Ganymede is the largest moon in the Solar System - bigger than the planet Mercury - with a surface split between dark, ancient cratered terrain and lighter bands of grooved ice. It is also the only moon known to generate its own magnetic field.
It is larger than Mercury, yet has less than half of Mercury’s mass.
It is the only moon known to generate its own magnetic field.
Its aurora behaviour points to a salty ocean roughly 150 km beneath the surface.
A moon with its own magnetic shield
Deep inside Ganymede, molten iron churns and acts as a dynamo, generating a magnetic field - something no other moon is known to do. That field carves out a small magnetic bubble inside Jupiter’s vast one, complete with its own auroras. The way those auroras rock back and forth, seen by Hubble, revealed a salty ocean buried under the ice.
Physical characteristics
Diameter
5,268 km
Mass
1.480e+23 kg
Surface gravity
1.43 m/s²
Density
1.94 g/cm³
Tidally locked
Yes
Mean temperature
-160 °C
Geometric albedo
0.43
Atmosphere and composition
Atmosphere
A very thin oxygen exosphere, detected by the Hubble Space Telescope.
Composition
Roughly half rock and half water ice, fully layered: a molten iron core, rocky mantle, and thick shells of ice - likely with a salty ocean sandwiched between them.
Interior structure
Iron coremodelled
Molten enough to run a dynamo - Ganymede is the only moon with its own magnetic field.
Rocky mantlemodelled
Silicate rock between metal below and ice above.
Ocean and high-pressure icemodelled
Possibly several stacked oceans sandwiched between exotic ice phases.
Ice crustmodelled
Old dark terrain and younger grooved bright terrain.
How we know: Galileo found the magnetic field; Hubble’s aurora observations and gravity data support the buried ocean. ESA’s JUICE arrives in the 2030s to map it.
Discovery
Discovered by
Galileo Galilei
Year
1610
How
one of the first astronomical telescopes
Missions
Voyager 2NASA · Flyby probe · 1977 · active
Launched 16 days before its twin, Voyager 2 is the only spacecraft ever to visit all four giant planets - Jupiter, Saturn, Uranus and Neptune. It entered interstellar space in 2018 and is still returning data.
Voyager 1NASA · Flyby probe · 1977 · active
The most distant human-made object, now roughly 25 billion km from the Sun. After historic flybys of Jupiter, Saturn and Titan, Voyager 1 became the first spacecraft to enter interstellar space in 2012.
GalileoNASA · Orbiter · 1989–2003 · completed
The first spacecraft to orbit an outer planet, Galileo circled Jupiter from 1995 to 2003 and dropped a probe into its atmosphere. In 2003 it was deliberately flown into Jupiter so it could never crash into - and contaminate - Europa’s possible ocean.
JunoNASA · Orbiter · 2011–2025 · completed
Juno looped over Jupiter’s poles from 2016 to 2025, peering beneath the clouds to map the planet’s interior, gravity and magnetic field. Its extended mission added daring close flybys of Ganymede, Europa and volcanic Io.