Dione

The Cassini spacecraft has detected a thin atmosphere that includes oxygen and ozone on Saturn's moon, Dione – see www.jpl.nasa.gov/news/news.cfm?release=2012-056&cid=release_2012-056. Admittedly, it is a faint atmosphere but enough to get news of it published in Geophysical Research Letters. Oxygen, it seems, is common in the Saturn system, already detected on the moon Rhea and in the rings around the planet. Dione's oxygen is thought to derive from either solar photons or energetic particles from space bombarding it's water/ice surface and liberating oxygen molecules. A similar argument m ight perhaps be made for an electrical charge.