Definition: Jupiter is orbited by over 60 moons; the four largest being Io, Europa, Ganymede and Callisto. Known as the Galilean Satellites, each of these massive moons has different characteristics. The first and nearest to Jupiter, Io, has intense volcanic activity while the second, Europa is covered by a layer of ice. Scientists believe that there may be liquid water under the ice. Ganymede is the third furthest of these moons from Jupiter and is the largest moon in our solar system. Callisto is the furthest from the planet and is the third largest moon in our solar system. All four are visible through binoculars but, as their orbits are slightly different, sometimes they are seen together, sometimes one or more of them may be hidden by the planet and sometimes they may appear separated, on different sides of Jupiter.