The object that slammed into Earth 66 million years ago and triggered the extinction event that wiped out nearly all dinosaurs was an asteroid that originally formed beyond the orbit of Jupiter, according to geochemical evidence from the impact site in Chicxulub, Mexico.

The August 15th inScience 1Published findings suggest that the mass extinction was the result of a series of events that began at the birth of the solar system. Scientists had long suspected that the Chicxulub impactor, as it is known, was an asteroid from the outer solar system, and these observations support the case.

The Cretaceous-Paleogene (K/Pg) extinction event was the fifth in a series of mass extinctions that have occurred over the last approximately 540 million years: the period during which animals spread across Earth. The event wiped out more than 60% of species, including all non-avian dinosaurs.

Since 1980, evidence has accumulated that the extinction was caused by a city-sized object hitting Earth. Such an impact would have Enormous amounts of sulfur, dust and soot were thrown into the air, which partially blocked the sun and led to a drop in temperature. A layer of rare iridium metal, rare on Earth but more common in asteroids, was deposited around the world at the start of the extinction event. In the 1990s, scientists described 2the impact site, a huge hidden crater near Chicxulub on Mexico's Yucatán Peninsula.

“We wanted to identify the origin of this impactor,” says Mario Fischer-Gödde, an isotope geochemist at the University of Cologne in Germany. To find out what it was and where it came from, he and his colleagues collected samples of K/Pg rocks from three locations and compared them with rocks from eight other impact sites from the last 3.5 billion years.

Ruthenium signature

The team focused on isotopes of the ruthenium metal. Ruthenium is extremely rare in Earth rocks, says Fischer-Gödde, so samples of it from an impact site provide “the pure signature” of the impactor. There are seven stable isotopes of ruthenium, and celestial bodies have characteristic mixtures of them.

In particular, looking at ruthenium isotopes can help researchers distinguish between asteroids that formed in the outer solar system - beyond Jupiter's orbit - and those with an origin in the inner solar system. When the solar system formed from a molecular cloud about 4.5 billion years ago, temperatures in the inner region were too high for volatile chemicals like water to condense. As a result, asteroids that formed there had low levels of volatilities and became rich in silicate minerals. Asteroids that formed further out became “coal-rich,” containing lots of carbon and volatile chemicals. The ruthenium isotopes were unevenly distributed throughout the cloud, and this heterogeneity is preserved in asteroids.

Fischer-Gödde's team found that the ruthenium isotopes in the Chicxulub impactor matched well with a carbon-rich asteroid from the outer solar system and not with silicate-rich asteroids from the inner solar system.

Previous studies have also suggested that the impactor was a carbon-rich asteroid, says Sean Gulick, a geophysicist at the University of Texas at Austin. But the latest work “is a really elegant way to get to some of those answers and get several of the same answers with one methodology,” he adds.

Not a comet

The ruthenium isotopes also provide evidence against another hypothesis: that the Chicxulub impactor was a comet and not an asteroid. “The idea that it was a comet goes way back in the literature,” says William Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. The hypothesis was tested in a controversial 2021 study 3revived, who argued that the impactor was part of a long-period comet that had broken up under the gravitational influence of the Sun.

But Fischer-Gödde says the ruthenium isotope data doesn't match a comet. Gulick agrees. He adds that geochemical evidence from the Chicxulub impact site has never been consistent with a comet, and the latest study "really helps clarify that."

Bottke adds that the comet hypothesis also “runs into difficulties” when considering the dynamics of the solar system. “Sizable carbon-rich asteroids are more likely to hit Earth than comets,” he says. In a 2021 study, he and his colleagues argued that the impactor likely came from the main asteroid belt between Mars and Jupiter.

According to their ruthenium isotopes, most of the other impactors that Fischer-Gödde's team studied appear to have originated in the inner solar system. The only exceptions were the oldest, from between 3.2 billion and 3.5 billion years ago, which looked more like the Chicxulub impactor. It could be that "something interesting was happening in the asteroid belt at that time, such as a large asteroid breaking up in a good place to bring objects to Earth," Bottke says.