Moons · Space Codex
Ganymede
Bigger than the planet Mercury and the only moon with its own magnetic field.
The largest moon in the Solar System
At a glance
Key points
A salty ocean is thought to be sandwiched between layers of ice deep inside.
- Larger than Mercury
- The only moon with its own magnetic field
- JUICE will orbit it after arriving in 2031
Interior
Ganymede is layered like an onion: an iron core that drives its magnetic field, a rocky mantle and a thick shell of ice. In 2015 Hubble watched its auroras rock back and forth less than expected as Jupiter's field swept past. The damping points to a conducting, salty ocean roughly 100 km deep under about 150 km of ice.
Surface
About a third of the surface is old, dark, heavily cratered terrain. The rest is younger, brighter ice cut by long parallel grooves where the crust was stretched and faulted. Bright frost caps the poles, where the moon's magnetic field channels charged particles onto the ice.
Exploration
Galileo discovered the magnetic field in 1996 and Juno flew close in 2021. ESA's JUICE arrives at Jupiter in July 2031 and will finish its mission in orbit around Ganymede, the first spacecraft to orbit a moon other than Earth's.
What we still don't know
How deep the ocean lies, whether it touches rock or is trapped between ice layers, and how the moon's field has survived.
Missions
- 1989–2003
Galileo
NASACompletedFirst Jupiter orbiter (1995–2003) and first probe into a giant planet's atmosphere.
- 2011–
In Jupiter orbit since Jul 2016 on an extended mission. NASA's status page is inconsistent; perijoves are planned into 2027.
- 2023–
Earth flyby Sep 2026, another in Jan 2029; Jupiter arrival Jul 2031, then orbit Ganymede.
First-hand · our 3D model
How we render it
Ganymede is a sphere wrapped in a global map built from USGS and NASA spacecraft mosaics.
It is drawn at about 10% of Jupiter's radius, enlarged from its true 3.8% so it stays visible. It orbits in Jupiter's equatorial frame, inclined 0.2°, with a 7.16-day period kept in true ratio to its neighbours, and keeps one face towards Jupiter.
Moon sizes and orbits are compressed so a whole system fits on screen, but never reordered: each moon is drawn at 0.72 × (true size ratio)^0.6 of its planet, capped so no large moon is drawn bigger than it really is, and orbit distances are log-compressed into a band that clears the planet's rings. Orbital periods keep their true ratios, every moon is tidally locked, and on lighter devices only the two largest moons of each planet are drawn.
Related entries
Sources
- JPL Solar System Dynamics: satellite physical parameters (opens in a new tab)
- JPL Solar System Dynamics: satellite mean orbital elements (opens in a new tab)
- NASA NSSDC Jovian Satellite Fact Sheet (opens in a new tab)
- NASA Science: Ganymede (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.