From the dazzling shimmer of Europa’s icy plains to the smog-shrouded skies of Titan, the moons of our solar system are as wondrous and diverse as the planets they orbit. These natural satellites — or lunae, as they’re known scientifically — come in all shapes and compositions, from cratered rockballs to geologically active worlds that rival planets in their own right. In this deep-space guide, we’ll rank the major moons by size and mass, exploring what makes each one extraordinary.
What Exactly Is a Moon?
Scientifically speaking, a moon is a natural satellite — a celestial body that orbits a planet or dwarf planet due to gravity. As of today, astronomers have identified more than 200 moons in our solar system, ranging from massive, planet-like spheres to captured asteroids.
The word “moon” derives from the Old English mōna, but in astronomy, Earth’s moon is Luna; Jupiter’s largest moon is Ganymede; and Saturn’s most famous is Titan. These worlds don’t merely decorate the night skies of their planets — they tell stories about the formation of our solar system, the chemistry of early planetary disks, and possibly, the conditions that foster life.
1. Ganymede — King of the Moons
When it comes to both size and mass, Ganymede reigns supreme. Orbiting Jupiter, Ganymede (scientific name: Jupiter III) has a diameter of 5,268 kilometers (3,273 miles) — making it even larger than the planet Mercury. It’s also the most massive moon, with a mass of about 1.48 × 10²³ kilograms. Ganymede’s surface is a mix of water ice and silicate rock, marked by ridges, grooves, and bright patches of newer terrain. What makes Ganymede truly fascinating is its magnetic field, a feature unique among moons. Generated by a liquid iron-nickel core, this field interacts with Jupiter’s magnetosphere, creating dazzling auroras that dance across its poles. Recent evidence from NASA’s Juno spacecraft suggests Ganymede hides a subsurface ocean buried beneath 100 miles of ice — possibly containing more water than all of Earth’s oceans combined. If life could ever exist beyond Earth, Ganymede would be one of the top contenders.
2. Titan — The Methane World of Saturn
Titan (Saturn VI) takes second place in both size and mass. Measuring 5,151 kilometers (3,200 miles) in diameter, Titan is nearly as big as Ganymede but significantly denser, weighing in at 1.35 × 10²³ kilograms.
Titan is unique for its thick nitrogen atmosphere — the only moon in the solar system with one comparable to Earth’s. Within this golden haze lie methane lakes and hydrocarbon rivers, sculpting an alien landscape that mirrors Earth’s hydrological cycle but with methane and ethane instead of water.
NASA’s Cassini mission revealed dunes made of organic sand, icy volcanoes that may spew ammonia, and potentially, a hidden ocean of liquid water and ammonia beneath its crust. Titan’s chemistry is tantalizingly “prebiotic,” resembling conditions that might have led to life on early Earth.
Future missions, such as NASA’s Dragonfly drone, aim to explore Titan’s surface in detail — perhaps even sampling the chemistry of its lakes.
3. Callisto — The Ancient Crater World
Callisto (Jupiter IV) ranks third, with a diameter of 4,821 kilometers (2,995 miles) and a mass of 1.08 × 10²³ kilograms. Unlike its siblings Ganymede and Europa, Callisto is geologically inactive — a relic from the early solar system.
Its surface is a record of cosmic bombardment, pockmarked with countless impact craters. The massive Valhalla Basin, nearly 4,000 kilometers wide, stands as one of the largest impact structures in the solar system.
Despite its inactivity, Callisto might still hide a salty ocean deep beneath its crust. Its relatively low radiation levels and stable surface make it a potential future outpost for human exploration or even a base for further journeys into the Jovian system.
4. Io — The Volcanic Inferno
Fourth in our ranking is Io (Jupiter I), the most volcanically active body in the solar system. It measures 3,643 kilometers (2,263 miles) in diameter and has a mass of 8.9 × 10²² kilograms.
Io’s interior is heated by tidal forces generated from its elliptical orbit around Jupiter, squeezed by the gravitational pull of both the giant planet and neighboring moons Europa and Ganymede. This internal friction fuels over 400 active volcanoes, which constantly resurface Io’s terrain with sulfur and lava.
