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Dinosaur-killing Chicxulub asteroid came from the edge of the solar system
We now know the origin of the asteroid that wiped out the dinosaurs.
By analysing the chemical signatures of rocks from the end of the Cretaceous Period, scientists have found that it came from beyond Jupiter.
The prime suspect in the extinction of the dinosaurs was no ordinary asteroid, researchers have revealed.
Comparisons between the chemical record left behind by the strike 66 million years ago and known meteorite samples suggest that the Cretaceous asteroid was a carbonaceous chondrite. This type of asteroid is one of the oldest known, having formed billions of years ago in the early solar system.
As these chondrites can only come from asteroids found beyond Jupiter, it suggests that the dinosaur-killing asteroid must have had its origins there too.
Dr Ashley King, a scientist who studies meteorites at the Natural History Museum, adds that this research is a further nail in the coffin for other ideas about what caused the mass extinction at the end of the Cretaceous. Ashley was not involved in the research, published in the journal Science.
“This is a neat piece of research that manages to confirm what kind of asteroid struck the Earth 66 million years ago,” he says. “The researchers managed to detect tiny amounts of ruthenium, an element which is very rare in the Earth’s rocks but much more common in asteroids.”
“The levels of ruthenium they detected are much more characteristic of being delivered by an asteroid strike, rather than a comet or massive volcanic eruptions. Future research might be able to reveal exactly what kind of carbonaceous asteroid this was, further narrowing down where it came from.”
While Chicxulub may look beautiful today, it was at the epicentre of the asteroid strike 66 million years ago. © Karen Hogan/ Shutterstock
What do we know about the asteroid that killed the dinosaurs?
While no human was alive to witness the asteroid strike that ended the reign of the dinosaurs and the Mesozoic Era, scientists have been able to find out a surprising amount about it.
The first clues that an asteroid was responsible for the end of the dinosaurs were discovered in the late 1970s, when the father and son team of Luis and Walter Alvarez found high levels of iridium in rocks dated to the end of the Cretaceous.
Iridium is quite rare in Earth’s rocks, but common in asteroids. This made an impact from space the most likely cause of the dinosaur extinction event, and although this idea met resistance at first it was gradually accepted over the following decade.
We also know where this asteroid strike took place – Mexico. The 150-kilometre-wide Chicxulub crater, the second largest on Earth, suggests that an asteroid as wide as 15 kilometres across fell at a steep angle and instantly vaporised everything nearby. This would have immediately plunged the world into a nuclear winter.
The force of the impact was so great that it altered the shape of the planet, as well as transforming rock into glass-like spherules. These spherules were scattered across the world, where they are often found together with the distinctive layer of iridium from the asteroid.
Some of these spherules got into the gills of dying fish, fossils of which have been used to reveal that the asteroid impacted during springtime in the northern hemisphere. This is possible to know based on where the lines of growth in the fish’s bones stop, which can be read somewhat like rings in a tree trunk.
Until now, however, scientists didn’t know where the asteroid had come from. While it was suggested in the mid-2000s that it might have originated among the Baptistina asteroids in the main asteroid belt, more recent evidence has ruled this out.
With the Chicxulub strike long predating the modern camera networks that allow scientists to trace meteorites, the authors of this study turned instead to the chemical record it left behind.
The Chicxulub impactor was a carbonaceous chondrite, similar to the more recent Winchcombe meteorite (pictured). © The Trustees of the Natural History Museum, London.
Cracking a 66-million-year-old cold case
To try and provide a better idea of what type of asteroid the Chicxulub space rock was, the team took samples from rocks in Spain, Italy and Denmark dated to the end of the Cretaceous. These were supplemented by samples recording other impacts ranging from 3.5 billion to 36 million years ago.
The different forms, or isotopes, of ruthenium in the different samples were measured and then compared to different types of meteorites.
The more recent strikes, ranging from 460 to 36 million years ago, were caused by impactors similar to the ordinary chondrites found in museum collections. These are named because they’re the most common type of meteorites recovered on Earth.
“Ordinary chondrites are silicate- and metal-rich rocks, which makes them much more likely to survive entering the atmosphere,” Ashley says. “It’s thought that they formed in the inner solar system, being close enough to the early sun that there wasn’t much water or carbon available for them.”
The Chicxulub impactor was an exception, however. Its ruthenium isotope values were most similar to the average value of the carbonaceous chondrites. While this study was unable to narrow it down to a specific type of these meteorites, Ashley believes this could be possible in the future.
“Carbonaceous chondrites are vulnerable to being chemically modified as they enter Earth’s atmosphere, which might explain some of the variation between the different types,” he explains. “If the researchers could measure even finer levels of ruthenium, they might be able to narrow it down to a particular type.”
“Different chondrites have different levels of water in them, and generally, the more water that’s in them, the further out they formed. While we’re not there yet, this might one day be able to give us some candidate asteroids for where the Chicxulub impactor came from.”
