Earth's atmosphere contains about 78 percent nitrogen and 20.9 percent oxygen—but it was not always that way. When the earth first formed some 4.65 billion years ago, it was surrounded by gases rich in hydrogen compounds, ammonia, methane, and water—a composition similar to that of the nebula from which it was born. How the earth got its oxygen has been a continuing controversy; it is now believed that it is not a remnant of the primitive atmosphere. Rather, it is now thought that gases, water vapor, and carbon dioxide, released from the interior of the earth about 4.65 billion years ago, heated up as a result of meteoric collisions and radioactive decay, replacing the primitive atmosphere.
In addition, two processes allowed oxygen-rich conditions to evolve: (1) the breakup of water molecules into hydrogen and oxygen in the upper atmosphere by ultraviolet radiation from the sun, and (2) photosynthesis, after green plants evolved. Sediments formed prior to 2.5 billion years ago were deposited under oxygen-poor conditions, but those deposited later indicate that oxygen molecules began to increase in concentration rapidly after about 2 billion years ago, to form an oxygen-rich atmosphere. Life and the atmosphere evolved together as oxygen molecules increased in number; small amounts high in the atmosphere were split into oxygen atoms by solar ultraviolet radiation, which then combined with oxygen molecules (O2) to form ozone (O3). The ozone layer itself absorbs ultraviolet radiation, which is lethal to life on earth. Apparently as a result of its formation, the ozone layer allowed more complex forms of life to evolve on earth. Oxygen content reached a critical level about 700 million years ago as life began to rapidly evolve into the great variety of life forms characteristic of the earth today.
See also: Atmosphere; Atmospheric Cycles; Climatology
Berner, Elizabeth K., and Robert A. Berner. 1996. Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, NJ: Prentice Hall; Laing, David. 1991. The Earth System: An Introduction to Earth Science. Dubuque, IA: Wm. C. Brown; Lutgens, Frederick K., and Edward J. Tarbuck. 2001. The Atmosphere: An Introduction To Meteorology, 8th ed. Upper Saddle River, NJ: Prentice Hall.
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