Astronomers: a fragment of a comet, not an asteroid, killed the dinosaurs

Some 66 million years ago, a catastrophic event occurred that wiped out three-quarters of all plant and animal species on Earth, including the destruction of dinosaurs. A wandering asteroid in the asteroid belt was considered the most likely culprit. However, in a new article published in Scientific Reports, Harvard astronomers offer an alternative: a special type of comet – originating from a field of debris at the edge of our solar system known as the Oort Cloud – which has been deflected by the gravity of Jupiter. towards the sun. The Sun’s powerful tidal forces then tore pieces of the comet, and one of the larger fragments of this “comet shrapnel” eventually collided with Earth.

The most widely accepted explanation for what triggered this catastrophic mass extinction is known as the “Alvarez Hypothesis,” according to the late physicist Luis Alvarez and his geologist son, Walter. In 1980, they proposed that the extinction event could have been caused by a massive asteroid or comet hitting Earth. They based this conclusion on their analysis of sedimentary layers at the Cretaceous-Paleogene boundary (the K-Pg boundary, formerly known as the KT boundary) found around the world, which included unusually high concentrations of iridium – a metal more commonly found in asteroids than on Earth. (That same year, Dutch geophysicist Jan Smit independently came to a similar conclusion.)

Since then, scientists have identified a probable impact site: a large crater in Chicxulub, Mexico, on the Yucatan Peninsula, first discovered by geophysicists in the late 1970s. created was large enough (between 11 and 81 kilometers, or 7 to 50 miles) to melt, shock, and eject granite from deep within the Earth, likely causing megatsunami and ejecting vaporized rocks and sulphates into the atmosphere. This in turn had a devastating effect on the global climate, leading to mass extinction.

This hypothesis was reinforced in 2016, when a scientific drilling project led by the International Ocean Discovery Program took core samples from the crater tip ring, confirming that the rock had been subjected to immense pressure on a period of a few minutes. Just last year, an article published in Nature Communications concluded that the impactor struck at the worst possible angle and caused the maximum damage. It was estimated that the impact would have released energy more than a billion times higher than the atomic bombs dropped on Hiroshima and Nagasaki in 1945.

The latter theory came about when co-author Amir Siraj, an undergraduate astrophysics student at Harvard, began to look at asteroid impact rates for Earth-like exoplanets, which has him leads to study the impact rates of comets on these systems. He performed numerical simulations to calculate the flow of so-called long-lived comets in our own solar system, because scientists know a lot more about our system. “What I found most striking was that a significant fraction of the Earth-crossing events were directly preceded by remarkably close encounters with the Sun, which came from a class of comets captured in orbits. of high eccentricity due to their gravitational interactions with the Jupiter-Sun system, ”Siraj told Ars.

Further investigation revealed that comets 10 to 60 kilometers (6 to 37 miles) in size would be torn into smaller fragments by sufficiently powerful tidal forces, like what happened to Comet Shoemaker-Levy. 9 when it crashed into Jupiter. in 1994. “Basically I have found that these events occur so often and produce such a large number of fragments that they result in a Chicxulub-sized impact rate on Earth that is an order of magnitude higher to populations of background asteroids or comets, “Siraj told Ars. “It was interesting because from a statistical point of view, the K-Pg impact is in contradiction with the impact rates of populations of asteroids or background comets, but consistent with the rate that I calculated for this new dynamic path. “

Siraj and co-author Avi Loeb concluded from their analysis that Jupiter’s gravitational field was strong enough to deflect many long period comets from the Oort Cloud, bringing them very close to the Sun. These comets are known as “sun grazers”; about 20% of long-lived comets become sun grazers, according to the authors. And the powerful tidal force of the Sun can in turn shatter them into fragments.

Siraj likened the effect to a pinball machine. “When you have these sun grazers, it’s not so much the melting that continues, which is a fairly small fraction of the total mass, but the comet is so close to the Sun that the part closest to the Sun is feels stronger. gravitational pull than the farthest part of the Sun causing a tidal force, ”he said.“ You get what’s called a tidal disturbance event, so these big comets that s ‘approaching very close to the Sun divide into smaller comets. And basically, when they come out, there’s a statistical chance that these little comets will hit Earth. “

Siraj and Loeb’s calculations showed that there was an increase in the likelihood of long-period comets impacting Earth by a factor of 10, and that the new rate was associated with the age of l Chicxulub impactor, making it a viable theory of its origin. “Our article provides a basis for explaining the occurrence of this event,” Loeb said. “We’re suggesting that in fact, if you smash an object as you approach the Sun, it could result in an appropriate event rate and also the type of impact that killed the dinosaurs.” Each of these events would produce “a collection of smaller fragments that pass through Earth’s orbit,” the authors wrote.

Their findings also show that the unusual makeup of Chicxulub’s impactor – carbonaceous chondritis – indicates that it originated from the Oort cloud and not from the main asteroid belt. It is a rare composition for main belt asteroids, but it is common among long period comets. The authors also report other impact craters of similar composition, including the Vredefort crater in South Africa – the result of an impact about 2 billion years ago – and the Zhamanshin crater in Kazakhstan, of an impact over the last million years. These timescales are consistent with calculations by Siraj and Loeb, which indicate that such objects are expected to strike Earth every 250,000 to 730,000 years.

A 2007 nature article proposed that the Chicxulub impactor originated from the “Baptisina family” of asteroids, fragments created in an asteroid belt collision about 160 million years ago. These asteroids have this rare composition of carbonaceous chondrites, in agreement with the analysis of the Chicxulub crater. Wide-field Infrared Survey Explorer (WISE) data casts doubt on this possibility in 2011, placing the date of Baptisina fragmentation at barely 80 million years ago – too late to explain the Chicxulub crater and the extinction of the K-Pg an event.

The calculations of Siraj and Loeb dig further holes in this possibility. “Our hypothesis … predicts a greater proportion of impactors with carbonaceous chondritic compositions than one would expect from main-belt asteroid meteorite falls,” the authors wrote.

An alternative theory that Siraj and Loeb have yet to address is known as the Multiple Impact Hypothesis. There are several other smaller craters roughly the same age as Chicxulub that have since been unearthed, suggesting that there may have been more than one fragment of a comet that hit Earth 66 million ago. years. “It’s an interesting question,” Siraj told Ars. “Further work will be needed to better understand whether there are any implications of this model for the multiple impacts hypothesis.”

Next, the pair will look at future observations from the Vera Rubin Observatory in Chile – which will see first light next year – to confirm their theory, hoping the data will provide evidence of comets undergoing tidal disturbances. “We should see smaller fragments coming to Earth more frequently from the Oort cloud,” Loeb said. “I hope we can test the theory by having more data on long-lived comets, getting better statistics, and maybe seeing some evidence of some fragments.”

DOI: Scientific Reports, 2021. 10.1038 / s41598-021-82320-2 (About DOIs).

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