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At least 39 natural satellites are known to orbit Jupiter. They are conveniently divided into three groups. The four largest : Io, Europa, Ganymede, and Callisto : were discovered by Galileo in 1610, shortly after he invented the telescope, and are known as the Galilean satellites. 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 entirely 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.
A second 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 ring.
The satellites in these two groups are regular satellites, that is, their orbits are relatively circular, near equitorial, and prograde, i.e., moving in the same orbital direction as the planet. The final group consists of the 31 satellites with orbits outside that of Callisto. Of these, only the five closest to Callisto : Themisto (diameter: 5 mi/8 km); Leda (diameter: 6 mi/10 km), Himalia (diameter: 106 mi/170 km), Lysithea (diameter: 15 mi/24 km), and Elara (diameter: 50 mi/80 km) : are regular. The remainder are irregular in that their orbits are elliptical, inclined to that of the planet, and retrograde, i.e., motion opposite to that of the planet's rotation: Ananke (diameter: 12 mi/20 km), Carme (diameter: 19 mi/30 km), Pasiphae (diameter: 22 mi/36 km), Sinope (diameter: 18 mi/28 km), Iocaste (diameter: 3 mi/5 km), Praxidike (diameter: 4.5 mi/7 km), Harpalyke (diameter: 2.8 mi/4.5 km), Isonoe (diameter: 2.5 mi/4 km), Erinome (diameter: 2 mi/3.2 km), Taygete (diameter: 3 mi/5 km), Chaldene (diameter: 2.4 mi/3.8 km), Kalyke (diameter: 3.2 mi/5.2 km), Megaclite (diameter: 3.4 mi/5.5 km), Callirrhoe (diameter: 5.3 mi/8.5 km), S/2000 J 11 (diameter: 2.5 mi/4 km), S/2001 J 1 (diameter: 5 mi/8 km), S/2001 J 2 (diameter: 5 mi/8 km), S/2001 J 3 (diameter: 5 mi/8 km), S/2001 J 4 (diameter: 3.7 mi/6 km), S/2001 J 5 (diameter: 2.5 mi/4 km), S/2001 J 6 (diameter: 2.5 mi/4 km), S/2001 J 7 (diameter: 3.7 mi/6 km), S/2001 J 8 (diameter: 2.5 mi/4 km), S/2001 J 9 (diameter: 2.5 mi/4 km), S/2001 J 10 (diameter: 2.5 mi/4 km), S/2001 J 11 (diameter: 3.7 mi/6 km). The odd orbits of these satellites indicate that they were captured after Jupiter's formation. Because they are small, irregularly shaped, and clustered into small families, 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 rings : Halo, 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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