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Around 66 million years ago a hurtling object tens of miles wide smashed into our planet, wiping out three-quarters of plant and animal species on Earth (including the dinosaurs).
But while the traces of the impact are still here, including a crater off the coast of Mexico that's 93 miles wide and 12 miles deep, one mystery has remained: where the object came from.
A new research paper by Avi Loeb and Amir Siraj of Harvard University might have the answer – and it’s all to do with the gas giant Jupiter, the largest planet in the solar system.
The researchers say a huge chunk of comet from the edge of our solar system may have been "nudged" by Jupiter – and then smashed into Earth.
Loeb and Siraj say 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.
A portion of the comet could then come close to the sun and break up into smaller pieces, increasing the probability that one of these fragments hits Earth on the way back.
Siraj said: "The solar system acts as a kind of pinball machine. Jupiter, the most massive planet, kicks incoming long-period comets into orbits that bring them very close to the sun."
These "sungrazer" comets can experience powerful forces that break apart pieces of rock, producing cometary shrapnel, Siraj said.
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"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."
The new calculations increase the chances of long-period comets impacting Earth by a factor of about 10.
The research also shows that about 20% of long-period comets become sungrazers.
In their study, published on Monday in Nature’s Scientific Reports, the researchers say their new rate of impact is consistent with the age of the crater, known as Chicxulub, providing a satisfactory explanation for its origin and other impactors like it.
Loeb said: “Our paper provides a basis for explaining the occurrence of this event.
“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.”
Evidence found at the Chicxulub crater suggests the rock was composed of carbonaceous chondrite.
But the pair’s hypothesis might also explain this unusual composition.
Carbonaceous chondrites are rare amongst main-belt asteroids, but possibly widespread amongst long-period comets, potentially providing additional support to the cometary impact hypothesis.
The researchers say their theory can be tested by further studying other similar craters, and even ones on the surface of the moon to determine the composition of the impactors.
Space missions sampling comets could also help.
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