How could liquid water exist out near Jupiter, so far away from heat-giving sunlight? Because tidal forces caused by massive Jupiter & its many moons make Europa & Callisto stretch & release - the same effect our own Moon (with some help from the Sun) has on Earth's oceans, causing them to swell & release - what we call our oceanic tides.
This stretching & releasing of Europa & Callisto causes their interiors to heat up - like when you bend a wire back & forth.
It's this internal heat, caused by tidal forces, that creates enough energy for liquid water to exist way out in the solar system, far from the energy-giving Sun.