Exosphere starts where the Thermosphere ends with thermopause. It roughly begins with the exobase. Being the upper limit of the atmosphere, it merges into space. The layer extends to about 10,000 km above the surface of the earth. Depending upon the solar activity that the region is subjected to, the lower boundary of the Exosphere ranges between 310–620 miles. Its upper boundary, on the other hand, is about 193,121 km above the earth’s surface. However, this roughly sums up to half the distance between the earth and the moon. Exobase is also referred to as ‘the critical level of escape’, as light and fast-moving molecules are known to escape into the outer space from it, unless they collide with each other. That visible part of the Exosphere from the earth is often referred to as ‘geocorona’. It is typically characterized by its luminosity and extends to about 62,000 miles from the surface of the earth.
As discussed above, Exosphere is a thin, atmosphere-like volume that surrounds a planetary body where molecules are gravitationally bound to it; but where the density is too low for them to behave as a gas by colliding with each other. Also Exosphere is the outermost layers of the atmosphere. Due to these reasons, Exosphere derives its name from an ancient Greek term ‘exo’ meaning ‘outside’ and ‘sphaira’ meaning ‘sphere’.
The Exosphere is also considered as a planetary space and hence it is almost a vacuum. The air is so thin that it does not transfer much heat to the objects in the air. Generally, scientists measure the temperature by the average speed of the molecules or atoms in a gas. The temperature is very hot when the particles are moving very fast. Temperature in the Exosphere is very fast as the particles move very quickly. However, one would not feel hot in the atmosphere even when the temperature is too high, as there are no particles in it which would heat up due to the hot air in the surrounding.
Air in the Exosphere is thin and hence Exosphere is composed of the lightest atmospheric gases, namely hydrogen, helium, carbon dioxide and atomic oxygen. These gases are found at the base of the Exosphere. As the molecular collisions are reduced because of the low gas density, these gases escape from it into space. However, among these gases, the prime components are hydrogen and helium that are only present at extremely low densities.