Saturn's Coolest #9: Rhea
Until recently I wouldn't have put Rhea, or the similar moon Dione, into a top-ten list of Saturn's interesting moons. It's a moon. It has craters and mountains that make for a wispy appearance at a distance. It's otherwise pretty well-behaved and looks kind of generic as moons go. But the Cassini orbiter found out something weird about Rhea that we can't, at this point, explain. The whole Saturn system has a certain amount of charged particles around it, basically resulting from the planet's magnetic field. The charged particles follow a known and expected pattern around each moon.
Except Rhea. When Cassini flew through the ring plane near Rhea - that is, through Rhea's orbit, above its equator - there was a drop-off in magnetic particles. Huh, the scientists said. This means there's probably a dim ring of uncharged particles around Rhea. That's pretty cool; let's take a picture the next time the orbiter goes by. So the orbiter went by again, they snapped pictures - no ring. Nothing. Just a gap in the charged particle density around Rhea.
Why? We don't know. Some kind of magnetic field around Rhea that repels the charged particles? Probably. Why and how did it get there? We don't know. But this mystery is enough to make Rhea - Saturn's second-largest moon - interesting, and earns her a spot in my top ten.