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Composition Atmosphere and Structure

The Past And Future Of The Atmosphere

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.



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James Marti


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Infrared radiation

—Radiation similar to visible light but of slightly longer wavelength. We sense infrared radiation as heat.


—Region of the atmosphere above about 48 mi (80 km) with elevated concentrations of charged atoms and molecules (ions).

Lapse rate

—The rate at which the atmosphere cools with increasing altitude.


—The third layer of the atmosphere, lying about 30–48 mi (50–80 km) in height and characterized by small lapse rate.

Ozone hole

—The sharp decease in stratospheric ozone over Antarctica that occurs every spring.


—A layer of the upper atmosphere above an altitude of 5–10.6 mi (8–17 km) and extending to about 31 mi (50 km), depending on season and latitude. Within the stratosphere, air temperature changes little with altitude, and there are few convective air currents.


—The top layer of the atmosphere, starting at about 48 mi (80 km) and stretching up hundreds of miles into space. Due to bombardment by very energetic solar radiation, this layer can possess very high gas temperatures.


—The layer of air up to 15 mi (24 km) above the surface of the Earth, also known as the lower atmosphere.

Ultraviolet radiation

—Radiation similar to visible light but of shorter wavelength, and thus higher energy.

X-ray radiation

—Light radiation with wavelengths shorter than the shortest ultraviolet; very energetic and harmful to living organisms.

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Science EncyclopediaScience & Philosophy: A-series and B-series to Ballistic Missiles - Categories Of Ballistic MissileComposition Atmosphere and Structure - Atmospheric Structure, The Past And Future Of The Atmosphere - Composition of the atmosphere