The giant moon with a magnetic heart
Ganymede
Moon of Jupiter · discovered 1610 by Galileo Galilei
Ganymede is the largest moon in the Solar System, wider even than the planet Mercury. It is the third of Jupiter's Galilean moons from the planet and a world of rock and ice whose surface records a turbulent geological past.
Ganymede is the only moon known to generate its own magnetic field, and beneath its icy crust scientists think lies a vast saltwater ocean. That combination makes it a key target for ESA's JUICE mission, which will become the first spacecraft ever to orbit a moon other than our own.
Key facts
- Diameter
- 5,268 km1.52× Earth's Moon
- Mass
- 1.48 × 10²³ kg
- Surface gravity
- 1.428 m/s²0.146 g
- Density
- 1,942 kg/m³
- Distance from Jupiter
- 1,070,412 km≈ 15.0 Jupiter radii
- Orbital period
- 7.16 daysSame as its rotation (tidally locked)
- Discovered
- 1610Galileo Galilei
- Surface
- Ancient dark terrain and younger bright grooved ice
- Atmosphere
- Very thin exosphere of oxygen, with traces of water vapour
- Surface temperature
- Roughly −180 °C to −110 °C
- Notable feature
- Galileo Regio, a dark plain about 3,200 km across
- Named after
- Ganymede, the Trojan youth who became cupbearer to the gods
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
Ganymede's surface is a patchwork of two very different terrains. Dark regions, covering about a third of the moon, are ancient, heavily cratered and darkened by dust and organic-rich material. The largest, Galileo Regio, spans about 3,200 kilometres.
The remaining two thirds is lighter, younger terrain carved into long parallel grooves and ridges, such as those in Uruk Sulcus. Scientists think these formed as the icy crust stretched and fractured, possibly while Ganymede's interior was heating and expanding long ago.
Ganymede also bears palimpsests, flat circular scars where ancient craters have relaxed until only a faint outline remains, as well as bright young craters like Tros, whose rays of fresh ice stand out against the darker ground.
Interior and composition
Ganymede is made of roughly equal parts rock and water ice and is fully differentiated, with an iron-rich core, a rocky mantle and a thick shell of ice. Motion in its liquid outer core is thought to drive the moon's magnetic field, discovered by the Galileo spacecraft in 1996.
Evidence for an ocean came from Galileo's magnetic measurements and from Hubble observations in 2015 showing that Ganymede's auroras rock less than expected, a sign that a salty, conducting layer is damping them. That ocean may lie about 150 kilometres below the surface, sandwiched between layers of ice, and could hold more water than all of Earth's oceans.
Orbit and rotation
Ganymede orbits Jupiter in about 7.15 days and is tidally locked. It anchors the outer end of the 1:2:4 Laplace resonance with Io and Europa, completing one orbit for every two of Europa's and four of Io's. Today it experiences only modest tidal heating, but scientists suspect the resonance may have been different in the past, which could have helped form its grooved terrain.
Its own magnetic field carves out a small bubble, or mini-magnetosphere, inside Jupiter's much larger one, producing auroral ovals around Ganymede's poles.
Origin
Ganymede formed in the disc of material around the young Jupiter, far enough from the planet for water ice to be abundant. Enough heat was released during its formation and early history for the moon to separate into distinct layers, unlike its neighbour Callisto, which appears only partly differentiated.
Exploration
Galileo Galilei spotted Ganymede in January 1610. Since then a series of flybys has slowly revealed its complexity, and the first dedicated orbiter is on its way.
Voyager 1 and 2 (1979)
The Voyagers mapped large parts of Ganymede and revealed its striking mix of dark cratered regions and bright grooved terrain.
Galileo (1995–2003)
Galileo's six close flybys discovered Ganymede's magnetic field and gathered the first evidence of a subsurface ocean.
Juno (2021)
Juno passed about 1,000 kilometres above Ganymede in June 2021, returning the closest views in two decades and mapping its magnetosphere.
JUICE (launched 2023)
ESA's Jupiter Icy Moons Explorer is scheduled to reach Jupiter in 2031 and enter orbit around Ganymede in 2034, studying its ocean, ice shell and magnetic field up close.
The moons of Jupiter
Surprising facts about Ganymede
- Ganymede is bigger than the planet Mercury, but because it is part ice it has less than half Mercury's mass.
- It is the only moon known to have its own internally generated magnetic field, complete with auroras.
- Its buried ocean may contain more water than all of Earth's oceans combined.
- JUICE is set to become the first spacecraft to orbit a moon other than Earth's.
- Under a dark sky, Ganymede is bright enough that it would be visible to the naked eye if Jupiter's glare did not drown it out.
Keep exploring
- Callisto, moon of Jupiter: a battered relic of the early solar system
- Europa, moon of Jupiter: an ocean hidden under ice
- Titan, moon of Saturn: lakes of methane under orange skies
- Jupiter: the planet guide
- Watch Ganymede orbit Jupiter in the interactive 3D solar system
- All the major moons of the Solar System