CHEMICAL COMPOSITION OF THE ATMOSPHERE
Up to about 90 kilometers gravitational settling causes no significant separation of the atmospheric gases by atomic weight. No separation occurs because the atomic and molecular constituents are mixed by air currents and random thermal motion. The chemical composition of the atmosphere therefore remains nearly uniform, with 77 percent nitrogen, 21 percent oxygen, nearly 1 percent argon, 0.03 percent carbon dioxide, and almost 1 percent water vapor (which varies up to several percent in the troposphere). The atmosphere has minute traces of other gases, including neon, krypton, xenon, methane, ammonia, nitrous oxide, carbon monoxide, and ozone.
Above about 90 kilometers or so the constituents are not well mixed; the heavier molecules and atoms settle toward the bottom, the lighter ones diffusing to the top. At extreme heights a rarefied layer of helium extends from about 600 to 1000 kilometers, topped by a very tenuous hydrogen layer that merges into interplanetary space.
The chemical composition of the atmosphere is not static. The present composition results from a balance between those processes that introduce a particular molecule into the atmosphere and those that remove the molecule from the atmosphere. Probably the most meaningful example is that of oxygen since it is essential for our existence.
Atmospheric oxygen is almost entirely produced in photosynthesis, primarily by green plants in shallow seas and to a lesser extend by plant life on land. A little oxygen comes from the direct dissociation of atmospheric water molecules by ultraviolet photons from the sun. Oxygen is chemically quite an active molecule, combining readily with a number of different atoms. Thus the formation of oxides in rocks removes oxygen from the atmosphere. Breathing by animal life also depletes atmospheric oxygen. If the supply of oxygen were shut off, it would take only a few tens of thousands of years to remove the major portion of oxygen now existing in the atmosphere.
The abundance of the other molecules in the atmosphere is also controlled by various "production and destruction" processes. And as in the evolution of the earth's surface, the atmosphere has also changed with time. Clearly, if atmospheric oxygen is due to the existence of life, then oxygen would not have been present prior to the emergence of life. The origin of the primitive earth's atmosphere is probably the result of outgassing by volcanoes and the escape of gases from the crust. The gaseous emission from presentday volcanoes includes water vapor, carbon dioxide, nitrogen, inert gases, and small amounts of methane, ammonia, and sulfur compounds. It is estimated that on a lifeless earth with no significant amounts of liquid water the dominant atmospheric constituent would be carbon dioxide in a very dense atmosphere. This estimate is based on the fact that a large amount of carbon dioxide is trapped in carbonate rocks on the earth's surface. Carbon dioxide is the main component of the atmospheres of Venus and Mars, with the Venusian atmosphere's being some hundred times denser that ours is. About 2 billion years ago the transition began to an oxygen-nitrogen atmosphere. The amount of oxygen grew from a trace to the present 21 percent as a result of the development of oxygenproducing photosynthesis by green plants.
