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viernes, 23 de julio de 2010

"Fresh" Crater Found in Egypt; Changes Impact Risk?


A small impact crater discovered in the Egyptian desert could change estimates for impact hazards to our planet, according to a new study.

One of the best preserved craters yet found on Earth, the Kamil crater was initially discovered in February during a survey of satellite images on Google Earth. Researchers think the crater formed within the past couple thousand years.

The Italian-Egyptian team that found the crater in pictures recently visited and studied the 147-foot-wide (45-meter-wide), 52-foot-deep (16-meter-deep) hole. The team also collected thousands of pieces of the space rock that littered the surrounding desert.

Based on their calculations, the team thinks that a 4.2-foot-wide (1.3-meter-wide) solid iron meteor weighing 11,023 to 22,046 pounds (5,000 to 10,000 kilograms) smashed into the desert—nearly intact—at speeds exceeding 2.1 miles (3.5 kilometers) a second.

There are no hard numbers for how many meteors this size might currently be on a collision course with Earth, but scientists think the potential threats could be in the tens of thousands.

Current impact models state that iron meteors around this size and mass should break into smaller chunks before impact.

Instead, the existence of the newfound crater implies that up to 35 percent of these iron giants may actually survive whole—and thus have greater destructive power.

Egypt Crater Still Shows Splatter

Estimating impact hazards to Earth isn't an exact science, since only 176 impact craters have been discovered so far, according to the Earth Impact Database, a resource maintained by the Geological Survey of Canada.

Most models are based on the number of impact craters on the moon, which has almost no atmosphere and so doesn't experience the same erosion processes as those on Earth.

"Current models predict that around a thousand to ten thousand such craters should have formed [on Earth] in one million years," said study co-author Luigi Folco, a scientist with the University of Siena in Italy.

"The reason why they are rare, however, is that, on Earth, weathering rates are high—small craters are usually easily eroded or buried."

Folco and colleagues were particularly surprised to find that the newfound, bowl-shaped crater has a prominent splatter pattern of bedrock shot up by the original impact blast.

Known as ejecta rays, these features are more often seen on other planets and moons with thin atmospheres.

The exact age of the Egyptian crater is still uncertain, the team reported this week in the online edition of the journal Science. Geologic evidence points to a relatively recent event, Folco said—although it's unlikely that any humans were around to witness the impact.

"During our field work we could see that some of the bedrock material ejected from the crater overlies prehistoric structures in the area," Folco said.

"We know from literature that the human occupation of this region ended about 5,000 years ago, with the onset of hyperarid conditions. Therefore we think that the impact occurred afterwards."

Meteor Threat Greater Than Realized

If future meteors like the Egyptian rock are more likely to remain intact, their energy on impact would be more focused, causing greater damage, said John Spray, a crater expert with the University of New Brunswick who isn't connected to the study.

Still, the probability of such a meteor hitting something critical for society, such as a major city, would be reduced, because the falling rocks would not be as spread out.

"Overall, the threat from impacts is probably greater than people realize, but historically there is very little information on this, and we just have not been collecting data for all that long," Spray said.

"Our knowledge is very limited, so events such as these are quite important for helping us understand the frequency and nature of impacts that affect our planet."

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