Moons of Jupiter

Jupiter has 62 confirmed moons, giving it the largest retinue 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 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.

Eight of Jupiter's moons 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 spheroidal in shape, 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.

Jupiter's other 54 or 55 moons are tiny irregular satellites, whose prograde and retrograde orbits are much farther from Jupiter and have high inclinations and eccentricities. These moons were likely captured by Jupiter from solar orbits. There are 13 recently-discovered irregular satellites that have not yet been named, plus a 14th whose orbit has not yet been established; if it is, the number of secured moons will rise to 63. The four largest moons, known as the "Galilean moons", are Io, Europa, Ganymede and Callisto.

Galilean moons

The orbits of Io, Europa , and Ganymede , some of the largest satellites in the Solar System, form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three large moons to distort their orbits into elliptical shapes, since each moon receives an extra tug from its neighbors at the same point in every orbit it makes. The tidal force from Jupiter, on the other hand, works to circularize their orbits.

The eccentricity of their orbits causes regular flexing of the three moons' shapes, with Jupiter's gravity stretching them out as they approach it and allowing them to spring back to more spherical shapes as they swing away. This tidal flexing heats the moons' interiors via friction. This is seen most dramatically in the extraordinary volcanic activity of innermost Io (which is subject to the strongest tidal forces), and to a lesser degree in the geological youth of Europa's surface (indicating recent resurfacing of the moon's exterior).

Classification of moons

Before the discoveries of the Voyager missions, Jupiter's moons were arranged neatly into four groups of four, based on commonality of their orbital elements. Since then, the large number of new small outer moons has complicated this picture. There are now thought to be six main groups, although some are more distinct than others.

A basic sub-division is a grouping of the eight inner regular moons, which have nearly circular orbits near the plane of Jupiter's equator and are believed to have formed with Jupiter. The remainder of the moons consist of an unknown number of small irregular moons with elliptical and inclined orbits, which are believed to be captured asteroids or fragments of captured asteroids. Irregular moons that belong to a group share similar orbital elements and thus may have a common origin, perhaps as a larger moon or captured body that broke up.

Regular moons

These are split into two groups:

• • Inner satellites or Amalthea group — they orbit very close to Jupiter: Metis, Adrastea, Amalthea, and Thebe. The innermost two orbit in less than a Jovian day, while the latter two are respectively the fifth and seventh largest moons in the system. Observations suggest that at least the largest member, Amalthea, did not form on the present orbit, but that it was formed farther from the planet, or that it is a captured Solar System body.[23] These moons, along with a number of as-yet-unseen inner moonlets, replenish and maintain Jupiter's faint ring system. Metis and Adrastea help to maintain Jupiter's main ring, while Amalthea and Thebe each maintain their own faint outer rings.The inner group of four small moons all have diameters of less than 200 km, orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree.

• • Main group or Galilean moons — the four massive satellites: Ganymede, Callisto, Io, and Europa. With radii that are larger than any of the dwarf planets, they are some of the largest objects in the Solar System outside the Sun and the eight planets in terms of diameter. Respectively the first, third, fourth, and sixth largest natural satellites in the Solar System, they contain almost 99.999% of the total mass in orbit around Jupiter. The inner moons also participate in a 1:2:4 orbital resonance. Models suggest that they formed by slow accretion in the low-density Jovian subnebula—a disc of the gas and dust that existed around Jupiter after its formation—which lasted up to 10 million years in the case of Callisto. These four moons, discovered by Galileo Galilei and by Simon Marius in parallel, orbit between 400,000 and 2,000,000 km, and include some of the largest moons in the Solar System.

Irregular moons

The irregular satellites are substantially smaller objects with more distant and eccentric orbits. They form families with shared similarities in orbit (semi-major axis, inclination, eccentricity) and composition; it is believed that these are at least partially collisional families that were created when larger (but still small) parent bodies were shattered by impacts from asteroids captured by Jupiter's gravitational field. These families bear the names of their largest members. The identification of satellite families is tentative, but the following are typically listed:

• Prograde satellites:

• • Themisto is the innermost irregular moon and not part of a known family.This is a single moon belonging to a group of its own, orbiting halfway between the Galilean moons and the Himalia group.

• • The Himalia group is spread over barely 1.4 Gm in semi-major axis, 1.6° in inclination (27.5 ± 0.8°), and eccentricities between 0.11 and 0.25. It has been suggested that the group could be a remnant of the break-up of an asteroid from the main asteroid belt. A tightly clustered group of moons with orbits around 11,000,000–12,000,000 km from Jupiter.

• • Carpo is the outermost prograde moon and not part of a known family. Another isolated case; at the inner edge of the Ananke group, it revolves in the direct sense.

• The irregular retrograde satellites are thought to have originally been asteroids that were captured by drag from the tenuous outer regions of Jupiter's accretion disk while the Solar system was still forming, and were later shattered by impacts. They are far enough from Jupiter that their orbits are significantly disturbed by the gravitational field of the Sun.

• • S/2003 J 12 is the innermost of the retrograde moons, and is not part of a known family.

• • The Carme group is spread over only 1.2 Gm in semi-major axis, 1.6° in inclination (165.7 ± 0.8°), and eccentricities between 0.23 and 0.27. It is very homogeneous in color (light red) and is believed to have originated from a D-type asteroid progenitor, possibly a Jupiter trojan. A fairly distinct group that averages 23,404,000 km from Jupiter with an average inclination of 165 degrees.

• • The Ananke group has a relatively wider spread than the previous groups, over 2.4 Gm in semi-major axis, 8.1° in inclination (between 145.7° and 154.8°), and eccentricities between 0.02 and 0.28. Most of the members appear gray, and are believed to have formed from the breakup of a captured asteroid.This group has rather indistinct borders, averaging 21,276,000 km from Jupiter with an average inclination of 149 degrees.

• • The Pasiphae group is quite dispersed, with a spread over 1.3 Gm, inclinations between 144.5° and 158.3°, and their eccentricities between 0.25 and 0.43.[16] The colors also vary significantly, from red to grey, which might be the result of multiple collisions. Sinope, sometimes included into Pasiphae group,[16] is red and given the difference in inclination, it could have been captured independently; Pasiphae and Sinope are also trapped in secular resonances with Jupiter.A dispersed and only vaguely distinct group that covers all the outermost moons.

• • S/2003 J 2 is the outermost moon of Jupiter, and is not part of a known family.