Did a comet fragment kill the dinosaurs? Unlikely, say researchers
The global extinction event that meant curtains for dinosaurs was probably not triggered by a comet fragment impacting Earth, several planetary scientists now say.
This most recent hypothesis to explain how a comet fragment (or fragments) could have crashed in the Gulf of Mexico near Chicxulub has been disputed. The recent paper supporting the hypothesis in the journal Scientific reports on nature postulates that a fragment (or fragments) of a large, long-lived comet were pushed into orbits skimming the sun by Jupiter. This, in turn, they write, led to an impact that caused the extinction of dinosaurs and three-quarters of all living species on Earth.
In their article, Harvard University astronomers Amir Siraj and Avi Loeb argue that on close passages to the Sun, such large comets are gravitationally disturbed, producing fields of cometary shell shards, one of which is part of it probably sank into the gulf about 66 billion years ago.
But David Kring, senior scientist at USRA’s Lunar and Planetary Institute (LPI) in Houston, remains skeptical.
“A surviving fragment from the Chicxulub impactor is similar to meteorite fragments of asteroids and does not look like the only sample we have of a comet,” Kring told me. Additionally, Kring says the impactor chemical traces in the debris ejected from Chicxulub crater resemble the chemical compositions of known meteorite fragments of asteroids.
There is also the problem of the abundance of iridium.
Most of Earth’s iridium is sequestered in our planet’s core and mantle, Kring writes in an essay. So any abnormally large amount found in the earth’s crust would imply that there was a new supply being delivered to the surface via some sort of impactor, notes Kring.
Now known colloquially among scholars as the K-Pg border, to mark the Cretaceous-Paleogene (K-Pg) mass extinction event, it is now widely accepted that the border is characterized by globally high concentrations. of iridium, the authors of a new article appear in the journal Scientists progress, noted last week. This iridium anomaly “thus reflects the global dispersion of meteorite material following the hypervelocity impact of an asteroid about 12 km in diameter”, write the authors.
“The fragment of the K-Pg meteorite that I found was compact and similar to carbonaceous chondrites which are certainly of asteroid origin; there’s no reason to believe comets are the same, ”Frank Kyte, a geophysicist at the University of California, Los Angeles (UCLA) told me. On the contrary, Kyte says that comets contain a lot of volatile ice, so they can contain much less iridium than meteorites.
In fact, the comet’s history conflicts with the iridium values found at more than 100 K-Pg boundary sites around the world, Philippe Claeys, geochemist at the Free University of Brussels (VUB) told me. in Belgium.
Still, Siraj, a Harvard University undergraduate studying astrophysics at the Harvard & Smithsonian Center for Astrophysics, and the lead author of the article, told me there was no conflict. with their model and the iridium concentration at the K-Pg boundary. In fact, a thorough and up-to-date analysis of the iridium data favors the impactor mass used in our model, he told me.
Even so, “Siraj and Loeb’s article makes no mention of iridium,” Stephen Kane, a planetary astrophysicist at the University of California, Riverside, told me.
Still, Siraj says their model is not refined, but rather can accommodate a wide range of impactor sizes at orders of magnitude en masse.
If so, Kane reckons, the projected size parameters of these objects become so large that they no longer provide meaningful results.
Why not asteroids?
Siraj and Loeb claim that asteroids that produce Chicxulub-sized events do not occur more than once every 750 million years or so, Kring notes. “Yet Chicxulub-sized events from a population of near-Earth objects are nominally estimated to occur on average once every 100 million years,” he writes.
Yet Siraj and Loeb’s model begins with a long period comet of about 60 km in diameter which, when disturbed, produces a fragment of a comet with a diameter of about 7 km, which they propose to be l Chicxulub impactor, notes Kring. The article does not demonstrate how a comet about 7 km away can produce Chicxulub, while computer simulations published by other investigators show that a much larger object is needed, Kring writes.
“[Thus], the diameter of the modeled comet is far too small to produce the Chicxulub impact crater, ”Kring told me.
Siraj and Loeb note that cometary fragments less than 7 km will hit Earth as well. The authors noted in their article that one of these cometary fragments may even have touched Earth in the past million years, possibly producing the 14 km-diameter Zhamanshin crater in Kazakhstan.
If so, writes Kring in his essay, it would require comet fragments to survive for over 60 million years without being ejected from the solar system, further disturbed, and / or colliding with other bodies. It only pushes the boundaries of credulity.
And why reinvent the Chicxulub impactor wheel when asteroid impactors might work after all?
If a fragment of a long-lived comet hit Earth, it would likely do so at speeds of up to 50 km per second, compared to 20 km per second for asteroid impactors, Simone Marchi, senior scientist at the Southwest Research Institute of Boulder, Colo, told me. Therefore, he says, a cometary impactor longer than 5-10 km coupled with such a high impact velocity would likely produce a much larger crater than Chicxulub.
In other words, due to its impact velocity, the comet impactor glove might simply never fit.
“So, it may not be necessary to invoke a cometary origin for the K-Pg impact to begin,” Marchi said.
It is all sobering to wonder if the dinosaurs themselves would have given so much thought to the cause of their own demise.
As for who is right about the true origin of the Chicxulub impactor?
I think it’s just such a random event, that trying to force it into some sort of statistical probability doesn’t really prove anything, Kyte says.
Or as Kring puts it, “While we cannot prove that the impactor was not a comet, the available data suggests it was an asteroid. “