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New study blames dino extinction on a comet fragment, not an asteroid

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Feb 19, 2021, 3:57 PM EST

By now we've all pretty much memorized what happened to cause the extinction of the dinosaurs some 66 million years ago. It's a middle school science staple, and we can likely quote it by heart.

The prevailing, and most widely accepted theory, is that back in the Late Cretaceous, a cataclysmic six-mile-wide asteroid struck Earth, causing a climate-altering event that wiped out nearly every living dinosaur and flying reptile on the planet, delivering a major dent in its crust, and rendering our world nearly uninhabitable for thousands of centuries.

For the best evidence of this doomsday space rock, scientists point to the massive 90-mile Chicxulub Crater found beneath the Yucatán Peninsula and the Gulf of Mexico. This deadly impactor struck at just the right angle to inflict maximum damage when it collided with the planet, thus ending the Age of Dinosaurs. 

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But a new study highlighted in The New York Times tosses a wrinkle in this conclusion by suggesting that it was the broken fragment of a blazing comet that flew too close to the sun that actually smacked Earth, and not an asteroid.

Researchers headquartered at the Center for Astrophysics, in Cambridge, Mass., have submitted a study published in the online journal Scientific Reports proposing that a giant cosmic snowball is the real villain in this sad scenario. 

Their explanantion indicates that a long-period comet form the fringes of our solar system was torn apart by intense tidal forces of the Sun and the lethal shards were slingshot across Earth's orbit where one enormous sliver landed.

“To this day, the origin of the Chicxulub impactor remains an open question,” said lead author Amir Siraj, an undergraduate studying astrophysics at Harvard.

Their hypothesis explains the comet-like composition of the largest confirmed impact crater in Earth’s history in addition to the biggest one within the last million years, predicting a far greater proportion of impactors with carbonaceous chondritic compositions than previously expected from meteorite falls of main-belt asteroids.

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While a mountain of evidence remains suggesting that the Chicxulub impact led to the mass extinction event, the nature and real identity of the Chicxulub impactor is still poorly understood. One current scenario hypothesizes that it was the breakup of the local Baptisina asteroid family that caused the formation of the Chicxulub impactor. But spectroscopic analysis has shown these were of a siliceous (stony) mineralogical composition, proving it to be an unlikely culprit of the Chicxulub space object, which had a carbonaceous chondritic composition.

Samples returned from Comet Wild 2 back in 2006 revealed clues that the icy object has a similar carbonaceous composition, indicating that this makeup might be much more widespread in comets than ever believed. The Cambridge team also speculates that South Africa's Vredefort crater and the Zhamanshin crater in Kazakhstan could also be the remains of cometary impacts. 

“The fact that long-period comets are likely to be made of the material — carbonaceous chondrites — that is deep in these craters is in support of our model,” Harvard astrophysicist and study co-author Dr. Avi Loeb noted.

But not everyone is on board this theory of sun-splintering comets peppering prehistoric Earth.

One opponent is Natalia Artemieva, a senior scientist at the Planetary Science Institute in Tucson, Ariz., who waves off this cometary-origin hypothesis. In her estimation, these four-mile-wide comet fragments presented in the research paper wouldn't have been big enough to create the huge Chicxulub crater.

For this contradiction she directs attention to the concentrations of iridium deposited in the aftermath of the impact, something that is visible in geological strata across the globe.

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“The projectile size should be consistent not only with the crater size but also with the global amount of iridium,” Dr. Artemieva explained. “This is certainly the case in the standard (stony asteroid) scenario, but not for a tiny cometary impact.”

For now Siraj and Loeb are sticking to their story and are confident that future exploratory missions to comets could reinforce their research. Upcoming powerful telescopes like one planned for the Vera C. Rubin Observatory will also aid astronomers in compiling a more detailed roster of asteroids, comets, and other assorted near-Earth objects.

“Ultimately, the more we look to nature, the closer we can get to answering fundamental questions about the world around us — about the past, but also about the future,” Siraj added. “That’s the beauty of science.”