Europa is the 6th closest moon to Jupiter and the smallest out of the 4 Galilean Moons.
The surface of Europa is just smooth ice water. Underneath there is believed to be liquid water which is heated by heat from the core. The surface is believed to have flipped around nearly 80° and this would not have been possible if the ice was attached to solid crust. In fact, the surface actually rotates faster than the core!
There is also a thin atmosphere of Oxygen, detected by the Hubble Space Telescope. Scientists think that some aquatic life may have risen under the ice in the warmer parts.
The surface of the moon is covered in dark lines. These are believed to have been caused by underground eruptions which then crack the crust revealing the warmer layers underneath. Since the ice moves freely, it cause fractures to the surface.