Last updated on March 15, 2014 at 21:56 EDT

Scientists Probe Mysterious Possible Comet Strikes On Earth

December 14, 2009

An investigation by the University of Kansas’ Adrian Melott and colleagues reveals a promising new method of detecting past comet strikes upon Earth and gauging their frequency


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.


“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.”




Melott and fellow researchers examined data from ice cores extracted in Greenland to compare atmospheric chemistry during the Tunguska and Younger Dryas events. In both instances, Melott’s group found evidence that the Haber process “” whereby a nitrogen fixation reaction produces ammonia “” may have occurred on a large scale.

“A comet entering the atmosphere makes a big shock wave with high pressure “” 6,000 times the pressure of air,” said Melott. “It can be shown that under those conditions you can make ammonia. Plus the Tunguska comet, or some fragments of it, landed in a swamp. And any Younger Dryas comet presumably hit an ice sheet, or at least part of it did. So there should have been lots of water around for this Haber process to work. We think the simplest way to explain the signal in both objects is the Haber process. Comets hit the atmosphere in the presence of a lot of water and you get both nitrate and ammonia, which is what both ice cores show.”




“There’s a whole program to watch for near-Earth asteroids as they go around the sun repeatedly, and some of them have close brushes with the Earth,” said Melott. “But comets are a whole different ball game. They don’t do that circular thing. They come straight in from far, far out “” and you don’t see them coming until they push out a tail only a few years before they would enter the inner solar system. So we could be hit by a comet and only have a few years’ warning “” possibly not enough time to do anything about it.”

Image Caption: Artist’s concept of a large asteroid impacting Earth. Credit: Don Davis.

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