7 largest moons in the solar system, explained by size and origin

Size, not quantity, is what separates a handful of moons from the rest of the solar system's satellites. Hundreds of moons have been catalogued, and the overwhelming majority are small, irregular lumps of rock and ice — many of them captured asteroids only a few kilometers across. Seven are different. Each is massive enough to have collapsed into a sphere under its own gravity and settled into distinct internal layers. Several have gone further, developing an atmosphere, a buried ocean, or eruptions that still rework the surface.

Ganymede, orbiting Jupiter, has a greater diameter than Mercury. Titan, at Saturn, carries a nitrogen atmosphere thicker than the air at Earth's surface and holds seas of liquid methane and ethane near its poles. Io, also circling Jupiter, is the most volcanically active body yet found. Earth's own satellite ranks only fifth, despite being unusually large next to the planet it circles.

Ranking the largest moons in the solar system by diameter does more than settle a trivia question. Diameter tracks closely with how much internal heat a world retains, how long it stays geologically active, and whether it can hold on to gas at its surface. It also explains why so much current planetary exploration points at moons rather than planets. The European Space Agency's JUICE spacecraft is on its way to Ganymede. NASA's Europa Clipper launched toward Jupiter in October 2024, and NASA's Dragonfly rotorcraft is being built to fly through Titan's dense air.

What follows runs through all seven in order of measured diameter, from Ganymede down to Triton, the moon Neptune appears to have captured from the Kuiper Belt. Each entry covers how big the moon is, which planet it orbits, and what it is made of. It also names the feature that keeps each one on mission planners' lists — a self-generated magnetic field, a hydrocarbon sea, a crater record four billion years old, or nitrogen geysers. Five of the seven are thought to hold liquid water beneath their crusts.

Illustration: Quartz

Ganymede measures about 5,268 kilometers across, making it the largest moon in the solar system and wider than the planet Mercury. It circles Jupiter once every seven days, tidally locked so that the same hemisphere always faces the planet. It was one of the four satellites Galileo Galilei spotted through a telescope in 1610.

Ganymede is the only moon confirmed to generate its own magnetic field, produced by a dynamo in its liquid iron core. That field carves out a small magnetosphere inside Jupiter's far larger one. It funnels charged particles toward the poles, where they light up as auroras. The Hubble Space Telescope has tracked those auroral belts. The way they rock back and forth in response to Jupiter's magnetic environment points to a conductive layer beneath the surface.

That layer is a salty ocean, sealed under an ice shell and thought to hold more water than all of Earth's oceans combined. Ganymede's interior is more clearly layered than those of the other Galilean moons, with rock, metal and several phases of ice sorted into separate shells. The ocean probably sits sandwiched between ice layers rather than resting on rock. That arrangement limits the mineral chemistry available to any potential biology, without ruling out habitability.

The surface records two different histories. Roughly a third of it is dark, ancient terrain saturated with impact craters. The rest is brighter ground cut by long parallel grooves and ridges, formed as tectonic stress fractured and stretched the crust. A thin oxygen atmosphere clings to the moon, far too tenuous to breathe or to hold heat.

The European Space Agency's JUICE spacecraft, launched in 2023, is scheduled to reach the Jupiter system in 2031 and later settle into orbit around Ganymede. That maneuver would make it the first spacecraft to orbit a moon other than Earth's own.

Illustration: Quartz

Titan is the second largest moon in the solar system, about 5,149 kilometers across, and the only moon known to have a dense atmosphere. It orbits Saturn, and the Dutch astronomer Christiaan Huygens identified it in 1655.

That atmosphere presses down at the surface with roughly 1.5 times the pressure of Earth's, and nitrogen makes up most of it. Sunlight breaking apart methane in the upper layers produces a thick orange haze. The haze kept the surface hidden from telescopes and from the Voyager flybys.

Titan is the only world besides Earth confirmed to have stable liquid standing on its surface. The liquid is methane and ethane rather than water. Rivers cut channels through the terrain, rain falls, and seas pool near the north pole. Kraken Mare, the largest of those seas, covers an area estimated to exceed that of the Caspian Sea.

NASA's Cassini spacecraft orbited Saturn from 2004 to 2017 and mapped the seas with radar that penetrated the haze. Cassini also carried the European Space Agency's Huygens probe. The probe descended by parachute and touched down in 2005, the first landing on a moon in the outer solar system and still the only one. Pictures taken during the descent showed drainage networks feeding a plain littered with rounded pebbles of water ice.

