Jupiter is 5 times farther from the Sun than we are, so it and its moons only receive 1/(5*5) = 4% of the sunlight that we do. Thus the surface of Europa is very cold, and covered with ice several miles thick. However, Europa is warmed from the inside out by friction from gravitational tidal forces. (These are the same forces by which the Moon causes tides on Earth, but on Europa they work a bit differently.)

The gravitational pull of an object increases as one gets closer to it, so on the Moon-facing side of Earth the Moon pulls most strongly, and it hauls up a big mound of water: a tide. In order for our tides to cause friction, the difference in the Moon's pull on different pieces of the Earth must be constantly changing. The average force is not important- only the differences (stretches) between separate parts of an object are important, and the stretches must constantly change in order to get friction and heating. You can test this with a rubber ball: Putting a large rock on top stretches it but does not heat it up, but beating it repeatedly with a mallet does heat it up. Only changing stretches cause the ball to heat up.

In Earth's case, the Moon's force differences change because of the Earth's rotation. When our side of the Earth faces the Moon, the Moon pulls our side more strongly than the opposite side of the Earth. Half a day later, the Moon pulls the opposite side more strongly than it pulls our side. Ocean tides slosh around the surface, following the Moon and Sun, and causing slight frictional heating as they go. Rocks also stretch and heat a little in response to tidal forces, but liquids are most effective at converting tidal forces into heat.

Europa always keeps the same side towards Jupiter, so rotation cannot produce the force difference changes. (Europa used to rotate in the distant past, but tidal forces gradually slowed it- will skip that explanation). Europa gets its force difference changes from its slightly non-circular orbit around Jupiter: As it swings closer the forces on each side increase, but the forces on the side closest to Jupiter increase more than the forces on the outward side. Jupiter's mass is enormous (about 25,000 times more than our Moon), so in its great gravity field even a slight deviation from a circular orbit can produce enough changing stretch forces to gradually heat its moons from the inside out.

The presence of three other large moons constantly changing their positions relative to Europa also adds tidal heating. Europa gets enough tidal heating to melt the deep ice and maintain an ocean which probably has more water than Earth's. There might also be hydrothermal vents and hot springs on the bottom of Europa's ocean. In the absence of sunlight, any life would have to subsist entirely on chemicals cooked up by the hot springs. On Earth whole ecosystems of strange creatures thrive near hot springs in the deep ocean, without getting any energy from the sun.

The next moon in from Europa is Io, which gets even more gravitational torture from being closer to Jupiter. There is so much tidal heating that all water has boiled away- no ocean. In fact, Io is the most volcanically active body in the solar system. Volcanoes spew sulfur in great fountains over 100 miles tall.

In the 20 years between visits from the Voyager and Galileo probes, enough lava flowed to noticeably change the map of Io.

The other large moons, Ganymede and Callisto, seem to have oceans of water. Since they are farther from Jupiter and experience less tidal heating, their oceans are probably smaller and deeper than Europa's. Europa has an icy surface with many cracks, resembling the ice over the Arctic Ocean. Ganymede and Callisto give no obvious surface clues about their oceans, but the Galileo probe can detect them indirectly. Saltwater conducts electricity, and this causes a salty ocean to produce a certain magnetic field in response to Jupiter's changing magnetic field. Galileo can detect the magnetic fields of the moons as it flies close, and this is our only clue about possible deep oceans inside these other moons.

Well there is a fair amount of water in the solar system. In fact, Hydrogen, the Hydrogen of H₂O (water) is the MOST ABUNDANT element in the UNIVERSE. And oxygen is not far behind (its the 4th most abundant element). So the potential is, if conditions are right for the Hydrogen and Oxygen to come together to form H₂O (water).

Comets are made up of water in the solid state (ice) and it is estimated that there are trillions and trillions of comets in the solar system. So water is quite abundant as a solar system material!!!

All of the Galilean satellites of Jupiter except Io have a lot of water present as ice or perhaps as liquid (water). Because these bodies are so far from the sun, its too cold to have liquid water AT THE SURFACE. However, deeper within the moons (Europa, Callisto and Ganymede) it is hotter because like on Earth, heat from the deep interior left over from when the planets and moons formed is escaping towards the surface. Hence at a depth a few kilometers ( 1 mile = 1.6 kilometer) below the surface, temperatures are high enough so that water ice becomes water liquid!.

There are below ground level large oceans of water on Europa and probably also on Callisto and Ganymede !!!

Since hydrogen and oxygen are very common in the solar system, it is probably no great surprise that water in its various forms is often found on planets and their satellites. Many of the planets and moons in the outer solar system have water ice or water vapor.

What is exciting about Europa is that there might be liquid water under the ice. While water ice does not support life very well, liquid water does (and I believe is necessary for all the types of life we know about on Earth), and if there is liquid water on Europa then perhaps there is some form of life there as well.

Finding life of any kind on another planet or moon would be an incredible discovery. Scientists at NASA and elsewhere are expending a lot of effort onlooking for evidence of extra-terrestrial life; it is a topic of great interest to many people.