An oxidizing atmosphere is a (planetary) atmosphere which oxidizes immersed (surface) compounds. It sometimes refers to an O2-rich atmosphere, for example the atmosphere of modern Earth. Most of the Earth’s atmospheric oxygen is generated by biological photosynthesis; however, oxidation can also be an effect of photochemistry initiated by stellar UV radiation. The Martian atmosphere, for instance, is an oxidizing atmosphere. Its main constituents are CO2, N2, Ar, and H2O, but the OH radicals and oxygen atoms produced by photolysis result in surface oxidation and the formation of O2, O3 and H2O2.