Europa may be far from the Sun, but it orbits in a very busy neighborhood.
Europa orbits Jupiter, so there is naturally some tidal flexing from that, but the other 3 large Gallilean moons (Io, Ganymede & Callisto) orbit close by each with their own orbit period. Each time they pass, these moons also tug on Europa, pulling it to and fro and flexing the moon generating heat.
The surface water of Europa evaporates then freezes solid when exposed to the vacuum, so Europa is covered with a layer of ice, but underneath the ice is believed to be one of the largest liquid water oceans in the solar system.
A secondary source of heat may be Jupiter’s powerful magnetic field which may agitate molecules within Europa enough to cause an increase in temperature. Europa’s neighbor moon, Io, is closer to Jupiter and receives even more energy from Jupiter’s magnetic field, running a powerful flux tube between Io and the giant planet.
All of this tidal heating from the gravitational interaction with Jupiter and its other large moons keeps enough water liquid under the ice to be measured by radar reflection and sensors from probes like Galileo.