If there is life on any of the gas giant moons (particularly Europa and Enceladus) it's tidal heating that sustains it. There are basically four ways a planet can be heated:
1. Latent heat of formation. As the planet forms, denser materials sink down to the core and lighter materials float up to the top. This releases their gravitational potential, heating the planet. Over time though, this heat radiates into space. Icy moons are too old for this to be enough heat to support a liquid ocean.
2. Radioisotope decay. Particularly stuff like Potassium, on long time scales, decays and releases heat energy. This is a significant source of heating on Earth. Less significant on a small moon since radioactive isotopes are pretty rare, all things considered.
3. Solar heating. Sun hot. Planet kinda hot.
4. Tidal heating. The planet gets physically flexed by interacting orbital resonances between another moon and the host planet. This can be powerful enough to drive large amounts of volcanic activity on the surface of Io.
Europa Clipper is a mission that will fly high-precision magnetometers over Europa relatively soon (by NASA standards). It's possible that those magnetometers could pick up on induced magnetic fields from the interaction of charged salt ions moving in Europa's ocean with the magnetic field of Jupiter. If that signal can be detected, it may be possible to map the sub-surface ocean and start to do some sort of astro-oceanography.
Most definitions of life are very arbitrary. When it comes to astrobiology, we mostly look for things that look like us because if we didn't, the search space would be incomprehensibly large and frankly there's not a lot we could say.
1. Latent heat of formation. As the planet forms, denser materials sink down to the core and lighter materials float up to the top. This releases their gravitational potential, heating the planet. Over time though, this heat radiates into space. Icy moons are too old for this to be enough heat to support a liquid ocean.
2. Radioisotope decay. Particularly stuff like Potassium, on long time scales, decays and releases heat energy. This is a significant source of heating on Earth. Less significant on a small moon since radioactive isotopes are pretty rare, all things considered.
3. Solar heating. Sun hot. Planet kinda hot.
4. Tidal heating. The planet gets physically flexed by interacting orbital resonances between another moon and the host planet. This can be powerful enough to drive large amounts of volcanic activity on the surface of Io.
Europa Clipper is a mission that will fly high-precision magnetometers over Europa relatively soon (by NASA standards). It's possible that those magnetometers could pick up on induced magnetic fields from the interaction of charged salt ions moving in Europa's ocean with the magnetic field of Jupiter. If that signal can be detected, it may be possible to map the sub-surface ocean and start to do some sort of astro-oceanography.