Jupiter’s four major moons range from roughly a quarter to just over two-fifths the diameter of Earth, making even the largest of them far smaller than our planet in overall size. Ganymede, the biggest, stretches about 5,268 kilometers across and actually outpaces Mercury in diameter, yet it still amounts to only about 41 percent of Earth’s width. The size comparison gets more interesting once you look beyond diameter alone, because these moons pack strikingly different interiors, oceans, and even magnetic fields into their relatively compact frames.
The Four Galilean Moons Lined Up Against Earth
The moons that matter most in any size discussion are the four Galilean satellites, discovered in 1610 and visible through a decent pair of binoculars. In order from closest to Jupiter outward, they are Io, Europa, Ganymede, and Callisto. Here is how each stacks up against Earth’s roughly 12,742-kilometer diameter:
- Ganymede: About 5,268 km in diameter, or 41 percent of Earth’s width. It is the largest moon in the entire solar system and bigger than Mercury, though it has far less mass because its interior is loaded with ice.
- Callisto: About 4,821 km across, roughly 38 percent of Earth’s diameter. Nearly the same size as Mercury, Callisto is the most heavily cratered object in the solar system.
- Io: About 3,643 km in diameter, or 29 percent of Earth’s. That makes it slightly larger than Earth’s own Moon, which comes in around 3,474 km.
- Europa: The smallest of the four at roughly 3,122 km across, about 25 percent of Earth’s diameter. Europa is a bit smaller than our Moon, yet it has become one of the most studied objects in the solar system for reasons that have nothing to do with its size.
Diameter alone can be misleading. Earth is far denser than any of these moons, so the mass gap is enormous. Ganymede, despite being wider than Mercury, contains only about 2.5 percent of Earth’s mass. Europa weighs in at less than one percent. If you dropped all four Galilean moons onto a scale together, they would still amount to barely 6 percent of Earth. The reason is straightforward: a large portion of the outer Galilean moons is water ice and other light material, while Earth is dominated by iron and silicate rock all the way through.
Why Diameter and Mass Tell Different Stories
Io is the moon that behaves most like a rocky planet. Its mean density is close to that of Earth’s Moon, despite a completely different bulk composition. Research comparing the two bodies found that Io is much richer in iron and volatile elements than our Moon, with estimated iron-to-silicon ratios roughly two to three times higher, even though both bodies end up at similar overall densities.
Ganymede and Callisto, by contrast, are roughly half ice by mass. Ganymede’s solid ice outer shell alone is estimated to be 890 to 920 kilometers thick, accounting for 46 to 48 percent of the moon’s total mass.1Icarus. Core Sizes and Internal Structure of Earth’s and Jupiter’s Satellites That ice layer is thicker than the distance from New York to Atlanta. On Earth, all the water on the surface, in glaciers, and in underground aquifers would form a layer just a few kilometers thick if spread evenly over the planet. So while Ganymede is physically smaller than Earth in every linear dimension, its proportion of water dwarfs anything on our planet.
Europa sits in between. It is mostly rock and metal by mass, but it wears a comparatively thin ice-and-water shell estimated at 120 to 140 kilometers thick, making up only about 7 to 8 percent of its total mass.1Icarus. Core Sizes and Internal Structure of Earth’s and Jupiter’s Satellites Thin by Ganymede’s standards, but that shell likely hides a liquid ocean of staggering volume.
Europa’s Ocean Versus Earth’s Oceans
Europa’s subsurface ocean is probably the single most surprising size comparison between Jupiter’s moons and Earth. Despite being only a quarter of Earth’s diameter, Europa likely holds about twice the volume of liquid water found in all of Earth’s oceans combined.2PubMed. Europa as an abode of life Earth’s oceans contain roughly 1.335 billion cubic kilometers of water. Europa, a body you could almost hide behind the Moon, may contain something on the order of two to three times that amount in a single global ocean sandwiched between the ice crust above and a rocky seafloor below.
This is possible because Europa’s ocean, if it exists as models predict, extends deep beneath the ice shell rather than sitting in shallow basins the way Earth’s oceans do. Earth’s average ocean depth is about 3.7 kilometers. Europa’s ocean could be 60 to 150 kilometers deep depending on which structural model you use, fed by the thickness constraints of that 120-to-140-kilometer ice-and-water shell. The result is a small world that is, by volume of liquid water, potentially the wettest place in the solar system.
