66 million years ago, a huge celestial object fell off the coast of what is now Mexico, causing a catastrophic “impact winter” that wiped out three-quarters of life on Earth, including dinosaurs. A pair of astronomers at Harvard University say they have solved long-standing mysteries surrounding the nature and origin of the Chicxulub crater impact.
Their analysis suggests that it was a comet originating from a region of icy debris at the edge of the solar system, that Jupiter was responsible for its crashing on Earth, and that similar impacts can be expected every 250 to 750 million years.
The study, published this week in the journal Scientific Reports, rejects an old theory that maintains that this object was a fragment of one of the thousands of asteroids that form the so-called Main Belt of our solar system.
“Jupiter is very important because it is the most massive planet in our solar system,” Amir Siraj, one of the study’s authors, said. “Jupiter ended up being a kind of “pinball machine” that “propels those long-period comets into orbits that bring them closer to the Sun,” he explained.
The so-called “long period comets” come from the Oort cloud; a kind of gigantic spherical crust that surrounds the solar system like a bubble. It is made up of frozen debris the size of mountains or more. Long-period comets take about 200 years to orbit the sun and are also called “sun grazers” because of how close they pass to the star.
Massive Comet tides
Coming from the coldest reaches of the solar system, comets are colder than asteroids and are characterized by the impressive traces of gas and dust they produce when they melt. However, Siraj said, the evaporation effect caused by solar heat on such comets is nothing compared to the massive tides they experience when one side faces the Sun.
“As a result, these comets experience a tidal force so great that the largest of them would break into thousands of pieces, each one large enough to produce a Chicxulub-size impact or a dinosaur-killing event on Earth”, he said.
Siraj and study co-author Avi Loeb developed a statistical model that showed the probability that long-period comets could impact Earth that is consistent with the age of the Chicxulub crater and other similar impacts.
Beautiful to see
Another line of evidence in favor of comets for the formation of Chicxulub: only a tenth of all the asteroids in the Main Belt, which is located between Mars and Jupiter, are formed by carbonaceous chondrites (a kind of meteorites) while most of comets have them.
Evidence suggests that the Chicxulub crater and others like it, such as Vredefort in South Africa, was impacted about 2 billion years ago, and Zhamanshin, in Kasakistan (one million years ago), had carbonaceous chondrites. The hypothesis can be tested by studying other craters, some on the Moon, or even sending probes to sample comets.
“It must have been beautiful to see that rock arrive 66 million years ago and whose size was larger than the island of Manhattan,” Loeb said, even though the ideal would be to learn to follow these objects and find ways to deflect them if necessary.
Loeb added that he is excited that the Vera Rubin Observatory in Chile will be operational next year. Its telescope would allow us to appreciate the long-period alterations of comets “and it will be extremely important to make forecasts for the next 100 years and to know if something bad could happen to us.”
Really don’t know when the next one is coming
Although both researchers estimate that events like Chicxulub occur every few million years, that “is statistical.” “You say ‘on average,’ ‘it’s every so often,’ but you never know when the next one will come,” Loeb said. “The best way to know is to observe the sky,” he concluded.