|MadSci Network: Earth Sciences|
The question could be rephrased this way: what happened to the rest of the atmosphere? This presupposes that we can figure out what the Earth's original atmosphere was like. To do that, we have comparative evidence, which includes the compositions of the atmospheres of other planets, the makeup of meteoroids and comets, which could bring atmospheric components to the Earth, and the gases that come out of the Earth's interior, via volcanoes, geysers, etc. There is also information about Earth's ancient atmosphere preserved in the rocks. The evidence suggests that Earth began with an atmosphere dominated by methane and hydrogen, with moderate amounts of water vapor, nitrogen, hydrogen sulfide, ammonia, and argon. Now, we have much more nitrogen, oxygen, and carbon dioxide, but less hydrogen, methane, hydrogen sulfide, and ammonia.
Nitrogen is not very reactive, and the diatomic molecule has great difficulty escaping the force of Earth's gravity. As a result, nitrogen tends to build up in the atmosphere. What about the other compounds? Methane is tasty to many microorganisms and it is also quite reactive in the presence of oxygen. Most of the methane is gone. Hydrogen either combined with oxygen to make water, or flew off into space, because the hydrogen molecule is extremely light. Some of the water became oceans as soon as the Earth cooled enough for liquid water to be stable at the surface. Hydrogen sulfide reacts with oxygen, as does ammonia. So, it seems that oxygen is in large part responsible for the changes in the Earth's atmosphere, yet the earliest atmosphere is not thought to have contained much oxygen. Where did the oxygen come from? Carbon dioxide is a major component of gas escaping from the Earth's interior. Plants and photosynthetic bacteria use photosynthesis to turn carbon dioxide into oxygen. The oxygen destroyed the methane, much of the hydrogen, the hydrogen sulfide, and the ammonia. If it wasn't for plants, our atmosphere would be like that of Venus: choked with carbon dioxide.
Plants are not the only sink for carbon dioxide: this chemical is one of the building blocks of the mineral calcite, which makes limestone, and most of this material is precipitated in the solid state by sea creatures, large and small, that make their shells out of the stuff. This is why the atmosphere contains far less than 1 % carbon dioxide. There is 21 % oxygen in the modern atmosphere, but why not more, when there was once a superabundance of carbon dioxide for plants to turn into oxygen and carbohydrates? Oxygen is very reactive, and animals use it for respiration, which helps explain why there isn't even more oxygen than there is in our atmosphere.
To summarize, nitrogen dominates our current atmosphere basically because it didn't fly away into space, react with other gases to form liquids or solids, or get taken up by biological systems.
It is no exaggeration to say that life has had a profound effect on Earth's atmosphere, its seas, its soils, and even its rocks down to depths as great as several miles. The whole surface of the planet has been irrevocably altered.
Much of this information is summarized in introductory historical geology textbooks, such as
Dott, R. H., Jr., and Batten., R. L., 1988, Evolution of the Earth, 4th edition, New York, McGraw-Hill Book Co., 643 p.
David Kopaska-Merkel Geological Survey of Alabama P.O. Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 FAX (205) 349-2861 www.gsa.state.al.us
Try the links in the MadSci Library for more information on Earth Sciences.