Comet, not asteroid, wiped out dinosaurs?

A speeding comet, rather than an asteroid, slammed into the Earth 65 million years ago and sparked the extinction of the dinosaurs, scientists claim.
Many scientists believe the 180 km Chicxulub crater in Mexico was made by the impact that caused the extinction of dinosaurs and about 70 per cent of all species on Earth.
The crater was probably blasted out by a faster, smaller object than previously thought, according to the new study presented at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.
Evidence of the space rock's impact comes from a worldwide layer of sediments containing high levels of the element iridium, dubbed the Cretaceous-Paleogene (K-Pg) boundary, which could not have occurred on Earth naturally.

The new study suggests the often-cited iridium values are incorrect. Scientists compared these values with levels of osmium, another element delivered by the impact.
Their calculations suggest the space rock generated less debris than previously thought, implying the space rock was a smaller object, 'LiveScience' reported.

The researchers concluded in order for the smaller rock to have created the giant Chicxulub crater, it had to have been going exceedingly fast.
Their results indicate the impact is more compatible with a long-period comet, which can take hundreds, thousands or sometimes millions of years to orbit the Sun once.
It is possible that a rapidly moving asteroid could have caused the Chicxulub impact crater, but the fastest-moving objects that have been observed are mostly comets, researchers said.