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.
|The layers of Earth's atmosphere
|The atmosphere seen from space
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 km. 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 km, 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 km. 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,
PHENOMENA AND STRUCTURES