Dinosaur-killing space rock ‘was a comet’

UK: The space rock that hit Earth 65 million years ago and is widely implicated in the end of the dinosaurs was likely a speeding comet. That is the conclusion of research which suggests the 180km-wide Chicxulub crater in Mexico was carved out by a smaller object than previously thought.

Many scientists consider a large and relatively slow moving asteroid to have been the likely culprit.

Details were outlined at the 44th Lunar and Planetary Science Conference. But other researchers were more cautious about the results.

“The overall aim of our project is to better characterise the impactor that produced the crater in the Yucatan peninsula [in Mexico],” Jason Moore, from Dartmouth College in New Hampshire, told BBC News.

The space rock gave rise to a global layer of sediments enriched in the chemical element iridium, in concentrations much higher than naturally occurs; it must have come from outer space.

However, in the first part of their work, the team suggests that frequently quoted iridium values are incorrect.

Using a comparison with another extraterrestrial element deposited in the impact – osmium – they were able to deduce that the collision deposited less debris than has previously been supposed.

The recalculated iridium value suggests a smaller body hit the Earth. So for the second part of their work, the researchers took the new figure and attempted to reconcile it with the known physical properties of the Chicxulub impact.

For this smaller space rock to have produced a 180km-wide crater, it must have been travelling relatively quickly. The team found that a long-period comet fitted the bill much better than other possible candidates.

“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 Dr Moore. “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.”

Co-author Mukul Sharma, also from Dartmouth College, told BBC News: “You would need some special pleading for an asteroid moving very rapidly – although it is possible. But of the comets and asteroids we have looked at in the skies, the comets are the ones that are moving very rapidly.”

Long-period comets are balls of dust, rock and ice that are on highly eccentric trajectories around the Sun. They may take hundreds, thousands or in some cases even millions of years to complete one orbit.

The extinction event 65 million years ago is now widely associated with the space impact at Chicxulub. It killed off about 70% of all species on Earth in just a short period of time, most notably the non-avian dinosaurs.

BBC NEWS