Satellites & Moons

More than 20 satellites have been discovered orbiting Saturn. Their diameters range from 20 to 5,150 km (12 to 3,200 mi). They consist mostly of the lighter, icy substances that prevailed in the outer parts of the gas and dust nebula from which the solar system was formed, where radiation from the distant Sun could not evaporate the frozen gases. The five larger inner satellites— Mimas, Enceladus, Tethys, Dione, and Rhea—are roughly spherical in shape and composed mostly of water ice. Rocky material may constitute up to 40 per cent of Dione's mass. The surfaces of the five are heavily cratered by meteoroid impacts. Enceladus has a smoother surface than the others, the least cratered area on its surface being less than a few hundred million years old. (Possibly Enceladus is still undergoing tectonic activity; see Plate Tectonics.) Astronomers suspect that Enceladus supplies particles to the E ring, which neighbours the satellite's orbit. Mimas, far from being smooth, displays an impact crater the diameter of which is one-third of the diameter of the satellite itself. Tethys also bears a large crater and a valley 100 km (62 mi) in width that stretches more than 2,000 km (1,200 mi) across the surface. Both Dione and Rhea have bright, wispy streaks on their already highly reflective surfaces. Some scientists conjecture these were caused either by ice ejected from craters by meteoric impacts, or by fresh ice that has migrated from the interior.

Several small satellites have been discovered immediately outside the A ring and close to the F and G rings. Possibly four so-called Trojan satellites of Tethys and one of Dione have also been discovered. The term Trojan is applied to bodies such as satellites or asteroids occurring in regions of stability that lead or follow a body in its orbit around a planet or the Sun. See Solar System.

The outer satellites Hyperion and Iapetus also consist mainly of water ice. Iapetus has a very dark region that contrasts with most of its surface, which is bright. This dark region and the rotation of the satellite are the cause of the variations of brightness that were noticed by Cassini in 1671. Phoebe, the farthest satellite, moves in a retrograde orbit that is highly inclined to Saturn's equator. Phoebe is probably a cometary body captured by Saturn's gravitational field.

Between the inner and outer satellites orbits Titan, Saturn's largest moon. Its diameter is 5,150 km (3,200 mi), larger even than the planet Mercury. The diameter of Titan is not known, however, because a dense orange haze hides the surface. The thickness of Titan's atmosphere is probably about 300 km (186 mi). Titan has a nitrogen atmosphere with traces of methane, ethane, ethyne (acetylene), ethene (ethylene), hydrogen cyanide, carbon monoxide, and carbon dioxide. On the surface, the temperature is about -182° C (-296° F), and methane or ethane may be present in the forms of rain, snow, ice, and vapour. The interior of Titan probably consists of equal amounts of rock and water ice. No magnetic fields have been detected. The southern hemisphere appears to be slightly brighter. In recent years, observations by the Hubble Space Telescope have provided some further information, indicating a large elevated area on the surface. The idea that Titan may have liquid oceans of ethane and other hydrocarbons recently gained support when scientists using the Keck I telescope at Mauna Kea, Hawaii, produced images showing dark areas. More details may be obtained by the Cassini spacecraft, due to reach Saturn in 2004 and drop a probe with cameras attached into Titan's atmosphere.

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