Dinosaur-killing rock traced to population of "dark primitive asteroids"
About 66 million years ago, a gigantic object crashed into the Earth, triggering a mass extinction that took out the dinosaurs. Now, scientists from Southwest Research Institute say they’ve traced the culprit back to its point of origin, identifying it as a “dark primitive asteroid.”
The end of the Cretaceous period came to a sudden end when an impactor some 10 km (6.2 miles) wide slammed into what is now the Yucatan Peninsula in Mexico. That would have set off a global cascade of cataclysms, including tsunamis, earthquakes, wildfires, ocean acidification, and enough debris in the air to block the Sun for up to 18 months. Three quarters of all life on Earth was wiped out.
But what exactly struck the Earth, and where did it come from? Some scientists suggest it was a comet from the icy Oort cloud at the very fringes of the solar system. Others believe it was more likely an asteroid from the belt between Mars and Jupiter.
Recent research supports the latter story – rock samples and drill cores taken from the site of the crater indicate the impactor was of a carbonaceous chondrite composition. These carbon-rich rocks are made of pristine materials left over from the birth of the solar system, and don’t reflect much light. As such, they’re often called “dark primitive” asteroids.
But while these objects do commonly drift near Earth they’re usually far too small to cause the amount of damage seen at Chicxulub. So for the new study, the researchers set out to find this impactor’s bigger “siblings.”
“To explain their absence, several past groups have simulated large asteroid and comet breakups in the inner solar system, looking at surges of impacts on Earth with the largest one producing Chicxulub crater,” says Wiliam Bottke, co-author of the study. “While many of these models had interesting properties, none provided a satisfying match to what we know about asteroids and comets. It seemed like we were still missing something important.”
There is one place where these larger rocks lurk – the outer asteroid belt. But objects there are on fairly stable orbits, and rarely dislodge and head towards Earth. To find out how often they might, the team simulated a main asteroid belt of 130,000 virtual space rocks, and tracked them over hundreds of millions of years. And surprisingly, they found that asteroids of about the size of Chicxulub strike Earth at least 10 times more often than was previously thought – an average of once every 250 million years or so.
According to the model, thermal forces acting on the asteroids over millions of years can slowly jostle them around, until they reach regions that act like gravitational “escape hatches” where they can slip loose. From there, they can end up on orbits that potentially cross paths with Earth.
Almost half of the rocks that struck in the simulation were carbonaceous chondrites too. Altogether the models show that there’s a good chance that the Chicxulub impact was caused by one of these dark primitive asteroids.
“This result is intriguing not only because the outer half of the asteroid belt is home to large numbers of carbonaceous chondrite impactors, but also because the team’s simulations can, for the first time, reproduce the orbits of large asteroids on the verge of approaching Earth,” says Simone Marchi, co-author of the study. “Our explanation for the source of the Chicxulub impactor fits in beautifully with what we already know about how asteroids evolve.”
The research is due to be published in the journal Icarus.
Source: Southwestern Research Institute
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