The collision that wiped out the dinosaurs from the face of the Earth began at the very edges of our solar system, according to a study published in Nature’s Scientific Reports (1). It turns out that the object that hit planet Earth didn’t come from the asteroid belt around Mars and Jupiter, as previously believed, but from the remote Oort cloud, a collection of icy objects further away than anything else in the solar system.
Researchers know that the extinction of dinosaurs was caused when the “Chicxulub impactor” hit just off Mexico’s coast and left a crater 93 miles across and 12 miles deep. Its impact was devastating and felt worldwide. This massive object not only brought the reign of the dinosaurs to an abrupt end but also triggered the extinction of almost three-quarters of plant and animal species on the planet.
What researchers don’t know is where this object originated and how it came to strike our planet with such force. They don’t even know whether to class it as a comet or an asteroid! It is believed that the impactor could have been a piece of a bigger asteroid originating from the asteroid belt between Mars and Jupiter, but now undergraduate Amir Siraj and astronomer Avi Loeb, from Harvard University, have a different theory to explain the origins of this catastrophic event.
So, what really happened?
After analysing how objects fly around in the solar system as well as computer simulations about the influence of gravity, they suggested that whatever wiped out dinosaurs came from the Oort cloud. The most remote region in our solar system, the Oort cloud is believed to be like a giant bubble surrounding the solar system, made of icy-comet like objects that can be as large as mountains.
According to the researchers, when a piece of a comet from the Oort cloud was knocked off course by Jupiter’s gravity, it was sent flying towards the sun. “Basically, Jupiter acts as a kind of pinball machine,” said Siraj. “Jupiter kicks these incoming long-period comets into orbits that bring them very close to the sun.”
As they’re catapulted towards the sun, these comets – known as ‘sungrazers’ – experience great forces that can break them into smaller pieces. Comet shrapnel, if you like. And, in this case, one of these smaller pieces headed straight to Earth. “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. “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.”
This is all important data, but what really made it for the Harvard researchers was the actual composition of the comet that hit our planet. Pieces found at the Chicxulub crater have been classed as carbonaceous chondrites, which are much more likely to be found in the Oort cloud than the asteroid belt.
Nevertheless, the researchers are cautious. This idea needs to be further tested, possibly by looking at other comet craters around the world or even similar ones on the Moon. And, as the cherry on top of the cake, when the Vera Rubin Observatory in Chile opens in the next few months, researchers will be able to watch comets live and see how they behave.
“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”, said Professor Loeb. For the researcher, understanding what happened is vital to solve the mystery of Earth’s history and could be fundamentally important if something like this was to ever threaten our planet again. “It must have been an amazing sight, but we don’t want to see that again,” he joked.
(1) Siraj, A., Loeb, A. Breakup of a long-period comet as the origin of the dinosaur extinction.Sci Rep11, 3803 (2021). https://doi.org/10.1038/s41598-021-82320-2