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Originally published Sunday, March 6, 2005 at 12:00 AM

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Interstellar dust cloud may have iced Earth

Like an automobile passing through a dust storm, Earth's passage through a giant cloud in space could have led to global freezing that virtually...

Los Angeles Times

Like an automobile passing through a dust storm, Earth's passage through a giant cloud in space could have led to global freezing that virtually wiped out life on the planet hundreds of millions of years ago.

Two research papers published this year in Geophysical Research Letters outline a scenario in which Earth iced over after the solar system passed through a cloud of interstellar dust so large that it took as long as 500,000 years to push all the way through.

Scientists believe Earth has gone through multiple freezing periods, known as glaciations. Most theories have focused on the idea that large asteroids kicked up so much dust into the atmosphere that the sun's rays were blocked, triggering a planet-wide freeze.

But the new research looks at two other possibilities. The first proposes a thick cloud of interstellar dust so dense it screened out the sun. The second scenario, considered more likely, proposes a less-dense cloud that stripped away Earth's protective ozone layer, allowing harmful cosmic rays to reach the planet, poisoning life.

"Computer models show dramatic climate change can be caused by interstellar dust accumulating in Earth's atmosphere," said Alex Pavlov, a researcher at the University of Colorado's Laboratory for Atmospheric and Space Physics and lead author of the papers.

According to Pavlov, there is evidence that at least two glaciations covered much of Earth from 600 million to 800 million years ago, turning it into a huge snowball.

"The big mystery revolves around how they are triggered," Pavlov said.

He suggested a way to test the new theories: Analyze rocks for increased levels of uranium 235, which is not produced on Earth but is present in space clouds.

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