Io and Europa are the most interesting Galilean moons.

"An artist’s concept shows the internal structure of Jupiter’s moon Io. Data from NASA’s Juno spacecraft suggests that Io does not have a shallow global magma ocean and is consistent with a mostly solid mantle (represented by green hues), with substantial melt (yellows and oranges), overlying a liquid core (red/black). Credit: NASA/Caltech-JPL/SwRI, edited." (ScitechDaily, Juno’s Shocking Discovery: Io’s Interior Isn’t What We Expected)

It is not what astronomers thought. There is no solid magma structure on that moon. The tidal forces form magma pockets inside that moon. And that makes Io interesting. It is known for its volcanic activity. And those findings tell that the planet must not have a common magma layer for volcanism. And that means also old planets can have volcanic activity. At least if they have a massive moon that moves particles inside them. 

"Cutaway illustration of Europa’s icy crust, subsurface ocean and possible vents that transport material to the surface. Credit: NASA" (ScitechDaily, We Asked a NASA Expert: Is There Potential for Life on Europa? [Video])

"Is there potential for life on Europa? This icy moon of Jupiter has a massive ocean hidden beneath its icy crust — one that might have the right ingredients for life. Currently en route, our upcoming Europa Clipper mission will explore this mysterious world, searching for clues about its potential habitability. Watch as a NASA scientist explains more. Credit: NASA" (ScitechDaily, We Asked a NASA Expert: Is There Potential for Life on Europa? [Video])

The Io is quite a large moon. It's smaller than our moon and gravity on its surface is lower than the moon. The lightweight composition of Io makes it possible for Jupiter's tidal forces to move things inside it. So the tidal forces can also create friction that forms the magma pockets under that moon's surface. The lightweight form of Io means that particles have space to move inside it. 

Same way Europa moon can keep its undersurface ocean liquid. The icy shell protects water that the tidal waves keep always in move. The magnetic particles in that water create a magnetic field that keeps water molecules in one form because it protects Europa from the cosmic rays. The Europa may be a harbor for lifeforms. The moon is very light and it's possible that there are some primitive organisms. The Europa clipper spacecraft can give an answer to that question. 

https://scitechdaily.com/we-asked-a-nasa-expert-is-there-potential-for-life-on-europa-video/

https://scitechdaily.com/junos-shocking-discovery-ios-interior-isnt-what-we-expected/

https://en.wikipedia.org/wiki/Europa_(moon)

https://en.wikipedia.org/wiki/Io_(moon)

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

There are two astrobiological models about lifeforms that can survive there. 

In some models, there can be some lifeforms that get their oxygen from electrolysis. Those organisms can have electric organs that can break water molecules by electricity that can be like electric eel's electric cells. 

There is also a model of the silicone-based lifeform. That kind of lifeform can be the silicone cell that is controlled by the DNA. Those organisms could be moon or planet-sized things that can be far from the organisms that we know on Earth. They can be far slower than organisms on Earth. The thing why I mentioned those things as organisms is that they are DNA-controlled structures that can send the silicone bites that carry their DNA even to space.