The existence of this ocean is not yet confirmed by direct measurement, but the evidence from magnetic field data, surface geology, and gravitational measurements is strong enough that NASA’s Europa Clipper mission, launched in 2024, is specifically designed to characterize it. If the ocean is confirmed at the volumes currently estimated, Europa would redefine what “small” means in the context of habitability. A moon one-quarter Earth’s width could harbor more habitable liquid water than our entire planet.
Ganymede’s Metallic Core and Magnetic Field
Ganymede is the only moon in the solar system known to generate its own magnetic field, a feature usually associated with much larger bodies like Earth. Generating a sustained magnetic field requires a liquid metallic core that convects vigorously enough to act as a dynamo, and research into Ganymede’s interior suggests that if this field is maintained by thermal convection, the core must be relatively large and strongly enriched in sulfur compared to other rocky-body cores.3Icarus. Size and compositional constraints of Ganymede’s metallic core for driving an active dynamo
Modeling of Ganymede’s internal structure estimates the radius of its iron core at roughly 580 to 650 kilometers, with an iron-sulfur core extending to 820 to 900 kilometers.1Icarus. Core Sizes and Internal Structure of Earth’s and Jupiter’s Satellites For comparison, Earth’s inner core alone is about 1,220 kilometers in radius, and its outer core extends to roughly 3,480 kilometers. Ganymede’s core is a fraction of Earth’s, but it still manages to do something Mercury, Venus, and Mars cannot: sustain an internally generated magnetic field in the present day. The field is weak, roughly a percent of Earth’s, but its mere existence in a body 41 percent Earth’s width is one of the more counterintuitive findings in planetary science.
The Atmosphere Gap
If you are wondering whether any of Jupiter’s moons come close to Earth in atmospheric terms, the answer is a resounding no. Europa has a detected atmosphere of molecular oxygen, but calling it an atmosphere is generous. The surface pressure is about a hundred-trillionth of Earth’s sea-level pressure.4Nature. Detection of an oxygen atmosphere on Jupiter’s moon Europa That is not a thin atmosphere; it is barely distinguishable from the vacuum of space. The oxygen comes not from biological activity but from charged particles in Jupiter’s magnetosphere slamming into Europa’s icy surface, splitting water molecules. The lighter hydrogen escapes easily, leaving a wisp of oxygen behind.
Io has an even more exotic atmosphere, one made largely of sulfur dioxide belched out by its hundreds of active volcanoes. But it is similarly tenuous, collapsing and re-forming as Io moves in and out of Jupiter’s shadow. Ganymede and Callisto have even thinner traces of oxygen and carbon dioxide. None of these envelopes would register as an atmosphere by any standard a person on Earth would recognize. The practical implication is that surface conditions on all four Galilean moons are defined by radiation exposure and vacuum, not by weather or air pressure.
Io and the Puzzle of Similar Sizes, Wildly Different Worlds
One of the more interesting aspects of comparing Jupiter’s moons to Earth is how two bodies of nearly the same size can be utterly unlike each other. Io and Earth’s Moon are close in diameter, separated by less than 200 kilometers, and they share a broadly similar average density. Yet they could not be more different in behavior or composition.
Io is the most volcanically active body in the solar system, with eruptions driven by the immense tidal forces Jupiter exerts on it. Research on Io’s bulk chemistry found that despite density similarities with our Moon, Io is far richer in both iron and volatile materials, with iron-to-silicon ratios estimated at roughly 0.40 to 0.66 compared to the Moon’s 0.22 to 0.24.5Planetary and Space Science. Resemblance and difference between the constitution of the Moon and Io The two bodies have, as the researchers put it, absolutely different bulk compositions despite their superficial resemblance in size and density.
This serves as a useful reminder that diameter is a crude way to compare worlds. Io is constantly resurfacing itself with lava flows, has no impact craters because they get buried, and loses about a ton of material per second to Jupiter’s magnetosphere. Our Moon, at nearly the same size, is geologically dead and has craters that have lasted billions of years. The difference comes down to environment: Jupiter’s gravity produces the tidal heating that keeps Io molten inside, while Earth’s gravity does nothing comparable to our Moon.
