There are 67 confirmed moons of Jupiter, giving Jupiter the largest number of moons with reasonably secure orbits of any planet in the Solar System. The most massive of them, the four Galilean moons, were discovered in 1610 by Galileo Galilei and were the first objects found to orbit a body that was neither Earth nor the Sun. From the end of the 19th century, dozens of much smaller Jovian moons have been discovered and have received the names of lovers, conquests, or daughters of the Roman god Jupiter, or his Greek equivalent, Zeus.
The Galilean moons are by far the largest and most massive objects in orbit around Jupiter, with the remaining 63 moons and the rings together comprising just 0.003% of the total orbiting mass. Eight are regular satellites, with prograde and nearly circular orbits that are not greatly inclined with respect to Jupiter’s equatorial plane. The Galilean satellites are nearly spherical in shape, due to having planetary mass, and so would be considered dwarf planets if they were in direct orbit about the Sun. The other four regular satellites are much smaller and closer to Jupiter; these serve as sources of the dust that makes up Jupiter’s rings. The remainder of Jupiter’s moons are irregular satellites, whose prograde and retrograde orbits are much farther from Jupiter and have high inclinations and eccentricities. These moons were probably captured by Jupiter from solar orbits. 16 irregular satellites, discovered since 2003, have not yet been named.
The moons’ physical and orbital characteristics vary widely. The four Galileans are all over 3,100 kilometres (1,900 mi) in diameter; the largest Galilean, Ganymede, is the ninth largest object in the Solar System, after the Sun and seven of the planets (Ganymede being larger than Mercury). All other Jovian moons are less than 250 kilometres (160 mi) in diameter, with most barely exceeding 5 kilometres (3.1 mi). Orbital periods range from seven hours (taking less time than Jupiter does to spin around its axis), to some three thousand times more (almost three Earth years).
The four largest moons, Io, Europa, Ganymede, and Callisto, are each distinctive worlds. Io is the most volcanically active body in the solar system, with its surface covered by sulfur in different colourful forms. As Io travels in its slightly elliptical orbit, Jupiter’s immense gravity causes “tides” in the solid surface that rise 100 m (300 feet) high, generating enough heat for volcanic activity and to drive off any water. Io’s volcanoes are driven by hot silicate magma. It has a core, and a mantle of at least partially molten rock, topped by a crust of solid rock coated with sulfur compounds. Europa’s surface is mostly water ice, and there is evidence that it may be covering an ocean of water or slushy ice beneath. Europa is thought to have twice as much water as does Earth. Life forms have been found thriving near subterranean volcanoes on Earth and in other extreme locations that may be analogues to what may exist on Europa. Ganymede is the largest moon in the solar system, and is the only moon known to have its own internally generated magnetic field. Europa and Ganymede both have a core; a rock envelope around the core; a thick, soft ice layer; and a thin crust of impure water ice. Callisto’s surface is extremely heavily cratered. However, the very few small craters on Callisto indicate a small degree of current surface activity. Layering at Callisto is less well defined and appears to be mainly a mixture of ice and rock. The moons all keep the same face towards Jupiter as they orbit, meaning that each moon turns once on its axis for every orbit around Jupiter.
Pioneers 10 and 11 (1973 to 1974) and Voyager 1 and Voyager 2 (1979) offered striking color views and global perspectives from their flybys of the Jupiter system. From 1995 to 2003, the Galileo spacecraft made observations from repeated elliptical orbits around Jupiter, passing as low as 261 km (162 miles) over the surfaces of the Galilean moons. These close approaches resulted in images with unprecedented detail of selected portions of the surfaces.
Credits: NASA, Wikipedia