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Exosphere

The exosphere (/ˈɛksoʊsfɪər, ˈɛɡzoʊ-/; from Ancient Greek ἔξω (éxō) 'outer, outside' and -sphere) is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that, as in outer space, the molecules are essentially collision-less. In the case of bodies…

Earth's exosphere, Exosphere of other celestial bodies & Overview