Jupiter has 39 orbiting satellites (like a Solar System in miniature).

The 4 largest moons (Io, Europa, Ganymede, Callisto) are comparable in size to our Moon and Mercury.

The mean density of these worlds decreases with increasing distance from the planet, which means that the proportion of ice to rock increases.

The inner moon, Io, is the most volcanically active place in the Solar System. The surface of Io is colored yellow, orange, red, and black according to the local heating of sulfur compounds. Several large plumes are always observed.

The heating is caused by tidal stresses that result from the competing gravitational forces of Jupiter and Europa and Ganymede (an example of an orbital resonance: the orbital periods of Europa and Ganymede are 2 and 4 times that of Io, respectively).