Its surface colors — fiery yellows, oranges, and reds — result from sulfur compounds, making Io look like a living painting of chaos. Despite its beauty, Io is a hostile world, with temperatures swinging drastically and a thin, toxic atmosphere of sulfur dioxide.
5. Earth’s Moon — The Familiar Giant
Our own Moon (Luna) ranks fifth, but it’s no less remarkable. With a diameter of 3,475 kilometers (2,159 miles) and a mass of 7.35 × 10²² kilograms, Luna is the fifth largest moon and among the densest. Formed about 4.5 billion years ago, likely from a colossal collision between Earth and a Mars-sized body named Theia, the Moon has profoundly shaped life on Earth. Its gravitational influence stabilizes our planet’s tilt, moderating the climate and creating the ocean tides that have influenced biological evolution. The Moon’s surface tells a story of cosmic history — vast basaltic plains (maria), highlands, and impact craters. Humanity’s exploration of Luna, from Apollo 11 in 1969 to Artemis missions planned for the mid-2020s, continues to deepen our understanding of planetary formation and habitability.
6. Europa — The Ocean World Beneath the Ice
Next is Europa (Jupiter II), with a diameter of 3,122 kilometers (1,940 miles) and a mass of 4.8 × 10²² kilograms. Beneath its dazzling, cracked ice crust lies one of the most promising environments for extraterrestrial life — a global subsurface ocean of liquid water kept warm by tidal heating.
Europa’s surface is one of the smoothest in the solar system, crisscrossed by dark streaks and chaotic terrains formed by shifting ice plates. Scientists believe this icy shell is 10–30 kilometers thick, floating atop an ocean that may be 60–150 kilometers deep.
The upcoming Europa Clipper mission (set to launch this decade) will perform multiple flybys, scanning for organic molecules, plumes, and potential biosignatures. If life exists elsewhere in our solar system, Europa may be where we find it first.
7. Triton — Neptune’s Captured Wanderer
Triton (Neptune I) is the seventh largest moon, with a diameter of 2,710 kilometers (1,680 miles) and a mass of 2.14 × 10²² kilograms. It orbits Neptune in the opposite direction of the planet’s rotation — a retrograde orbit that suggests Triton was once a dwarf planet captured from the Kuiper Belt. Triton’s surface is a mix of frozen nitrogen, water ice, and methane. Voyager 2’s 1989 flyby revealed geysers of nitrogen gas erupting from beneath its surface, hinting at internal heat and possible subsurface oceans. At only 38 Kelvin (–235°C), Triton is among the coldest known objects in the solar system, yet it may still harbor cryovolcanic activity. Its youthful surface implies it’s geologically active, making it one of the most intriguing outer solar system worlds.
8. Titania — Queen of Uranus’ Moons
Titania (Uranus III), with a diameter of 1,578 kilometers (981 miles) and a mass of 3.5 × 10²¹ kilograms, is the largest moon of Uranus. Discovered by William Herschel in 1787, Titania combines ancient craters with vast canyons stretching hundreds of kilometers — evidence of past tectonic or cryovolcanic activity.
Its surface is a blend of water ice and rocky material, and its reddish tint hints at space weathering from charged particles. There’s also speculation that Titania could possess a subsurface ocean, similar to Europa or Ganymede, though confirmation awaits future Uranian missions.
9. Rhea — The Icy Relic of Saturn
Rhea (Saturn V), the second largest moon of Saturn, comes next with a diameter of 1,527 kilometers (949 miles) and a mass of 2.3 × 10²¹ kilograms. Its bright, heavily cratered surface reflects light like snow, earning it one of the highest albedos in the solar system. Rhea’s interior seems largely composed of water ice with a small rocky core. Cassini data once suggested the possibility of a thin atmosphere of oxygen and carbon dioxide, created by surface ice chemistry, though it’s far too thin to support life. One of Rhea’s mysteries is its possible ring system — faint debris that may orbit the moon. If confirmed, it would make Rhea the only moon known to have rings.
