2.5. Exosphere

The uppermost layer of the atmosphere above the thermosphere is known as the exosphere. This is the highest layer but very little is known about it. It lies beyond 400km to 1000s of kms where it merges with outer space. At such great height the density of atoms is extremely low. It

is largely home to Helium and Hydrogen. Temperature increases with height and may cross 50000C.

Stratification of atmosphere can also be done on the basis of chemical composition. According to International Space Symposium 1962, atmosphere can be divided into two broad layers, namely Homosphere and Heterosphere. Former is the lower layer and extends up to 88km from the earth’s surface. The proportions of the component gases are uniform at different levels. The three main-sub divisions of Homosphere are troposphere, stratosphere and mesosphere. Heterosphere extends beyond 88 km to more than 3500 km. Here, atmosphere is not uniform in its composition. It is also referred to as thermosphere as temperature rises with height. In this sphere, gases are arranged in roughly spherical shells. The innermost of these is a Nitrogen layer, found at heights between 100 and 200km; this is succeeded in turn by layers of Oxygen (200-1100km) and Helium (1100-3500km); and finally beyond 3500km only Hydrogen exists.

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