According to researchers from Dartmouth University, it may not have been a slow-moving asteroid that struck the planet 65 million years ago, bringing about the end of the reign of the dinosaurs, but instead it may have been a fast-moving comet.

Speaking at the 44th Lunar and Planetary Science Conference last week, Dartmouth University paleoecologist Jason Moore discussed how the impact that occurred 65 million years ago deposited a global layer of sediments that contained the element known as iridium, but the oft-quoted values for the concentration of this iridium are incorrect. Using iridium's next-door neighbor on the periodic table, osmium — which was also deposited during the impact — Moore and his colleagues found that the impact deposited far less than previously thought.

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"You'd need an asteroid of about 5 km diameter to contribute that much iridium and osmium. But an asteroid that size would not make a 200 km-diameter crater," said Dr. Moore, referring to the roughly 200 km-wide Chicxulub crater, centred off the northern tip of the Yucatan Peninsula.

"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."

Professor Mukul Sharma, who is one of Moore's colleagues, 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."

However, other researchers aren't so sure.

"I think it's some very interesting work," said Brandon Johnson, a physicist at Purdue University, according to LiveScience, but he added: "There's a possibility that a lot of the impacted material could have been ejected at escape velocity, so we couldn't find it on Earth."

With large impacts, some of the material from the collision — both from the asteroid and from the planet it strikes — can be thrown away from the impact site so violently and quickly that it never falls back to the ground, and is instead launched into space. For example, the Tissint meteorite, which fell to Earth in July of 2011, was found to have been once part of the planet Mars.

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Dr. Gareth Collins, a researcher at Imperial College London who studies impact cratering, apparently agrees with Johnson, calling the work "thought-provoking", but also saying: "I don't think it is possible to accurately determine the impactor size from geochemistry."

"Geochemistry tells you — quite accurately — only the mass of meteoritic material that is distributed globally, not the total mass of the impactor. To estimate the latter, one needs to know what fraction of the impactor was distributed globally, as opposed to being ejected to space or landing close to the crater."

"The authors suggest that 75% 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%," he told BBC News.

Moore and his colleagues were willing to concede this, but were also quick to point out that there have been several studies in recent years that suggest that the object lost only between 11% and 25% of its mass in the collision.

Asteroids and comets have been found to some typical speeds as they fly through space, with asteroid velocities averaging out to around 17 km/s and comets around 51 km/s. However, faster and slower objects are known. Asteroids 2008 FF5 and 2011 BT59, for example, travel at over twice the 'typical' speed for asteroids. So, although it is possible that a comet was responsible, it could easily have been a faster-moving asteroid.

"The evidence that they have for a high velocity impact is marginally positive," said Jay Melosh, a geophysicist at Purdue University who helped create the Earth Impact Effects calculator. "However, the probability that that high velocity impact is a comet is very low."

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