High-energy radiation from deep space may be burning a hole in our ozone layer. A new study appearing in the August 13 Physical Review Letters finds a correlation between so-called cosmic rays and ozone depletion and shows with experiments how cosmic rays could destroy ozone. The results may help atmospheric scientists improve their models of ozone depletion.

For 20 years, scientists have recognized that active chlorine molecules from man-made chlorofluorocarbons (CFCs) destroy ozone. Conventional wisdom holds that ultraviolet light from the sun releases the active chlorine from the CFCs. That theory, however, has a problem: during the polar springtime, when ozone depletion is greatest, giant clouds of ice block the sun's ultraviolet light. In recent years, researchers have theorized that these clouds could house CFCs and that some process deep inside them breaks CFCs down into active chlorine. But how this process could happen without ultraviolet light has remained a mystery.

Now Leon Sanche and Qing-Bin Lu of the University of Sherbrooke in Canada think they know what's causing the release of active chlorine. The two believe that cosmic rays from deep space are penetrating the clouds and knocking loose electrons. The electrons interact with the CFCs to liberate the active chlorine molecules. Those molecules, according to Sanche and Lu, can reside inside an icy polar cloud until springtime, when it dissolves and releases them into the atmosphere.

In support of their theory, Sanche and Lu found a strong relationship between cosmic ray ionization and ozone depletion in data taken by ground stations, balloons and satellites. They also simulated a polluted polar cloud in the laboratory and bombarded it with electrons like those released by cosmic rays. Their results show that electrons are about a million times more likely to interact inside the cloud than anyone previously believed.

"What [Sanche and Lu] observed is extremely interesting," says Rob Compton of the University of Tennessee. Indeed, Compton believes that the duo's results may force atmospheric scientists to revise their models of ozone depletion.