The Comet –“That Forever Changed Planet Earth”

Chicxulub impactor
“It must have been an amazing sight, but we don’t want to see that again,” said Harvard astrophysicist, Avi Loeb about the comet that created the the Chicxulub crater off the coast of Mexico that spans 93 miles and runs 12 miles deep that forever changed Earth’s evolutionary history when it crashed 66 million years ago.

The Scene at Impact
The scene of the massive impact that brought the reign of the dinosaurs to an abrupt and calamitous end by triggering their sudden mass extinction, along with the end of almost three-quarters of the plant and animal species living on Earth, has been described by  mass-extinction authority, Peter Brannen in Ends of the World: “it would have been a pleasant day one second and the world was already over by the next,” about the Mount Everest sized object that blasted a hole in the ground, releasing the equivalent of 100 million megatons of TNT on the ancient continent of Pangaea.
“The asteroid itself was so large that, even at the moment of impact, the top of it might have still towered more than a mile above the cruising altitude of a 747,” observed Brannen. “In its nearly instantaneous descent, it compressed the air below it so violently that it briefly became several times hotter than the surface of the sun,” hitting Earth with enough force enough to lift a mountain back into space at escape velocity.
The Enduring Puzzle –Solved?
The enduring puzzle suggests Loeb is where did the massive object originate, and how did it come to strike Earth? Now, Loeb and Harvard University astrophysics undergraduate student Amir Siraj using statistical analysis and gravitational simulations have put forth a new theory that could explain the origin and journey of this catastrophic object. They calculate that a significant fraction of long-period comets originating from the Oort cloud, an icy sphere of debris at the edge of the solar system, can be bumped off-course by Jupiter’s gravitational field during orbit.
“The solar system acts as a kind of pinball machine,” explains Siraj, who is pursuing bachelor’s and master’s degrees in astrophysics, in addition to a master’s degree in piano performance at the New England Conservatory of Music. Jupiter, Siraj explains, the most massive planet, “kicks incoming long-period comets into orbits that bring them very close to the sun, nicknamed “sungrazers”—can experience powerful tidal forces that break apart pieces of the rock and ultimately, produce cometary shrapnel.”
A Sungrazing Event
“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 says. “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.”
The new calculations from Siraj and Loeb’s conjecture increase the chances of long-period comets impacting Earth by a factor of about 10, and show that about 20 percent of long-period comets become sungrazers –a rate consistent with the age of Chicxulub, that “provides a basis for explaining the occurrence of this event,” Loeb says. “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.”
It Came from the Oort Cloud
Evidence found at the Chicxulub crater suggests the rock was composed of carbonaceous chondrite –some of which compositions are considered to be close to that of the solar nebula from which the Solar System formed and are but possibly widespread amongst long-period comets that originate in the Oort Cloud a giant, thick-walled bubble made billions, or even trillions of icy space debris the sizes of mountains and sometimes larger.–Siraj and Loeb’s cometary hypothesis might also explain this unusual composition.
Other Tidally Disrupted Impacts
Other Earthly impact craters display the same composition, including an object that hit about 2 billion years ago and left the Vredefort crater in South Africa, which is  one of the largest ever to strike Earth since the Hadean Eon some four billion years ago, and the impactor that left the Zhamanshin crater in Kazakhstan, which is the largest confirmed crater within the last million years. The researchers say that the timing of these impacts support their calculations on the expected rate of Chicxulub-sized tidally disrupted comets.
Siraj and Loeb say their hypothesis can be tested by further studying these craters, others like them, and even ones on the surface of the moon when the new Vera Rubin Observatory in Chile becomes operational next year allowing astronomers to observe tidal disruption of long-period determine the composition of the impactors.
“We should see smaller fragments coming to Earth more frequently from the Oort cloud,” Loeb says. “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.”

Jackie Faherty, astrophysicist, Senior Scientist with AMNH via Harvard CfA, Nature: Breakup of a long-period comet as the origin of the dinosaur extinction and Peter Brannen’s Ends of the World (Kindle edition).

Image credit –The Day the Dinosaurs Died, with thanks to BBC Bancroft

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