Exosphere

Theexosphereis a thin, atmosphere-likevolumesurrounding a planet or natural satellite. Its molecules are gravitationally bound to that body. The density is so low that the molecules are essentially collisionless.In the case of bodies with substantial atmospheres, such as Earth's atmosphere, the exosphere is the uppermost layer,

The lightest atmospheric gases such ashydrogenandheliumexist throughout the exosphere. Heavier ones likecarbon dioxideand atomicoxygenare 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 thethermopausebecause it is in between the exosphere and thethermosphere.The molecules are moving very fast, so the temperature in this zone is thought to be thousands of degrees celsius.