Iapetus

Saturn's moon Iapetus is one huge ball of ice.  Most such bodies have rocky cores, but scientists believe Iapetus is water ice through and through.

That doesn't mean Iapetus is a simple ball, however.  It's equator sports a mountain range with peaks 12 miles high.  It's also an active world, at least potentially.  The Cassini spacecraft has snapped images of "landslides"-- actually ice slides-- that seem to have occurred recently.