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Sixty-six million years ago, the story of life on Earth took a dramatic turn when an asteroid collided with what is now the Yucatán Peninsula in Chicxulub, Mexico. The aftermath of the collision resulted in the extinction of an estimated 75 percent of animal species, including most dinosaurs, except for birds. But the asteroid itself is virtually gone.
In a new study published Thursday in the journal Science, researchers have pieced together the chemical identity of the asteroid that sparked the planet’s fifth mass extinction. The dino-killer was a rare clay-rich mud ball containing materials from the dawn of the solar system, the findings suggest.
Although the Chicxulub asteroid landed tens of millions of years ago, learning more about this ancient space rock is important because it is “part of a bigger picture of understanding the dynamic nature of our solar system,” said study co-author Dr. Steven Goderis, research professor of chemistry at the Vrije Universiteit Brussel.
Constructing a theory for the extinction of non-avian dinosaurs
Scientists hypothesized in 1980 that a collision with a giant space rock had led to the dinosaurs’ demise. At the time, researchers did not find the asteroid itself; instead, they found a thin layer of the metal iridium in rocks around the world from 66 million years ago. Iridium is rare in the Earth’s crust, but abundant in some asteroids and meteorites.
Some members of the broader scientific community were skeptical of the hypothesis. In 1991, however, scientists discovered that the Chicxulub crater was the right age to have been formed by a massive asteroid impact that coincided with the demise of the dinosaurs. Over the years, researchers have gathered more and more evidence that the asteroid impact did indeed trigger the catastrophic extinction.
The asteroid was huge — likely between 6 and 9 miles (9.7 and 14.5 kilometers) in diameter. But its colossal size is what caused it to largely disappear. The rock, about the size of Mount Everest, hurtled toward Earth at 15.5 miles per second (25 kilometers per second), according to NASA.
“Basically, all of that kinetic energy is converted into heat,” Goderis said. “When the thing hits the target, it’s not going to explode; it’s going to vaporize.” The impact created a dust cloud that consisted of the asteroid itself and the rock it landed on. The dust spread across the globe, blocking out sunlight and lowering temperatures for years, resulting in a mass extinction.
As for the asteroid, “there’s nothing left except this chemical trail that’s been deposited all over the world,” Goderis said. “That forms this little clay layer that you can recognize all over the world, and it’s basically the same point in time, 66 million years ago.”
Chemical composition of dinosaur-killing asteroid revealed
Asteroids (and the smaller meteoroids that break off from them) come in three main varieties, each with their own chemical and mineral makeup: metallic, stony, and chondritic. In the new study, Goderis and his colleagues, including the study’s lead author, Dr. Mario Fischer-Gödde of the University of Cologne in Germany, examined the chemical composition of the thin clay layer to unlock the asteroid’s secrets.
The researchers sampled 66-million-year-old rocks from Denmark, Italy and Spain and isolated parts containing the metal ruthenium. (Like iridium, ruthenium is more common in space rocks than in Earth’s crust.) The team also analyzed ruthenium from other asteroid impact sites and meteorites. The chemical makeup of the ruthenium from 66 million years ago matched the chemical makeup of ruthenium present in a certain type of chondritic meteorite, the scientists found.
“We found that there’s a perfect overlap with carbonaceous chondrite signatures,” Goderis said. Therefore, the asteroid that killed off the dinosaurs was likely a carbonaceous chondrite, an ancient space rock that often contains water, clay and organic (carbon-bearing) compounds.
Although carbonaceous chondrites make up the majority of rocks in space, only about 5 percent of meteorites that fall to Earth fall into this category. “There’s quite a bit of diversity in carbonaceous chondrites, and some of them can smell,” Goderis said. But in hell, when the Chicxulub impactor landed, Goderis said, “you probably wouldn’t have had time to do a good sniff.”
What the findings mean for the future
Impacts the size of Chicxulub happen only once every 100 million to 500 million years. But because there’s still a chance Earth could encounter another asteroid or giant meteorite, Goderis said it’s good to “know the physical and chemical properties of these objects to think about how we can protect ourselves” from a collision with a large space rock.
Goderis cited the 2022 DART mission, or the Double Asteroid Redirection Test, in which NASA sent a spacecraft to deliberately deflect an asteroid off course. Understanding how different types of asteroids interact with the physical forces around them would be crucial to an effective planetary defense operation.
“The carbonaceous chondrite is going to react completely differently than a regular chondrite — it’s much more porous, it’s much lighter, and it’s going to absorb much more of an impact if you send an object at it. So, we need to learn more about this to have a corresponding reaction,” Goderis said.
Dr. Ed Young, a professor of cosmochemistry at the University of California, Los Angeles, who was not involved in the study, agreed with the findings.
He said the discovery “adds richness to our understanding of what happened” when the dinosaurs went extinct. Young noted that the researchers’ assessment that the asteroid was a carbonaceous chondrite “is a robust conclusion.”
Kate Golembiewski is a freelance science journalist from Chicago who studies zoology, thermodynamics, and death.
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