The Moons of JUPITER

Jupiter has 63 moons that we know of, and they’re incredibly diverse. Most are small, rocky, inert worlds. A few are probably asteroids that were gravitationally captured when they wandered too close to Jupiter. Many of these captured moons have tilted or backward orbits, and a few of the smaller, inner moons helped form Jupiter’s faint rings.

Four of Jupiter’s largest moons – the Galilean moons – are about the size Earth’s moon or larger. With ice-covered oceans, sulfur-spewing volcanoes, magnetospheres and surfaces covered with traces of geological activity, these moons are fascinating worlds in their own right. Three of these moons may even have internal liquid-water oceans.

Other giant planets orbiting other stars may have similar kinds of moons, and because those moons may harbor life – or at least boast environments friendly to life – they’re important places to explore.

Although Juno’s mission is to explore Jupiter itself, it will also study how the moons influence the planet and its magnetosphere. For example, the gravity of the Galilean moons alters Jupiter’s shape in subtle ways. The planet’s magnetic field also sweeps up particles that are ejected from the moons’ surfaces, filling the magnetosphere.

Inner Moons
These four small moons probably provide most of the material for Jupiter’s wispy rings.    


Dotted with erupting volcanoes, Io is the most geologically active moon in the solar system.


Below Europa’s icy surface may be a liquid ocean – a possible environment for life.


As the largest moon in the solar system, Ganymede is also the only one with its own magnetic field.


Almost as big as Mercury, Callisto is one of the most cratered surfaces in the Jovian system.

Galilean Moons
Jupiter's four largest moons are called the Galilean satellites, after Italian astronomer Galileo Galilei, who observed them in 1610.

Outer Moons
More than 50 small moons have orbits farther from Jupiter than Callisto. Most of these outer moons have odd orbits, looping around at large angles relative to Jupiter’s equator or circling in the opposite direction of Jupiter’s rotation. Because moons that formed with Jupiter should be orbiting in the same direction as its rotation and roughly along its equator, these peculiar orbits suggest that these moons were asteroids captured by Jupiter’s gravity.    

Trojan Moons
Thousands of small rocks called the Trojan moons orbit the sun along with Jupiter. Unlike normal moons, the Trojan moons don’t orbit Jupiter. These objects sit in two special locations in the solar system, where the Sun’s and Jupiter’s gravity have canceled out in a way that creates gravitationally stable spots. If Jupiter’s orbit is a clock face, and the planet is at 12 o’clock with the Sun at the center, the Trojan moons are at 10 o’clock and 2 o’clock.