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Jupiter has 63 moons, more than any other planet in our Solar System. Its moons range from the largest one in the Solar System to one only about one kilometer in diameter. The planet’s moons are divided into different categories. The two basic groups are regular and irregular satellites. The regular satellites have less eccentric orbits and are closer to the planet. Regular satellites can be further divided into inner satellites and the Galilean moons. The regular satellites – comprised of Metis, Amalthea, Thebe, and Adrastea – are closest to Jupiter.
Metis is the closest moon to Jupiter at only 128,000 kilometers from the gas giant. Adrastea orbits Jupiter in less than a third of an Earth day. Amalthea, which was discovered in 1892, is the reddest object in the Solar System and the third moon from Jupiter. Thebe is the final inner moon. Scientists believe that material from some of these moons contributes to different planetary rings. For instance, the think that dust from Thebe and Amalthea help form the Gossamer ring.
The Galilean moons were originally named the Medici family who were Galileo’s patrons. They were renamed after Galileo though and the moons themselves were named by Simon Marius – who claimed to have seen them first – after lovers of Zeus (Jupiter’s equivalent) in Greek mythology. The Galilean moons are Ganymede, Io, Callisto, and Europa. Ganymede is the only one named after a male figure in mythology, but according to Greek mythology he was also a lover of Zeus. Ganymede is the largest of all the moons with a diameter of 6,262 kilometers. Io is the most volcanic celestial object in the Solar System with over 400 active volcanoes. Europa is an icy moon with an almost flawlessly smooth surface and the second Galilean moon from the Sun.
The rest of the satellites are classified as irregular satellites. They tend to be smaller than the regular moons and irregularly shaped. These have also been classified into several different groups. Some of Jupiter’s other moons include Sinope, Carme, Leda, and Elara. The two smallest moons of Jupiter are S/2003 J 9 and S/2003 J 12. Both of these moons are approximately one kilometer in diameter. Although the first moons of Jupiter were discovered in 1610, many of them were only discovered in the past decade or two. This is especially true of many of the smaller moons, which were too hard to detect before with poorer telescopes.
Astronomy Cast has an article on Jupiter’s moons.