We just learned where the asteroid that ended dinosaurs came from

The Cretaceous period ended in a terrible disaster that had its origins beyond Jupiter.

An asteroid more than six miles wide slammed into prehistoric Central America, kicking off a global heat pulse and years of winter that wiped out more than 60 percent of known species. It was famously the end of the non-avian dinosaurs like Tyrannosaurus rex and Triceratops, as well as flying pterosaurs, seagoing mosasaurs, and other reptiles.

Now geologists have identified where the devastating asteroid came from. The immense chunk of rock wasn’t orbiting nearby but had journeyed across our solar system on its collision course.

The impact left behind a giant crater known as Chicxulub beneath the Mexican coast and One of the first signs of the impact geologists noticed was a global spike in a metal called iridium in the layer of rock dividing the Cretaceous from the following period, the Paleogene. The iridium-rich layer is known as the K/Pg boundary, and a similar metal in the same rocks has provided the geological fingerprint for where the asteroid came from. The key metal is ruthenium.

Just like iridium, ruthenium is a metal that’s rare in Earth’s crust but is often found in meteorites and asteroids. The extinction boundary rocks have elevated ruthenium levels. What makes ruthenium key, however, is that levels of isotopes, or different versions of the element, vary between meteorites from different parts of our solar system.

“The idea for this study was born on the rationale that if different types of meteorites can be distinguished according to their ruthenium isotope compositions, and if the enrichment of elements such as ruthenium in the boundary layer is of extraterrestrial origin, the ruthenium isotope data from the boundary layer samples would provide information on the type of impactor,” says study author and University of Cologne geologist Mario Fischer-Gödde. 

Meteorites from near the sun, for example, have different chemical signatures than those from the outer part of our solar system. It’s these variations that allowed Fischer-Gödde and colleagues to determine where the Chicxulub impactor came from. 

The new analysis, published today in Science, identifies the extinction-triggering asteroid as a carbonaceous chondrite meteorite that formed in the outer part of our solar system. Experts refer to such chunks of space rock as C-type asteroids.

“This is fantastic work,” says Arizona State University astrophysicist Steven Desch, who was not involved in the new study. The new data provide striking evidence that the extinction-triggering asteroid was a carbonaceous chondrite, he notes, and not a comet or other potential impactor. (Did a second asteroid strike Earth during the demise of the dinosaurs?)

The ruthenium signature left behind by the Chicxulub asteroid differed from those of several other impact craters included in the study. The other samples, between a range of 36 and 470 million years old, were most consistent with S-type asteroids that formed in the inner part of the solar system. “That’s an amazing discovery,” Desch notes, as the data narrows down where other meteorites that have left their mark on the Earth originated. Impactors have come from different parts of our solar system rather than a single reservoir.

In addition to identifying the Chicxulub asteroid’s origins, the new study underscores that it truly was the asteroid impact that caused the disaster at the end of the Cretaceous. Huge volcanoes called the Deccan Traps erupted in ancient India before and after the asteroid’s arrival and until recently have been considered a competing extinction trigger.

But patterns of iridium, ruthenium, and similar elements in the  boundary layer are inconsistent with the basalt rock formed by the prehistoric eruptions and instead, best match a massive space rock impact. In fact, previous research indicates that greenhouse gases emitted by the Deccan Traps likely softened the impact winter that followed the asteroid strike and mitigated its after-effects. (Read more about how volcanoes helped life bounce back after the dino-killing asteroid strike.)

How the asteroid went from floating in space to triggering a catastrophe for life on Earth is still murky. In the early part of our solar system’s history, Fischer-Gödde says, gravity pulled most asteroid-sized space rocks together to form planets and moons. The Chicxulub asteroid must have somehow escaped this fate. “The asteroid of the [Chicxulub] impact was stored in a stable orbit until 66 million years ago,” Fischer-Gödde says. At some unknown point prior to impact, the migration of Jupiter through space may have pulled the asteroid out of its orbit and sent it on a one-in-a-million shot towards Earth.

The findings make the impact at the end of the Cretaceous all the more unique in Earth’s history. “About 80 percent of all meteorites hitting the Earth derive from S-type asteroids,” or those from the inner part of the solar system, Fischer-Gödde says. The dino-killer was different, a distant part of the solar system that came for an unfortunate visit.

Birds were the only dinosaurs to survive, and even groups thought of as survivors, such as mammals and lizards, suffered steep losses. Life on Earth would not be the same today without the impact, a rare chance event that wiped away many forms of ancient life and allowed the survivors, including our earliest primate ancestors, to flourish.