Astrum presents “What NASA Discovered on Ganymede”.
That is the largest moon in the Solar System, larger than Mercury and Titan, and the largest of the four “Galilean” moons of Jupiter.
However it’s density is much lower than Mercury’s.
The Moon has an unusually deep ocean layer dozens of kilometers deep, with a hundred times (or more) as much salt water as Earth’s oceans. And there are several unusual ice layers.
But Ganymede is also the only moon in the Solar System with its own magnetic field, which might be produced by tidal forces. Because of the extreme depth of the ocean, in comparison to Europa’s, it is considered unlikely that forces from the interior would create enough energy to trigger simple life.
Wikipedia diagram: Galilean moon Laplace diagram, click for attribution (animated gif).