Exosphere

Outermost layer of an atmosphere

The exosphere (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 with substantial atmospheres, such as Earth's atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with outer space. It is located directly above the thermosphere.

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