Europa is roughly the size of the Moon and is 85% water by weight. An icy crust reflects most of the light falling on the satellite. In 1979, the Voyager spacecraft flew past Europa and discovered a series of cracks and glacier ripples (see picture) on the surface. This led astronomers to believe liquid water existed under this icy crust. Scientists speculate that tidal effects caused by its close proximity to Jupiter allows liquid water to exist under the surface.

By studying the types of salts seen on Europa scientists can put constraints on the 'saltiness' of the interior ocean, and whether life can be supported by it. On the Earth hydrated salts - such as those discovered on Europa - are formed by salt water evaporating to form white salt crystals. The salts themselves usually come from volcanic activity. On Europa, the salts seen on the surface are formed by flash evaporation, freezing and sublimation as water pushes out of the interior through cracks on the surface and into the vacuum of space. Unlike their white counterparts on Earth, the salts on Europa are red and yellow.