Last update: October 4, 2025

The Moons of Jupiter: A Celestial Archipelago of Over 90 Diverse Worlds

Jovian system showing Jupiter and its four Galilean moons aligned with their relative sizes

A solar system in miniature

With its 95 confirmed moons as of 2025, including 4 discovered by Galileo (1564-1642) in 1610, Jupiter forms a true miniature planetary system. This gas giant, 318 times more massive than Earth, exerts such a powerful gravitational influence that it captures passing asteroids and comets. The Jovian system is a family of satellites with characteristics as varied as the planets of the solar system. The JUICE (ESA) and Juno (NASA) missions now reveal ocean worlds, active volcanoes, and complex atmospheres, providing an ideal study ground for exobiology and comparative planetology.

What are the key features that distinguish this system?

Geological diversity: From Io's sulfur volcanoes to Europa's geysers, and Ganymede's unique magnetic field (the only moon with one), each satellite tells a different story of planetary evolution.
Dynamic interactions: Orbital resonances (e.g., the 1:2:4 ratio between Io, Europa, and Ganymede) generate tidal forces capable of heating their interiors, as modeled by Stanton Peale (1937-2023) in 1979.
Analogy with exoplanets: Icy moons like Europa (global ocean beneath 15-25 km of ice) are considered priority targets for the search for extraterrestrial life, according to criteria defined by the COSPAR.

Comparison of the Galilean moons: contrasting worlds

The diameters of irregular moons (< 200 km) are often estimated from their absolute magnitude and assumed albedo.
For non-spherical moons, triaxial dimensions are indicated (e.g., Amalthea).
The 4 Galilean moons (Ganymede, Callisto, Io, Europa) represent 99.997% of the total mass of Jupiter's satellites.

Comparison of Jupiter's main moons
Satellite	Diameter (km)	Distance from Jupiter (10³ km)	Orbital period (days)	Notable features	Type/Group
Ganymede	5,262.4 ± 0.2	1,070.4	7.1546	Largest moon in the solar system. Own magnetic field (0.718 μT). Salty ocean between two ice layers (estimated depth: 100 km). Mixed surface of bright and dark terrains.	Regular (Galilean)
Callisto	4,820.6 ± 0.2	1,882.7	16.6890	Most cratered surface in the solar system. Valhalla crater (3,800 km in diameter). Possible subsurface ocean at 150 km depth. Albedo: 0.22.	Regular (Galilean)
Io	3,643.2 ± 0.3	421.8	1.7691	Most volcanically active body in the solar system (> 400 active volcanoes). Composition: silicate and sulfur compounds. Max temp: 2,000 K. Over 100 mountains (some taller than Everest).	Regular (Galilean)
Europa	3,121.6 ± 0.2	671.1	3.5512	Global subsurface ocean (100 ± 20 km deep). Young surface (30-80 Ma) with ice fractures. High albedo: 0.67. Tidal pressures generating internal heat.	Regular (Galilean)
Amalthea	166.7 × 98.3 × 93.6	181.37	0.4982	Elongated shape. Reddish surface (sulfur deposits ejected by Io). Surface temp: 110 ± 10 K. Giant craters (Pan, Gaea) relative to its size.	Regular (Amalthea group)
Himalia	85 ± 5	11,460	250.56	Largest member of the Himalia group. Prograde irregular orbit. Composition similar to C-type asteroids. Discovered in 1904 by Perrine.	Irregular (Himalia group)
Elara	80 ± 5	11,740	259.64	Second-largest moon in the Himalia group. Albedo: 0.04. Rotation period: ~12h. Possible captured object from the asteroid belt.	Irregular (Himalia group)
Thebe	98.6 × 97.6 × 82.6	221.89	0.6745	Zethus crater (40 km in diameter, ~40% of the moon's diameter). Contributes to the outer gossamer ring. Markedly irregular shape.	Regular (Amalthea group)
Pasiphae	58 ± 2	23,620	743.63	Largest retrograde moon. Highly inclined orbit (151.4°). Composition: likely a captured asteroid. Family of 17 irregular moons.	Irregular (Pasiphae group)
Metis	43 ± 2	127.96	0.2948	Innermost moon. Orbits in 7h05m. Main source of Jupiter's main ring. Surface covered in deep impact craters.	Regular (Amalthea group)
Adrastea	16.4 × 14.0 × 12.0	128.98	0.2983	Smallest moon in the Jovian system. Irregular shape. Orbits at the outer edge of the main ring. Discovered by Voyager 2 images.	Regular (Amalthea group)
Carpo	3 ± 0.5	17,150	458.62	Prograde orbit inclined at 51°. Possible collision fragment. Discovered in 2003. Absolute magnitude: 16.9.	Irregular (isolated)
S/2003 J 2	2 ± 0.5	28,570	982.5	Extreme irregular moon. Highly eccentric orbit (e=0.38). Belongs to the Pasiphae group. Rotation period unknown.	Irregular (Pasiphae group)
Valetudo	1.0 ± 0.3	19,000	533.3	"Suicidal" orbit crossing retrograde moons. High future collision risk. Named after the Roman goddess of health.	Irregular (isolated)
S/2018 J 2	1.5 ± 0.3	23,550	722.3	Discovered in 2018. Belongs to the Pasiphae group. One of the smallest known moons of Jupiter. Retrograde orbit.	Irregular (Pasiphae group)

Sources: NASA Solar System Exploration (October 2025); JPL Small-Body Database; Scott S. Sheppard (2025), Jupiter's Moons: A Comprehensive Review, Annual Review of Astronomy and Astrophysics; Data from HST and VLT/ESO (2024).

Challenges of future exploration

Three major missions will revolutionize our understanding of the Jovian system by 2035:

JUICE (launch 2023, arrival 2031): Will study Ganymede, Europa, and Callisto with a penetrating radar (RIME) capable of probing up to 9 km beneath the ice.
Europa Clipper (NASA, 2024): 45 flybys of Europa to analyze its chemical composition (MISE spectrometer).
JUICE-L project (proposed for 2035): European lander aiming to collect samples from Europa's surface.

As astrophysicist Michel Blanc (1954-) from IRAP notes: "Exploring Jupiter and its moons is like traveling back in time to observe the conditions of the early solar system while preparing for the study of exoplanetary systems."

Jupiter as a model for exoplanets

Hot Jupiters (giant exoplanets close to their star) could host similar moon systems. Simulations by Nikku Madhusudhan (1981-) (University of Cambridge) suggest that up to 27% of these planets could have habitable moons in the CHZ. The JWST detected potential moon signatures around Kepler-1625b in 2024, 8,000 light-years away.

The Jovian system reminds us that the search for life is not limited to planets: moons, with their hidden oceans and internal energy sources, could be the most likely cradles for extraterrestrial life in our galaxy.