Jupiter's natural satellites: Meaning (information, definition, explanation, facts)

Jupiter has many natural satellites.

The Voyager 1 mission discovered 3 moons in 1979, bringing the total then known to 16 (17 if you count Themisto, which had been found in 1975). The total rested there until 1999, since when sensitive ground-based detectors have found a further 47 (or 46) tiny moons in long, eccentric, generally retrograde orbits around Jupiter. They average 3 kilometres in diameter, and the largest is 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. The total number of known moons of Jupiter now stands at 63, currently the most of any planet in the solar system. Many additional tiny moons may exist that have not yet been discovered.

On October 6 1999, the Spacewatch programme discovered one more asteroid, 1999 UX₁₈. But it was soon realised that this was in fact a new moon of Jupiter, Callirrhoe. One year later, between November 23 and December 5, 2000, the team led by Scott S. Sheppard and David C. Jewitt of the University of Hawaii begins a systematic search for small irregular moons of Jupiter. The other members, as time goes by, include Yanga R. Fernández, Eugene A. Magnier, Scott Dahm, Aaron Evans, Henry H. Hsieh, Karen J. Meech, John L. Tonry, David J. Tholen (all from the University of Hawaii), Jan Kleyna (Cambridge University), Brett J. Gladman (University of Toronto), John J. Kavelaars (Hertzberg Institute of Astrophysics), Jean-Marc Petit (Observatoire de Besançon) and Rhiannon Lynne Allen (University of Michigan / University of British Columbia).

They used the world's two largest CCD cameras, mounted on two of the 13 telescopes atop Mauna Kea in Hawaii: the Subaru (8.3 m diameter) and the Canada-France-Hawaii (3.6 m). The 2000 observations revealed ten new moons, putting the count at 28 (Themisto had been rediscovered at the beginning of 2000).

The following year, on December 9-11, 2001, eleven other moons were discovered, bringing the total to 39. The year 2002 bore less fruit, netting only one moon, S/2002 J 1. However, four months later, between February 5 and 9, 2003, 23 more moons were found.

References:

• David C. Jewitt, Scott S. Sheppard, An abundant population of small irregular satellites around Jupiter, Nature, Vol 423, p. 261, May 2003
• David C. Jewitt, Scott S. Sheppard and Carolyn Porco, Jupiter's Outer Satellites and Trojans, Nov 2003
• New Satellites of Jupiter Discovered in 2003

Note that the outer satellites do not follow the simple period / axis relationship suggested by Kepler's third law because of the gravitational influence of the sun distorting the orbits. The table below is sorted in order of increasing period, which is not the same as the order of increasing mean orbital radius.

The orbits of the unnamed moons are still susceptible of improvement.

The interior groups seem natural: Amalthea's, the Galilean moons. Themisto is isolated in space. The Himalia group is "tight", spread over barely 1.4 Gm in semi-major axis and 1.6° in inclination (27.5±0.8°). The eccentricities vary between 0.11 and 0.25. S/2003 J 20 and S/2003 J 12 are two other isolated cases, and so is S/2003 J 2, the most exterior moon.

What is left of the outer retrograde irregular satellites of Jupiter can be grouped in three families, based on shared orbital characteristics and bearing the name of the largest member in each case. These families are clumps in semi-major axis, but also in inclination and in eccentricity. In the two plots below, Carme's group is in orange and Ananke's in yellow.

Inclinations (°) vs semi-major axes (Mm) for the outer satellites

Inclinations (°) vs eccentricities for the outer satellites

Carme's group is obvious, centered on a = 23 404 Mm; i = 165.2±0.3° and e = 0.238–0.272. Only S/2003 J 10 seems somewhat apart, because of its great eccentricity.

Ananke's group is centered on a = 21 276 Mm; i = 149.0±0.5° and e = 0.216–0.244; but its borders are less definite. The eight core members (S/2003 J 16, S/2003 J 21, Euanthe, Orthosie, Harpalyke, Praxidike, Thyone, S/2003 J 22, Ananke, Iocaste) are well-clumped, but the attribution of the remaining eight members to the group is debatable to varying degrees.

Pasiphaë's group, finally, picks up the remainder, with the exception of the moons at the inner and outer limits of the groups (S/2003 J 12 and S/2003 J 2); it is centered on a = 23 624 Mm; i = 151.4±6.9° and e = 0.156–0.432 (note the much larger spreads). If it is real, it must be ancient to explain the dispersion of its membership.

Naming notes

Some asteroids share the same names as moons of Jupiter: 9 Metis, 38 Leda, 52 Europa, 85 Io, 113 Amalthea, 239 Adrastea, 1036 Ganymed.

Note that the satellites discovered between 1904 and 1951 (Himalia, Elara, Pasiphaë, Sinope, Lysithea, Carme and Ananke) were not officially named until 1975, well after their discoverers had passed away. They were simply known by their Roman numeral designations (Jupiter VI through Jupiter XII). See Naming of natural satellites.