SATELLITES AND RINGS Early on, when
SATELLITES were forming around Jupiter, heat radiating from the planet was much greater.
Hence the
satellites that formed are rockier near Jupiter and icier farther away. This trend is evident among the four large satellites discovered by Galileo Galilei in 1620 and called the Galilean moons. The regular, circular, equatorial orbits of these satellites suggest that they did indeed form from a cloud of small
particles circling the planet. The satellites, named Callisto, Ganymede, Europa, and Io, are described in separate entries. In addition to the Galilean moons, Jupiter has several smaller satellites and rings. Amalthea, the largest of the four satellites interior to Io's orbit, is irregularly shaped, about 265 km (165 mi) long, and 150 km (93 mi) wide. Its surface is dark and red and continually bombarded by the energetic particles of Jupiter's magnetosphere, including material originally from Io. Voyager 1 photographed (1979) a narrow flat ring orbiting the planet about halfway from the surface out to Amalthea. A doughnut-shaped "halo" ring lies immediately inside that main ring, and a flat, faint "gossamer" ring extends far out beyond the main ring. Jupiter's rings are very diffuse. The ring particles, which resemble dark, reddish soot, must generally be about as big as the wavelength of lightÑthat is, only a few microns. Particles this size are susceptible to drag that makes them spiral down toward Jupiter, and to pumping by the magnetosphere, creating the "halo" ring, as they pass inside the main ring. The ring system would be depleted of material in only 100,000 years if there were no continuing supply. Images made by the Galileo spacecraft resolved this problem by showing that the gossamer ring starts with its outer edge precisely at the orbit of satellite Thebe, and a denser part of the gossamer ring starts at the orbit of satellite Amalthea. Moreover, the main ring lies just inside the orbits of the two innermost satellites, Metis and Adrastea. Bombardment of all four satellites by micrometeoroids keeps the rings supplied with freshly ejected material. The eight outer satellites of Jupiter are small, dark, stony objects that closely resemble the Trojan asteroids (see Trojans). This evidence, combined with their highly eccentric and inclined orbits near the limit of Jupiter's gravitational sphere of influence, suggests that the outer satellites were captured from interplanetary orbits. Such capture may have resulted in their clustering at two distinct distances from Jupiter and the retrograde (opposite Jupiter's rotation) motion of the outer four satellites.