From: Phil Berardelli, Science
Published August 9, 2009 10:21 AM

Do Clouds Come From Outer Space?

Most of Earth's clouds get their start in deep space. That's the surprising conclusion from a team of researchers who argue that interstellar cosmic rays collide with water molecules in our atmosphere to form overcast skies.

As common as clouds are on Earth, the processes that produce them are not well understood. Scientists think particles of dust or pollen can serve as nuclei for water droplets, which in turn gather by the trillions into clouds. That would help explain how clouds form over urban areas: Fine particles called aerosols are emitted from the exhaust pipes of millions of vehicles and work their way into the atmosphere, where they are thought to attract water molecules. But it doesn't explain how clouds formed in preindustrial society--or how they form today over vast stretches of rainforest and ocean.


That's where cosmic rays come in. The idea goes like this: High-speed cosmic ray particles--protons and neutrons of still-mysterious origins that travel at nearly the speed of light--collide with water molecules in the atmosphere, stripping away electrons from those molecules and converting them into electrically charged ions. The ions then begin attracting other water molecules, which eventually form clouds.

The theory seems to hold water in the lab. In 2006, physicist Henrik Svensmark of the Technical University of Denmark in Copenhagen and colleagues produced aerosols artificially in an atmospheric chamber by bombarding water molecules with a particle beam. "More ions resulted in more aerosols," Svensmark says.

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