Blasts from the Past
A new investigation reveals a promising new method of detecting past comet strikes upon Earth and gauging their frequency.
Aired December 20, 2009
2 minutes 3.7 MB) | Download mp3
Scientists probe the rate of comet strikes upon the Earth. From the University of Kansas, this is Research Matters. I'm Brendan Lynch.
It's the stuff of a Hollywood disaster epic: A comet plunges from outer space, splitting the sky with a devastating shockwave. But this isn't a disaster movie plotline, as Adrian Melott, KU professor of physics and astronomy, explains.
Adrian Melott: Comet impacts might be much more frequent than we suspect. 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 large impacts on the Earth.
Now, an investigation by Melott and colleagues reveals a promising new way of detecting past comet strikes and gauging their frequency. The research shows a signature of nitrate and ammonia to be found in ice cores tied to suspected impacts.
Adrian Melott: Now we have another potential marker for extraterrestrial events in ice. You don't just look for nitrates, you also look for ammonia.
Melott studied two possible comet strikes. One, in 1908 is known as the Tunguska Event. The other happened 13,000 years earlier and is known as the Yonger Dryas event. In both, Melott's group examined ice cores and found evidence that the Haber process - whereby a nitrogen fixation reaction makes ammonia - may have occurred on a large scale.
Adrian Melott: A comet entering the atmosphere makes a big shockwave with high pressure - 6,000 times the pressure of air. 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. 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.
For more on comet strikes, log onto Research Matters dot K-U dot E-D-U. For the University of Kansas, I'm Brendan Lynch.