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Source of cosmic dust found

By Bjorn Carey
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( -- Researchers believe they have identified the main source of the cosmic dust that gets dumped on Earth -- meteoroids. A new study shows that grains of dust left in meteoroid trails are larger than previously thought.

Meteoroids are chunks of rocky debris -- ranging in size from a grain of sand to a boulder -- that zip through the solar system. Some make their way to Earth and buzz through the atmosphere, leaving fiery streaks known as shooting stars, along with clouds of dust particles.

For a long time, scientists thought that these particles were just a few nanometers in size. Now, Andrew Klekociuk of the Australian Antarctic Division and his colleagues have studied the dust cloud from a recent large meteoroid and determined that the dust particles are actually 10 to 20 micrometers -- a thousand times larger than previously thought.

On September 3, 2004 a large meteor -- estimated to have an original mass of a million kilograms -- crashed through the Earth's atmosphere, releasing more energy than a 15 kiloton nuclear bomb.

Not only did this meteor leave bright streaks of fire as it tore through the atmosphere, it left behind a trail of meteoritic 'smoke' or dust. The dust cloud stretched across altitudes from 35 to 11 miles (56 to 18 kilometers) and hung in the sky for weeks.

"There is some uncertainty in the total influx of meteoric matter, but it is probably on the order of 40 +/- 20 thousand tons per year," Klekociuk told "Our event represented a mass influx of about one thousand tons."

To determine the size of the particles, Klekociuk and his colleagues used an atmospheric Light Detection and Ranging (LIDAR) instrument, which is basically an optical version of radar.

Using the LIDAR, they measured the size and shape of the dust particles by determining how the particles scattered and absorbed polarized light from a laser beam. These observations were matched up to theoretical models for particles of different sizes, shapes, and compositions, said Klekociuk.

This research is detailed in the August 25 issue of the journal Nature.

Previously, it was thought that large dust particles from meteoroids were only formed as the particles in the cloud coagulate over a period of weeks, eventually settling on the Earth. However, in the case of the meteoroid studied, the dust particles were large from the beginning.

"In the case of our object, most of the mass of the remnant particles were in a size range that is unlikely to have formed from the normal way in which the condensation and coagulation process have been viewed," Klekociuk said.

Depending on their size and overall number, cosmic dust and other particles in the atmosphere have the potential to change Earth's climate. They can reflect sunlight, which cools the Earth, absorb sunlight, which warms the atmosphere, and act as a blanket for the planet by trapping any heat it gives off. They can also facilitate the formation of rain clouds.

However, it's unlikely that dust from this 2004 event caused any of this to happen, at least for longer than a few weeks.

"It is likely that the dust from our event, by virtue of its size, had a residence time in the stratosphere of a few weeks," Klekociuk said. "After that the particles would have drifted down to the troposphere and may have been 'washed out' by rain."

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