Ganymede’s diameter is about 5,268 kilometers, which makes it roughly 41 percent the width of Earth. That alone makes it the largest moon in the solar system and, remarkably, bigger than the planet Mercury. But size and mass tell very different stories here: Ganymede packs only about 2.5 percent of Earth’s mass into that frame, because it is built from fundamentally lighter stuff. Understanding why a world so physically large can be so much less massive than Earth gets at the heart of what makes Ganymede one of the most scientifically fascinating objects orbiting Jupiter.
Diameter, Mass, and Why They Don’t Scale Together
If you could set Ganymede next to Earth, it would look like a slightly undersized tennis ball beside a regulation basketball. At 5,268 km across, Ganymede edges out Mercury’s 4,879 km diameter by a comfortable margin. Yet Mercury is more than twice as massive. The reason is density. Earth’s average density sits around 5.5 grams per cubic centimeter, thanks to a large iron-nickel core and a rocky silicate mantle. Ganymede’s density is only about 1.94 g/cm³, barely twice that of water. It is made of roughly equal parts ice and rock by mass, a composition shared with its neighbor Callisto.1Encyclopedia of the Solar System. Ganymede and Callisto
This ice-rock mix means that Ganymede’s volume tells you something Earth’s volume does not. On Earth, a large fraction of the interior is compressed iron and silicate under enormous pressure, driving up the density. On Ganymede, a thick shell of water ice hundreds of kilometers deep dominates the outer layers. If you squeezed all that ice out and kept only the rocky and metallic material, what remained would be much smaller than it looks from the outside. In practical terms, Ganymede’s surface gravity is only about 1.4 m/s², or roughly one-seventh of what you feel standing on Earth. You could jump several meters high with a casual effort.
What the Inside Looks Like
Despite being an icy moon, Ganymede has a layered interior that is more Earth-like in structure than most objects its size. Data from NASA’s Galileo spacecraft showed that Ganymede is fully differentiated, meaning its heavier materials sank toward the center and lighter materials floated outward over time. At the center sits a metallic core estimated at somewhere between 400 and 1,300 km in radius, surrounded by a silicate mantle, which is itself wrapped in an ice shell roughly 800 km thick.2Nature. Gravitational constraints on the internal structure of Ganymede
That ice shell alone is enormous. Eight hundred kilometers is about the distance from New York to Chicago. Earth has nothing comparable; our planet’s water exists almost entirely as a thin layer on the surface and vapor in the atmosphere. On Ganymede, water in various forms of ice (and likely liquid in between) constitutes a massive fraction of the moon’s total volume.
This differentiation makes Ganymede notably different from its sister moon Callisto, which is almost the same size but appears to have its ice and rock still mostly mixed together rather than sorted into neat layers.1Encyclopedia of the Solar System. Ganymede and Callisto Why two moons of similar size ended up so structurally different is one of the enduring puzzles of Jovian science. Recent modeling suggests the answer comes down to where each moon formed within Jupiter’s surrounding disk of gas and dust: the disk was warmer closer to Jupiter, where Ganymede formed, and cooler farther out where Callisto assembled. That temperature difference, combined with Ganymede’s slightly larger final size, may have given Ganymede enough internal heat to melt and separate its materials while Callisto never quite got there.3The Planetary Science Journal. Conditions for Accretion Favoring an Unmelted Callisto and a Differentiated Ganymede
The Only Moon With Its Own Magnetic Field
Earth generates a magnetic field through churning liquid iron in its outer core. Ganymede does something similar, which is extraordinary for a moon. It is the only moon in the solar system known to produce its own intrinsic magnetic field. Galileo mission measurements pegged the field strength at about 750 nanoteslas at the surface, tiny compared to Earth’s field but remarkable for an icy world.4Icarus. The production of Ganymede’s magnetic field
Researchers believe this field is generated by dynamo action in Ganymede’s metallic core, essentially molten or partially molten iron (possibly mixed with iron sulfide) stirring around and generating electric currents that sustain a magnetic field.5Nature. The magnetic field and internal structure of Ganymede How that dynamo has kept running for billions of years is still debated. One possibility is that the core formed gradually rather than all at once, with dense iron-rich melt slowly settling onto a growing central mass and continuously stirring the liquid metal enough to keep the dynamo alive.6PubMed Central. Powering Ganymede’s dynamo with protracted core formation Another line of modeling suggests that maintaining the dynamo requires either a very sulfur-poor or very sulfur-rich core composition, alongside a silicate mantle that can shed heat efficiently.4Icarus. The production of Ganymede’s magnetic field
For comparison, Mercury has a weak magnetic field too, and Mars once did but lost it billions of years ago. Ganymede’s field is embedded within Jupiter’s enormous magnetosphere, creating a complex interaction where Ganymede has its own mini-magnetosphere carving out a bubble inside Jupiter’s much larger one. No other moon in the solar system does this. It is one of the things that makes Ganymede feel less like a typical satellite and more like a small planet that happens to orbit another planet rather than a star.
