Dark origins of massive ‘planet killer’ asteroid that wiped out the dinosaurs

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Since the late 1970s, scientists have been increasingly sure that the asteroid that killed the dinosaurs landed near the Yucatán Peninsula in Mexico.

But now they think they know where it came from, too.

The object that smashed into the Earth at over 67,000 mph, forming the 93-mile-wide Chicxulub impact crater, was completely destroyed by the collision.

But it left a tell-tale layer of layer of iridium-rich clay at the the boundary between the Cretaceous and Paleogene periods, 66 million years ago.

Iridium, a metal that’s normally only found deep underground on Earth, is known to be common in asteroids.

Research by scientists from the Southwest Research Institute in Colorado, suggests that the killer object was almost certainly a giant dark primitive space rock from the outer reaches of the asteroid belt between Mars and Jupiter.

Dark asteroids are so called because their chemical composition makes them almost invisible in the blackness of space.

By simulating the effect of Jupiter’s gravity on the asteroid belt, the researchers demonstrate that the Cretaceous-era planet killer was almost certainly one of them.

David Nesvorný, who led the new study, said he had a “suspicion” that the outer half of the asteroid belt — where the dark primitive asteroids are most commonly found – might be source of many major terrestrial impactors, but, he said “I did not expect that results to be so definitive.”

"We find in the study that some 60% of large terrestrial impactors come from the outer half of the asteroid belt … and most asteroids in that zone are dark/primitive," he told Live Science. "So there is a 60% probability that the next one will come from the same region."

Jessica Noviello, who is a NASA fellow at the Universities Space Research Association at Goddard Space Flight Centre, described the team’s work as "excellent," adding: "I think they make a good argument for why [the Chicxulub asteroid] could have come from that part of the solar system."

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That's not the only argument though. In February this year, at team led by Harvard University’s Avi Loeb published a paper stating that the Chicxulub impactor was more likely a fragment of a comet originating in the Oort cloud, field of frozen rock and ice larking at the furthest reaches of the Solar System.

They say the comet was pulled in by Jupiter’s gradational tide and shattered as it came too close to the Sun.

“Basically, Jupiter acts as a kind of pinball machine,” said Amir Siraj, who worked with Loeb on the research. “Jupiter kicks these incoming long-period comets into orbits that bring them very close to the sun.”

But it’s when object like this swing around the Sun that they become a threat to Earth-dwellers: “On the journey back to the Oort Cloud there’s an enhanced probability that one of these fragments hit the Earth,” Siraj explained.

Based on their model, Siraj and Loeb say the expected “rate of impact” of comet fragments like this is almost 10 times more likely than previously thought.

It’s still only about once every 250-730 million years, so the probability of us being wiped out by a comet fragment or an asteroid is still fairly low.

That may be the least of our problems. While the twilight of the dinosaurs is the best-known mass extinction in history, its death toll is dwarfed by that of an event known as The Great Dying, which took place just over 250 million years ago.

There’s no space rock to blame for that one. The extinction event that killed some 70% of life on land and a horrifying 90% of life in the oceans was caused by a huge excess of carbon dioxide in the atmosphere, leading to runaway climate change.

Geoscientist Professor Uwe Brand from Brock University in Canada warns:"Right now our emissions are 10 to 20 times higher than what happened at the end of the Permian mass extinction, which was the largest and biggest mass extinction”.

Makes something as simple as a runaway comet seem almost comforting.

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