Sentence:  "Scientists weren't even sure Dione would be big enough to hang on to an<WSD> exosphere</WSD>" ('Cassini Detects Hint of Fresh Air at Dione', Cassini Solstice Mission, JPL, Mar. 02, 2012) 
Meaning of the word marked with <WSD> tags:  an extremely thin atmosphere, as on Saturn's moon Dione