What is Jupiter's largest moon?
Quick answer
Ganymede is Jupiter’s largest moon and the largest moon in the solar system. At about 5,268 km (3,273 mi) across, it’s even bigger than the planet Mercury, though only about half as massive. It’s the only moon known to have its own magnetic field, and scientists think a salty ocean lies beneath its icy crust.
Key facts
- Diameter
- About 5,268 km (3,273 mi)
- Discovered
- January 1610, by Galileo Galilei
- Orbit
- About 7.2 days around Jupiter
- Surface
- Water ice and rock
Bigger than a planet
| Body | Diameter |
|---|---|
| Ganymede | 5,268 km |
| Titan (Saturn) | 5,150 km; see the largest moon of Saturn |
| Mercury (planet) | 4,879 km |
| Callisto (Jupiter) | 4,821 km |
| Io (Jupiter) | 3,643 km |
| Earth’s Moon | 3,475 km |
Ganymede is less massive than Mercury because it’s made largely of ice, while Mercury is dense rock and metal.
A moon with a magnetic field
Ganymede has a molten iron core that generates its own magnetic field, creating faint auroras near its poles; see what causes the northern lights. By watching how those auroras wobble, scientists using the Hubble Space Telescope found evidence in 2015 of a saltwater ocean under the surface, possibly holding more water than all of Earth’s oceans.
Jupiter’s four great moons
Ganymede is one of the four Galilean moons, discovered in 1610; see who discovered Jupiter’s four largest moons.
| Moon | Known for |
|---|---|
| Io | The most volcanically active body in the solar system |
| Europa | An icy crust over a global ocean; a top target in the search for life |
| Ganymede | Largest moon; its own magnetic field |
| Callisto | One of the most heavily cratered surfaces known |
Missions to Ganymede
The European Space Agency’s JUICE (JUpiter ICy moons Explorer) launched in April 2023. It’s due to reach Jupiter in the early 2030s and later become the first spacecraft to orbit a moon other than our own, circling Ganymede. NASA’s Juno flew close by in 2021.
For Jupiter itself, see what the largest planet is.
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