the outermost portion of a planet's, esp. the earth's, atmosphere, consisting of a hot layer of light atoms often moving at escape velocity
Origin of exosphereexo- + sphere
- The outermost region of a planet's atmosphere.
- The outermost region of the earth's atmosphere, lying above the thermosphere and extending thousands of kilometers into space, from which molecules having sufficient velocity can escape the earth's gravitation.
OriginSee also: exosphère
exo- + -sphere