Europa is Jupiter’s fourth largest moon and the sixth largest moon in the solar system. Based on observations of its smooth and icy surface, it is hypothesized that there could be an underground ocean of liquid water which may harbor life. At this distance from the sun it would be unlikely that any liquid water would form, but because of Europa’s slightly elliptical orbit around the very massive Jupiter, it is possible that tidal friction could occur and generate enough energy to melt ice below the surface. This is supported by the large craters on the surface which are filled with relatively fresh water ice and the geological activity of the planet, which is similar to Earth’s plate tectonics.
If there is in fact a sub-surface ocean on Europa, one hypothesis proposes that the ocean there is approximately twice the volume of Earth’s oceans and several kilometers below the surface ice. Another hypothesis proposes that the surface ice is even thinner. Astrobiologists are particularly interested in it because of the possibility of extraterrestrial life.
(images source: solarsystem.nasa.gov, www.jsg.utexas.edu)