Below the crust, Titan likely holds an ocean of liquid water mixed with ammonia. That would place it alongside the ice-shelled moons of Jupiter among the bodies worth examining for habitable conditions. Surface temperatures sit near minus 179 degrees Celsius, far too cold for water to stay liquid in the open.

NASA's Dragonfly mission is scheduled to launch in 2028 and land on Titan in 2034. The nuclear-powered rotorcraft is designed to fly between sites and sample the moon's organic chemistry at each one, rather than study a single fixed location.

Illustration: Quartz

Callisto measures about 4,821 kilometers across, ranking third among the largest moons in the solar system, and its surface is the most densely cratered of any object yet surveyed. Nothing has erased the impact record there for roughly four billion years.

The outermost of Jupiter's four Galilean satellites, Callisto sits far enough from the planet to have escaped the tidal squeezing that keeps Io and Europa geologically restless. Without that internal heat engine, its crust froze early and stayed put. Craters accumulated layer upon layer, until new impacts could only land on top of older ones.

Valhalla dominates the visible hemisphere — a multi-ring impact structure whose concentric fractures spread outward for hundreds of kilometers from a bright central plain. The rings formed when a large body punched through the icy crust and the surrounding material rebounded in waves that froze in place. Asgard, a smaller basin on the same moon, repeats the pattern at reduced scale.

Data from NASA's Galileo orbiter pointed to something beneath that static exterior. The way Callisto responds to Jupiter's rotating magnetic field suggests a layer of electrically conductive salty water tens of kilometers down. If that reading holds, Callisto is a third ocean-bearing world in the Jupiter system, alongside Ganymede and Europa, despite showing no surface activity.

Its bulk composition runs close to half rock and half ice. Its density indicates the interior never fully separated into a dense core and a lighter mantle, as Ganymede's did. Callisto is therefore a partially differentiated body — an intermediate case between a fully layered moon and a uniform mix of rock and ice.

Distance from Jupiter brings a practical advantage. Callisto orbits outside the harshest part of the planet's radiation belts, and NASA studies of crewed missions to the outer planets have treated it as a plausible staging point for that reason. It completes one orbit every 16.7 days and keeps the same face toward Jupiter throughout.

Illustration: Quartz

Io erupts more than any other world yet observed, with hundreds of active volcanoes spread across a surface about 3,643 kilometers wide. It ranks fourth among the largest moons in the solar system and orbits closer to Jupiter than any of the planet's other large moons.

The energy comes from tidal heating. A gravitational relationship with Europa and Ganymede keeps Io's orbit slightly elongated, so Jupiter's pull stretches and squeezes the interior on every circuit. The flexing generates friction deep in the rock. That heat has nowhere to go but up, feeding magma to the surface almost continuously.

Plumes climb to heights no terrestrial eruption approaches. NASA's Voyager spacecraft and, later, the Galileo orbiter recorded eruptions throwing material more than 300 kilometers above the ground. Loki Patera, a lava lake roughly 200 kilometers across, is among the most persistent heat sources on the moon, and it brightens and dims on a rough cycle astronomers have followed from Earth.

Sulfur and sulfur dioxide frost coat much of the terrain, which is why images show a mottled spread of yellow, orange, white and red rather than the gray and brown of a cratered world. Impact craters are essentially absent. Lava flows and fallout bury them faster than they accumulate, and the surface can look measurably different from one visit to the next.

Io is also close to dry. Whatever water it began with was largely lost to heat and to the radiation it sits in, which sets it apart from the ice-covered moons farther out.

The radiation environment is severe. Io orbits deep inside Jupiter's belts, absorbing doses that would disable most spacecraft quickly, and no lander has been sent. NASA's Juno probe has made a series of close flybys since 2023 instead, imaging lava lakes and plumes from a distance. One lap around Jupiter takes Io about 1.77 days — the quickest circuit of the four large Jovian moons.

Illustration: Quartz

Earth's moon is the fifth largest moon in the solar system, about 3,474 kilometers across, and it is the largest satellite relative to the planet it orbits — roughly a quarter of Earth's diameter. No other planet carries a companion so close to its own size.

That ratio sits at the center of the leading account of how the moon formed. The giant-impact hypothesis holds that a body roughly the size of Mars, often called Theia, struck the young Earth about 4.5 billion years ago. Debris thrown into orbit by that collision later gathered into a single satellite. Rock samples brought back by NASA's Apollo missions between 1969 and 1972 support the idea. Their chemistry closely resembles Earth's mantle rather than that of an unrelated world.

