Ganymede, as imaged by the Galileo spacecraft. The large dark oval region at top right is called Galileo Regio, after the moon's discoverer. (Galileo Project, JPL, NASA, apod000620)

     Ganymede is too far from Earth to be seen as anything other than a dot in most telescopes, but is easily large enough to be considered a planet if it orbited the Sun instead of Jupiter. It is the largest known satellite, having 2% greater diameter than Titan, and both satellites are larger than Mercury and close to three-quarters the diameter of Mars. Despite their large size the two moons are less than half the mass of Mercury, because their density is only a third that of the denser Terrestrial planets.
     The dark regions seen in images such as those above are heavily cratered, suggesting that they are "older", or relatively unchanged since the earliest stages of Solar System history; while the lighter regions have far fewer craters and are dominated by unusual grooves, suggesting that they are "younger", or in some way changed -- by geological activity of unknown nature -- since the era of heavy cratering ended over four billion years ago.

     Gravitational and magnetic studies of Ganymede by the Galileo spacecraft suggest that the satellite has undergone substantial differentiation, with a dense metallic core (shown in gray), a substantial rocky outer core (shown in light brown), a still larger mantle consisting of warm, soft ice (shown in dark blue), and a relatively thin rigid water ice crust (shown in white, with a brownish overlay based on surface images). This sort of complete differentiation -- as opposed to one in which heavier rock and metallic materials sank to the center but did not separate from each other -- implies substantial heating at some point in Ganymede's past, and is undoubtedly related to the geological activity seen in the lighter-colored "grooved" areas on the surface. Since the metallic core is almost certainly solid, a partially liquid or slushy layer of icy materials may be responsible for Ganymede's weak magnetic field. (Galileo Project, NASA, JPL)