• The planet Jupiter's four largest moons are called the Galilean satellites, after Italian astronomer Galileo Galilei, who observed them in 1610. 

• Io is the most volcanically active body in the solar system.

• Europa's surface is mostly water ice, and there is evidence that it may be covering an ocean of water or slushy ice beneath.

• Europa is thought to have twice as much water as does Earth. 

• Ganymede is the largest moon in the solar system (larger than the planet Mercury), and is the only moon known to have its own internally generated magnetic field.

• Callisto's surface is extremely heavily cratered and ancient -- a visible record of events from the early history of the solar system.

Retrieved from:

http://solarsystem.nasa.gov/planets/jupiter/moons