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The Question

(Submitted July 18, 1997)

I want to know more about the gamma-rays that do make it through the atmosphere. Are these the same as the 'cosmic rays' that are constantly bombarding earth and have a bearing on mutation? Do any gamma-rays get through, what happens to them, are they measured down here? Would the quality of our atmosphere have any bearing on how many rays get through? I know that rays do not cause the ozone hole, but does the hole in the ozone let more rays in? Do our increased levels of CO2 in the atmosphere cause changes in the 'cosmic ray' barrier?

The Answer

Very few gamma-rays make it through the atmosphere. The atmosphere is as thick to gamma-rays as a twelve-foot thick plate of aluminum. Gamma-rays are very very unlikely to go through that much material. However, they can strike the material and produce 'secondary' particles which are more penetrating, and can go through the material.

Most of the cosmic rays which reach the Earth's surface are 'secondary cosmic rays', produced by gamma-rays or (much more commonly) 'primary cosmic rays' hitting the top of Earth's atmosphere. These primary cosmic rays are high energy particles (such are protons and the nuclei from iron atoms) moving at very close to the speed of light. These primary cosmic rays have a hard time even getting to the top of our atmosphere--the Earth's magnetic field deflects most of them away. If Earth didn't have a magnetic field, there would be many more primary cosmic rays hitting the atmosphere, and many more secondary cosmic rays hitting us.

There is a page in Imagine the Universe! about observations of the light produced when cosmic rays and gamma-rays hit the top of the atmosphere. It is at:

http://imagine.gsfc.nasa.gov/docs/science/how_l2/cerenkov.html

The cosmic rays are not very sensitive to the quality of the air (the chemical composition--how the nitrogen, oxygen, carbon and other elements in the air are joined together to make ozone, smog and other chemicals). They are more affected by the quantity of the air, because most interactions depend only on the nuclei of the atoms, and not on entire molecules. Three O2 molecules and two O3 (ozone) molecules look exactly the same to a cosmic ray. A carbon atom looks only slightly different from an oxygen or nitrogen atom, so the increased CO2 level has almost no effect. Nothing we do is likely to significantly change the number of cosmic rays hitting Earth.

David Palmer
for Ask an Astrophysicist

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