About 66 million years ago, something huge hit our planet off the coast of what is now the Yucatan Peninsula. It is estimated to have been between 10 and 15 kilometers in diameter, created a 90 million megaton explosion, sent shock waves through the planet and a giant tsunami. And, according to a leading theory, it was responsible for a mass extinction, killing the dinosaurs.
But there has always been debate about what exactly it was: was it a comet or an asteroid?
Now, new study published in the journal Science showed that it was indeed an asteroid — specifically, one called a carbonaceous asteroid — that came from beyond the orbit of Jupiter.
The study authors made the discovery by studying an isotope of ruthenium, a rare element belonging to the platinum group, or PGE.
Ruthenium is one of the rarest elements on Earth, occurring at just 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, and rocks smashed together, the molten sea destroyed much of it.
However, it is found in some asteroids, especially those beyond Jupiter, where it has been preserved in a kind of cold storage.
Our solar system has a lot of debris left over from its formation. Comets are dusty and icy debris, while asteroids are mostly rock. Every so often, meteorites, small pieces of rock, fall to Earth. What scientists know about the composition of asteroids comes mostly 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 (stony); and M-types (metallic).
Cosmic Fingerprint
The asteroid that likely killed the dinosaurs hit Earth 66 million years ago, between the Cretaceous and Paleogene eras, known as the K-Pg boundary. While scientists can’t study the asteroid itself because it was destroyed, they can study the isotopes left behind — in this case, ruthenium.
“The isotopic signatures we measure can be seen as 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 huge impact, we vaporize the rocks and the asteroid itself, but this fingerprint is preserved.”
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But they also looked at ruthenium isotopes in five other samples from the past 541 million years, as well as samples from those about 3.2 billion to 3.5 billion years old and another from two carbonaceous meteorites. They also took measurements at places in Europe where debris from the event can be found, known as distal sites.
They found that ruthenium isotopes at the K-Pg boundary closely correspond to carbonaceous meteorites.
“All the results clearly show that no matter which site you look at… they all consistently give the same isotopic signature of the C-type asteroid material,” Fischer-Gödde said. “That’s why we can be quite confident about that.”
The paper rules out the presence of a comet, but Fischer-Gödde notes that we have not yet been able to take a sample from the comet’s nucleus.
“I’m a scientist. I consider all the possible outcomes, complexities, etc.,” he said.
Paul Wiegert, a professor in the Department of Physics and Astronomy at Western London University in Ontario, said the study, in which he was not involved, makes a compelling case.
“This is a very interesting article,” Wiegert said.
He added that the strong presence of ruthenium is exactly what scientists expect to find in asteroids, so finding it in Earth’s crust is good evidence.
“I would say that’s pretty convincing,” he said. “Ruthenium is very rare in most of the solar system. It’s pretty rare in the Earth’s crust, for example. … I think they’ve made that connection in a very convincing way.”
Fischer-Gödde also approaches this discovery philosophically.
“A large C-type asteroid impact in recent Earth history, let’s say in the last 500 million years… is a really rare and exceptional event, and this 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 come out of their caves. They would have been eaten by dinosaurs.”
