About 66 million years ago, a massive boulder hit the earth on what is now the Yucatan peninsula. The impact wiped out about 75% of all life on earth. It was best known for the event that wiped out the dinosaurs.
While common scientific thinking has pointed to an asteroid as an impactor, a new research letter states that it could actually be a comet.
The so-called Chicxulub impactor has excavated a crater with a diameter of 150 km and a depth of about 20 km. The impact was devastating. There was a tsunami, global forest fires, and an immediate greenhouse effect, followed by prolonged periods of cooler temperatures. Researchers are still trying to put it all together.
One question has not yet been answered: Was the Chicxulub impactor an asteroid or a comet? A new study says it was a comet.
The Chicxulub Crater in Mexico. Photo credit: Wikipedia / NASA
The study is entitled “The Decay of a Longstanding Comet as the Origin of the Extinction of the Dinosaurs”. The authors are Amir Siraj, student of astrophysics at Harvard University, and Avi Loeb, professor of Harvard astronomy. The paper is published in Nature’s Scientific Reports.
Previous research showed that the impactor was an asteroid from the main belt. However, some of the evidence casts doubt on this conclusion. Evidence at the point of impact suggests that the impactor was a carbonaceous chondrite. But these types of asteroids are rare in the main belt. They may only make up about 10% of the asteroids there.
However, carbonaceous chondrite material could be widespread in the Oort Cloud.
The Oort Cloud is home to long-period comets, comets with orbital periods of more than 200 years. Some of these long-period comets can be perturbed by gravity and drifted off course by Jupiter.
The well-known solar system with its 8 planets occupies a tiny space in a large spherical shell that contains billions of comets – the Oort cloud. Photo credit: Wikimedia Commons
“The solar system acts as a kind of pinball machine,” Siraj explained in a press release. “Jupiter, the most massive planet, throws incoming long-period comets into orbits that bring them very close to the sun.”
Astronomers call these comets “sun willows”. Sun willows can have very close encounters with the sun, and that means trouble. The sun’s strong gravity can tear them apart, causing a shower of splinters. With so many parts, there is an increased chance of colliding with a planet.
“In a sunburn event, the part of the comet that is closer to the sun will feel a stronger pull than the part that is further away, resulting in a tidal force across the object,” says Siraj. “You can get what is called a tidal disruption event where a large comet breaks up into many smaller pieces. Crucially, on the journey back to the Oort cloud, the probability increases that one of these fragments will hit the earth. “
This figure from the study shows the impact rate for tidal LPCs and the impact rate for intact LPCs and MBAs. It also shows the range of rates that would explain the observed effects of Chicxulub. Photo credit: Siraj / Loeb, 2021.
The scientist couple used statistical analyzes and gravitation simulations in their work. They found that Jupiter’s gravity could disrupt a significant number of Oort Cloud comets and send them closer to the sun. Their calculations show that it is ten times more likely that one of these comets or their fragments will hit the earth than previously assumed. Their work also shows that about 20 percent of long-period comets could be sun grazers.
Loeb and Siraj say their calculations agree well with the time of the Chicxulub impact. They say their work explains these effects and could explain the origin of other impactors in the solar system as well.
“Our paper provides a basis for explaining how this event occurred,” Loeb said. “We suggest that if you break an object near the sun, it could result in a reasonable rate of occurrences and also the type of impact that killed the dinosaurs.”
Siraj and Loeb say their longstanding cometary solution explains some other effects on Earth as well.
Although the Chicxulub impact was an enormously catastrophic event that marked the course of life on Earth, it did not leave the largest crater. The largest impact crater on earth is the Vredefort crater in South Africa. It’s 300 km in diameter and was created when an impactor hit Earth a little over two billion years ago.
The Vredefort crater dome from space on the STS-51-I mission. It was created by the impact of a large object about two billion years ago. The new research letter claims it could be the result of a longtime comet torn apart by the tidal forces of the sun, with one of the fragments hitting Earth. By Júlio Reis (User: Tintazul) – [1], Public Domain, https://commons.wikimedia.org/w/index.php?curid=400487
There is also the Zhamanshin crater in Kazakhstan. It’s nine miles in diameter and was created by an impact about a million years ago. The Zhamanshin impact was the most recent impact event large enough to cause a nuclear winter. However, it wasn’t big enough to cause mass extinction.
Loeb and Siraj say their fragmented comet hypothesis could explain these two effects as well. Their study shows that an LPC in the Chicxulub area should hit Earth every 250 to 730 Myr and that smaller fragments from one that has been disturbed by tides should hit Earth approximately every 0.25 to 0.73 Myr.
However, your hypothesis needs to be tested. The authors say that further investigation of these three craters and similar craters will help either strengthen or weaken the hypothesis. Ideally, it would also help study craters on the moon. Comet sampling missions could also help.
More targeted observations of comets could also help. The upcoming Vera Rubin Observatory will be particularly effective at detecting transient and variable events, including asteroids and comets moving through the inner solar system. When long-period comets are disturbed by Jupiter’s gravity and sent to the Sun only to be torn apart by tidal forces, the Vera Rubin should be able to detect them. It should also see all of the fragments that were sent to Earth.
The LSST or Vera Rubin Survey Telescope under construction in Cerro Pachon, Chile. Image Credit: LSST
“We should see that smaller fragments come to earth from the Oort cloud more frequently,” says Loeb. “I hope we can test the theory by having more data on long-period comets, getting better statistics, and maybe seeing evidence for some fragments.”
If the couple’s hypothesis proves correct, the repercussions can be severe. This means that we can expect a long-period comet to dissolve again in the inner solar system and that the earth could be in danger. Fortunately, we can be more careful about these potential impacts and are even working out ways to prevent catastrophic effects.
For now, at least, the hypothesis sheds light on a critical event in the history of the earth. An event that would have been really great. From a safe distance.
“It must have been an amazing sight, but we don’t want to see it again,” said Loeb.
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