An impression of life on an ice moon. Some natural satellites, such as the Galilean moon Europa (of Jupiter), are covered in shells of water ice. Astronomers believe that underneath the icy crust of some of these worlds are oceans of liquid water, unable to freeze because of their high salt content and because of tidal heating from the nearby giant planet. Since water is thought to be a requirement for life, it is possible that these vast saline oceans, permanently hidden from sunlight, are teeming with life. This illustration depicts such a hypothetical scene, with the underside of the icy crust shown at the top. The lifeforms might be bioluminescent. The shafts of light in this illustration are not rays from a sun - the ice is too thick. Instead, these light rays are cast by colonies of bioluminescent lifeforms living in the icy ceiling.