Comet, not asteroid, said dinosaur killer

HANOVER, N.H., April 4 (UPI) -- While an asteroid is usually considered the culprit partially responsible for the extinction of dinosaurs, two U.S. researchers name a comet as chief suspect.

Jason Moore and Mukul Sharma of the Department of Earth Sciences at Dartmouth said they believe a high-velocity comet was the extraterrestrial source of the impact 65 million years ago that killed off almost all the dinosaurs and some 70 percent of all other species living on Earth.

That puts them at odds with most of the scientific community that considers an asteroid impact as the creator of the deeply buried and partially submerged, 110-mile wide Chicxulub crater off of Mexico's Yucatan Peninsula.

They've based that belief on the levels of iridium and osmium, elements common in space rock such as asteroids, found in rock layers dating to the dinosaur extinction.

However, new evidence suggests lower iridium/osmium levels than previously believed at the layer of rock known as the K-Pg boundary, marking the end of the Cretaceous period, the epoch of the dinosaurs, and the beginning of the Paleogene period, with its notable absence of the large lizards.

Moore and Sharma said a comet explanation reconciles the conflicting evidence of a huge impact crater with the revised, lower iridium/osmium levels at the boundary.

"We are proposing a comet because that conclusion hits a 'sweet spot.' Comets have a lower percentage of iridium and osmium than asteroids, relative to their mass, yet a high-velocity comet would have sufficient energy to create a 110-mile-wide crater," Moore said.

"Comets travel much faster than asteroids, so they have more energy on impact, which in combination with their being partially ice means they are not contributing as much iridium or osmium."