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Moon of Jupiter
Ganymede
The biggest moon in the solar system, with a hidden ocean and its own magnetic field
Key facts
- Diameter
- About 3,270 miles / 5,260 km (mean radius 2,631 km), the largest moon in the solar system
- Distance from Jupiter
- About 665,000 miles / 1,070,400 km on average
- One orbit
- About 7.16 Earth days; it rotates once per orbit, keeping one face to Jupiter
- Discovered
- 7 January 1610 by Galileo Galilei (Simon Marius also claimed it)
- Named after
- Ganymede, a Trojan prince in Greek mythology who became cupbearer to the gods
- Surface
- Water ice and rock; old dark cratered terrain and younger bright grooved terrain
- Surface temperature
- About −203 °C to −121 °C
- Magnetic field
- The only moon known to generate its own, discovered by Galileo in 1996
- Ocean
- Strong evidence for a salty ocean beneath the ice, possibly holding more water than Earth's surface
- Atmosphere
- Extremely thin oxygen, detected by Hubble
- Visited by
- Pioneer 10 and 11, Voyager 1 and 2, Galileo, New Horizons (distant), Juno; JUICE arrives 2031
About Ganymede
Ganymede is a moon of Jupiter. It is the biggest moon we know. It is bigger than the planet Mercury. Ganymede is made of rock and ice. Its surface is grey and brown. Some parts are dark and very old. Some parts have long bright grooves. Deep down there is a hidden sea. The sea is salty and very cold. Ganymede has its own magnetic field. It makes glowing lights near its poles. A probe called Juno flew past it. One day a new probe will orbit it. You can see Ganymede with a small telescope.
Ganymede is the largest moon in the whole solar system. It is about 5,260 km across, wider than the planet Mercury. Ganymede orbits Jupiter at a distance of about 1,070,000 km. One orbit takes just over seven days to complete.
Like our Moon, it keeps the same face towards its planet. Galileo Galilei discovered it in January 1610 with an early telescope. It is named after a prince from Greek myths who served the gods. Ganymede is a sphere of rock and ice with an iron core.
Its surface has two very different kinds of terrain. Dark regions are old and covered in craters. Brighter regions are cut by long grooves and ridges. Scientists think a salty ocean lies deep beneath the icy crust.
It may hold more water than all of Earth's oceans. Ganymede is the only moon with its own magnetic field. This field makes glowing auroras dance around its poles. Its gravity is strong enough to hold a very thin oxygen atmosphere.
Spacecraft such as Voyager, Galileo and Juno have flown past it. Europe's JUICE probe will reach Jupiter in 2031. It will then become the first spacecraft to orbit a moon of another planet.
Ganymede is the largest and most massive moon in the solar system. JPL gives its mean radius as 2,631 km, so it is about 5,260 km across: larger than the planet Mercury and the dwarf planet Pluto, though with less than half of Mercury's mass because it is roughly half water ice. Its mean density is 1.94 g/cm³. It orbits Jupiter at about 1,070,400 km, once every 7.16 days, rotating synchronously so the same hemisphere always faces the planet, and it sits in a 1:2:4 orbital resonance with Europa and Io. Galileo Galilei recorded it on 7 January 1610; Simon Marius claimed an independent discovery and later supplied the mythological name, Ganymede being the Trojan prince taken to Olympus as cupbearer to the gods.
The surface divides into two terrains. Dark, heavily cratered regions cover about a third of the moon and are around four billion years old. Brighter grooved terrain covers the rest and is somewhat younger, its long ridges and furrows probably produced by tectonic stretching of the ice shell. Surface temperatures range from roughly −203 °C to −121 °C. Hubble detected a tenuous oxygen atmosphere in the 1990s.
Ganymede is differentiated, with a metallic iron core, a rocky mantle and a thick icy outer shell. The Galileo spacecraft discovered in 1996 that it generates its own magnetic field, the only moon known to do so, which drives auroras near the poles. In 2015 Hubble observations of how those auroras rock back and forth provided strong evidence for a salty subsurface ocean, which may hold more water than all of Earth's surface water and could be layered with ice in several tiers.
Pioneer 10 (1973), Pioneer 11 (1974) and Voyagers 1 and 2 (1979) flew past. Galileo made six close flybys between 1996 and 2000, the nearest at 264 km. New Horizons imaged it from 3.5 million km in 2007, and Juno passed within about 1,040 km on 7 June 2021. ESA's JUICE, launched on 14 April 2023, is due to reach Jupiter in July 2031 and later enter orbit around Ganymede itself, the first spacecraft ever to orbit a moon other than our own. A common error is to call Ganymede a planet-sized world with an atmosphere like Titan's; its oxygen envelope is vanishingly thin, far too sparse to breathe.
Did you know?
- Ganymede is wider than the planet Mercury, yet it is still counted as a moon because it orbits Jupiter rather than the Sun.
- Its hidden salty ocean may hold more water than all the oceans, lakes and rivers on Earth's surface put together.
- Ganymede is the only moon known to make its own magnetic field, which paints glowing auroras around its poles.
Neighbours around Jupiter
Questions children ask
Why is Ganymede a moon if it is bigger than Mercury?
Being a planet is about what you orbit, not how big you are. Mercury orbits the Sun, so it is a planet. Ganymede orbits Jupiter, so it is a moon, even though it is about 5,260 km across and Mercury is smaller.
Could there be life on Ganymede?
Nobody knows. Its salty underground ocean has water, and possibly contact with rock, which are two things life needs. But the ocean is buried under a very thick ice crust. ESA's JUICE mission will study it closely from 2031 to find out more.
Can I see Ganymede from my garden?
Yes. Ganymede is one of the four Galilean moons, and all of them show up as tiny dots beside Jupiter through binoculars or a small telescope. They shift position from night to night as they orbit, just as Galileo noticed in 1610.
What does the surface of Ganymede look like?
It is a patchwork. Dark areas are ancient, around four billion years old, and pitted with craters. Brighter areas are striped with long grooves and ridges made when the icy crust stretched and cracked. Juno photographed both kinds up close in 2021.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- What makes Ganymede different from every other moon we know?
- Which planet does Ganymede orbit?
- Roughly how long does Ganymede take to orbit Jupiter once?
Mercury
Titan
The Moon