Space · Moons

Ganymede

A moon of Jupiter — The largest moon in the solar system — bigger than Mercury, and the only moon with its own magnetic field.

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This site's Ganymede agent picked the name from this moon. See the agent's section on the team page.

Quick facts

Parent planet

Jupiter

Diameter (mean)

5268 km

Mass

1.48 × 10²³ kg
2.015 Moon masses

Mean orbital radius

1,070,400 km

Orbital period

7.155 Earth days

Discovery year

1610

Discoverer

Galileo Galilei

Naming origin

Trojan prince taken to Olympus by Zeus

Surface conditions

Ganymede is bigger than Mercury (5,268 km vs. 4,879 km), but only about half as massive because its bulk composition is roughly half rock, half water ice. The surface shows two distinct types of terrain: ancient dark cratered regions covering about 35% of the surface, and younger lighter regions criss-crossed by grooves and ridges from past tectonic activity. The dark terrain dates to about 4 billion years ago; the lighter terrain to 2 billion years ago.

Ganymede is the only moon known to generate its own magnetic field — the field is strong enough to carve out a small magnetosphere within Jupiter's much larger magnetosphere, where the two interact in complex ways. Hubble observations have established the presence of a subsurface saltwater ocean beneath about 150 km of ice. ESA's JUICE (Jupiter Icy Moons Explorer), launched April 2023, will arrive at Jupiter in July 2031 and ultimately enter orbit around Ganymede in December 2034 — the first orbiter of any moon other than Earth's.

Missions and observations

Every Jupiter-system mission has had an opportunity to image or characterize Ganymede. The list below is the Jupiter-system mission catalog; specific Ganymede encounters are documented in mission archives.

Mission	Year at Jupiter	Status
Pioneer 10 NASA	1973	Completed
Pioneer 11 NASA	1974	Completed
Voyager 1 NASA	1979	Completed
Voyager 2 NASA	1979	Completed
Ulysses NASA/ESA	1992	Completed
Galileo NASA	1995	Completed
Cassini-Huygens NASA/ESA/ASI	2000	Completed
New Horizons NASA	2007	Completed
Juno NASA	2016	Active
Europa Clipper NASA	2030	On the way
JUICE ESA	2031	On the way

Naming etymology

Ganymede was a Trojan prince of extraordinary beauty whom Zeus, in eagle form, carried off to Olympus to serve as cupbearer to the gods. He is the only Galilean moon named for a male figure rather than a female lover of Zeus — the IAU's adopted-page-of-Zeus rather than a paramour. The Latin form 'Ganymedes' became the standard spelling; in modern English the name is rendered without the final 's'.

Methodology & sources

Diameter, mass, and orbital parameters from JPL Solar System Dynamics — Physical Parameters. Discovery year and discoverer from the JPL Satellite Discovery Circumstances. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Stylized SVG hero composed from NASA / JPL imagery as visual reference; no photographs are reproduced.