Cosmic rays sow the seeds of cloud growth.

The lowly raindrop that splatters on the ground may have an origin far loftier than the clouds. According to a new atmospheric theory, cosmic rays streaming in from distant reaches of our galaxy help form one of the necessary ingredients of cloud droplets.

Cosmic rays crash through the lower layers of the atmosphere like tiny tractor-trailers with broken brakes. Moving at close to the speed of light, these nuclear fragments smash into air molecules hard enough to knock electrons loose. This well-documented process creates negatively and positively charged ions.

Atmospheric researchers have often ignored the role of such ions because they are relatively rare. The new theory, however, suggests that ions play a central role in creating aerosols--the minute but important airborne particles that can serve as the cores of growing cloud droplets.

Richard P. Turco of the University of California, Los Angeles and his colleagues present this idea in the March 1 Geophysical Research Letters.

"If this mechanism works, and I think it does, it would be a major source of aerosols in Earth's atmosphere," says Turco.

Many aerosols are liquid droplets containing dissolved sea salt, sulfuric acid, organic molecules from trees, and other compounds. Over the continents, dust and soot are also aerosols. Both liquid and solid specks help clouds develop by encouraging the condensation of water vapor, which does not occur readily without an original seed particle of some sort in the air.

Despite the importance of aerosols, scientists remain uncertain about how they develop.

Turco and his coworkers suggest that ions created by cosmic rays form the nucleus of many aerosol particles. In this process, water with an extra proton and other positive ions combine with neutral molecules to make small clusters. Similar clusters grow around negative ions, such as a derivative of sulfuric acid. Soon, the negative and positive clusters clump to form a larger particle stable enough to persist as an aerosol.

The standard theory of aerosol formation holds that clusters form through neutral molecules coming together. In contrast, Turco's group argues that in many cases, the ion idea more accurately reflects aerosols observed in nature and in the laboratory.

Ion clusters grow in a wide range of conditions, whereas neutral molecules won't form clusters until the right circumstances arise, at which time a burst of aerosols develops. The researchers cite measurements showing that aerosols do not typically form in great bursts.

David S. Covert, an atmospheric chemist at the University of Washington in Seattle, says that the ion mechanism may play an important role in aerosol formation. "I'm sure it's significant, but I can't say whether it's the dominant process."

Covert notes that other research groups have also proposed new mechanisms for aerosol formation. The outstanding questions hamper climate predictions because aerosols help set Earth's temperature by absorbing and reflecting radiation. To judge the merits of these competing aerosol ideas, scientists must both repair to their labs and take to the skies.