Each of Io, Europa, and Ganymede
always keeps its same face pointing
toward Jupiter (just like the Earth's
Moon).
Each of Io, Europa, and Ganymede is
on slightly elliptical orbits,
which leads to some tidal stretching
(also just like the Earth's Moon).
Remember that tidal stretching is really
just an application of gravity: the closer
two bodies are to each other, the greater
the magnitude of the tidal stretching.