| Ganymede |
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One of the four Galilean moons of Jupiter (number III) and the largest natural satellite in the solar system. The first high-resolution images of Ganymede were returned by Voyagers 1 and 2. Images showing even finer detail were obtained by Galileo. There are several different types of terrain, notably dark areas that are heavily cratered and a lighter grooved terrain that constitutes around 60 per cent of the surface photographed. Galileo images of the dark area suggest it is surface that has been changed by various episodes of shearing and furrowing. Galileo also revealed many small craters on the finely grooved areas. One of the most significant discoveries made by Galileo was that Ganymede has a substantial magnetic field, which is stronger at its surface than the fields of Mercury, Venus or Mars. Data from Galileo's trajectory, combined with the magnetic, field, suggest that Ganymede must have a molten iron-rich core. Overall Ganymede's density is about twice that of water. It is likely that the core is surrounded by a rocky mantle, overlain by a thick layer of ice. See also: Table 6. |