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).