Jupiter has lots of moons but perhaps Europa is the most interesting of all. Why? Because life might exist in its deep water oceans. The surface of Europa is water ice making it bright and hence more easily to spot in the sky. Scientists believe that there is a vast and deep water ocean below this thick icy surface. Why? Because Europa has cracks and streaks all over its surface. It seems like there are ice chunks in the surface that continuously shift and refreeze. One of the logical explanations for the cracks and moving ice chunks is that there is an ocean below this surface. How could be liquid water below this surface when it's so far away from the sun? Well there is another source of energy that heats up Europa. Jupiter and other moons near Europa strain it through their gravitational force. Somehow like the tidal effects of our moon on Earth. It's like hitting a tennis ball with a racket. The hitting literally warms up the ball. In this analogy gravity is like the racket hitting Europa. These strains crack the surface and heat up Europa's interiors making it possible for a hidden ocean to exist beneath its surface. It's estimated that Europa has more water than Earth because its ocean is thought to be very deep. A lot of Europa's material below the ocean is rocks similar to the ones that are under Earth's oceans. This could make the water salty; A good place for life to flourish.
Sorry for this long-ass answer!