Kansas scientists probe mysterious possible comet strikes on Earth

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.

" 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."

Melott studied two possible cometary airbursts with Brian Thomas, assistant professor of physics and astronomy at Washburn University, Gisela Dreschhoff, KU adjunct associate professor of physics and astronomy, and Carey Johnson, KU professor of chemistry.

In June 1908, a puzzling explosion rocked central Siberia in Russia; it came to be known as the "Tunguska event." A later expedition found that 20 miles of trees had been knocked down and set alight by the blast. Today, scientists have coalesced around the idea that Tunguska's devastation was caused by a 100-foot asteroid that had entered Earth's atmosphere, causing an airburst.

Some 13,000 years earlier, an occurrence thought by some researchers to be an extraterrestrial impact set off cooler weather and large-scale extinctions in North America. The "Younger Dryas event," as it is known, coincided with the end of the prehistoric Clovis culture.

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."

Melott cautions that the results are inconclusive because the ice cores are sampled at five-year intervals only, not sufficient resolution to pinpoint peaks of atmospheric nitrates and ammonia, which rapidly would have been dissipated by rains following a comet strike.

But the KU researcher contends that ammonia enhancement resulting from the Haber process could serve as a useful marker for detecting possible comet impacts. He encourages more sampling and analysis of ice cores to see where the nitrate-ammonia signal might line up with suspected cometary collisions with the Earth.

Such information could help humankind more accurately gauge the danger of a comet hitting the Earth in the future.

"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."

Source: University of Kansas

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Dec 14, 2009
I can't say anything about the "Younger Dryas event", but even majority of those who think that Tunguska was caused by a space-impact have ruled out "a comet" (there are many reasons why they had to rule out). So,...

Andrei Ol'khovatov

Dec 14, 2009
As Carl Sagan said: "extraordinary claims require extraordinary evidence". The theory is interesting, but it will take decades of collecting other possible comet-impact "proxies" and comparing them to determine if it is correct.

Dec 14, 2009
It seems we still suffer from a few quaint Victorian assumptions concerning our planet, its climate, and how we interact with the cosmos. There are large gaps in understanding how prehistoric development of humans was influenced by cosmic events. One of the greatest mysteries of prehistoric human civilization is the apparently global culture that existed before some great water disaster destroyed and dispersed it.

A ocean impact by a large enough comet would create waves large enough to destroy every coastal civilization on earth, and it may be this event that created both the widespread flood myths and the odd climate variations seen in the younger dryas.

Dec 15, 2009
Rather than AGW, NEO's is where we should be spending our money to protect the earth from instant disaster.

Dec 15, 2009
Andrei: The article does say that it is believed that Tunguska "was caused by a 100-foot asteroid" in line with the recent simulations, and the most recent expeditions to gather evidence. It would seem that he included the event in this article because the previous thinking was focussed on a comet as the explosive material. Perhaps they should have used the 6th century tree growth paralysis as a better example, (Prof. Mike Bailie)

Dec 15, 2009
Birgir: That quote is becoming extra ordinary on PhysOrg ;-)

After the recent comet materials research I expect that better markers for comet related events will be found and will (sooner rather than later, I hope) prove to be as valuable as iridium is for identifiying terrestrial and suboceanic asteroid cratering.

See also: http://www.physor...250.html

Dec 15, 2009
Arkaleus: There is plenty of circumstancial evidence supporting the flood stories from ancient cultures, and those used in moderm doctrines, but it may turn out that more than one event actually ocurred to produce similar stories that are told by different cultures all over the globe today.

Ref: Shiva Crater (see the other craters that Chatterlee is investigating, some he says may relate to the biblical/hindu flood sagas)

See also: http://www.physor...629.html

Dec 15, 2009
Nartoon: I know I am going to regret sharing this, but here goes ... NEO activity may well help the AGW case if it can be shown that previous warm and cold spells can be related to NEO impacts.

According to Prof. Mike Bailie, a comet may (yes, it's still a very big MAY) have exploded in the atmosphere causing a cloud of materials that shaded much of the Earth's surface from sunshine for a period between two and ten years.

The term 'Dark Ages' has been thought of as part of the 'Middle Ages' centuries by historians, but it may have been a very short but acute period of hardship followed by a slow recovery which has been documented in a very haphazard manner (in Europe) causing much confusion and doubt since.

Also, I think you may want to double-check the investment being made on Wide Field Surveys.

The more recent telescopes are designed to keep an eye out for NEO's, by tracking light/radiation from "smaller" cold objects within our solar system. Ref: VISTA, WISE

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