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Crumbling Comet Has NASA Scientists Looking For Crater Chains

Concentric impact rings of the Aorounga Crater in Chad. Did this location on Earth experience an encounter with a fragmented comet? Image credit: NASA/JPL
by Phil Berardelli
Washington DC (SPX) May 15, 2006
In a remote windswept area named Aorounga, in Chad, there are three craters in a row, each about 10 kilometers (6.2 miles) in diameter. "We believe this is a crater chain formed by the impact of a fragmented comet or asteroid about 400 million years ago in the Late Devonian period," said Adriana Ocampo of NASA headquarters.

Ocampo and colleagues discovered the chain in 1996. The main crater, Aorounga South, had been known for many years. It sticks out of the sand and can be seen from airplanes and satellites. A second and possibly third crater were buried, however, lying hidden until radar onboard a space shuttle mission penetrated the sandy ground, revealing their ragged outlines.

"Here on Earth, crater chains are rare," Ocampo said, but they are common in other parts of the solar system.

NASA's Voyager 1 spacecraft discovered the first crater chains. In 1979, when the probe flew past Jupiter's moon Callisto, its cameras recorded a line of craters - at least 15 of them, evenly spaced as if someone had strafed the moon with a machine gun. Eventually, Voyager found eight such chains on Callisto and three more on Ganymede.

At first, the chains presented a puzzle. Scientists wondered if they were volcanic. Had an asteroid skipped along the surface of Callisto like a stone skipping across a pond?

They solved the mystery in 1993 with the discovery of Comet Shoemaker-Levy 9. SL-9 was not a single comet, but a string of pearls - 21 cometary fragments created a year earlier when Jupiter's gravity ripped the original, single body apart.

SL-9 struck back in 1994, crashing into Jupiter. Onlookers watched titanic explosions in the giant planet's atmosphere. Scientists quickly realized what would have happened if Jupiter's surface was solid: a chain of craters.

Astronomers since have realized fragmented comets and rubble-pile asteroids are commonplace. Comets fall apart rather easily. Sunlight alone can shatter their fragile nuclei.

There also is mounting evidence many seemingly solid asteroids are assemblages of boulders, dust and rock held together by feeble gravity – such as Itokawa, the asteroid visited last November by the Japanese probe Hayabusa. When these bodies hit planets and moons, they create crater chains.

In 1994, researchers Jay Melosh and Ewen Whitaker announced they had found two crater chains on the Moon. One, on the floor of the crater Davy, is an almost-perfect line of 23 pockmarks each a few miles in diameter. This proved crater chains exist in the Earth-Moon system.

Earth tends to hide its craters, however. "Wind and rain erode them, sediments fill them in, and the tectonic recycling of Earth's crust completely obliterates them," Ocampo said. On the Moon, there are millions of well-preserved craters, but on Earth, "so far we've managed to find only about 174."

Amateur astronomer Emilio González actually used Google Earth to find other chains in March of this year. Google Earth is a digital map of the planet made of stitched-together satellite images. Using the online tool, a researcher can zoom in and out, fly around and inspect the landscape in impressive detail.

González began by calling up Kebira impact crater in Libya—the Sahara's largest. Because it was so easy to see, he said, "I decided to look around for more."

Minutes later he was using the search facility to look down on the Libya-Chad border when another crater appeared - and then another. They both had multiple rings and a central peak, the telltale splash of a high-energy impact. "It couldn't be this easy," he said.

At least one of the craters had never been cataloged before, and both lined up with the Aorounga crater, about 200 kilometers (125 miles) away. In less than 30 minutes, González had found two impact candidates and possibly had multiplied the length of the Aorounga chain.

Hours of additional searching produced no new results. "Beginner's luck," he said.

Ocampo said she doubts the new craters are related to Aorounga. "They don't appear to be the same age," she said, but added she cannot rule it out either.

"We need to do some fieldwork," she said. To prove a crater is a crater —and not, for example, a volcano — researchers must visit the site to look for signs of extraterrestrial impact such as shatter cones and other minerals forged by intense heat and pressure. Such geological study also can reveal the age of an impact site, marking it as part of a chain or an independent event.

Civil war in Chad and the possibility of war between Chad and Sudan currently are preventing scientists from mounting an expedition. So meanwhile, researchers are scrutinizing candidate chains in Missouri and Spain – although nothing new has been confirmed.

"The history of Earth is shaped by impacts," Ocampo said. "Crater chains can tell us important things about our planet."

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Palomar Movie Tracks Crumbling Comet
Palomar Mountain CA (SPX) May 14, 2006
Astronomers tracking 73P/Schwassmann-Wachmann's passage through the nearby part of the solar system have collected enough images to create a movie of the comet's disassembly.

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