Overview of Jupiter :: Jupiter's Moons
Jupiter's 4 Galilean moons, in a composite image comparing their sizes and the size of Jupiter (Great Red Spot visible). From the top they are: Callisto , Ganymede, Europa and Io. Jupiter has at least 63 moonsThe four large moons, known as the "Galilean moons", are Io, Europa, Ganymede and Callisto. Ganymade is the largest moon in the solar system.
The orbits of Io, Europa, and Ganymede, form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three moons to distort their orbits into elliptical shapes, since each moon receives an extra tug from its neighbors at the same point in every orbit it makes. If this resonance is not present , tidal forces would tend to circularize the moons' orbits over time.
A picture of Jupiter and its moon Io taken by Hubble. The black spot is Io's shadow.
The tidal force from Jupiter, on the other hand, works to circularize their orbits. This constant tug of war causes regular flexing of the three moons' shapes, Jupiter's gravity stretches the moons more strongly during the portion of their orbits that are closest to it and allowing them to spring back to more spherical shapes when they're farther away. This flexing causes tidal heating of the three moons' cores. This is seen most dramatically in Io's extraordinary volcanic activity, and to a somewhat less dramatic extent in the geologically young surface of Europa indicating recent resurfacing.