Ganymede is the largest moon in the Solar System, but its most intriguing feature may be hidden beneath its ice: observations of its auroras provide evidence for a deep, salty ocean. Here are 10 facts about what scientists have measured, what they infer, and what remains unknown.
1. Ganymede is larger than Mercury by diameter
At about 5,260 kilometers (3,270 miles) across, Ganymede is the Solar System’s largest moon. It is larger than Mercury by diameter, though it is a moon orbiting Jupiter—not a planet. NASA lists the diameter as an approximate measurement.
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2. Galileo discovered it in 1610
Galileo Galilei first observed Ganymede on January 7, 1610. His observations of Jupiter’s four large moons—Ganymede, Io, Europa and Callisto—showed that celestial bodies could orbit something other than Earth, helping challenge the Earth-centered picture of the cosmos. NASA’s Ganymede facts page recounts the discovery.
3. A hidden saltwater ocean is strongly indicated
Ganymede’s ocean has not been directly seen or sampled. The evidence comes in part from Hubble observations of the moon’s auroras. As Jupiter’s magnetic field changes, Ganymede’s auroras should rock; their observed motion was smaller than expected. A salty, electrically conductive ocean beneath the ice could damp that motion. This is an inference from magnetic effects, not a direct detection of liquid water. NASA described the Hubble observations in 2015.
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4. The estimated ocean would be immense
NASA reported estimates of an ocean about 100 kilometers (60 miles) thick beneath an approximately 150-kilometer (95-mile) mostly icy crust. These are model-based estimates, not measurements taken by a probe inside Ganymede. The inferred ocean may contain more water than all the water on Earth’s surface combined. NASA’s 2015 report explains the estimate and its basis.
5. It is the only known moon with its own magnetic field
Ganymede generates an intrinsic magnetic field and a magnetosphere of its own—the only moon known to do so. Its magnetic environment sits inside Jupiter’s much larger magnetosphere, where the two interact. NASA’s Galileo spacecraft made close flybys and established this distinctive feature through spacecraft observations. NASA summarizes Ganymede’s exploration history.
6. Its auroras help scientists probe the hidden interior
Ganymede’s magnetic field produces auroras near its poles. Their motion responds to changes in Jupiter’s magnetic field, so tracking how much they rock offers a way to investigate the moon’s magnetic environment. The reduced rocking observed by Hubble is consistent with a conductive salty ocean below the surface. The auroras are observable; the ocean remains an interpretation of those observations. NASA’s Hubble release describes how the auroras informed the ocean inference.
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7. Scientists model Ganymede as a layered world
NASA describes a likely interior with a metallic iron core, a rocky mantle and an outer region made mostly of ice. Models also allow liquid-water and ice layers to be stacked at depth. These layers have not been directly inspected; they are an interior picture inferred from measurements and modeling. NASA’s facts page outlines the modeled structure.
8. Its surface has dark cratered and bright grooved regions
Ganymede’s surface preserves two striking kinds of terrain. NASA estimates that heavily cratered, dark terrain covers about 40 percent of the surface, while brighter grooved terrain covers about 60 percent. The long ridges and grooves point to large-scale deformation of the icy crust, though the percentages describe mapped terrain rather than a direct measure of its age. NASA provides the terrain estimates and description.
9. It has a tenuous oxygen atmosphere
Hubble found evidence for oxygen around Ganymede, which is thought to originate from the icy surface. The atmosphere is extremely thin and is nothing like Earth’s breathable air. NASA notes the evidence for oxygen and the atmosphere’s tenuous nature.
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10. Its orbit is part of a repeating three-moon rhythm
Ganymede completes one orbit around Jupiter in about 7.155 Earth days and is tidally locked, keeping the same face pointed toward the planet. Its orbit is linked to those of Europa and Io in a repeating resonance: for every one Ganymede orbit, Europa completes two and Io completes four. NASA lists the orbital period and resonance.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesWhat Juno found—and what it does not prove
NASA’s Juno spacecraft flew about 1,046 kilometers (650 miles) above Ganymede on June 7, 2021. In data reported in 2023, the spacecraft detected surface signatures including hydrated sodium chloride, ammonium chloride and sodium bicarbonate, along with possible aliphatic aldehydes. These chemical clues can help researchers study Ganymede’s origins and surface history; they are not evidence of life, nor proof that Juno sampled ocean water. NASA’s Juno report details the flyby and detections.
Ganymede is the primary scientific target of ESA’s JUICE mission, which will also study Europa and Callisto. That focus reflects the moon’s unusual magnetic field, possible ocean and complex surface. The evidence summarized here does not establish JUICE’s current operational status or schedule; ESA’s mission page is the source to consult for current milestones.
Does the ocean mean Ganymede has life?
No life has been detected on Ganymede. The ocean is inferred rather than directly observed, and the evidence does not establish whether it has the conditions needed for life. A subsurface ocean makes Ganymede scientifically interesting as a possible habitable environment, but that possibility is a question for future investigation, not a discovery.
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