Jupiter’s Swarm of Tiny Moons
Beyond the four Galilean satellites, Jupiter has a large and growing catalog of smaller moons. As of recent surveys, the planet has well over 90 confirmed moons, most of them irregular satellites on distant, tilted, and often retrograde orbits. These objects are overwhelmingly tiny. The two largest irregular moons, Himalia and Elara, have estimated radii of about 85 kilometers and 43 kilometers respectively.6The Astrophysical Journal. CAPTURE OF IRREGULAR SATELLITES AT JUPITER To put that in perspective, Himalia would fit comfortably inside a single mid-sized metropolitan area. Compared to Earth’s radius of 6,371 kilometers, these are specks.
The known population of these small irregular moons is likely just a fraction of the actual total. Survey work on retrograde Jovian irregulars estimates that there are, within a factor of two, around 600 such moons down to very faint detection limits.7The Planetary Science Journal. The Population of Kilometer-scale Retrograde Jovian Irregular Moons Most of these are probably only a kilometer or two across, objects smaller than many asteroids. Capture simulations support this picture, suggesting that Jupiter’s gravitational pull snagged far more small bodies than large ones, and that the original captured population may have been whittled down over time by collisions that broke moons apart.6The Astrophysical Journal. CAPTURE OF IRREGULAR SATELLITES AT JUPITER
These irregular moons are thought to be captured objects rather than bodies that formed alongside Jupiter, which explains their chaotic orbits. Formation models suggest that as material accumulated in the disk of gas and dust around young Jupiter, planetesimals passing through were ablated and captured, losing significant fractions of their mass in the process.8Astronomy & Astrophysics. Formation of moon systems around giant planets: Capture and ablation of planetesimals as foundation for a pebble accretion scenario The result is a population of beaten-down remnants, nothing like the planet-scale Galilean moons that assembled from that same disk early in the solar system’s history.
How the Galilean Moons Compare to Other Solar System Bodies
Placing Jupiter’s moons on the broader map of the solar system helps put their size into context. Ganymede and Callisto are both larger than Mercury in diameter, though Mercury is substantially denser and more massive than either. Io and Europa are both roughly Moon-sized: Io slightly larger, Europa slightly smaller. All four Galilean moons are smaller than Mars, which at about 6,779 kilometers in diameter sits roughly halfway between Ganymede and Earth.
Saturn’s moon Titan, at about 5,150 kilometers in diameter, is the only other moon that competes with Ganymede for size, and Titan has the distinction of possessing a thick nitrogen atmosphere that none of Jupiter’s moons can match. Neptune’s Triton, at roughly 2,707 kilometers across, is comparable to Europa in scale. The pattern across the outer solar system is that giant planets tend to host a handful of moon-sized to small-planet-sized satellites plus a swarm of captured debris, and Jupiter’s system is the archetype of this arrangement.
What makes the Jovian system stand out is not raw size but diversity packed into a tight orbital space. Within a span of about 1.9 million kilometers from Jupiter, you get the most volcanically active world known, a moon with a likely ocean holding twice Earth’s water volume, the only moon with its own magnetic field, and one of the most ancient and cratered surfaces in the solar system. Earth is bigger than all of them, but no single planet hosts four satellites this individually remarkable in such close quarters.
Gravity on the Surface
Surface gravity follows from a moon’s mass and radius, and on all four Galilean moons it is far weaker than Earth’s. Ganymede’s surface gravity is about 1.43 meters per second squared, roughly one-seventh of Earth’s 9.8 m/s². Europa’s is similar at about 1.31 m/s². Io comes in at around 1.80 m/s², the highest among the four, thanks to its rocky and relatively dense interior. Callisto’s is about 1.24 m/s². For practical purposes, you would weigh about 85 percent less on any of these moons than you do on Earth.
Low gravity has real consequences for what these worlds can and cannot do. None of them can hold onto a substantial atmosphere at their temperatures and distances from the sun. Gas molecules move fast enough relative to the escape velocity to slowly leak away over geological time. Earth’s gravity, combined with its magnetic field, holds a thick nitrogen-oxygen atmosphere in place. The Galilean moons lack both the gravitational pull and, with the exception of Ganymede’s weak field, the magnetic shielding to do the same. This is one of the most direct ways that the size gap between Jupiter’s moons and Earth translates into a fundamental difference in the kind of world each one can be.