There are at least 63 known moons orbiting Jupiter as of 2015. Europa, Ganymede, Io and Callisto are the largest four of these moons and are known as the Galilean moons. Ganymede is also the largest moon in the solar system; its diameter is 3,260 miles.
Galileo Galilei discovered the four largest moons at the same time as the German astronomer Simon Marius claimed to have done. However, Marius did not publish his observations and, as a result, credit remained with Galileo.
Io is the most volcanically active moon in the solar system; its surface is blanketed in sulfur. Europa's surface is chiefly composed of water ice, and Callisto is heavily cratered.