Surface Gravity and What It Would Feel Like
Standing on Ganymede would feel dramatically different from standing on Earth. At roughly one-seventh Earth gravity, a person who weighs 70 kg on Earth would feel as if they weighed about 10 kg. Walking would be more like bounding, and dropping a rock would look like slow motion. This low gravity is a direct consequence of Ganymede’s low density. Even though it has a diameter 41 percent of Earth’s, its mass is only about 2.5 percent of Earth’s, so the gravitational pull at the surface is weak.
Compare that to the Moon, which has about one-sixth Earth gravity despite being much smaller in diameter. The Moon is denser (about 3.3 g/cm³) because it is mostly rock and lacks Ganymede’s enormous ice fraction. So Ganymede and the Moon end up with similar surface gravity for different reasons: Ganymede is bigger but fluffier, the Moon is smaller but denser.
Almost No Atmosphere, and a Frozen Surface
Earth’s atmosphere is thick enough to keep surface temperatures livable, create weather, and block harmful radiation. Ganymede has something loosely called an atmosphere, but it is vanishingly thin. Observations from the Hubble Space Telescope found tenuous amounts of molecular oxygen and water vapor around the moon, with the oxygen column density hovering in the low trillions of molecules per square centimeter and the water vapor varying depending on which hemisphere faces the Sun.7Icarus. Ganymede’s atmosphere as constrained by HST/STIS observations By any practical standard, this is a near-vacuum. You could not breathe, and the “atmosphere” provides essentially no insulation or pressure.
Surface temperatures reflect that. Millimeter-wavelength observations of Ganymede yield brightness temperatures around 90 to 99 Kelvin, depending on the wavelength, which translates to roughly minus 174 to minus 183 degrees Celsius.8IOP Publishing. Ganymede’s Surface Properties from Millimeter and Infrared Thermal Emission Earth’s average surface temperature is about 288 Kelvin (15°C), so Ganymede’s surface is nearly 200 degrees colder. The surface itself is a mix of bright, grooved icy terrain and older, darker regions pocked with craters.
A Hidden Ocean Beneath the Ice
One of the most intriguing possibilities about Ganymede is that a liquid water ocean may exist beneath its thick ice shell. The evidence is indirect but accumulating. Ganymede’s magnetic field produces auroral bands, and observations of how those auroras rock back and forth in response to Jupiter’s rotating magnetic field suggest an electrically conductive layer beneath the surface, most likely saltwater. The ice shell above this hypothetical ocean could be well over a hundred kilometers thick, meaning the ocean would sit deep inside the moon, sandwiched between layers of ice.
If this ocean exists, it would contain more water than all of Earth’s oceans combined, simply because Ganymede is so large and its ice-water layer so deep. Whether that ocean is habitable in any sense is a separate and much harder question. The ocean floor would rest against high-pressure ice rather than rock, which limits the chemical interactions that some researchers think are important for life. Still, the sheer volume of liquid water makes Ganymede a target for further investigation.
Tidal Heating and the Laplace Resonance
Ganymede orbits Jupiter as part of a gravitational dance with two other large moons, Io and Europa, in what is called the Laplace resonance. For every orbit Ganymede completes, Europa completes two and Io completes four. This lockstep relationship means the three moons regularly tug on each other, and those gravitational tugs can pump energy into a moon’s orbit and interior through tidal flexing.
Today, the tidal heating in Ganymede is modest, especially compared to Io (which is the most volcanically active body in the solar system) and Europa. But modeling of how the moons evolved into their current orbital configuration suggests that Ganymede may have experienced periods of much more intense tidal heating in the past. One study found that certain resonance configurations could have pumped Ganymede’s orbital eccentricity high enough to produce tidal heating several hundred times greater than what occurs now.9Icarus. Tidal Evolution into the Laplace Resonance and the Resurfacing of Ganymede That kind of internal energy release could explain why parts of Ganymede’s surface appear to have been resurfaced, with younger grooved terrain overwriting older cratered regions. It may also have contributed the heat needed for Ganymede to fully differentiate its interior, separating ice from rock from metal in the way that Callisto never managed.
