Jupiter's Rings Fact Sheet
The following are additional facts related to the discovery of the source of the rings of Jupiter, announced by the Galileo spacecraft scientific team at a press conference held at Cornell University, Ithaca, NY, Sept. 15.
Voyager program -- Voyager 1 and Voyager 2 were launched by NASA in 1977. In 1979 Voyager 1 obtained the first images of a ring system around Jupiter. The same year, most of the important observations of Jupiter's rings were provided by Voyager 2 after it went into the shadow of the giant planet and looked back toward the sun and saw a system of three rings. In just one image the spacecraft captured a faint outer ring, apparently made up of fine, microscopic particles. This was named the gossamer ring.
Galileo spacecraft -- Launched by NASA in 1989, the Galileo spacecraft has provided the first close images of asteroids (Gaspra, 1991; Ida and its moon, Dactyl, 1993). The spacecraft also captured images of the Comet Shoemaker-Levy 9 as it hit Jupiter. Galileo arrived at Jupiter on Dec. 7, 1995, and released an atmospheric probe that plunged into Jupiter's atmosphere and relayed information on the structure and composition of the planet. In the 2 1/2 years since then, Galileo has orbited the planet, studying Jupiter and its four largest moons-Callisto, Europa, Ganymede and Io. Its current extended journey, called the Galileo Europa Mission, will continue through the end of 1999.
Jupiter -- The solar system's largest planet and the fifth planet from the sun (distance: 778.3 million kilometers (483.6 miles), it is a gaseous planet with an atmosphere composed mostly of hydrogen and helium with traces of methane, ammonia and other gases.
Rings of Jupiter -- Galileo's high-quality images have provided greatly improved information about the rings' structure. The rings are very tenuous; only about one-millionth of the surface area is covered, and the rings contain many particles of microscopic size.
Gossamer Ring -- The ring actually consists of two faint, fairly uniform rings, one enclosing the other, visibly spreading from the outer boundary of the main ring and fading somewhere beyond 221,000 kilometers (136,000 miles) from the center of Jupiter, at Thebe's orbit. The denser, enclosed gossamer ring extends radially inward from Amalthea's orbit at 181,000 kilometers (112,000) from Jupiter's center, while the fainter ring is situated interior to Thebe's orbit at 222,000 kilometers (138,000 miles). As Amalthea's orbit is crossed, the ring's brightness drops to one-fifth, while near Thebe's path the drop is by a factor of three. Both rings have cross-sections that are crudely rectangular. A striking feature of both rings is that each is banded with top and lower edges brighter than the centers.
Main Ring -- The brightest of the Jovian rings reaches from the halo's outer boundary across 6,440 kilometers (4,000 miles) to 128,940 kilometers (80,000 miles) just interior to Adrastea's orbit at 128,980 kilometers. At its outer edge the main ring takes nearly 1,000 kilometers (600 miles) to develop its full brightness. The ring's brightness noticeably decreases around 127,850 kilometers (79,000 miles) in the vicinity of Metis. The precise location and nature of the main ring's outer periphery might shift slightly from image to image.
Halo -- The innermost component of the Jovian ring system is a toroidal halo extending radially from about 92,000 kilometers (57,000 miles) to about 122,500 kilometers (76,000 miles). Its brightness decreases with the height of the equatorial plane and decreases as the planet is approached. The halo is actually a nimbus of fine particles that 'blooms' vertically at the main ring's inner boundary and continues toward the planet and vertically.
Moons of Jupiter -- Sixteen moons have been discovered since 1610. In addition to Io, Europa, Ganymede, and Callisto, the four largest Galilean moons, Jupiter has two groups of small satellites. Four orbit closer than Io, and eight orbit well beyond Callisto. The four moons that the Galileo imaging science team identify as ring moons, supplying the debris that form the ring systems, are the inner moons, Metis, Adrastea and Thebe, discovered by Voyager 1, and Amalthea, first seen through a telescope by Edward Barnard in 1892. Galileo images have provided much more detailed shape, color and photometric information on these four inner small satellites than was provided by the Voyager images. The satellites are in synchronous rotation and their major geologic features are impact craters. The new Galileo observations are the first time that Metis, Adrastea and Thebe have been seen as more than points of light.
Metis -- This moon is embedded within the main ring. Its longest dimension is about 60 kilometers (37 miles) across.
Adrastea - This moon, the smallest of the four satellites, skims along the main ring's outer edge, but details of its actual shape are not available. Its longest dimension is 20 kilometers (12 miles) across.
Amalthea - This moon, the largest of the four inner moons, lies at the outer periphery of the inner gossamer ring. A digital image map shows crater rims and bright markings. The largest crater, Pan, appears to be bowl-shaped and 90 kilometers (54 miles) wide. A bright spot at the south pole is associated with a smaller crater named Gaea. Amalthea's longest dimension is 247 kilometers (154 miles) across.
Thebe -- This moon is near the outer periphery of the outer gossamer ring. Galileo images show the satellite to have bright spots near the rim of one large crater. Thebe's longest dimension is 116 kilometers (72 miles) across.