Ganymede is the largest moon in the Solar System. Completing an orbit in roughly seven days, it is the seventh moon and third Galilean moon from Jupiter. It is larger in diameter than the planet Mercury but has only about half its mass. It has the highest mass of all planetary satellites with 2.01 times the mass of the Earth's moon.
Ganymede is composed primarily of silicate rock and water ice. It is has an iron-rich, liquid core. A saltwater ocean is believed to exist nearly 200 km below Ganymede's surface, sandwiched between layers of ice. Its surface comprises two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity brought about by tidal heating.
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Io is the most volcanically active body in the solar system. It's surface is covered by sulfur in different colorful forms. As Io travels in its slightly elliptical orbit, Jupiter's immense gravity causes 'tides' in the solid surface 100 meters (300 feet) high on Io, generating enough heat to give rise to the volcanic activity and drive off any water. Io's volcanoes are driven by hot silicate magma. 
Europa's surface is mostly water ice, and there is evidence that it may be covering an ocean of water or slushy ice. Europa is thought to have twice as much water as does Earth. This moon intrigues astrobiologists because of its potential for having a 'habitable zone.' Life forms have been found thriving near subterranean volcanoes on Earth and in other extreme locations that may be similar to what may exist on Europa. 
The most distant of Jupiter's Galilean Moons, Callisto shows the highest density of impact craters in the Solar System, but harbors no volcanoes or even any large mountains. It is thought that the surface is billions of years old. The first hint that something interesting might be happening beneath the surface came from Galileo's measurements of Callisto's magnetic field. Dr. Krishan K. Khurana of UCLA and colleagues discovered that the magnetic field fluctuated in time with Jupiter's rotation. The best explanation was that Jupiter's powerful magnetic field was creating electrical currents somewhere within Callisto, and those currents in turn created a fluctuating magnetic field around Callisto.