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

About 66 million years ago, a catastrophic event took place that wiped out three-quarters of all plant and animal species on earth, and especially destroyed the dinosaurs. A deviating asteroid from the asteroid belt is 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 landfill at the edge of our solar system, known as the Oort Cloud – by Jupiter’s gravity has been thrown off course. to the sun. The Sun’s powerful tidal forces then tore pieces off the comet, and one of the larger fragments of this ‘comet shrapnel’ eventually collided with Earth.

The most widely accepted explanation for what caused the catastrophic mass extinction is known as the ‘Alvarez hypothesis’, after the late physicist Luis Alvarez and his son the geologist Walter. In 1980, they suggested that the event of extinction may have been caused by a massive asteroid or comet hitting the earth. They based this conclusion on their analysis of sedimentary layers at the Cretaceous-Paleogene boundary (the K-Pg boundary, formerly known as the CT boundary) found around the world, which include extremely high concentrations of iridium – a metal that is more common in asteroids than on Earth. (In the same year, the Dutch geophysicist Jan Smit came to a similar conclusion independently.)

Since then, scientists have identified a likely impact site: a large crater in Chicxulub, Mexico, in the Yucatan Peninsula, first discovered by geophysicists in the late 1970s. The impact it 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, probably causing a megatsunami and evaporating rock and sulphates are emitted into the atmosphere. This in turn had a devastating effect on the world climate, leading to mass extinction.

The hypothesis was further strengthened in 2016 when a scientific drilling project led by the International Ocean Discovery Program took nuclear samples from the crater’s peak ring, confirming that the rock was subjected to tremendous pressure for a few minutes. Just last year, an article published in Nature Communications concluded that the impact hit the worst possible angle and caused maximum damage. It is estimated that the impact would have released more than a billion times higher energy than the atomic bombs dropped on Hiroshima and Nagasaki in 1945.

This latest theory arose when co-author Amir Siraj, an undergraduate student in astrophysics at Harvard, began looking at the asteroid impact rates for Earth-like exoplanets, which in turn led to a study of the impact rates on the systems. He did numerical simulations to calculate the flood of so-called long-term comets in our own solar system, as scientists know much more about our system. ‘What I found most striking was that a significant fraction of the Earth’s crossing events were directly preceded by remarkably close encounters with the sun, which came from a class of comets orbiting in high eccentricity around their gravitational interaction with the Jupiter-Sun system, ”Siraj told Ars.

Further investigation revealed that comets within the range of 10 to 60 kilometers (between 6 and 37 miles) would be torn apart by sufficiently strong tidal forces into smaller fragments, similar to what happened to the comet Shoemaker-Levy 9 when it was on Jupiter crashed. in 1994. ‘It is of great importance that I have found that these events occur so frequently and produce such a large number of fragments that they result in an impact rate of Chicxulub size on Earth that is an order of magnitude greater as the background asteroid or comet populations, ‘Siraj told Ars. “It was interesting because from a statistical point of view, the K-Pg impact is at odds with the impact rates of the asteroid or comet populations, but agrees with the pace I gained for this new dynamic path.”

Siraj and co-author Avi Loeb concluded from their analysis that Jupiter’s gravitational field was strong enough to bump many such long-term comets out of the Oort cloud, bringing them very close to the sun. Such comets are known as ‘sunbeams’; About 20 percent of long-term comets become sunbathers, according to the authors. And the powerful tidal force of the Sun can break it to pieces again.

Siraj compares the effect to a pinball machine. “If you have this solar pasture, it is not so much the melting, which is a fairly small fraction in relation to the total mass, but the comet is so close to the sun that the part that is closer to the sun feels stronger gravity pulls as the portion farther from the sun, which causes a tidal force, “he said.” You get what is called a tidal disturbance event, so these large comets that come very close to the sun break up into smaller comets. And on their way out, there’s a statistical chance that these smaller comets will hit the earth. ‘

Siraj and Loeb’s calculations showed that there was an increase in the probability that long-range comets would affect a factor of 10 on Earth, and that new tempo jibes with the age of the Chicxulub impact, making it a viable theory of its origin. “Our paper provides a basis for explaining the occurrence of this event,” Loeb said. “We suggest that if you break up an object while it is approaching the sun, it could cause the appropriate pace of events and also the kind of impact that the dinosaurs killed.” At each such event, a collection of smaller fragments would emerge that would cross the Earth’s orbit, ‘the authors wrote.

Their findings also provide evidence that the unusual composition of the Chicxulub impact – carbonaceous chondrite – indicates that it comes from the Oort cloud and not from the main steroid belt. It is a rare compound for asteroids on the headband, but it is common for long-term comets. The authors also point to other impact craters with a similar composition, especially the Vredefort crater in South Africa – the result of an impact about 2 billion years ago – and the Zhamanshin crater in Kazakhstan, due to an impact recently million years. These time frames are in line with the calculations of Siraj and Loeb, which indicate that such objects must fall to Earth every 250,000 to 730,000 years.

A 2007 Nature article suggests that the Chicxulub impact may have come from the “Baptisina family” of asteroids, fragments that formed about 160 million years ago in a collision with the asteroid. These asteroids have the rare composition of carbonaceous chondrites, consistent with the analysis of the Chicxulub crater. Data from the Wide-field Infrared Survey Explorer (WISE) questioned this possibility in 2011, but the date of the Baptisina fragmentation was only 80 million years ago – too late for the Chicxulub crater and the K-Pg extinction. to declare. event.

Siraj and Loeb’s calculations put further holes in the possibility. “Our hypothesis … predicts to a greater extent impactors with carbonaceous chondritic compositions than would be expected by meteorite falls of main belt asteroids,” the authors wrote.

One alternative theory that Siraj and Loeb have yet to address is known as the multiple impact hypothesis. There are several other, smaller craters about the same age as Chicxulub that have since been discovered, suggesting that there may have been more than one comet fragment that hit Earth 66 million years ago. “This is an interesting question,” Siraj told Ars. “Future work will be needed to better understand what the implications of this model are for the multiple impact hypothesis.”

The couple will then look to future observations of 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 experiencing the tidal disturbance. “We will have to see smaller fragments coming from the Oort cloud to Earth regularly,” Loeb said. “I hope we can test the theory by having more data on long-term comets, getting better statistics and maybe seeing evidence for some fragments.”

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

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