The space rock believed to have wiped out dinosaurs 65 million years ago when it collided with earth was a comet, not an asteroid, according to a controversial new study.
Geochemistry tests performed on the 112-mile Chicxulub crater on Mexico's Yucatan peninsula - believed to be the impact site - analyzed levels of osmium and iridium. Levels of iridium, in particular, were lower than scientists previously thought. Researchers then concluded that a much smaller rock led to the extinction of dinosaurs and 70 percent of other species on Earth. The extraterrestrial rock, most likely a comet, would have been travelling very fast.
"You'd need an asteroid of about [three miles] diameter to contribute that much iridium and osmium. But an asteroid that size would not make a [112-mile] diameter crater," researcher from Dartmouth College Jason Moore told 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.”
Unlike rocky asteroids, comets are comprised of a loose collection of ice, dust and small rocky particles. Comets range in size from a few kilometers to tens of kilometers across. They also have a tail, formed when heat from the sun vaporizes its ice and sends dust particles flying in the solar wind.
Not every scientist agrees with Moore's hypothesis.
Dr. Gareth Collins, who researches impact cratering at Imperial College London, did not come to the same conclusion.
"I don't think it is possible to accurately determine the impactor size from geochemistry," he said. "The authors suggest that 75 percent of the impactor mass is distributed globally, and hence arrive at quite a small-sized impactor, but in reality this fraction could be lower than 20 percent." He added that this could still allow for the extinction event to have been caused by an asteroid.
Moore and his colleagues detailed their findings at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.