Exosphere

The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite. Its molecules are gravitationally bound to that body. The density is so low that the molecules are essentially collisionless.Steigerwald, William 2015. NASA's LADEE Spacecraft Finds Neon in Lunar Atmosphere. NASA. In the case of bodies with substantial atmospheres, such as Earth's atmosphere, the exosphere is the uppermost layer,

The lightest atmospheric gases such as hydrogen and helium exist throughout the exosphere. Heavier ones like carbon dioxide and atomic oxygen are near the base. There are two borders to the exosphere; the lower boundary and the upper boundary. The lower boundary is about 500-800 km (300-500 miles) above the surface and is called the exobase.  It is also called the thermopause because it is in between the exosphere and the thermosphere. The molecules are moving very fast, so the temperature in this zone is thought to be thousands of degrees celsius.

 References 

Category:Layers of Earth's atmosphere