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see atmosphereatmosphere
[Gr.,=sphere of air], the mixture of gases surrounding a celestial body with sufficient gravity to maintain it. Although some details about the atmospheres of other planets and satellites are known, only the earth's atmosphere has been well studied, the science of
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(eks -ŏ-sfeer) See atmospheric layers.



the outer and most rarefied layer of the atmosphere, where the mean free path of the particles is so large that they can be dispersed (diffused) into interplanetary space. The mass of the “air” in the exosphere is close to 10–10 times the mass of the atmosphere. Light gases, such as H and He, are the most quickly dispersed.

The exosphere begins at altitudes of 450 to 800 km, and its upper boundary is several thousand km above the earth’s surface, where the concentration of particles is the same as in interplanetary space. The exosphere consists of ionized gases (plasma), and the ratio of charged particles to neutral particles is close to unity where it begins; in the upper half of the exosphere, the air is almost completely ionized. The lower and middle parts of the exosphere are composed mainly of O and N atoms; with increasing altitude, the relative concentration of light gases, especially ionized hydrogen, grows rapidly. The gas kinetic temperature of the exosphere, which increases somewhat with altitude, is 1500° to 3000° K. An increase in solar activity warms the exosphere and increases its thickness. The magnetic field of the earth, which has an intensity of 0.3 oersted in the lower part of the exosphere and 10–2 to 10–3 oersted at its upper boundary, strongly affects the physical processes within it. The radiation belts of the earth are located for the most part in the exosphere.


Khvostikov, I. A. Fizika ozonosfery i ionosfery. Moscow, 1963.
Risbeth, H., and O. K. Garriott. Vvedenie v fiziku ionosfery. Leningrad, 1975. (Translated from English.)
Akasofu, S. I., and S. Chapmen. Solnechno-zemnaia fizika, part 1. Moscow, 1974. (Translated from English.)



An outermost region of the atmosphere, estimated at 300-600 miles (500-1000 kilometers), where the density is so low that the mean free path of particles depends upon their direction with respect to the local vertical, being greatest for upward-traveling particles. Also known as region of escape.


exosphereclick for a larger image
The upper part of the thermosphere. It is the topmost part of the atmosphere and is approximately 300 to 600 miles (500–1000 km) above the earth's surface. In this region, the atmosphere is extremely tenuous because of high temperature and low density; some of the atoms and molecules escape the earth's gravitational field.