Moons of Jupiter

Io is the most volcanically active body in the Solar System — dramatically more so than Earth. Over 400 active volcanoes have been identified on its surface, with eruption plumes of sulfur dioxide reaching hundreds of kilometres into space. Some plumes are so large they were photographed from orbit by the Voyager spacecraft in 1979, a complete surprise to scientists who had expected a dead, cratered world like our Moon.

The energy driving this activity comes not from radioactive decay but from tidal heating. Io is caught in a gravitational tug-of-war between Jupiter, Europa, and Ganymede — their 1:2:4 orbital resonance constantly flexes Io’s interior, converting gravitational energy into frictional heat. The result is a world that resurfaces itself continuously; impact craters are buried almost as fast as they form, making Io’s surface one of the youngest in the Solar System.

Io’s surface is painted in striking yellows, reds, oranges, and whites — deposits of various forms of sulfur and silicate rock baked by constant volcanic activity. Its atmosphere, thin and constantly replenished by eruptions, is composed primarily of sulfur dioxide.

Beneath Europa’s cracked, frozen shell lies what scientists believe to be the largest ocean in the Solar System — a global body of liquid saltwater estimated at roughly twice the total volume of all Earth’s oceans combined, though this figure is model-derived and carries significant uncertainty. This ocean has remained liquid for billions of years, kept from freezing by the same tidal heating that drives Io’s volcanism, just at a lower intensity.

Europa’s surface is extraordinarily smooth — the flattest in the Solar System — and crosshatched by reddish-brown lines called lineae: fractures where the ice shell has split and refrozen. In regions called chaos terrain, blocks of ice appear to have shifted, rotated, and refrozen in new positions, suggesting the shell is thin enough in places for the ocean below to reach the surface. Water vapour plumes have been tentatively detected near the south pole, potentially offering a direct sample of the ocean without drilling.

NASA’s Europa Clipper spacecraft launched in October 2024 and is currently en route, expected to arrive in 2030. It will conduct dozens of close flybys to assess the ocean’s chemistry, ice shell thickness, and habitability — the most detailed investigation of a potentially life-bearing world ever attempted.

Ganymede is the largest moon in the Solar System and would qualify as a planet in its own right if it orbited the Sun directly — at 5,268 km across it is larger than Mercury, though only about half as massive due to its composition of roughly equal parts rock and water ice. It is the only moon anywhere in the Solar System known to generate its own intrinsic magnetic field, creating a small but distinct magnetosphere embedded within Jupiter’s vastly larger one.

This magnetic field creates auroral ovals near Ganymede’s poles, observed in ultraviolet by the Hubble Space Telescope. Careful analysis of how these auroras “rock” back and forth in response to Jupiter’s oscillating magnetic field revealed something remarkable: the rocking was far smaller than expected, which could only be explained by a highly electrically conductive layer beneath the surface damping Jupiter’s influence — almost certainly a salty global ocean. That ocean may contain more water than all of Earth’s.

ESA’s JUICE (Jupiter Icy Moons Explorer) spacecraft is currently en route and will enter orbit around Ganymede in 2034, becoming the first spacecraft ever to orbit a moon other than our own.

Callisto is the outermost of the Galilean moons and in almost every respect the opposite of Io. Sitting outside the orbital resonance that drives its siblings’ internal activity, it receives virtually no tidal heating. Its surface is the most heavily cratered in the Solar System — a frozen record of the intense bombardment phase of the early Solar System, essentially unchanged for four billion years. Not a single geological process has erased those ancient scars.

Its surface is dominated by enormous multi-ring impact basins, the largest of which — Valhalla — spans 3,800 km across, making it one of the largest impact structures in the Solar System. Despite this geologically dead exterior, data from the Galileo spacecraft suggests that Callisto too may harbour a subsurface ocean, though this remains less certain than for Europa or Ganymede.

Callisto’s distance from Jupiter’s intense radiation belts — far safer than the inner moons — combined with its possible subsurface resources makes it a serious candidate for a future human base for exploring the Jovian system.

Himalia is by far the largest of Jupiter’s irregular satellites — at around 170 km it utterly dwarfs the rest of the captured swarm, most of which are under 5 km across. Discovered in 1904 by Charles Perrine at Lick Observatory, it leads the Himalia group: four prograde irregular moons (Himalia, Leda, Lysithea, and Elara) sharing similar orbital parameters, thought to be collisional fragments of a single captured C-type asteroid.

The clearest images we have of Himalia came not from a dedicated mission but from NASA’s New Horizons spacecraft during its 2007 Jupiter flyby — brief exposures that revealed an elongated, potato-shaped body. Its surface is very dark, reflecting only about 5% of sunlight, and spectrally resembles the carbonaceous asteroids of the outer main belt — consistent with a captured origin rather than formation in the Jovian system.

Amalthea holds the distinction of being the reddest object in the entire Solar System — even Mars, often called the Red Planet, is less intensely red in colour. The source of this striking colouration is not fully understood but is believed to be a combination of sulfur compounds deposited from Io’s volcanic plumes — which coat the inner Jovian system — combined with complex organic compounds darkened over billions of years by Jupiter’s intense radiation environment.

Even more puzzling is Amalthea’s heat output: it radiates significantly more energy than it receives from the Sun. Unlike Io, this excess heat cannot be explained by tidal forces alone. The leading hypothesis is that electrical currents are induced in Amalthea’s interior as it moves through Jupiter’s powerful and rapidly rotating magnetic field — essentially acting as a natural electrical generator that dumps energy as heat. This electromagnetic heating mechanism is rare and makes Amalthea a scientifically unusual object.

Discovered in 2018 as part of a wide-field survey that also netted nine other new Jovian moons, Valetudo is one of Jupiter’s smallest and strangest satellites. It orbits in the prograde direction — the same direction Jupiter rotates — but lives in the orbital zone dominated by retrograde irregular moons that travel in the opposite direction. This is the celestial equivalent of driving the wrong way on a motorway.

The inevitable result is that Valetudo will, on a geological timescale, collide head-on with one or more of its retrograde neighbours. Astronomers believe this has already happened many times in the past. Valetudo is most likely the final surviving fragment of a once-larger prograde moon that has been progressively demolished by such collisions over billions of years — and the debris from those impacts may have contributed to forming several of the retrograde moons we observe today. It is a moon in the process of destroying itself simply by existing where it does.

S/2003 J 2 holds an unusual record: the longest orbital period of any confirmed moon of Jupiter at 981 days — nearly three Earth years to complete a single orbit. It travels in a retrograde direction at extreme distances from Jupiter, reaching well beyond 20 million kilometres at the farthest point of its eccentric orbit.

At roughly 2 km across it sits right at the edge of what ground-based telescopes can reliably detect as a distinct object. Discovered in 2003 during a wide-field survey, it required years of patient follow-up observations to confirm that it was genuinely bound to Jupiter rather than a small asteroid following a similar but heliocentric path nearby. It remains unnamed and poorly characterised — no spacecraft has passed anywhere near it. It is a reminder of how much is still unknown at the outermost fringes of even the best-studied planetary system beyond our own.