Jupiter's four Galilean moons
Io, Europa, Ganymede and Callisto were the first moons ever found orbiting another planet — proof the universe didn't revolve around the Earth. Together they outweigh every other moon in the Solar System combined, save our own.
Io
Most volcanic body knownSqueezed by Jupiter's immense gravity and the tug of its neighboring moons, Io is racked by tidal heating that powers hundreds of active volcanoes — more than any other body in the Solar System.
| Mean diameter | 3,643.2 km |
|---|---|
| Mass | 8.93 × 10²² kg |
| Mean orbital radius | 421,700 km |
| Orbital period | 1.769 days |
| Surface gravity | 1.796 m/s² |
| Surface temperature | −143 °C average |
| Discovered | 1610, Galileo Galilei |
Europa
Subsurface oceanBeneath Europa's smooth, cracked ice shell lies a global saltwater ocean thought to hold more water than all of Earth's oceans combined — making it one of the most promising places to search for life.
| Mean diameter | 3,121.6 km |
|---|---|
| Mass | 4.80 × 10²² kg |
| Mean orbital radius | 671,034 km |
| Orbital period | 3.551 days |
| Surface gravity | 1.315 m/s² |
| Surface temperature | −171 °C average |
| Discovered | 1610, Galileo Galilei |
Ganymede
Bigger than MercuryGanymede is the largest and most massive moon of any planet — larger than the planet Mercury, though less massive. It's also the only moon known to generate its own magnetic field.
| Mean diameter | 5,268.2 km |
|---|---|
| Mass | 1.4819 × 10²³ kg |
| Mean orbital radius | 1,070,412 km |
| Orbital period | 7.155 days |
| Surface gravity | 1.428 m/s² |
| Surface temperature | −163 to −113 °C |
| Discovered | 7 January 1610, Galileo Galilei |
Callisto
Ancient, untouched surfaceFarthest of the four from Jupiter, Callisto orbits outside the planet's main radiation belt and shows the oldest, most cratered surface in the Solar System — a landscape essentially unchanged for billions of years.
| Mean diameter | 4,820.6 km |
|---|---|
| Mass | 1.0759 × 10²³ kg |
| Mean orbital radius | 1,882,700 km |
| Orbital period | 16.689 days |
| Surface gravity | 1.235 m/s² |
| Surface temperature | −139 °C average |
| Discovered | 1610, Galileo Galilei |
How the four moons compare
Ordered by distance from Jupiter — the same order Galileo watched them shift night after night in January 1610.
| Moon | Diameter | Mass (kg) | Orbital radius | Orbital period | Surface gravity |
|---|---|---|---|---|---|
| Io | 3,643.2 km | 8.93 × 10²² | 421,700 km | 1.77 days | 1.80 m/s² |
| Europa | 3,121.6 km | 4.80 × 10²² | 671,034 km | 3.55 days | 1.31 m/s² |
| Ganymede | 5,268.2 km | 14.82 × 10²² | 1,070,412 km | 7.15 days | 1.43 m/s² |
| Callisto | 4,820.6 km | 10.76 × 10²² | 1,882,700 km | 16.69 days | 1.24 m/s² |