Rhea

"'The second-largest of Saturn's moons, Rhea is also the second-smallest body in the solar system to have reached hydrostatic equilibrium, which among other things means that it has a differentiated core.""Though often second-place, Rhea does have one potential claim to being absolutely unique in the solar system: several readings from the Cassini spacecraft in 2005 have indicated that Rhea might have its own system of rings, which would make it the only known moon to do so. Only future exploration, and the first-hand reports of your explorers, will be able to confirm this romantic idea.'"