Ganymede
A moon of Jupiter
The largest moon in the Solar System, wider than the planet Mercury: a ball of rock and ice whose crust shows two ages side by side, old dark regions crowded with craters and younger bright bands scored with parallel grooves, where the ice was pulled apart and renewed. It is the only moon known to make its own magnetic field, and a salty ocean lies deep under its ice.
It goes round Jupiter in a little over seven days, always with the same face towards it, exactly as Europa makes two laps and Io four: from that side the giant hangs still in the sky, some 7 degrees wide. The map comes from the Voyager and Galileo probes; Juno flew past at 1,000 km in 2021, and ESA's JUICE is due to go into orbit round it in 2034.
Key figures
- Diameter
- 5,268 km, 0.413 × Earth
- Mass
- 1/40 of Earth's
- Surface gravity
- 1.43 m/s², 15% of Earth's
- Distance from Jupiter
- 1,070,400 km
- One orbit of Jupiter
- 7.15 days
- Rotation
- 7.15 days, the same face always towards Jupiter
- Temperature at the equator
- below -113 °C
- Ocean under the ice
- About 100 km deep, under some 150 km of ice
- Magnetic field
- Its own, the only moon known to have one (found by Galileo, 1996)
- Discovered
- 1610, by Galileo Galilei