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The Question

(Submitted January 05, 1997)

Is it possible that the primary cause of ozone depletion are cosmic rays as it enters the atmosphere? And the big "hole" of the ozone layer at Antarctica is the result of the geomagnetic effect on cosmic rays, even though they are isotropic?

The Answer

The ozone layer is the result of an equilibrium between the creation and destruction of ozone by UV light (UV creates ozone by ionizing free oxygen atoms which then combine with oxygen molecules to create ozone). Cosmic rays have been studied since the 1930s and we have no reason to believe that cosmic rays have not always existed. So if cosmic rays destroyed ozone at a significant level, then there would be no ozone layer (or at least a layer significantly smaller than the UV-only interaction picture would predict). More specifically, cosmic rays are made up of high energy particles and photons, which only react well with free electrons and dense material with high atomic numbers (note that high-Z atoms have a lot of electrons which are effectively "free" given the high energy of the cosmic rays). So in order for cosmic rays to have been significant in the creation of ozone holes, the flux of cosmic rays would have had to have increased by orders of magnitude, which has not occurred (to my knowledge).

Andy Ptak
for the Ask an Astrophysicist team

Note added in 2001 August: It is thought that man-made chlorofluorocarbons (CFCs), such as Freon, are the major destroyer of the ozone layer. The prevailing theory is that ultraviolet light breaks down CFCs, releasing active chlorine, which destroys ozone molecules. However, a recent study (http://focus.aps.org/story/v8/st8) suggests that cosmic rays may also break down CFCs. If this is correct, cosmic rays do play a part in creating ozone holes, but only because there are man-made CFC molecules for them to break down.

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