10. Iapetus — The Two-Tone Mystery
Iapetus (Saturn VIII), with a diameter of 1,471 kilometers (914 miles) and a mass of 1.8 × 10²¹ kilograms, rounds out our top ten. What sets Iapetus apart is its distinct color contrast: one hemisphere is dark as coal, the other bright as snow.
This stark two-tone appearance likely results from dark material deposited by infalling dust from the outer Saturnian moons, combined with sublimation and redeposition processes.
Iapetus also boasts an equatorial ridge up to 20 kilometers tall, making it resemble a cosmic walnut. The moon’s slow rotation — one day lasts 79 Earth days — and its inclined orbit add to its list of celestial oddities.
Honorable Mentions: Beyond the Top Ten
While our ranking focuses on the largest and most massive moons, several smaller satellites deserve mention for their unique properties:
- Enceladus (Saturn II): Famous for its water-ice geysers, which feed Saturn’s E ring and hint at a subsurface ocean containing organic compounds.
- Dione (Saturn IV): Displays bright ice cliffs and potential internal activity.
- Oberon (Uranus IV): A heavily cratered world that might conceal frozen subsurface oceans.
- Charon (Pluto I): Technically orbiting a dwarf planet, but massive enough to form a binary system with Pluto.
- Miranda (Uranus V): Known for its chaotic surface with cliffs and canyons towering up to 20 kilometers high.
The Science Behind Ranking Moons
Ranking moons by size and mass isn’t just about numbers — it reveals deep insights into how planets and their satellites formed. Larger moons tend to have undergone differentiation, separating into core, mantle, and crust layers, while smaller ones remain more primitive.
For example, Ganymede’s dense, layered structure and magnetic field show it once had — and still has — significant internal heat. In contrast, Callisto’s uniform interior suggests it never fully melted. These differences tell us how varying levels of radiation, distance from the planet, and gravitational interactions shape a moon’s evolution.
Mass also determines gravitational influence — heavier moons can retain atmospheres (like Titan) or maintain liquid interiors due to pressure and heating effects. Comparing density helps distinguish ice-rich worlds from rocky ones, revealing their composition and origins.
Could Moons Harbor Life?
Perhaps the most thrilling revelation from moon studies is their potential to host life. Moons like Europa, Ganymede, Titan, and Enceladus all have ingredients necessary for habitability: liquid water, organic molecules, and energy sources. Europa and Enceladus, in particular, show evidence of active oceans in contact with rocky seafloors, where hydrothermal vents could sustain microbial ecosystems. Titan, though cold, boasts complex organic chemistry that could represent a different kind of life altogether — one not based on water. Future missions such as Europa Clipper and Dragonfly may soon answer one of humanity’s most profound questions: Are we alone in the universe?
The Future of Lunar Exploration
As our technology advances, the next few decades promise a golden era for moon exploration. NASA, ESA, and other space agencies plan missions to nearly every major moon listed here — from orbiters and landers to autonomous drones and even subsurface probes.
- Europa Clipper (NASA) will search for biosignatures and measure ocean depth.
- JUICE (ESA) — Jupiter Icy Moons Explorer — will study Ganymede, Callisto, and Europa.
- Dragonfly (NASA) will fly across Titan’s surface in the 2030s.
- Future Uranus and Neptune missions may finally bring close-up views of Titania, Oberon, and Triton.
These endeavors won’t just uncover new data; they’ll expand our cosmic perspective, bridging the gap between planetary science and astrobiology.
The Moons as Gateways to Understanding the Cosmos
Ranking moons by size and mass offers more than a chart of statistics — it paints a story of creation, evolution, and potential. From Ganymede’s planetary grandeur to Iapetus’s enigmatic duality, each moon reflects a chapter in the solar system’s grand narrative.
They remind us that even the smallest celestial bodies can hold vast mysteries, and that exploration — whether robotic or human — is not merely about discovery but about connection. When we look up at the night sky, we’re not just seeing distant rocks orbiting faraway giants; we’re glimpsing the building blocks of worlds — and perhaps, the seeds of life itself.