The surface divides into two kinds of terrain. The dark plains visible from Earth, called maria, are basalt flows that filled large impact basins with lava billions of years ago. The brighter highlands are older and far more densely cratered. The moon has no appreciable atmosphere and no global magnetic field, so nothing erodes those features or deflects the solar wind. Footprints and small craters survive for geological spans of time.

The same hemisphere stays turned toward Earth, a consequence of tidal locking, and one orbit takes about 27 days. Lunar gravity raises the ocean tides. It also steadies the tilt of Earth's rotational axis, which keeps seasonal climate patterns comparatively stable over long periods.

Permanently shadowed craters near the lunar poles never receive direct sunlight. Measurements from NASA instruments and from the Indian Space Research Organisation's Chandrayaan-1 orbiter point to water ice trapped inside them. That ice is one reason the south pole has become the focus of renewed exploration. NASA's Artemis program is working toward landing astronauts in that region, where accessible water could supply drinking water, breathable oxygen and rocket propellant.

Illustration: Quartz

Europa is the sixth largest moon in the solar system, about 3,122 kilometers across and slightly smaller than Earth's moon. Beneath its outer shell of water ice lies a global saltwater ocean. That ocean is why Europa ranks among the leading targets in the search for life beyond Earth.

Europa completes an orbit of Jupiter in about 3.5 days, and it is one of the four satellites Galileo Galilei spotted in 1610. Its surface is among the smoothest known, broken by long reddish-brown fractures called lineae that run for hundreds of kilometers. Large impact craters are scarce. That tells planetary scientists the crust is young and gets resurfaced, probably by material pushing up from below and by sections of the ice shell shifting against one another.

Measurements from NASA's Galileo orbiter in the 1990s built the strongest case for the ocean. The spacecraft's magnetometer detected a magnetic signature induced by Jupiter's field — the response expected from a layer of electrically conductive salty water. Estimates put the ocean's volume at roughly twice that of all Earth's oceans combined. The water stays liquid because Jupiter's gravity flexes the moon's interior and generates heat. Models of the ice shell above it range from a few kilometers thick to several tens of kilometers. Settling that figure remains one of the open problems in outer-planet science.

The Hubble Space Telescope has recorded signs of water vapor venting from Europa. The activity appears intermittent and far weaker than the geyser plumes Cassini watched at Saturn's moon Enceladus. Plumes matter because they offer a way to sample ocean material without drilling through the ice.

NASA's Europa Clipper launched in October 2024 and is due to reach the Jupiter system in 2030. Rather than orbit Europa directly, it will make dozens of close flybys, which limits its exposure to Jupiter's radiation belts. Its instruments are built to assess whether conditions for life exist there, not to detect organisms.

Illustration: Quartz

Triton is the seventh largest moon in the solar system at roughly 2,707 kilometers across, and the only large moon that orbits its planet backward. Its retrograde path runs opposite to Neptune's rotation, which marks it as a body Neptune captured rather than one that formed alongside it.

Moons that condense from the disk of material around a young planet orbit in the same direction the planet spins. Triton does not. Its composition — rock mixed with frozen nitrogen, methane and carbon dioxide — closely resembles Pluto and other Kuiper Belt objects. The leading explanation holds that Triton formed in that distant belt of icy worlds beyond Neptune and was pulled into orbit during a close encounter billions of years ago.

Voyager 2 flew past in 1989 and remains the only spacecraft to have studied Triton at close range. Its images showed a young surface with few impact craters, along with a wrinkled region known as cantaloupe terrain for its dimpled texture. The flyby also caught dark plumes above the southern polar cap, where nitrogen gas and dust vented several kilometers into a thin atmosphere and trailed downwind in long streaks.

Surface temperatures near minus 235 degrees Celsius rank Triton among the coldest places measured in the solar system. Nitrogen freezes solid at that temperature and coats much of the terrain in a bright, reflective frost. A liquid water ocean may survive beneath the crust, kept from freezing by internal heat and the tidal stress Neptune exerts.

Those same tidal forces are dragging Triton inward. Tens of millions of years from now, it will likely cross the point where Neptune's gravity pulls it apart into a ring, or else fall into the planet. No spacecraft has returned since the 1989 flyby, so most of what is known about Triton still comes from a single pass through the Neptune system.