The exosphere (Ancient Greek: ἔξω éxō "outside, external, beyond", Ancient Greek: σφαῖρα sphaĩra "sphere") is a thin, atmosphere-like volume surrounding a planetary body where molecules are gravitationally bound to that body, but where the density is too low for them to behave as a gas by colliding with each other. In the case of bodies with substantial atmospheres, such as the Earth's atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with interplanetary space. It is located directly above the Thermosphere.
Several moons, such as Earth's moon and the Galilean satellites, have exospheres without a denser atmosphere underneath. Here molecules are ejected from surface rocks and follow independent parabolic trajectories until they collide with the surface. Authors differ as to whether such moons are considered to have atmospheres or not. Smaller bodies such as asteroids, in which the molecules emitted from the surface escape to space, are not considered to have exospheres.
Earth's exosphere 
The main gases within the Earth's exosphere are the lightest atmospheric gases, mainly hydrogen, with some helium, carbon dioxide, and atomic oxygen near the base of the exosphere. Since there is no clear boundary between outer space and the exosphere, the exosphere is sometimes considered a part of outer space.
Lower boundary 
The lower boundary of the exosphere is known as exobase; it is also called the thermopause, as in Earth's atmosphere the atmospheric temperature becomes nearly a constant above this altitude. Before the term exobase was established the boundary was also called the critical level where barometric conditions no longer apply. The altitude of the exobase ranges from about 500 to 1,000 kilometres (310 to 620 mi) depending on solar activity.
The exobase can defined in one of two ways: .
If we define the exobase as the height at which upward traveling molecules experience one collision on average, then at this position the mean free path of a molecule is equal to one pressure scale height. This is shown in the following. Consider a volume of air, with horizontal area and height equal to the mean free path , at pressure and temperature . For an ideal gas, the number of molecules contained in it is:
where is the universal gas constant. From the requirement that each molecule traveling upward undergoes on average one collision, the pressure is:
where is the mean molecular mass of the gas. Solving these two equations gives:
which is the equation for the pressure scale height. As the pressure scale height is almost equal to the density scale height of the primary constituent, and since the Knudsen number is the ratio of mean free path and typical density fluctuation scale, this means that the exobase lies in the region where .
The fluctuation in the height of the exobase is important because this provides atmospheric drag on satellites, eventually causing them to fall from orbit if no action is taken to maintain the orbit.
Upper boundary 
In principle, the exosphere covers all distances where particles are still gravitationally bound to Earth, i.e. particles still have ballistic orbits that will take them back towards Earth. Theoretically, the upper boundary of the exosphere can be defined as the distance at which the influence of solar radiation pressure on atomic hydrogen exceeds that of the Earth’s gravitational pull. This happens at half the distance to the Moon (190,000 kilometres (120,000 mi)). The exosphere observable from space as the geocorona is seen to extend to at least 100,000 kilometres (62,000 mi) from the surface of the Earth. The exosphere is a transitional zone between Earth’s atmosphere and interplanetary space.
- Bauer & Lammer, Planetary Aeronomy: Atmosphere Environments in Planetary Systems, Springer, 2004.
- Gerd W. Prolss: Physics of the Earth's Space Environment: An Introduction. ISBN 3-540-21426-7