Jupiter, in astronomy: Its Moons and Rings

Its Moons and Rings

Astronomers have discovered 79 satellites orbiting Jupiter, but five of those, small satellites that were identified in 2003 and 2011 but have not been found since then, are considered lost. Jupiter's satellites are divided into six main groups (in order of increasing distance from the planet): Amalthea, Galilean, Himalia, Ananke, Carme, and Pasiphae. The first group is comprised of the four innermost satellites—Metis, Adrastea, Amalthea, and Thebe. The red color of Amalthea (diameter: 117 mi/189 km), a small, elongated satellite discovered (1892) by Edward Barnard, probably results from a coating of sulfur particles ejected from Io. Metis (diameter: 25 mi/40 km), Adrastea (diameter: 12 mi/20 km), and Thebe (diameter: 62 mi/100 km) are all oddly shaped and were discovered in 1979 in photographs returned to earth by the Voyager 1 space probe. Metis and Adrastea orbit close to Jupiter's thin ring system; material ejected from these moons helps maintain the rings.

The four largest satellites—Io, Europa, Ganymede, and Callisto—were discovered by Galileo in 1610, shortly after he invented the telescope, and are known as the Galilean satellite group. Io (diameter: 2,255 mi/3,630 km), the closest to Jupiter of the four, is the most active geologically, with 30 active volcanoes that are probably energized by the tidal effects of Jupiter's enormous mass. Europa (diameter: 1,960 mi/3,130 km) is a white, highly reflecting body whose smooth surface is covered with dark streaks up to 43 mi/70 km in width and from several hundred to several thousand miles in length. Ganymede (diameter: 3,268 mi/5,262 km), second most distant of the four and the largest satellite in the solar system, has heavily cratered regions, tens of miles across, that are surrounded by younger, grooved terrain. Callisto (diameter: 3,000 mi/4,806 km), the most distant and the least active geologically of the four, has a heavily cratered surface.

The eight inner satellites are regular, that is, their orbits are relatively circular, near equatorial, and prograde, i.e., moving in the same direction as the planet's rotation. The remainder are irregular in that their orbits are large, elliptical, inclined to that of the planet, and, in the case of nearly all the moons beyond Carpo, retrograde. (Jupiter's retrograde satellites are distinguished from the regular by the spelling of their names, which all end in the letter “e”.) In addition, most of the outer moons are much smaller.

Themisto (diameter: 5 mi/8 km) orbits Jupiter midway between the Galilean and next main group of satellites, the Himalias. The Himalia group consists of seven tightly clustered satellites with orbits outside that of Callisto—Leda (diameter: 6 mi/10 km), Himalia (diameter: 106 mi/170 km), Ersa (diameter 2 mi/3 km), Pandia (diameter 1 mi/2 km), Lysithea (diameter: 15 mi/24 km), Elara (diameter: 50 mi/80 km), and Dia (diameter: 2.5 mi/4 km). These eight satellites have prograde orbits. Situated between the Himalia and Ananke groups are Carpo (diameter: 2 mi/3 km) and S/2016 J2 (diameter: .6 mi/1 km), which like Themisto do not seem to belong to any of the main groups. Carpo has the most highly inclined orbit of any of the prograde satellites; S/2016 J2, the outermost prograde satellite, has an orbit that intersects those of a number of retrograde satellites.

The Ananke group comprises 20 satellites, which share similar orbits and range from .6 to 3 mi (1–5 km) in diameter except for two: Euporie, Eupheme, Jupiter LV, Jupiter LII, Thelxinoe, Euanthe, Helike, Orthosie, S/2017 J7, Jupiter LIV, S/2017 J3, Iocaste, S/2003 J16, Praxidike (diameter: 4.5 mi/7 km), Harpalyke, Mneme, Hermippe, Thyone, S/2017 J9, and Ananke (diameter: 12.5 mi/20 km). Like the Ananke group, the Carme group is remarkably homogeneous. It comprises 20 satellites, which share similar orbits and, except for one, range from .6 to 3 mi (1–5 km) in diameter: Herse, Aitne, Kale, Taygete, S/2003 J19, Chaldene, Erinome, Kallichore, S/2017 J5, S/2017 J8, Kalyke, Carme (diameter: 28 mi/46 km), S/2017 J2, Pasithee, Jupiter LI, Eukelade, Arche, Isonoe, S/2003 J9, and Eirene.

The most distant of the groups from the planet is the Pasiphae, which comprises 16 widely dispersed satellites that, except for three, also range from .6 to 3 mi (1–5 km) in diameter: S/2017 J6, Philophrosyne, S/2003 J23, Aoede, Callirrhoe (diameter: 5.5 mi/9 km), Eurydome, Kore, Cyllene, Jupiter LVI, Jupiter LIX, Pasiphae (diameter: 36 mi/58 km), Hegemone, Sinope (diameter: 23 mi/38 km), Sponde, Autonoe, and Megaclite. The odd orbits of the irregular satellites indicate that they were captured after Jupiter's formation. Because they are small, irregularly shaped, and clustered into groups, it is believed that they originated as parts of a larger body that either shattered due to Jupiter's enormous gravity or broke apart in a collision with another body.

Jupiter has three ringsHalo, Main, and Gossamer—similar to those of Saturn but much smaller and fainter. An intense radiation belt lies between the rings and Jupiter's uppermost atmospheric layers.

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