Exosphere
Full definition of exosphere
Noun
exosphere
(plural exospheres)- the uppermost layer of a planet's atmosphere
- an extremely thin atmosphere, as on Saturn's moon Dione
- "Scientists weren't even sure Dione would be big enough to hang on to an exosphere" ('Cassini Detects Hint of Fresh Air at Dione', Cassini Solstice Mission, JPL, Mar. 02, 2012)