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Vinony

exosphere

noun

  1. outermost layer of the atmosphere
L320345 on Wikidata ↗

Wiktionary

noun

Etymology: From exo- + -sphere.

  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', , JPL, Mar. 02, 2012)”