Exosphere

Origin

- + sphere + -

Full definition of exosphere

Noun

1. the uppermost layer of a planet's atmosphere
2. 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)