Ganymede is one of Jupiter’s moons and was discovered in 1861 by the famous Italian astronomer and physicist Galileo Galilei.

Diameter: 5268 km
Mass: 1.48 x 1023 kg
Composition: Ice and rock
Orbit: 1,070,400 km from Jupiter

Ganymede is the largest moon in the Solar System and the seventh moon and third satellite outwards from Jupiter. There are known to be 67 moons orbiting around Jupiter. The four largest moons are Ganymede, Io, Europa and Callisto, all of which were discovered by Galileo. He initially believed them to be stars, as the discovery of Ganymede was the first time that a moon had been seen orbiting a planet other than Earth. Ganymede is part of the group of four moons collectively known as the Galilean Satellites.

These moons were originally named the Medicean Stars by Galileo, after the Medici family. Only when more satellites were discovered were unique names attributed to them. Ganymede is named after a young Greek boy who was taken to Olympus by Zeus. It is the only Galilean moon to be named after a male.

The composition of Ganymede consists of a metallic iron core and a rocky mantle covered with a shell of ice which is also thought to contain some rock deposits. Ganymede generates a magnetic field which is emitted by the iron core. It has two distinct types of surface. A dark, cratered terrain mostly made up of a clay-like substance covers approximately one third of the surface. This is widely believed to be the original crust. A lighter and younger crust consisting of grooves and ridges covers the rest of the satellite. Some of these ridges peak at over 700 metres. Polar caps were also identified on Ganymede during a Voyager space mission. A significant area on Ganymede, known as Galileo Regio, has a series of concentric circles embedded on the surface.

The orbital period of Ganymede is seven days and three hours. In common with other moons, it is tidally locked. This means that the satellite always has one side facing its planet, in this case, Jupiter.

Since the 1970s, several missions have passed close to Ganymede. The first was Pioneer 10 in 1973, closely followed by Pioneer 11 a year later. Voyager 1 and 2 were able to collect data relating to the size of Ganymede and also noted its distinctive surface and terrain. In 1995, the Galileo spacecraft orbited around Jupiter, passing within 270km of Ganymede. It was in 1996 that the magnetic field was discovered. The last mission to Ganymede took place in 2007 and was undertaken by New Horizons en route to Pluto. It was on this occasion that the composition and topography were mapped out.

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