About 66 million years ago, something huge slammed into our planet off the coast of what is now the Yucatan Peninsula. About 10 to 15 kilometers in diameter, it created a 90 million megaton explosion, a planetary shock wave, and a giant tsunami. And, the leading theory goes, it caused the mass extinction that wiped out the dinosaurs.
But there has always been debate about what exactly it was: a comet or an asteroid?
Now, new research A paper published in the journal Science suggests that it was indeed an asteroid – specifically a carbonaceous type – that came from beyond the orbit of Jupiter.
The study's authors made the discovery while studying an isotope of ruthenium, a rare element belonging to the platinum group elements (PGE).
Ruthenium is one of the rarest elements on earthonly with 0.001 parts per million. However, it is believed that there is much more of it in the Earth's core. This is because when the Earth was formed 4.6 billion years ago, when rocks collided with each other, a molten sea destroyed much of it.
However, it is found in some asteroids, particularly those beyond Jupiter, where it is preserved in a kind of cold storage.
Our solar system contains a lot of debris left over from its formation. Comets are dusty and icy remnants, while asteroids are mostly rock. Meteorites, or small chunks of rock, fall to Earth from time to time. Much of what scientists understand about the composition of asteroids comes from them.
However, not all asteroids are made of the same material. There are three main classes of asteroid composition: C-types (carbonaceous); S-types (rocky); and M-types (metallic).
Cosmic fingerprint
The asteroid that is believed to have wiped out the dinosaurs slammed into Earth 66 million years ago between the Cretaceous and Paleogene periods, known as the K-Pg boundary. While scientists can't study the asteroid itself because it was destroyed, they can study the isotopes it left behind — in this case, ruthenium.
“The isotopic signatures we measure can be considered a kind of fingerprint,” said lead author Mario Fischer-Gödde, who is also a scientist at the Institute of Geology and Mineralogy at the University of Cologne. “So if there is a big impact, we vaporize the rocks and the asteroid itself, but this fingerprint remains.”
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But they also looked at ruthenium isotopes in five other samples from the last 541 million years, as well as samples from those that are about 3.2 to 3.5 billion years old, and one of the two carbonaceous meteorites. And they also took measurements from locations in Europe where debris from the event can be found, known as distal sites.
They found that ruthenium isotopes from the K-Pg boundary closely matched carbonaceous meteorites.
“All the results clearly show that no matter what site we are at… they all gave consistently the same isotopic signature of C-type asteroid material,” Fischer-Goedde said. “That's why we can be confident.”
The paper rules out the possibility of a comet, but Fischer-Goedde noted that we have yet to collect a sample from the comet's nucleus.
“I'm a scientist. I look at all the possible outcomes, the complexities, and so on,” he said.
Paul Wiegert, a professor in the department of physics and astronomy at Western University in London, Ontario, said he thought the study, which he was not involved in, made a compelling case.
“It's a very interesting article,” Wiegert said.
He said the strong traces of ruthenium are exactly what scientists expect to find in asteroids, so its discovery in the Earth's crust is good evidence.
“I would say it's pretty compelling,” he said. “Ruthenium is very rare in most solar system objects. For example, in the Earth's crust it's pretty rare… I think they've made the connection in a pretty compelling way.”
Fischer-Gödde also views this discovery philosophically.
“A collision with a large C-type asteroid in recent Earth history, say in the last 500 million years… is a really rare and unique event, and it's an event: we can call it a cosmic coincidence, but if it hadn't happened, we probably wouldn't be sitting here,” he said.
“Our ancestors probably would never have dared to crawl out of their caves. They would have been eaten by dinosaurs.”