The auroras on the skies of Jupiter's moons can tell that there is an ocean under those moon's icy shells.

Above: Galilean moons. 

Many times people ask how icy moons can have water. The reason for that is the magnetic field. Those moons are quite small things, and that's why people think that there is not enough radioactive material that keeps their nucleus warm and that causes a loss of the magnetic field. 

But the magnetic field doesn't need to have a melted nucleus. The magnetic nucleus and fast-rotating cloud of magnetic crystals can turn the icy moon into a generator, even if the nucleus of the moon is solid. In that model water that rotates the magnetic nucleus acts like a generator if there are magnetic iron bites in that liquid. 

That model requires that there is a water layer around the nucleus. And that water must rotate the entire nucleus. Then the iron bites that are in the water turn the structure into the generator. 

And the aurorae of Jupiter's moons prove that there is a magnetic field. The low gravitation and tidal forces keep water liquid. And that makes the magnetic field to those small objects without the radioactive isotopes. 

The reason that icy moons Galilean moons can have oceans that cover their entire surface is their weak gravitation. The gravitation on those moons is so weak, that water cannot turn to ice. 

There is the possibility that water that is under the ice shells is in the form that seems a little bit combination of liquid water and vapor. The water would be thin. 

Low gravitation and tidal forces from the gas planet keep water in liquid form And that water layer forms a water mantle for those icy moons. Because friction is extremely low, water rotates the nucleus of those moons at a very high speed. 

The tidal waves are keeping that water in the move. And when that water layer moves around the moon's nucleus it rises small metal bites to the ice shell. 

When the moon's nucleus rotates. It interacts with iron or some other magnetic metal bites that are under ice and in the water layer. That thing turns the moon into a giant generator.  That thing forms the magnetic field like similar effect forms on Earth. 

https://www.sciencealert.com/jupiters-largest-moons-all-have-aurorae-that-glow-deep-red-and-15x-brighter-than-ours

https://www.universetoday.com/44796/galilean-moons/

https://en.wikipedia.org/wiki/Galilean_moons

This same effect can use to create an energy supply for small-size satellites. 

By the way, the water that is loaded with iron bites can turn into generators for very small satellites. In that case, the water that is in low pressure would be put into the chamber and then put to rotate around the magnetic stick. 

That thing makes it possible to create power systems for satellites that don't require solar panels or nuclear isotopes. That kind of generator can keep the satellite's size small. Or the system can need small solar panels. That feeds magnets around the chamber making that water rotate the generator's heart. 

https://astronomyandtechnology.blogspot.com/