Composition Atmosphere and Structure

If any atmosphere was present after Earth was formed about 4.5 billion years ago it was probably much different than that of today. Most likely it resembled those of the outer planets—Jupiter, Saturn, Uranus, and Neptune—with an abundance of hydrogen, methane, and ammonia gases. The present atmosphere did not form until after this primary atmosphere was lost. One theory holds that the primary atmosphere was blasted from the earth by the sun. If the sun is like other stars of its type, it may have gone through a phase where it violently ejected material outward toward the planets. All of the inner planets, including Earth, would have lost their gaseous envelopes. A secondary atmosphere began to form when gases were released from the crust of the early Earth by volcanic activity. These gases included water vapor, carbon dioxide, nitrogen, and sulfur or sulfur compounds. Oxygen was absent from this early secondary atmosphere.

The large amount of water vapor released by the volcanos formed clouds that continually rained on the early Earth, forming the oceans. Since carbon dioxide dissolves easily in water, the new oceans gradually absorbed most of it. (Nitrogen, being unreactive, was left behind to become the most common gas in the atmosphere.) The carbon dioxide that remained began to be used by early plant life in the process of photosynthesis. Geologic evidence indicates this may have begun about two to three billion years ago, probably in an ocean or aquatic environment. Around this time, there appeared aerobic (oxygen using) bacteria and other early animal life, which consumed the products of photosynthesis and emitted CO₂. This completed the cycle for CO₂ and O₂: as long as all plant material was consumed by an oxygen breathing organism, the two gases stayed in balance. However, some plant material was inevitably lost or buried before it could be decomposed. This effectively removed carbon dioxide from the atmosphere and left a net increase in oxygen. Over the course of billions of years, a considerable excess built up this way, so that oxygen now makes up over 20% of the atmosphere (and carbon dioxide makes up less than 0.033%). All animal life thus depends on the oxygen accumulated gradually by the biosphere over the past two billion years.

Future changes to the atmosphere are difficult to predict. There is currently growing concern that human activity may be altering the atmosphere to the point that it may affect the earth's climate. This is particularly the case with carbon dioxide. When fossil fuels such as coal and oil are dug up and burned, buried carbon dioxide is released back into the air. As discussed earlier, carbon dioxide is a greenhouse gas—it acts to trap infrared (heat) energy radiated by the earth, warming up the atmosphere. What effect will this have on future temperatures? While no one has a definite answer, this is an area of active research, using computers to model the oceans, the atmosphere, and the land areas as a very complicated climate system.