The four largest moons orbiting Jupiter—Io, Europa, Ganymede, and Callisto—are known as the Galilean moons, named for their discoverer, Galileo Galilei. These are not small rocks, but substantial planetary-mass objects with a significant presence in the outer solar system. To understand their true scale, it is necessary to compare their dimensions directly to Earth and its satellite. This comparison reveals that Jupiter’s moons are immense, challenging the common perception of what a moon can be.
Establishing the Earth Baseline
Earth serves as the natural standard for comparison when discussing the size of other solar system bodies. Our planet has an average diameter of approximately 12,742 kilometers and a total mass of about 5.97 x 10^24 kilograms. These metrics define the scale of a terrestrial planet.
Earth’s own Moon is a large satellite relative to its parent planet, but its size is modest on a solar system scale. The Moon has a diameter of roughly 3,475 kilometers, making it about 27% the diameter of Earth. This provides a baseline before introducing the scale of the Jovian system.
Direct Comparison of the Galilean Moons to Earth
Jupiter’s four largest moons are massive, with Ganymede being the largest moon in the entire solar system. Ganymede has a diameter of approximately 5,262 kilometers. This means Ganymede measures about 41% of Earth’s total diameter and holds nearly 2.5% of Earth’s mass.
Callisto, the outermost of the four, is the next largest with a diameter of about 4,821 kilometers. This size is nearly 38% of Earth’s diameter, and its total mass is approximately 1.8% of Earth’s mass. This places Callisto in a different class than most other satellites.
Io is the innermost of the Galilean moons and has a diameter of roughly 3,642 kilometers. Io is marginally larger than Earth’s Moon and represents nearly 29% of Earth’s diameter. Its mass is about 1.5% of Earth’s mass, making it the densest of the four Galilean satellites.
Europa is the smallest of the four, with a diameter of approximately 3,122 kilometers, which is slightly smaller than Earth’s Moon. Europa’s diameter is about 24.5% of Earth’s diameter. Despite its smaller size among its siblings, Europa contains about 0.8% of Earth’s total mass.
Contextualizing Their Solar System Scale
The scale of Jupiter’s moons is clearer when compared to non-lunar bodies in the solar system. Ganymede, the largest moon, has a diameter of 5,262 kilometers, which makes it larger than the planet Mercury (4,880 kilometers). This makes Ganymede a planetary-scale object.
Callisto’s 4,821-kilometer diameter is only slightly less than that of Mercury. Io and Europa are both significantly larger than the dwarf planet Pluto. Pluto has a diameter of approximately 2,377 kilometers, meaning all four Galilean moons are larger and more massive than Pluto.
The comparison to Pluto highlights that the four Galilean moons collectively represent a substantial portion of the solar system’s mass outside of the Sun and the eight major planets. These satellites are massive worlds in their own right.
Physical Implications of Their Massive Size
The large size of the Galilean moons has directly influenced their geological structure and complexity. Their mass provides sufficient self-gravity to pull them into a spherical shape. This gravitational force also causes internal differentiation, where denser materials like rock and iron sink to form a core, separating from lighter outer layers of ice.
Ganymede’s large size is responsible for it being the only moon in the solar system known to generate its own magnetic field. This requires the presence of a liquid iron core, which is only possible due to the sustained internal heat driven by its large mass and gravitational interactions. Similarly, the size and mass of Europa and Ganymede provide the necessary conditions to retain vast, liquid subsurface oceans beneath their icy crusts.
Io’s mass, combined with the intense gravitational flexing from Jupiter, results in extreme tidal heating. This internal friction generates enough heat to make Io the most volcanically active world in the solar system. The massive scale of these satellites has allowed them to develop complex, multi-layered interiors and dynamic processes.