The
exosphere ( "outside, external, beyond", "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
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.