Smoothest surface in the Solar System

Jupiter's large icy moon Europa has the smoothest surface of any solid body in the Solar System. The only prominent relief on its surface are ridges a few hundred metres in height. The tidal forces which cause Io’s intense volcanic activity also affect Europa. There are very few impact craters, meaning the surface is geologically young. A subsurface ocean of salty water is believed to exist on Europa, backed up by evidence in images showing what looks like broken sea ice refrozen into position.

The nature of the surface of Europa was discovered by Voyager 2, during its flyby of Jupiter in July 1979. The Hubble Space Telescope has discovered plumes of water vapour surrounding Europa, almost certainly caused by cryovolcanic activity similar to what happens on Saturn's moon Enceladus.