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ex·o·sphere  (ks-sf?r)
1. The outermost region of a planets atmosphere.
2. The outermost layer of the earths atmosphere, lying above the thermosphere and extending thousands of kilometers into space, from which molecules having sufficient velocity can escape the earths gravitation.

exo·spheric (-sf?rk, -sfr-) adj.

exosphere [ˈɛksəʊˌsfɪə]
(Earth Sciences / Physical Geography) the outermost layer of the earths atmosphere. It extends from about 400 kilometres above the earths surface

exosphere  (ks-sf?r)
The outermost region of the Earths atmosphere, beginning at an altitude of approximately 550 km to 700 km (341 to 434 mi) and merging with the interplanetary medium at around 10,000 km (6,200 mi). The exosphere consists chiefly of ionized hydrogen, which creates the geocorona by reflecting far-ultraviolet light from the Sun. On the remote edges of the exosphere, hydrogen atoms are so sparse that each cubic centimeter might contain only one atom; furthermore, the pressure and gravity are weak enough that atoms in the exosphere can escape entirely and drift into space. Artificial satellites generally orbit in this region. See also mesospherestratospherethermospheretroposphere, See illustration at atmosphere.

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