From the far reaches of the solar system, a giant asteroid or comet plunged into Earth near what is now the Chesapeake Bay 35 million years ago (shown in this artist's conception). Credit: © The Virginian-Pilot.

ScienceDaily (Dec. 15, 2009) — It's the stuff of a Hollywood disaster epic: A comet plunges from outer space into the Earth's atmosphere, splitting the sky with a devastating shock wave that flattens forests and shakes the countryside.

But this isn't a disaster movie plotline.

"Comet impacts might be much more frequent than we expect," said Adrian Melott, professor of physics and astronomy at the University of Kansas. "There's a lot of interest in the rate of impact events upon the Earth. We really don't know the rate very well because most craters end up being destroyed by erosion or the comets go into the ocean and we don't know that they're there. We really don't have a good handle on the rate of impacts on the Earth."

An investigation by Melott and colleagues reveals a promising new method of detecting past comet strikes upon Earth and gauging their frequency. The results will be unveiled at the American Geophysical Union's Fall Meeting, to be held Dec. 14-18 in San Francisco.

The research shows a potential signature of nitrate and ammonia that can be found in ice cores corresponding to suspected impacts. Although high nitrate levels previously have been tied to space impacts, scientists have never before seen atmospheric ammonia spikes as indicators of space impacts with our planet.

"Now we have a possible new marker for extraterrestrial events in ice," Melott said. "You don't just look for nitrates, you also look for ammonia."

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