Jupiter's densely cratered moon Ganymede is the largest satellite in the Solar System and is believed to be composed mostly of rock and ice. Its small iron core generates a magnetic field about 1% as strong as the Earth's.
Photo: Ganymede taken by the Galileo probe (Galileo Project, JPL, NASA)
The Solar System's largest moon orbits Jupiter.
Sir Patrick Moore is amazed by the detail of Voyager 1's images.
Early in Voyager 1's mission, Sir Patrick Moore and his guest Dr Garry Hunt of University College London discussed some of the probe's findings, which included a ring around Jupiter and images of Jupiter's moons Amalthea, Callisto, Io, Europa and Ganymede. Since Sir Patrick and Dr Hunt spoke, Jupiter has been found to have three faint rings.
Jupiter's moons aren't the cold, dead worlds the experts expected.
Voyager scientists thought Jupiter's moons would be cold, dead worlds. They were amazed when the first close-up images from the spacecraft revealed four moons, each different from the next. The probes were launched in 1977.
Ganymede // (Jupiter III) is the largest moon of Jupiter and in the Solar System, and the only moon known to have a magnetic field. It is the seventh satellite outward from Jupiter and third of the Galilean moons, the first group of objects discovered orbiting another planet. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. Ganymede has a diameter of 5,268 km (3,273 mi), 8 % larger than the planet Mercury, but its mass is only 45 % that of Mercury. Ganymede is 2 % larger than Saturn's Titan (second-largest moon of the Solar System). At 2.02 times the mass of the Moon, it is the most massive planetary satellite. It is the ninth-largest object in the Solar System, and the largest without a substantial atmosphere.
Ganymede is composed of approximately equal amounts of silicate rock and water ice. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined. Its surface is composed of 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 due to tidal heating.
Ganymede's magnetic field is probably created by convection within its liquid iron core. The meager magnetic field is buried within Jupiter's much larger magnetic field and would show only as a local perturbation of the field lines. The satellite has a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone).Atomic hydrogen is a minor atmospheric constituent. Whether the satellite has an ionosphere associated with its atmosphere is unresolved.
Ganymede's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610. The satellite's name was soon suggested by astronomer Simon Marius, for the mythological Ganymede, cupbearer of the Greek gods and Zeus's lover. Beginning with Pioneer 10, spacecraft have been able to examine Ganymede closely. The Voyager probes refined measurements of its size, whereas the Galileo craft discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. After flybys of all three icy Galilean moons, the probe is planned to enter orbit around Ganymede.