Atmospheric Layers

The atmosphere is the gaseous outer portion of a planet. Atmospheres have been detected around all planets and many satellites. Some are very dense, and blend gradually into fluid envelopes which contain the bulk of the planet's mass, while others are extremely tenuous. The compositions vary from the hydrogen and helium dominated atmospheres of the gas giants to those found around the terrestrial planets and satellites, with substantial fractions of nitrogen, carbon dioxide, or sulfur dioxide. Trace elements, such as methane or ammonia, can be a negligible mass fraction of an atmosphere and yet play a substantial role in our observations of the atmosphere. All atmospheres are controlled by the same physical processes, and utilize a similar photochemistry. Most feature condensation and clouds, the upper layers are typically strongly modified by solar radiation, and variations in temperature and pressure lead to winds. We state below the basic layers of an atmosphere.

  • Thermosphere: a thermal classification of the atmosphere. In the thermosphere, temperature increases with altitude due to the strong incident UV solar flux. It includes the exosphere and part of the ionosphere. On Earth, the temperature rises to 1000K at 500 km, and is isothermal above this level.

  • Exosphere: the outermost layer of the Earth's atmosphere, ranging from 640 km to about 1,280 km high. The lower boundary of the exosphere is called the critical level of escape, where atmospheric pressure is very low (the gas atoms are very widely spaced, collisions are infrequent) and the temperature is very low.

  • Exobase: the lower boundary of the exosphere, located at 500 km above the surface on Earth. At this altitude, the product of the collisional cross-section and the integrated atmospheric number density is equivalent to one mean free path length (for a fast atom).

  • Ionosphere: starts at about 75 km high and continues for hundreds of km. It contains many ions, generated by the solar flux, and a plasma of free electrons.

  • Mesospause: the transition layer between the ionosphere and the mesosphere. On Earth and on Titan, the mesopause is a temperature minimum.

  • Mesosphere: usually an almost isothermal region. On Earth it is characterized by temperatures that quickly decrease as height increases, extending from 17 to 80 km above the surface.

  • Stratopause: the transition layer between the mesosphere and the stratosphere; a temperature maximum.

  • Stratosphere: characterized by a slight temperature increase with altitude and the absence of clouds. It extends between 17 to 50 km above the earth's surface. The Earth's ozone layer is located in the stratosphere; this layer absorbs a lot of ultraviolet solar energy. Only the highest clouds (cirrus, cirrostratus, and cirrocumulus) are found in the lower stratosphere.

  • Tropopause: the transition layer between the troposphere and the stratosphere; where the temperature structure becomes inverted. It is characterized by little or no change in temperature as altitude increases (i.e. a temperature minimum).

  • Troposphere: the lowest region in the Earth's (or any planet's) atmosphere. On the Earth, it goes from ground (or water) level up to about 17 km high. Weather and clouds form from trace elements of condensable gases in the troposphere, and the temperature generally decreases as altitude increases.