New study identifies the asteroid that wiped out the dinosaurs as a rare CO chondrite

Background: The Alvarez hypothesis and the Chicxulub crater

Alvarez supported his thesis with, among other things, the globally elevated iridium content in rocks of the Cretaceous-Paleogene boundary layer, which can only be explained by the impact of a large meteorite. Around 66 million years ago, about 75 percent of all animal and plant species perished, including the dinosaurs.

Now an international team, which also includes Christian Köberl from the University of Vienna, together with colleagues from France, Belgium, and Canada, has measured nickel isotopes in various ejecta layers around the globe and analyzed the composition of different carbonaceous chondrites. The results were published in the journal Science Advances. According to these findings, the "dino-killer" asteroid had the composition of a so-called CO chondrite.

CO chondrite from the outer asteroid belt

CO chondrites originate from the outermost part of the asteroid belt near Jupiter's orbit. They contain only little sulfur and, according to Köberl, are "relatively dry and have few volatile elements and water compared to other carbonaceous chondrites." Carbonaceous chondrites with high carbon content make up only two to three percent of all meteorites in total; around 86 percent of all meteorites are chondrites in general. Only a few percent of all meteorites examined on Earth belong to the group of so-called "primitive meteorites" — that is, those celestial bodies that have remained largely unchanged since their formation in the early solar system, meaning they have not been melted or thermally altered.

Köberl explained that such asteroids originate from the early solar system and are among the oldest objects that have remained unchanged since their formation. "If the orbit of such bodies is disturbed, they can be deflected into the inner solar system and also hit the Earth," said Köberl.

Already two years before the current publication, a team led by Köberl had shown using ruthenium isotopes that the asteroid was a carbonaceous chondrite. The new work now specifies this: according to the findings, it is a CO chondrite. Philippe Claeys from the Vrije Universiteit Brussel (VUB), a colleague of Köberl's, described the work as demanding. "In the KT clay layer of the planet, only a tiny fraction of the projectile is preserved, since the entire meteorite vaporized upon impact," said Claeys.

Significance for the theory of mass extinction

The dust hurled into the atmosphere led to a global "impact winter"; the release of volatile substances such as sulfur and carbon dioxide drastically altered the climate and ocean chemistry. Köberl explained why the ejecta layers are so important for research: "because in these ejecta layers higher concentrations of extraterrestrial material can be found than in the crater itself." They are therefore examined together with drill cores from the Chicxulub crater.

According to Köberl, the new insight does not change the existing theory about the triggers of the mass extinction — but it makes it less likely that the sulfur contained in the impactor was decisive. "The fine debris particles hurled into the atmosphere were probably the main cause," said Köberl. Claeys summed up the scale of what happened: "The fact that they were hit by such a rare projectile originating from such a distance really makes clear how unlucky the dinosaurs were."