In a searing apocalyptic moment 66 million years ago, the earth was transformed from a lush harbor into a nightmare world with a fiery wound that bleed soot into the sky. The extraterrestrial object that invaded our planet meant fate to dinosaurs and countless other species, even as its fallout opened new niches for our mammalian ancestors.
For decades, scientists have been debating the identity of the impactor that hit our planet on that fateful day and left a 90-mile scar called Chicxulub Crater under what is now Mexico’s Yucatán Peninsula.
Although an asteroid remains the main candidate, a team at the Center for Astrophysics in Cambridge, Massachusetts has suggested that the culprit could have been an icy comet that flew too close to the sun.
When long-period comets approach the sun from the outer reaches of the solar system, they can be torn apart by the star’s immense tidal forces. The resulting fragments may have been catapulted into orbit, providing “a satisfactory explanation for the origin of the impactor” that killed the dinosaurs, according to a study published in Scientific Reports Monday.
“To this day, the origin of the Chicxulub impactor remains an open question,” said Amir Siraj, an astrophysics student at Harvard who led the research. His model, he said, examines “that particular population of comets” that could have produced enough broken pieces – of the right size, at the right speed, and in the right trajectories – to “threaten Earth” in ways that would corresponds to current observation conditions. “
Other experts disagreed with the study’s methods and conclusions. “I believe your work has several intrinsic problems that counteract your hypothesis,” said Bill Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado.
From the point of view of Siraj and his co-author, Harvard theoretical physicist Avi Loeb, a cometary origin fills some gaps in our understanding of this ancient catastrophe that sparked what is known as the KT extinction, which meant the end of the Cretaceous and the beginning of the Tertiary.
The researchers cite evidence that the impactor was made from carbonaceous chondrites, a rocky material found in a class of primitive asteroids that date back to the birth of the solar system. Samples returned from Comet Wild 2 in 2006 showed evidence that the icy world was similar in composition, suggesting that this makeup “may be widespread in comets,” the researchers in the study said. The team speculates that the Vredefort Crater in South Africa and the Zhamanshin Crater in Kazakhstan may also be the remains of cometary impacts.
“The fact that long period comets are likely made of the material – carbonaceous chondrites – that reside deep within these craters supports our model,” said Dr. Loeb.
The researchers argue that including sun-fragmented comets in impact models increases the rate of dangerous objects on the Chicxulub scale by an order of magnitude and increases the likelihood that the Earth was hit by a comet fragment 66 million years ago.
Natalia Artemieva, a senior scientist at the Planetary Science Institute in Tucson, Arizona, rejected this cometary origin hypothesis. The four mile wide comet fragments envisaged in the study would have been too small to create Chicxulub crater, she said. She noticed the tell-tale tip of iridium that deposited on it after impact and is visible in geological layers around the world.
“The projectile size should not only match the size of the crater, but also the global amount of iridium,” explained Dr. Artemieva in an email. “This is certainly the case in the standard scenario (stony asteroid), but not for a tiny cometary impact.”
Dr. Bottke expressed several concerns about the study. For example, he said his model overestimated how often long-period comets would be pulled apart by the sun and how many dangerous fragments such encounters would produce.
Although Dr. Bottke is not convinced that the impactor has a meteoric origin, he noted that the explanation of the asteroid also raises many exciting and unresolved questions.
“The evidence we have for the KT impactor points to asteroids rather than comets, but is inconclusive,” said Dr. Bottke. “There is still scope if someone really wants it to be a comet. I just think that this case is really difficult to sort out. “
Siraj and Dr. Loeb aren’t the only scientists killing visions of comets killing dinosaurs. Two geoscientists, Mukul Sharma of Dartmouth College and Jason Moore of the University of New Mexico, have also hypothesized comet formation for the impactor.
“Assuming the modeling is correct, this paper provides independent evidence for our claim in 2013 that a comet (high speed, small) and no asteroid (slow, large) had an impact 66 million years ago,” wrote Dr. Sharma in an email. “Our claim was based on geochemical and geophysical evidence. So it is exciting to see this new research based on modeling comet / asteroid movement.”
“As a scientist, it’s really important to keep reevaluating your hypotheses,” said Dr. Moore, adding that if the new paper “stands up to the scrutiny of the entire community, it will provide further incentive to re-examine other existing data.” Sources and models for a cometary candidate. “
Siraj and Dr. Loeb said future samples returned from comets could shed more light on their hypothesis. Sophisticated telescopes like that of the Vera C. Rubin Observatory will also help scientists compile a more comprehensive catalog of comets, asteroids and other near-Earth objects.
These advances will limit theories about the source of the object that wiped out the dinosaurs and perhaps help humanity escape the same fate.
“The more we study nature, the closer we can get to answering fundamental questions about the world around us – the past, but also the future,” said Siraj. “That’s the beauty of science.”
