Dione is a mere 700 miles across. The Earth, by comparison, is 7926 miles across. Despite this, one scientists said of the moon:
"The concentration of oxygen in Dione's atmosphere is roughly similar to what you would find in Earth's atmosphere at an altitude of about 300 miles. It's not enough to sustain life, but — together with similar observations of other moons around Saturn and Jupiter — these are definitive examples of a process by which a lot of oxygen can be produced in icy celestial bodies that are bombarded by charged particles or photons from the Sun or whatever light source happens to be nearby."