Enceladus.
A satellite of Saturn, discovered by William Herschel in 1789. Voyager 2 returned images showing detail as fine as 2 kilometres. Large areas of the surface have no craters, and the density of craters in areas that do have them is relatively low. This is evidence that the surface of Enceladus has been completely remodelled by geological activity of some kind since it was first formed. Activity has almost certainly taken place within the last 100 million years; there is even a suggestion of current activity since the orbit of Enceladus coincides with Saturn's faint E ring. Eruptions on the satellite could be the source of the material of the ring, though there is no direct evidence.