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Jupiter: Moons and Rings
Moons
Jupiter reigns over 16 satellites: the four large Galilean moons (those spotted by Galileo with his telescope) and the 12 lesser moons. There is a seventeenth object that has been discovered but not yet confirmed. Most of Jupiter’s satellites are named for his lovers. Mythology records hundreds of them, so new names will never be a problem. As can be imagined, a very complex and powerful tidal relationship exists between Jupiter and its moons. The tidal drag created by the Galilean satellites, for example, is gradually slowing down Jupiter’s orbit. The tidal power works on the moons themselves, slowly forcing them away from the giant planet. There is a great deal of tidal resonance among the moons. Io, Europa, and Ganymede are locked into a 1:2:4 relationship. Callisto is working itself into this right now. In a few hundred million years, Callisto will be locked in as the newest addition in a 1:2:4:8 resonance.
These four Galilean moons, even without their resonance, are among the most fascinating of all celestial bodies. Io has active volcanism. Europa, which may have an atmosphere, has the possibility of a water environment capable of sustaining life. The Galileo spacecraft was launched to investigate these moons, particularly Europa, in an effort to lay to rest some of these long-standing questions.| Name | Distance (km) | Radius (km) | Mass (kg) | Discoverer (Year) |
| Metis | 128 | 20 | 9.56 x 1016 | Synnott (1979) |
| Adrastea | 129 | 10 | 1.91 x 1016 | Jewitt (1979) |
| Amalthea | 181 | 98 | 7.17 x 1018 | Barnard (1892) |
| Thebe | 222 | 50 | 7.77 x 1017 | Synnott (1979) |
| Io | 422 | 1,815 | 8.94 x 1022 | Galileo (1610) |
| Europa | 671 | 1,569 | 4.80 x 1022 | Galileo (1610) |
| Ganymede | 1,070 | 2,631 | 1.48 x 1023 | Galileo (1610) |
| Callisto | 1,883 | 2,400 | 1.08 x 1023 | Galileo (1610) |
| Leda | 11,094 | 8 | 5.68 x 1015 | Kowal (1974) |
| Himalia | 11,480 | 93 | 9.56 x 1018 | Perrine (1904) |
| Lysithea | 11,720 | 18 | 7.77 x 1016 | Nicholson (1938) |
| Elara | 11,737 | 38 | 7.77 x 1017 | Perrine (1905) |
| Ananke | 21,200 | 15 | 3.82 x 1016 | Nicholson (1951) |
| Carme | 22,600 | 20 | 9.56 x 1016 | Nicholson (1938) |
| Pasiphae | 23,500 | 25 | 1.91 x 1017 | Melotte (1908) |
| Sinope | 23,700 | 18 | 7.77 x 1016 | Nicholson (1914) |
| S/1999 J 1 (Possible Moon) | 23,000 | 3 | Unknown | Spacewatch (1999) |
Rings
Jupiter has rings, but they are much fainter and smaller than Saturn’s. Their composition (small-grained rocky material, no ice) contributes to their low albedo (reflectivity). Jupiter’s rings were discovered only after two of the scientists controlling the Voyager 1 mission insisted that the probe, after coming a billion km to Jupiter, may as well look around to see if there are any. Their discovery was a complete surprise. The Galieleo spacecraft discovered that the rings are continuously maintained by the addition of micrometeor debris on the four inner moons.