Ganymede (moon)

Ganymede is a natural satellite of Jupiter and the largest and most massive in the Solar System. Like Saturn's largest moon Titan, it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io, or the Moon due to its lower density compared to the three. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.

Ganymede is composed of silicate rock and water in approximately equal proportions. It is a fully differentiated body with an iron-rich, liquid metallic core, giving it the lowest moment of inertia factor of any solid body in the Solar System. Its internal ocean potentially contains more water than all of Earth's oceans combined.

Ganymede's magnetic field is probably created by convection within its core, and influenced by tidal forces from Jupiter's far greater magnetic field. Ganymede has a thin oxygen atmosphere that includes O, O₂, and possibly O₃ (ozone). Atomic hydrogen is a minor atmospheric constituent. Whether Ganymede has an ionosphere associated with its atmosphere is unresolved.

Ganymede's surface is composed of two main types of terrain, the first of which are lighter regions, generally crosscut by extensive grooves and ridges, dating from slightly less than 4 billion years ago, covering two-thirds of Ganymede. The cause of the light terrain's disrupted geology is not fully known, but may be the result of tectonic activity due to tidal heating. The second terrain type are darker regions saturated with impact craters, which are dated to four billion years ago.

Ganymede's discovery is credited to Simon Marius and Galileo Galilei, who both observed it in 1610, as the third of the Galilean moons, the first group of objects discovered orbiting another planet. Its name was soon suggested by astronomer Simon Marius, after the mythological Ganymede, a Trojan prince desired by Zeus (the Greek counterpart of Jupiter), who carried him off to be the cupbearer of the gods.

Beginning with Pioneer 10, several spacecraft have explored Ganymede. The Voyager probes, Voyager 1 and Voyager 2, refined measurements of its size, while Galileo discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moons Explorer (JUICE), which was launched in 2023. After flybys of all three icy Galilean moons, it is planned to enter orbit around Ganymede.