Most people are familiar with the tides in the Earth's oceans, which are raised by the Moon and, to a lesser extent, the Sun. In fact, there is a tide in the solid Earth as well, but it is too small to see. The Earth raises a tide on the Moon also. Because the Earth is so much larger than the Moon, Earth's tides have forced the Moon into a state of being tidally locked, where the Moon keeps one face always turned toward Earth. (How must the length of the lunar day and its orbit time around the Earth be related to make this so?).
Now the Galilean satellites, like our Moon, are tidally locked by their master. However, Jupiter's gravity is immense and raises huge tides. Plus, the inner satellites have other satellites whizzing by that also raise tides. This creates a constant flexing of these worlds that heats them up, driving their geology. Think of heating a rubber ball by squeezing it repeatedly. Io has the most active geology known in the Solar System because it is both very close to Jupiter and its neighbor moon Europa. Distant Callisto was not tidally heated, both because it was farther from Jupiter and because Ganymede is comparatively far away.
Click here for an image of Enceladus, an icy moon of Saturn only 500 km in diameter. Enceladus is the second-inner moon of nine major satellites. Sketch and/or describe its surface. Explain its geology.