Comet, Not Asteroid, Killed Dinosaurs in Mass Extinction Event

First Posted: Mar 23, 2013 09:12 AM EDT

Millions of years ago when dinosaurs ruled the Earth, a celestial object streaked through the sky, slamming into our planet. The resulting effects caused a mass extinction, wiping out thousands of species and paving the way for mammals. Now, though, scientists have found that it wasn't an asteroid that destroyed the dinosaurs. Instead, a comet may have been responsible. The announcement comes just in time for the the east coast to witness a meteor streaking through the sky on Friday night.

The study, which was presented at the 44th Lunar and Planetary Science Conference this week, examined the 112-mile Chucxulub crater in Mexico. Although scientists are sure that whatever caused the crater caused the extinction of the dinosaurs, there is still debate as to what exactly caused the impact.

In order to find out exactly what caused the mass extinction, the researchers examined the sediment within the crater. They found different iridium values than had been previously reported and then compared these values with levels of osmium, another element that was delivered by the celestial impact.

The researchers found that, surprisingly, whatever hit the Earth generated far less debris than had been previously thought. This implied that the missile was a much smaller object and therefore, must have been travelling at phenomenal speeds in order to create the giant crater.

"You'd need an asteroid of about 5km diameter to contribute that much iridium and osmium, But an asteroid that size would not make a 200km-diameter crater," said Jason more, one of the researchers, in an interview with BBC News. "So we said: how do we get something that has enough energy to generate that size of crater, but has much less rocky material? That brings us to comets."

Comets are balls of dust, rock and ice that are usually on highly eccentric trajectories around the sun. They can take hundreds, thousands or even millions of years to complete one orbit. They can also break apart when entering Earth's orbit, raining debris on the planet below.

Yet some doubt that this new finding shuts the door completely on the possibility of an asteroid. Gareth Collins, who researchers impact cratering at Imperial College, is one of those who pointed out some of the flaws in the work.

"Geochemistry tells you--quite accurately--only the mass of the meteoritic material that is distributed globally, not the total mass of the impactor. To estimate the latter, one needs to know what fraction of the impactor was distributed globally, as opposed to being ejected to space or landing close to the crater," said Collins in an interview with BBC News. Essentially, it's impossible to determine the size of the object that impacted the Earth through geochemistry alone.

Yet if the new research is correct, it could change the way scientists view these icy missiles. Asteroids may usually be larger than comets, but they're also slow-moving. This research reveals that a faster, smaller space rock may have caused a mass extinction that killed 70 percent of the world's species.

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