When you put these things together,
you discover that Io, Europa, and
Ganymede are all heated through
their continual tidal stretching.
This happens because Io gets a tidal
kick (gravitational tug) at the same
time of its orbit, every orbit, by
Jupiter and Europa and Ganymede.
Europa also gets stretched by Jupiter,
Io, and Ganymede every orbit at the
same time.
(The same is also true for Ganymede.)
These gravitational kicks have the result of
heating these moons. Io gets heated most --
why is that? -- then comes Europa, then
Ganymede.