EUROPA
Europa is Jupiter’s fourth-largest moon and the smoothest of all the celestial bodies. There are almost no craters, and despite a dense network of cracks and ridges covering this moon, none are higher or deeper than a few hundred metres. This suggests that Europa’s surface is geologically young and possibly floating on a liquid mantle. The Hubble Space Telescope has also spotted plumes of water vapour spewing 200 kilometres (124 miles) into the air from the south pole. This lends weight to the idea that Europa has a subsurface saltwater ocean covered by a layer of ice that may be just a few kilometres thick in places.