Alvarez hypothesis,” after the late physicist Luis Alvarez and his geologist son, Walter. In 1980, they proposed that the extinction event may have been caused by a massive asteroid or comet hitting the Earth. They based this conclusion on their analysis of sedimentary layers at the Cretaceous-Paleogene boundary (the K-Pg boundary, formerly known as the K-T boundary) found all over the world, which included unusually high concentrations of iridium—a metal more commonly found in asteroids than on Earth. (That same year, Dutch geophysicist Jan Smit independently arrived at a similar conclusion.)
Since then, scientists have identified a likely impact site: a large crater in Chicxulub, Mexico, in the Yucatan Peninsula, first discovered by geophysicists in the late 1970s. The impactor that created it was sufficiently large (between 11 and 81 kilometers, or 7 to 50 miles) to melt, shock, and eject granite from deep inside the Earth, probably causing a megatsunami and ejecting vaporized rock and sulfates into the atmosphere. This in turn had a devastating effect on global climate, leading to mass extinction.
That hypothesis was further bolstered in 2016, when a scientific drilling project led by the International Ocean Discovery Program took core samples from the crater’s peak ring, confirming that the rock had been subjected to immense pressure over a period of minutes. Just last year, a paper published in Nature Communications concluded that the impactor struck at the worst possible angle and caused maximum damage. It has been estimated that the impact would have released energy over a billion times higher than the atomic bombs dropped on Hiroshima and Nagasaki in 1945.
This latest theory came about when co-author Amir Siraj, an undergraduate in astrophysics at Harvard, began looking into the asteroid impact rates for Earth-like exoplanets, which in turn led him to study the comet impact rates on those systems. He ran numerical simulations to calculate the flux of so-called long-period comets in our own solar system, since scientists know much more about our system. “What I found most striking was that a significant fraction of Earth-crossing events were directly preceded by remarkably close encounters with the Sun, which came from a class of comets caught in high-eccentricity orbits due to their gravitational interactions with the Jupiter-Sun system,” Siraj told Ars.
Further investigation revealed that comets within the size range of 10 to 60 kilometers (between 6 and 37 miles) would be torn apart into smaller fragments by sufficiently strong tidal forces, similar to what happened to the comet Shoemaker-Levy 9 when it crashed into Jupiter in 1994. “Crucially, I found that these events happen so often and produce such a large number of fragments that they result in an impact rate of Chicxulub-size at Earth that is an order of magnitude higher than the background asteroid or comet populations,” Siraj told Ars. “This was interesting because from a statistical standpoint, the K-Pg impact is at odds with the impact rates from background asteroid or comet populations but consistent with the rate I derived for this new dynamical pathway.”
Siraj and co-author Avi Loeb concluded from their analysis that Jupiter’s gravitational field was strong enough to bump many such long-period comets from the Oort cloud off course, bringing them very close to the Sun. Such comets are known as “sun grazers”; about 20 percent of long-period comets become sun grazers, per the authors. And the Sun’s powerful tidal force in turn can break them into fragments.
Siraj likened the effect to a pinball machine. “When you have these sun grazers, it’s not so much the melting that goes on, which is a pretty small fraction relative to the total mass, but the comet is so close to the Sun that the part that’s closer to the Sun feels a stronger gravitational pull than the part that is farther from the Sun, causing a tidal force,” he said. “You get what’s called a tidal disruption event, so these large comets that come really close to the Sun break up into smaller comets. And basically, on their way out, there’s a statistical chance that these smaller comets hit the Earth.”
Siraj and Loeb’s calculations showed that there was an increase in the likelihood of long-period comets impacting Earth by a factor of 10, and that new rate jibes with the age of the Chicxulub impactor, making this a viable theory of its origin. “Our paper provides a basis for explaining the occurrence of this event,” Loeb said. “We are suggesting that, in fact, if you break up an object as it comes close to the Sun, it could give rise to the appropriate event rate and also the kind of impact that killed the dinosaurs.” Each such event would produce “a collection of smaller fragments that cross the orbit of the Earth,” the authors wrote.
Their findings also offer evidence that the unusual composition of the Chicxulub impactor—carbonaceous chondrite—indicates it originated from the Oort cloud and not from the main asteroid belt. It’s a rare composition for main-belt asteroids, but it’s common among long-period comets. The authors also point to other impact craters with similar composition, most notably the Vredefort crater in South Africa—the result of an impact some 2 billion years ago—and the Zhamanshin crater in Kazakhstan, from an impact within the last million years. Those time frames are in line with Siraj and Loeb’s calculations, which indicate such objects should strike Earth every 250,000 to 730,000 years.
A 2007 Nature paper proposed that the Chicxulub impactor may have originated from the “Baptisina family” of asteroids, fragments created in an asteroid belt collision some 160 million years ago. These asteroids have that rare composition of carbonaceous chondrites, in keeping with analysis of the Chicxulub crater. Data from the Wide-field Infrared Survey Explorer (WISE) cast doubt on this possibility in 2011, however, placing the date of the Baptisina fragmentation at just 80 million years ago—too late to account for the Chicxulub crater and the K-Pg extinction event.
Siraj and Loeb’s calculations poke further holes in that possibility. “Our hypothesis… predicts a larger proportion of impactors with carbonaceous chondritic compositions than would be expected from meteorite falls of main-belt asteroids,” the authors wrote.
One alternative theory that Siraj and Loeb have yet to address is known as the multiple impact hypothesis. There are several other, smaller craters about the same age as Chicxulub that have since been discovered, suggesting that there may have been more than one comet fragment that hit the Earth 66 million years ago. “It is an interesting question,” Siraj told Ars. “Future work will be required to better understand if there are any implications of this model for the multiple impact hypothesis.”
Next, the pair will look toward future observations from the Vera Rubin Observatory in Chile—which will see first light next year—to confirm their theory, hoping that data will yield evidence of comets experiencing tidal disruption. “We should see smaller fragments coming to Earth more frequently from the Oort cloud,” said Loeb. “I hope that we can test the theory by having more data on long-period comets, getting better statistics, and perhaps seeing evidence for some fragments.”
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