Jupiter's outer moons and their highly inclined orbits.

Discovery

The Galilean moons. From left to right, in order of increasing distance from Jupiter: Io, Europa, Ganymede, Callisto.

The Galilean moons and their orbits around Jupiter.

The first claimed observation of one of Jupiter's moons is that of the Chinese astronomer Gan De around 364 BC.^[1] However, the first certain observations of Jupiter's satellites were those of Galileo Galilei in 1609.^[2] By March 1610, he had sighted the four massive Galilean moons with his 33x telescope: Ganymede, Callisto, Io, and Europa. No additional satellites were discovered until E.E. Barnard observed Amalthea in 1892.^[3] With the aid of telescopic photography, further discoveries followed quickly over the course of the twentieth century. Himalia was discovered in 1904,^[4] Elara in 1905,^[5] Pasiphaë in 1908,^[6] Sinope in 1914,^[7] Lysithea and Carme in 1938,^[8] Ananke in 1951,^[9] and Leda in 1974,^[10] By the time Voyagers reached Jupiter, a number of 13 moons had been discovered, while Themisto was observed in 1975,^[11] but lost until 2000. The Voyager missions discovered an additional three inner moons in 1979: Metis, Adrastea (moon), and Thebe.^[12]

For two decades no additional moons were discovered; but between October 1999 and February 2003, researchers using sensitive ground-based detectors found another 32 moons, most of which were discovered by a team lead by Scott S. Sheppard and David C. Jewitt.^[13] These are tiny moons, in long, eccentric, generally retrograde orbits, and average 3 kilometres in diameter, with the largest having barely 9 km across. All of these moons are thought to be captured asteroidal or perhaps cometary bodies, possibly fragmented into several pieces, but very little is actually known about them. Additional moons were discovered since then, but not yet confirmed, bringing the total number of observed moons of Jupiter at 63. This is currently the most of any planet in the Solar System, but additional tiny, undiscovered moons may exist.

Naming

The Galilean moons of Jupiter (Io, Europa, Ganymede and Callisto) were named by Simon Marius soon after their discovery in 1610. However, until the 20th century these fell out of favor, and instead being referred in the astronomical literature simply as "Jupiter I", "Jupiter II", etc., or as "the first satellite of Jupiter", "Jupiter's second satellite", etc. The names Io, Europa, Ganymede, and Callisto became popular in the 20th, while the rest of the moons, numbered, usually in Roman numerals V (5) through XII (12), remained unnamed.^[14] By a popular though unofficial convention, Jupiter V, discovered in 1892, was given the name Amalthea, first used by the French astronomer Camille Flammarion.^[15]

The other moons were, in the overwhelming majority of astronomical literature, were simply labeled by their Roman numeral (i.e. Jupiter IX) until the 1970s.^[16] In 1975, the IAU Task Group for Outer Solar System Nomenclature granted names to satellites V-XIII,^[17] and provided for a formal naming process for future satellites to be discovered. The practice was that newly discovered moons of Jupiter to be named after lovers and favorites of the mythological Jupiter (Zeus), and since 2004, after Zeus' descendants.^[18] All of Jupiter's satellites from XXXIV (Euporie) on are named after daughters of Zeus.

Some asteroids share the same names as moons of Jupiter: 9 Metis, 38 Leda, 52 Europa, 85 Io, 113 Amalthea, 239 Adrastea. Two more asteroids previously shared the names of Jovian moons until spelling differences were made permanent by the IAU: Ganymede and asteroid 1036 Ganymed; and Callisto and asteroid 204 Kallisto.

Groups

The orbits of Jupiter's irregular satellites, showing how they cluster into groups. Satellites are represented by circles that indicate their relative sizes. An object's position on the horizontal axis shows its distance from Jupiter in Gm (million km) and the fraction of the Hill sphere's gravitational influence radius (~53 Gm for Jupiter). The position on the vertical axis indicates its orbital inclination?he satellites above the axis are prograde, and the ones beneath are retrograde. The yellow lines indicate its orbital eccentricity (i.e. the extent to which its distance from Jupiter varies during its orbit).

Regular satellites

These are believed to have formed in situ, and are split into two groups:

• Amalthea group ? the inner moons that 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 the fifth, respectively the seventh largest moons in the system;

• Galilean moons ? the four massive satellites: Ganymede, Callisto, Io, and Europa. With radiuses 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: the first, third, fourth, and respectively sixth largest satellites. They contain more than 99.9% of the total mass of matter that revolves around Jupiter. The inner moons also participate in a 1:2:4 orbital resonance.

Irregular satellites

They are substantially smaller objects with more distant and eccentric orbits. It is thought that most of these satellites were bodies with originally heliocentric orbits that were captured by Jupiter's gravity. Typically, the following groupings are listed:

• Prograde satellites:

• Himalia group ? is "tight", 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.^[19]

• Themisto and Carpo are so far believed to be isolated in their own group.

Retrograde satellites: inclinations (°) vs eccentricities with Carme's (orange) and Ananke's (yellow) groups identified.

• Retrograde satellites ? are clumped in terms of semi-major axis, inclination, and eccentricity, and bear the name of their largest member:

• Carme group ? is "tight", spread over 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.^[20]

• Ananke group ? is not as "tight", with a spread 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.^[20]

• Pasiphae group ? is very dispersed, with a spread over 1.3 Gm, inclinations between 144.5° and 158.3°, and their eccentricities between 0.25 and 0.43.^[20] The colors also vary significantly, from red to grey, which might be the result of multiple collisions. Sinope, sometimes included into Pasiphae group,^[20] is red and given the difference in inclination, it could have been captured independently;^[19] Pasiphae and Sinope are also trapped in secular resonances with Jupiter.^[21]

• S/2003 J 12, and S/2003 J 2, the most interior and exterior of the irregular moons, which are thought to be isolated.

Evolution

formation

Table

The moons of Jupiter are listed below by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in blue; these are the four Galilean moons, which are comparable in size to Earth's moon. Irregular, captured moons are indicated by grey shading: light grey for prograde satellites, dark grey for retrograde.

See also

• Galilean moons
• Jupiter's moons in fiction
• Rings of Jupiter
• Natural satellites of Earth · Mars · Saturn · Uranus · Neptune

Notes

References

• Scott S. Sheppard, David C. Jewitt, Carolyn Porco Jupiter's outer satellites and Trojans, In: Jupiter. The planet, satellites and magnetosphere Edited by Fran Bagenal, Timothy E. Dowling, William B. McKinnon. Cambridge planetary science, Vol. 1, Cambridge, UK: Cambridge University Press, ISBN 0-521-81808-7, 2004, p. 263 - 280. Full text(pdf).

External links

• Jupiter's Moons by NASA's Solar System Exploration
• Jupiter satellite data
• 43 more moons orbiting Jupiter
• Cast: Jupiter's Moons
• Moons of Jupiter articles in Planetary Science Research Discoveries

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