What Is Ganymede?
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Ganymede is the largest moon in the solar system and the third of Jupiter's four Galilean moons. About 3,270 miles across, Ganymede is bigger than Mercury and Pluto. It's the only moon in the solar system known to have its own magnetic field. Ganymede also has a subsurface ocean between layers of ice.
Ganymede holds several solar system records: it's the largest moon, the only moon with its own magnetic field, and one of just a few worlds known to have a subsurface ocean. If Ganymede orbited the Sun directly instead of Jupiter, it would qualify as a planet. The European Space Agency's JUICE mission, launched in 2023, will arrive at Jupiter in 2031 to study Ganymede in unprecedented detail.
How big is Ganymede?
Bigger than Mercury. According to NASA, Ganymede is Jupiter's largest moon, and the largest moon in our solar system. It's even bigger than Mercury, and Pluto. Ganymede's diameter is about 3,270 miles, making it about 8% larger than Mercury (though only about 45% as massive, because Ganymede is made of less dense materials). If Ganymede orbited the Sun directly rather than Jupiter, it would be classified as a planet. Among moons, only Saturn's Titan comes close in size.
Why does it have a magnetic field?
Liquid iron in its core. Ganymede is the only moon in the solar system known to generate its own magnetic field. The field is produced by motion within a liquid iron core, similar to how Earth's magnetic field works but on a smaller scale. The magnetic field creates auroras on Ganymede that the Hubble Space Telescope has observed, and the interaction of this field with Jupiter's massive magnetic field produces complex effects. The discovery of Ganymede's magnetic field was a major surprise when NASA's Galileo spacecraft confirmed it in 1996.
Does Ganymede have an ocean?
Yes, a deep subsurface one. Like Europa, Ganymede has an interior ocean of liquid water. But Ganymede's ocean is much deeper and is sandwiched between layers of different ice, rather than being in direct contact with rock. The ocean may be more than 60 miles below the surface, far harder to access than Europa's. Scientists believe the ocean could contain more water than all of Earth's oceans combined. The exact structure (whether multiple ocean layers exist) is still being investigated.
What missions have visited?
Several past, more coming. NASA's Pioneer 10 and 11 first flew by Ganymede in 1973-74. The Voyager spacecraft followed in 1979, then the Galileo orbiter from 1995 to 2003 made detailed studies. Juno has performed close flybys in recent years. The European Space Agency's JUICE (JUpiter ICy moons Explorer) launched in 2023 and will arrive at Jupiter in 2031, eventually entering orbit around Ganymede in 2034. This will be the first spacecraft to orbit a moon of another planet.
Ganymede is the largest moon in the solar system and the third of Jupiter's four Galilean moons. At about 3,270 miles across, it's bigger than Mercury and Pluto. Ganymede is the only moon known to have its own magnetic field, generated by a liquid iron core. It also has a deep subsurface ocean between layers of ice. ESA's JUICE mission, launched in 2023, will arrive at Jupiter in 2031 and eventually orbit Ganymede.
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