What Juno Revealed
NASA’s Juno spacecraft, primarily a Jupiter orbiter, made a close flyby of Ganymede in June 2021, passing within about 1,038 km of the surface. This was the closest any spacecraft had come to Ganymede since Galileo in 2000. Juno’s radio tracking data, combined with older Galileo measurements, allowed scientists to update the moon’s gravity field. The results confirmed that Ganymede’s overall shape is broadly consistent with a body in hydrostatic equilibrium, meaning it has settled into the shape you would expect from its rotation and tidal forces. But hints of localized gravity anomalies at higher resolution suggest there are non-uniform features beneath the surface, possibly tied to variations in ice shell thickness or density.10PubMed Central. Juno’s Close Encounter With Ganymede—An Overview
A revised analysis of Ganymede’s moment of inertia, which reflects how mass is distributed from center to surface, came with wider error bars than earlier estimates. This means scientists are now less certain about the exact densities of Ganymede’s internal layers than they were after Galileo, though the basic layered structure is not in doubt.11Geophysical Research Letters. Gravity Field of Ganymede After the Juno Extended Mission The higher central value of the moment of inertia also hints that Ganymede may be slightly less sharply differentiated than the cleanest models suggest, meaning the boundaries between its layers could be more gradual than a simple three-layer cartoon implies.
How Ganymede Compares to Other Large Moons
Ganymede is the biggest moon, but it exists in a neighborhood of genuinely large worlds. Saturn’s Titan is a close second in diameter at about 5,150 km, and it has the distinction of being the only moon with a thick, hazy atmosphere. Callisto, Ganymede’s Jovian sibling, comes in at about 4,821 km. Io and Europa are smaller but far more geologically active (Io) or astrobiologically compelling (Europa) in their own right.
Against Earth’s Moon, the size gap is substantial. Our Moon is about 3,475 km in diameter, roughly two-thirds the width of Ganymede. But the Moon is denser and has stronger surface gravity per unit of volume. Earth’s Moon would look noticeably smaller next to Ganymede, yet it would feel more “rocky-planet-like” underfoot.
What sets Ganymede apart from all other moons is the combination of its size, its layered interior, its magnetic field, and the likely presence of a subsurface ocean. No other moon checks all four boxes. Titan has a thick atmosphere and possibly a subsurface ocean but no intrinsic magnetic field. Europa has a subsurface ocean and sits inside Jupiter’s magnetosphere but is much smaller and has no field of its own. Ganymede is, in a sense, the most planet-like moon we know of.
JUICE and the Next Chapter of Exploration
The European Space Agency’s JUICE (Jupiter Icy Moons Explorer) mission launched in April 2023 and is expected to arrive at Jupiter in 2031. After a series of flybys of Europa and Callisto, it will eventually enter orbit around Ganymede, making it the first spacecraft to orbit a moon other than Earth’s. One of the key instruments aboard is the Ganymede Laser Altimeter (GALA), which will map Ganymede’s topography in fine detail and measure how the moon’s shape changes over the course of its orbit due to tidal flexing from Jupiter.12Space Science Reviews. The Ganymede Laser Altimeter (GALA) on the Jupiter Icy moons Explorer (JUICE) Mission
Those tidal deformation measurements are the most direct way to test whether a liquid ocean exists beneath the ice. If the ice shell floats on liquid water, the surface will flex much more in response to Jupiter’s gravity than if the interior is solid all the way through. GALA will also study the roughness and structure of the surface, contributing to our understanding of how ice tectonics work on a world where “bedrock” is frozen water rather than silicate.13Advances in Space Research. The Ganymede Laser Altimeter (GALA) for the Jupiter Icy Moons Explorer (JUICE)
JUICE carries a suite of other instruments as well: radar to probe beneath the surface, magnetometers to study the magnetic field up close, and cameras to image the surface at much higher resolution than anything we currently have. By the time the mission ends in the mid-2030s, Ganymede should go from being a moon we know in broad strokes to one we understand in genuine detail. Given that this world is larger than Mercury, possesses its own magnetic field, and may harbor an ocean holding more water than Earth, the scientific payoff could reshape how we think about where in the solar system the conditions for interesting chemistry, and possibly life, might exist.