Jupiter's Moons

Galilean Satellites

These are the four Galilean satellites. They are named so because they were the only four moons that Galileo was able to see. They are also the largest of Jupiter's moons. From top-left in a clock-wise direction, they are Ganymede, Callisto, Europa, and Io.

If Ganymede were not bound to Jupiter, it would be considered a planet in its own right. It is actually bigger than Mercury, being 390 km (234 miles) larger in diameter. It has heavily cratered dark regions, with lighter expanses in-between. Geologists think that it used to have plates, like the Earth, but they froze together soon after Ganymede's birth.

Callisto, the outermost of the Galilean moons, is almost an exact twin of Mercury in size and appearance. Every square mile is covered with craters or other signs of bombardment. Other than that, there are no distinct characteristics

Europa, closer to Jupiter than Ganymede, is the smoothest natural body in the solar system. It resembles a billiard ball until seen very close-up. At that distance you can start to see dark, deep, and narrow cracks. In scale, though, the relief is no bigger than a line on a billiard ball made with a felt-tipped marker. Geologists think that Europa has liquid water underneath the icy surface - and possibly life.

Io, closer yet, is commonly compared to a pizza. Its volcanoes make it the most active world in the solar system. They spew out the sulfuric acid that gives Io its many colors. They also make Io one of the only three moons with an atmosphere in the solar system. Saturn's Titan and Neptune's Triton are the other two moons. Io is similar in size and composition to our moon. Io is caught in the middle of a tug-of-war between Jupiter and the other moons. Tension has melted the interior and raised the surface temperatures so high that scientists calculated that it generates the most heat for its size of any body in the solar system, except for the sun.

Other Moons

There are 59 other moons that have been discovered around Jupiter. All of them are smaller than the Galilean Satellites, and most of them are the size of small asteroids. Most have highly elliptical orbits and are highly inclined relative to Jupiter's equator. These are all very strong indications that the moons are not native to Jupiter, but rather they were captured. The largest piece of evidence for this is that over 80% of the non-Galilean Satellites orbit retrograde to Jupiter's spin.

The largest non-Galilean moon is Amalthea (right; image is a three-color composite taken by Voyager 1), being 262 km (163 miles) at its widest axis.

The innermost two moons, Metis and Adrastea, patrol the outer edge of the rings, their small gravity being enough to keep the particles from flying out. There are two other moons that are closer to Jupiter than Io; they are Amalthea and Thebe.

Nearly all of the moons that have been discovered in the last few years orbit retrograde to the direction of Jupiter's rotation , indicating that they are most likely not native to the system. They are also very small. Those that have not yet been given names will eventually be given names by the International Astronomical Union, the only official naming group.

Data for Jupiter's Moons

* This indicates that the moon orbits in a retrograde - the opposite direction to the planet's spin - motion.