Europa is yet another extreme environment. The surface
is covered with H2O to a thickness of between 80 and 170 kilometers.
There is ample evidence pointing to an ocean but no proof of one as the
behavior of the European Crust could still be explained by slush or warm
ice.
Tidal kneading a gravitational force produces the heat
required inside Europa to maintain a liquid ocean and the volcanism that
would be required to produce hydrothermal vents. Tidal kneading occurs
because the gravitational pull of Jupiter flexes Europa like a tennis ball
generating internal heat.
Photo of a Crater Feature on Europa Courtesy of NASA
Sources: Scientific America, NASA webpage