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Predicting Space Impacts on Earth and Their Frequency

Red_cometNew research 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. This could be a timely and important enhancement considering the disaster epic that played out on Jupiter this past summer.

In further evidence that space itself is an action movie (or at least that God watches Michael Bay movies), an explosion the size of the Pacific ocean has scarred Jupiter.  Yes, the entire ocean.  The explosion occurred on July 19 when an asteroid or comet slammed into the planet, and although Jupiter has no solid ground the gas can still get thick enough for things like "impacts" to happen.

The Jupiter impact event is another big red line underscoring Stephen Hawking's theory that one of the major factors in the possible scarcity of intelligent life in our galaxy is the high probability of an asteroid or comet colliding with inhabited planets. We have observed, Hawking pointed out in his lecture Life in the Universe, the collision of a comet, Schumacher-Levi (below), with Jupiter, which produced a series of enormous fireballs, plumes many thousands of kilometers high, hot "bubbles" of gas in the atmosphere, and large dark "scars" on the atmosphere which had lifetimes on the order of weeks. Last summer's July 19th event is a weak second place, but still totally awesome (and awesome if projected to a planet called Earth)."

0 As Stephen Hawking says, the general consensus is that any comet or asteroid greater than 20 kilometers in diameter that strikes the Earth will result in the complete annihilation of complex life - animals and higher plants. (The asteroid Vesta, for example, one of the destinations of the Dawn Mission, is the size of Arizona).

"Comet impacts might be much more frequent than we expect," says 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."

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 and his colleagues studied two possible cometary airbursts.

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 cautioned 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 Melott 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.

"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." 

Casey Kazan via EurekaAlert.


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The research is part of the Aquarius Project, which uses the largest supercomputer simulations to study the formation of galaxies like the Milky Way.

which rapidly would have been dissipated by rains following a comet strike.

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