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The atmosphere of the earth is classified into two layers: the heterosphere and the homosphere. The heterosphere is the outermost part of the atmosphere and comprises of light gases that are distributed according to their atomic weight and gravitational pull. The gases in this region are hydrogen and helium which are very light gases.
The atmosphere between the heterosphere and the earth's surface is referred to as the homosphere. This region comprises of constant gases like oxygen, nitrogen and argon which are the major gases. Nitrogen makes up 78% of this region and it is a very improtant gas that supports life because it is a major component of protein. Oxygen makes up about 21% of the atmosphere and it is very critican for the survaval of life. Argon only makes up 0.934% of the atmsophere. It is an inert and odourless gas.
Other variable gases that exist in small propotions in the atmosphere are methane, carbon dioxide, neon and also krypto.