Scientists Discover Why Female Dinosaurs Laid Smaller EggsReuters

The space rock that wiped out the dinosaurs about 65 million years ago may have been a speeding comet, rather than an asteroid, according to scientists.

Earlier studies have suggested that an asteroid slammed into Earth at Chicxulub in Mexico causing the Cretaceous-Tertiary (KT) extinction, which wiped out the dinosaurs as well as 70 percent of the Earth's species. But a new study by researchers from New Hampshire, U.S., has suggested that the 180 kilometer-wide Chicxulub crater might have been made by a relatively smaller object than a larger, slow-moving asteroid.

A key piece of evidence showing the space rock's impact is the presence of high levels of iridium in geological samples around the world from the time of the extinction. Iridium is very rarely found in Earth's crust, but is commonly found in asteroids. Based on this evidence, earlier studies had suggested that an asteroid strike might have killed the dinosaurs.

But researchers involved in the new study have suggested that the iridium values have been calculated incorrectly in previous studies. The research team compared the values of iridium with another extraterrestrial chemical element- osmium that was deposited as a result of the impact.

They recalculated the values and suggested that the space rock that hit Earth had deposited less debris than previously thought. This suggests that the space rock was a smaller body rather then an asteroid. For a smaller rock to have created a 180 km- wide, it must have travelled exceedingly fast, the researchers said.

"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," study author Jason Moore, a paleoecologist at Dartmouth College in New Hampshire, told BBC. "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 icy small bodies that release gas and dust. When they approach close to the sun, they display thin, fuzzy atmospheres called comas or tails. Researchers suggest that a long-period comet, which would take hundreds or thousands or sometimes millions of years to orbit the sun once, could have caused the Chicxulub impact.

Although it is possible that rapidly moving asteroid could have caused the Chicxulub impact crater, the research team suggest that the fast-moving objects observed in the skies are mostly comets, implying that a comet strike caused the Cretaceous-Tertiary (KT) extinction.

The details of the findings were presented at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.