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Monday, May 27, 2024 | Back issues
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Scientists Find Dinosaurs Brought to Extinction by Comet, Not Asteroid

Astrophysicists put forth a new theory to settle the old debate over whether an asteroid or a comet killed the dinosaurs 66 million years ago.

(CN) — Everyone has heard of the asteroid that killed the dinosaurs — or was it a comet? That’s what astrophysicists from Harvard aimed to find out in their new study.

Quick refresher: Asteroids are balls of rock which tend to live between the orbits of Mars and Jupiter until Jupiter’s powerful gravity field slingshots them toward the Sun. Comets, on the other hand, are primarily ice and dust from the far reaches of the solar system that begin to burn up, throwing off their signature tail, once they venture too close to the Sun.

Researchers from the Harvard-Smithsonian Center for Astrophysics published a new study Monday in the journal Scientific Reports that seeks to settle this debate once and for all.

They believe the object was a comet. Using statistical analysis and gravitational simulations, the authors calculated that a large number of long-period comets can be knocked off course by Jupiter’s gravity, flinging them toward the Sun. Long-period comets are thought to originate in the Oort cloud, an icy debris field at the outer edges of the solar system, named for Dutch Astronomer Jan Oort, who predicted the region’s existence in 1950.

"The solar system acts as a kind of pinball machine," explains Amir Siraj, an undergraduate in astrophysics at Harvard University, in a related statement. "Jupiter, the most massive planet, kicks incoming long-period comets into orbits that bring them very close to the sun."

Once comets come into the Sun’s proximity, they appropriately become known as “sungrazers.” These sungrazers experience incredible tidal forces brought on by the Sun’s gravity, breaking them apart and effectively turning them into a cosmic shotgun blast.

"In a sungrazing event, the portion of the comet closer to the sun feels a stronger gravitational pull than the part that is further, resulting in a tidal force across the object," Siraj said. "You can get what's called a tidal disruption event, in which a large comet breaks up into many smaller pieces. And crucially, on the journey back to the Oort cloud, there's an enhanced probability that one of these fragments hit the Earth."

And that’s exactly what the authors believe took place 66 million years ago, dooming not just the dinos but three-quarters of the other less famous life on Earth. According to their calculations, the chances of long-period comets impacting Earth is exponentially higher than previously thought. They believe 20% of all long-period comets will become sungrazers, sending a whole lot of shrapnel barreling towards Earth.

The object that impacted Earth all those years ago causing the Cretaceous-Paleogene extinction event is named Chicxulub. By studying other impact events over time, they determined the rate of impact is consistent with the age of Chicxulub, lending credence to their theory.

"Our paper provides a basis for explaining the occurrence of this event," said Avi Loeb, a Harvard astronomer, in a related statement. "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."

Previous studies of the Chicxulub impact site off the coast of Mexico found that whatever object created it was composed of carbonaceous chondrite. Asteroids from the belt are rarely composed of that substance, so it stands to reason the crater was caused by something else. Long-period comets, however, likely could be, though more space missions to sample comets are needed to be certain.

The largest confirmed crater in Earth’s history, located in South Africa, was formed about 2 billion years ago and also contains carbonaceous chondrite. So does an impact site in Kazakhstan, the largest crater formed in the past million years. 

The authors studied these sites and found that the timing of those impacts correlates well with the numbers they came up with. The new Vera Rubin Observatory in Chile with its impressive 3,200-megapixel camera may be able to witness these tidal disruption events in comets after it comes online next year.

Loeb thinks it’s vital to better understand the behavior of comets and their history of impacting Earth as it’s likely to happen again on a long enough timeline.

"We should see smaller fragments coming to Earth more frequently from the Oort cloud," Loeb said. "I hope that we can test the theory by having more data on long-period comets, get better statistics, and perhaps see evidence for some fragments."

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