Researchers identify source of dinosaur-killing asteroid – Houston Chronicle

The dinosaur-killing asteroid took a back road to Earth.

The 6-mile-wide space rock likely escaped from the outer asteroid belt before striking our planet 66 million years ago. Researchers previously thought this portion of the asteroid belt didn’t have many good exit routes to Earth. Now, they believe dinosaur-killing asteroids could hit the Earth once every 250 million years on average.

This discovery, published in the Icarus journal, is changing researchers’ understanding of how massive asteroids get bumped toward the Earth, how likely they are to impact us in the future and what role they might have played in the Earth’s evolution.

“In a sense, the asteroid belt becomes another place to try to probe the Earth’s early history,” said Bill Bottke, a co-author of the paper and director of the Southwest Research Institute’s department of space studies in Boulder, Colo.

It is believed an asteroid killed the dinosaurs because it struck the Earth and sent debris flying outside of Earth’s atmosphere. As this debris fell back to the planet, it caused friction that heated the atmosphere and made the surface temperature of Earth feel like an oven set to broil. This lasted several hours and essentially roasted everything on the planet, Bottke said.

Previous theories on the origin of this dinosaur-killing asteroid have had holes. Bottke said the suspected frequency of impact — once every billion years — was unsatisfying because it suggested getting a dinosaur-killing asteroid 66 million years ago was a fluke.

And these theories largely focused on superhighways connecting the asteroid belt with Earth.

Asteroids in these superhighways get a stronger kick from the gravity of Jupiter or other large planets, meaning they tend to move faster from the asteroid belt into planet-crossing orbits. Asteroids on back roads, however, receive weaker gravitational kicks from planets. It takes them longer to slip out of the asteroid belt. This means they are more likely to interact with the gravity of Mars, and this could put the asteroids in orbits that are more likely to hit Earth, Bottke said.

These back roads are a new finding for Bottke, who focused on a particular superhighway more than 10 years ago. He was working on a paper that suggested a large asteroid named Baptistina had broken apart in the inner asteroid belt, sending numerous 6-mile-wide, dinosaur-killing asteroids along one of these more easily spotted superhighways. However, a NASA spacecraft later determined that Baptistina likely wasn’t large enough to create enough of these 6-mile-wide chunks of rock.

“That was a bummer,” he said. “I thought we had it, but it just didn’t hold up.”

It took a supercomputer to find the back roads identified in the new paper. Researchers were given at least a month, not all at once but in a few days here and there, on NASA’s Pleiades supercomputer to model 130,000 asteroids.

They looked at these present-day asteroids and modeled what they would do in the next hundreds of millions of years. The asteroid belt hasn’t changed much since dinosaurs roamed the Earth, so modeling future asteroid movements can provide insight into what happened 66 million years in the past.

“What we found, to our surprise, is that a lot of the biggest asteroids in the asteroid belt escape not through the major superhighways,” Bottke said, “but they get out through the back roads. Collectively, those back roads are more important than the superhighways for delivering things to the Earth.”

And this theory better matches what geologists have found on Earth. The dinosaur-killing asteroid made landfall on what is now Mexico’s Yucatán peninsula. It created the Chicxulub crater, where geologists have found 66-million-year-old rock samples that indicate the asteroid was made of materials similar to carbonaceous chondrite meteorites landing on Earth.

There aren’t many rocks in the inner asteroid belt, where Bottke and others previously thought this asteroid originated, that are carbonaceous chondrite and large enough to have such a devastating impact.

There are, however, plenty of these asteroids in the outer asteroid belt.

The findings from this paper, which was led by David Nesvorný of the Southwest Research Institute, are focused on the Chicxulub crater, but Bottke believes the data might also answer questions about the Earth’s early evolution. The Earth is believed to be roughly 4½ billion years old.

About 3 billion years ago — or 1½ billion years after Earth’s formation — multiple asteroids as large as 20 miles wide are believed to have impacted the Earth. Around this same time, the Earth began going through some major changes. Earth’s atmosphere was getting its first short-lived whiffs of oxygen. And plate tectonics, Earth’s system for creating continents and cooling itself, may have started near this time.

Could a really, really large asteroid have caused volcanic activity on Earth by creating a corridor, if you will, from Earth’s mantle to the surface? Researchers don’t know. But the new asteroid modeling might help answer such questions.

“This is a process which is telling us about the solar system,” he said. “There is evidence that impacts may have affected the history of life on the Earth over billions of years.”

andrea.leinfelder@chron.com

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