The layers of Earth's atmosphere.
The atmosphere seen from space.
The composition of Earth's atmosphere at sea-level is approximately 78% nitrogen, 21% oxygen, 0.9% argon, and 0.04% carbon dioxide, with variable small amounts of water vapor and pollutants such as sulfur dioxide and carbon monoxide. Variations in temperature define the various layers of the atmosphere, which include, in ascending order, the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere.
Troposphere and stratosphere
The troposphere extends to a height of about 8 km at the poles and 18 km near the equator, and accounts for three-quarters of the atmospheric mass. At the top of the troposphere, where the temperature falls to -60°C, is the tropopause and, above this, the stratosphere. Temperatures within the stratosphere, where there is no vertical air movement, are at first steady and then rise to about 0°C at an altitude of about 50 kilometers. The heating within the stratosphere comes from the absorption of ultraviolet radiation from the Sun by molecules of ozone.
Above the stratosphere
Beyond the stratosphere is the mesosphere in which the temperature once again falls since ozone is less plentiful. At the top of the mesosphere, at a height of about 85 kilometers, the temperature is -90°C. Above lies the thermosphere in which oxygen and nitrogen absorb solar ultraviolet causing the temperature to rise to about 1,300°C at a height of 500 kilometers. The heating effect of this high temperature, however, is negligible since the density at such altitudes is only one million millionth that at sea level. "Shooting stars" (trails of incinerating meteors) and aurorae are produced in this region. Finally, beyond the thermosphere, is the exosphere which contains the Van Allen radiation belts, extends into Earth's magnetosphere, and merges with the near-vacuum of interplanetary space.
Evolution of Earth's atmosphere
The earliest terrestrial atmosphere probably formed from the vaporization of volatile materials in the outer layers of Earth's crust and was gradually altered by a number of factors, including the later presence of life, until it acquired its present composition (see Earth, early history).