Moons of Jupiter. Computer illustration showing the four largest moons of the planet Jupiter, known as Galilean moons because they were discovered by Galileo Galiei in 1610. They are Io, Europa, Ganymede and Callisto, and they exhibit some of the most remarkable geological characteristics in the Solar System. Io is covered with large volcanoes, while Europa's surface and crust are made almost entirely of water ice. Ganymede, is the largest moon in the Solar system, even larger than planet Mercury and the Dwarf planet Pluto. Callisto's crust is believed to be a mixture of ice and rocky material, peppered by thousands of bright impact craters.