Ganymede is the Solar System’s largest moon, and it is also the only moon known to have its own magnetic field. That field and the auroras around its poles give scientists indirect clues about a vast, likely salty ocean beneath the ice. Here are 10 facts that show what has been observed—and what remains an inference.
1. Ganymede is larger than Mercury by diameter
At about 5,260 kilometers (3,270 miles) across, Ganymede is the largest moon in the Solar System. It exceeds Mercury in diameter, though Mercury is far more massive. “Largest” here refers to size, not planetary status: Ganymede is a moon orbiting Jupiter. NASA’s Ganymede facts page gives the diameter.
2. Galileo discovered it in 1610
Galileo Galilei observed Ganymede on January 7, 1610, along with Jupiter’s other three large moons. Seeing celestial bodies orbit a planet other than Earth helped undermine the idea that everything in the heavens revolved around Earth.
3. A deep ocean is strongly indicated, but has not been directly seen
Scientists infer a salty ocean beneath Ganymede’s ice from how the moon responds to Jupiter’s changing magnetic field. In Hubble observations, Ganymede’s auroras rocked as the magnetic environment changed. The motion was smaller than expected if the moon had no electrically conductive layer below its surface; a salty ocean could conduct electricity and suppress some of that rocking. This is evidence from magnetic effects, not an image or sample of the ocean. NASA described the Hubble findings in 2015.
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4. The estimated ocean would be enormous
NASA reported estimates of an ocean about 100 kilometers (60 miles) thick beneath an approximately 150-kilometer (95-mile) mostly icy crust. The ocean is thought to contain more water than all the water on Earth’s surface. These are estimates of a hidden interior, not direct measurements of its thickness or volume. NASA’s 2015 report explains the estimate.
5. It has its own magnetic field
Ganymede is the only moon known to have an intrinsic magnetic field and a magnetosphere of its own. Galileo spacecraft observations established this distinctive feature. Ganymede’s magnetosphere sits inside Jupiter’s much larger magnetic environment, so the moon’s magnetic field interacts with its planet’s. NASA’s exploration overview summarizes spacecraft findings.
6. Its auroras help probe what lies beneath the ice
Ganymede’s magnetic field produces auroras near its poles. Their changing motion provided the key observation in the ocean inference: the auroras’ rocking, tracked by Hubble as Jupiter’s magnetic field varied, was consistent with a conductive salty ocean beneath the crust. The auroras are visible effects of magnetic interactions; they are not a direct view of the water below.
7. Its interior is modeled as a set of layers
NASA describes a likely metallic iron core, rocky mantle, and mostly icy exterior. Interior models also allow layers of liquid water and ice at depth. These structures are scientific models based on observations and physical calculations, not layers directly inspected by a spacecraft. NASA’s overview of Ganymede outlines the proposed structure.
8. Its surface contrasts old cratered ground with grooved terrain
NASA estimates that dark, heavily cratered terrain covers about 40 percent of Ganymede’s surface, while brighter grooved terrain covers about 60 percent. The ridges and grooves point to large-scale deformation of the crust. Their contrast preserves evidence of different surface histories across the moon. NASA’s facts page reports these approximate proportions.
9. It has a tenuous oxygen atmosphere
Hubble observations found evidence for oxygen around Ganymede, thought to originate from its icy surface. The atmosphere is extremely thin and bears no resemblance to Earth’s dense, breathable air. NASA’s Ganymede reference describes the oxygen atmosphere.
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10. Its orbit is part of a repeating three-moon rhythm
Ganymede takes about 7.155 Earth days to orbit Jupiter and is tidally locked, keeping the same face turned toward the planet. Its orbit is in resonance with Europa and Io: for each Ganymede orbit, Europa completes two and Io completes four. NASA lists the orbital period and resonance.
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Galileo and Juno
NASA’s Galileo spacecraft made several close flybys and established the presence of Ganymede’s intrinsic magnetosphere. On June 7, 2021, Juno flew about 1,046 kilometers (650 miles) above the surface, returning close images and infrared spectral data. NASA reported detections including hydrated sodium chloride, ammonium chloride, sodium bicarbonate, and possible aliphatic aldehydes. These chemical signatures can help scientists investigate Ganymede’s origins and surface history; they are not evidence of life. NASA’s 2023 Juno report details the findings.
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JUICE
NASA identifies Ganymede as the primary scientific target of ESA’s JUICE mission, which is also set to study Europa and Callisto. Mission timing and operational status can change, so consult ESA’s JUICE mission page for current milestones.
Could there be life on Ganymede?
A subsurface ocean makes Ganymede relevant to questions about habitable environments beyond Earth, but habitability is not the same as evidence of life. The ocean has not been sampled, and no life has been detected there. The most accurate conclusion is that Ganymede has a compelling, indirectly inferred ocean and remains a target for investigating its interior and potential habitability.
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