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Moons of Jupiter

Jupiter's moons: fire, ice and hidden oceans

Galileo spotted Jupiter's four biggest moons in January 1610, the first moons found beyond Earth. Jupiter now has 115 known moons, but these four, the Galilean moons, are worlds in their own right.

Jupiter's four largest moons side by side: yellow Io, white Europa, grey-brown Ganymede and dark Callisto

NASA/JPL/DLR, Galileo (PIA01299). Public domain.

Meet the moons

Io

Diameter
3,643 km
One orbit
1.8 days
Known for
over 400 active volcanoes

Io is the most volcanically active world known. Lava fountains and plumes of sulfur rise hundreds of kilometres high, and the yellow, orange and red colours come from sulfur compounds.

Its heat comes from tides. Io, Europa and Ganymede are locked in a rhythm (1:2:4 orbits) that keeps Io's orbit slightly oval. Jupiter's gravity squeezes and relaxes it every orbit, and the friction melts rock inside. NASA's Juno spacecraft made close flybys in 2023 and 2024.

Jupiter's moon Io in true colour: a yellow, orange and white globe spotted with dark volcanic vents
Io in true colour from NASA's Galileo spacecraft.NASA/JPL/University of Arizona, Galileo (PIA02308). Public domain.

Europa

Diameter
3,122 km
Ice shell
roughly 15 to 25 km thick (estimates vary)
Ocean
perhaps 60 to 150 km deep

Europa is slightly smaller than our Moon and covered in bright ice crossed by reddish cracks and ridges. Under the ice is a global ocean of salty water that may hold about twice as much water as all of Earth's oceans.

Its surface is young, with few craters, so the ice is still being renewed. Hubble and Galileo data hint at plumes of water vapour.

Europa's icy white and blue-grey surface criss-crossed by long reddish-brown cracks and ridges
Europa's cracked icy surface, from Galileo images reprocessed in 2014.NASA/JPL-Caltech/SETI Institute, Galileo data reprocessed 2014 (PIA19048). Public domain.

Europa Clipper

NASA's Europa Clipper, the largest planetary spacecraft NASA has built, launched on 14 October 2024 on a Falcon Heavy. After a Mars flyby in March 2025 and an Earth flyby in December 2026, it is due to reach Jupiter in April 2030. It will swoop past Europa 49 times, using ice-penetrating radar, cameras and a mass spectrometer to judge whether its ocean could support life.

Europe's JUICE (Jupiter Icy Moons Explorer), launched in April 2023, arrives in 2031 and will become the first spacecraft to orbit another planet's moon, Ganymede.

Artist's concept of the Europa Clipper spacecraft with its long solar arrays above Europa, with Jupiter behind
Artist's concept of Europa Clipper over Europa, with Jupiter behind.NASA/JPL-Caltech, artist's concept (PIA26442).

Ganymede

Diameter
5,268 km, the largest moon
One orbit
7.2 days
Special
its own magnetic field

Ganymede is bigger than the planet Mercury. It is the only moon known to make its own magnetic field, discovered by Galileo in 1996, which creates auroras around it.

By watching how those auroras rock back and forth, Hubble found evidence (2015) for a salty ocean deep below the surface, possibly sandwiched between layers of ice.

Ganymede seen close up by Juno: dark ancient terrain and lighter grooved bands, half in sunlight
Ganymede from NASA's Juno during its June 2021 flyby, the closest any spacecraft had come in more than 20 years.NASA/JPL-Caltech/SwRI/MSSS, Juno JunoCam (PIA24681), 7 Jun 2021. Public domain.

Callisto

Diameter
4,821 km
One orbit
16.7 days
Known for
the most heavily cratered surface

Callisto's dark surface is saturated with craters: it has barely changed in 4 billion years. Its giant Valhalla impact basin is surrounded by rings that reach out about 1,900 km from its centre.

Galileo's magnetic measurements suggest Callisto may also have a salty ocean deep inside. It sits outside Jupiter's worst radiation, making it a candidate for a future human outpost.

Callisto: a dark globe peppered with bright impact craters
Callisto in colour from NASA's Galileo.NASA/JPL/DLR, Galileo (PIA03456). Public domain.

Could life be here?

Our rough guide, from 0 (no chance) to 5 (among the best places to look). No life has been found beyond Earth.

Could life be here? Io

  • Plenty of heat and chemistry
  • No liquid water
  • Bathed in deadly radiation
  • Surface constantly repaved by lava

Verdict: Almost certainly not.

Could life be here? Europa

  • Global salty ocean in contact with rock
  • Chemical energy may come from the seafloor
  • Young, active surface
  • Ice shell kilometres thick
  • Harsh radiation on the surface

Verdict: One of the best places to look. Europa Clipper will judge whether it is habitable, not whether life is there.

Could life be here? Ganymede

  • Deep salty ocean
  • Ocean may be trapped between ice layers, away from rock

Verdict: Possible, but less promising than Europa.

Could life be here? Callisto

  • Possible deep ocean
  • Low radiation
  • Little sign of internal heat or activity

Verdict: Unlikely, but not ruled out.

Quick answers

How many moons does Jupiter have?

About 115 known moons as of 2026. The four large Galilean moons are Io, Europa, Ganymede and Callisto.

Is there an ocean on Europa?

Yes, almost certainly. Magnetic measurements by Galileo and the young cracked surface point to a global salty ocean under the ice.

When will Europa Clipper arrive?

In April 2030. It launched on 14 October 2024.

Why is Io so volcanic?

Tidal heating. Its slightly oval orbit, kept that way by Europa and Ganymede, makes Jupiter's gravity flex Io constantly, melting rock inside.

Sources and credits

  1. NASA Science, Jupiter moons.
  2. NASA, Europa Clipper mission.
  3. ESA, JUICE.
  4. Saur, J. et al. (2015), The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals, JGR Space Physics.

Photos credited to NASA, NOAA and other public-domain sources are used with thanks; BotBuhdy adds a small botbuhdy.com mark. Pictures labelled AI-generated illustration are artist-style illustrations made for this page, not photographs.