• The outermost region of a planet's atmosphere.
  • The outermost region of the earth's atmosphere, lying above the thermosphere and extending thousands of kilometers into space, from which molecules having sufficient velocity can escape the earth's gravitation.
  • the <xref>uppermost</xref> layer of a planet's <xref>atmosphere</xref>
  • an extremely thin atmosphere, as on Saturn's moon <xref>Dione</xref>
  • the outermost atmospheric layer
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