Ganymede
What is Ganymede?
Moons· Diameter 5,268 km. The largest moon in the solar system, bigger even than the planet Mercury, and the only moon that makes its own magnetic field. Europe's JUICE spacecraft is due to enter orbit around it in December 2034.
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Orbit and size
| Diameter | 5,268 km |
|---|---|
| Parent | Jupiter |
| Note | largest moon — bigger than Mercury |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Parent planet and sibling moons
Ganymede's name came from Marius, not Galileo
Galileo discovered Ganymede and Jupiter's three other large moons. The names we use, though, were given in 1614 by Simon Marius, a German astronomer who observed the moons at about the same time, following a suggestion from Kepler. Galileo called them the 'Medicean Stars' after his patrons, the Medici family, and told them apart by number. Marius claimed he had found them first, which led to a quarrel. In the end Galileo got the credit for the discovery, while Marius's names stuck.
All four names come from figures close to Zeus in Greek myth (Zeus is Jupiter in Roman myth). Ganymede was the Trojan boy who poured wine for the gods. Yet until the mid-twentieth century astronomers often preferred numbers such as 'Jupiter III'. As more and more moons of Jupiter were found, numbers alone became awkward, and the mythological names became standard.
Ganymede's magnetic field was discovered in 1996, on the Galileo spacecraft's first close flyby. The moon's small magnetic bubble was found sitting inside Jupiter's enormous one. After Galileo, no spacecraft came close for more than twenty years, until Juno passed 1,038 km away on 7 June 2021 and sent back new images. Juno also found traces of salts and organic compounds on the icy surface.
The moon in depth
At 5,268 km across, Ganymede is the largest moon in the solar system, bigger even than the planet Mercury (4,879 km). Because it contains a lot of ice, it has a little less than half of Mercury's mass. If it orbited the Sun instead of Jupiter, it would be big enough to be called a planet. Beneath its icy shell there is thought to be an ocean holding more water than all of Earth's seas.
Ganymede is the only moon that makes its own magnetic field, and that field gave away the hidden ocean. When the Hubble Space Telescope watched Ganymede's auroras, they rocked back and forth less than expected under the influence of Jupiter's magnetic field. The best explanation is a salty, electrically conducting ocean under the ice that damps the rocking.
Ganymede's surface has two kinds of terrain. One is dark and crowded with craters, about four billion years old. The other is brighter, younger and scored with long parallel grooves. The grooves formed where the crust was pulled apart, showing that Ganymede was once geologically active.
Europe's JUICE spacecraft will reach Jupiter in 2031, fly past several moons, and then enter orbit around Ganymede in December 2034. It will be the first spacecraft to orbit a moon other than our own. Zooming in on Jupiter in the orbit view shows Ganymede completing a lap about every seven days.
Frequently asked
- How big is Ganymede?
- The diameter of Ganymede is 5,268 km.
- What is notable about Ganymede?
- The note of Ganymede is largest moon — bigger than Mercury.
- Which planet does Ganymede orbit?
- Ganymede orbits Jupiter.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.