VII.

Moons

Saturn has at least 60 moons. They range up to 2,575 km (1,600 mi) in radius. 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 and where radiation from the distant Sun could not evaporate the frozen gases. The discovery of 12 of Saturn’s moons was reported as recently as May 2005 and 9 more were announced in June 2006. An additional 4 moons were reported in 2007. These moons are irregular in shape and small, ranging in diameter from 3 km (2 mi) to 7 km (4 mi). A number of the moons orbit Saturn in a direction opposite that of Saturn’s larger moons, suggesting that some of these recently discovered satellites were originally asteroids that were captured by Saturn’s gravitational field.

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 percent of Dione's mass. The surfaces of the five moons are heavily cratered by meteorite impacts.

Enceladus has a smoother surface than Saturn’s other moons, the least cratered area on its surface being less than a few hundred million years old. The detection of geysers on Enceladus suggests that liquid water below the surface is being heated by some source. Among the possibilities are tidal forces—the gravitational pull of Saturn and other moons. These tidal forces could cause friction that heats rocks within the interior of the moon. Heat released by the radioactive decay of rocks deep in the moon could also melt some of the ice. The Cassini spacecraft detected oxygen atoms in the geyser plumes that jet out from the moon’s southern polar region. The plumes reach about 418 km (about 260 mi) into space. Scientists concluded that the geysers were spewing out water molecules that then broke down into oxygen and hydrogen atoms. Astronomers think that Enceladus supplies most of the particles in the E ring, which neighbors Enceladus’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 meteorites, or by fresh ice that has migrated from the interior.

Several small moons have been discovered immediately outside the A ring and close to the F and G rings. Possibly four so-called Trojan or coorbital satellites of Tethys and one of Dione have also been discovered. Trojan satellites occur in regions of gravitational stability that lead or follow a body in its orbit around a massive central body, in this case, Saturn.

The outer satellites Hyperion and Iapetus also consist mainly of water ice. Iapetus has a very dark region in contrast to 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 large satellite, moves in a retrograde orbit (in the opposite direction of the orbits of most of the other satellites) that is at a sharp angle to Saturn's equator. Phoebe is probably a cometary body captured by Saturn's gravitational field.

Titan, Saturn's largest moon, orbits the planet between the inner and outer satellites. Titan’s radius is 2,575 km (1,600 mi), larger even than the planet Mercury. The moon appears nearly featureless to optical telescopes. A dense orange haze hides the surface, but astronomers have glimpsed distinct methane clouds. Titan’s atmosphere is largely composed of nitrogen with traces of methane, ethane, hydrogen cyanide, carbon dioxide, water vapor, and several other organic compounds. The Cassini spacecraft imaged the moon close-up using various wavelengths of light and mapped it with radar. Titan has a geologically young surface. The Huygens probe descended to the surface in January 2005 and sent back pictures of large rocklike objects probably made of ice.

Radar observations reported in 2006 revealed sandlike dunes remarkably similar to those found in the Sahara and the Namib Desert on Earth. In some cases the dunes are as high as 100 m (330 ft) and stretch for as long as 1,500 km (930 mi), running parallel to each other like those in the Sahara. The fine sandlike grains are probably made of ice or organic solids or a mixture of both, rather than the silicates that make up sand on Earth. Scientists theorized that the dunes are formed by winds created by the tidal forces that Saturn exerts on Titan. Scientists recently learned that those forces are 400 times more powerful than the tidal forces that the Moon exerts on Earth. Previously, scientists believed that there was little wind on Titan because the amount of sunlight it receives is insufficient to supply the energy for atmospheric circulation. Tidal forces exerted by Saturn, however, could be enough to create winds averaging nearly a kilometer per hour.

Titan’s north pole has areas that appear to be lakes based on radar images returned in July 2006 and afterward. The shapes and smooth surfaces of the bodies suggest they contain liquid, probably methane mixed with ethane. Such lakes may form and evaporate depending on Titan’s seasons. Other recent research indicates large methane storms can occur in the dense, cold atmosphere and may precipitate out as hydrocarbon rain. The topography on Titan shows evidence of channels and other drainage features similar to those created by water erosion on Earth. Radar also indicates a continent-like highland area named Xanadu has features resembling river courses, hills, and mountains over 1 km (0.6 mi) high. The elevated region is thought to be formed from rock-hard water ice and has a rugged terrain that suggests methane rains may have riddled the ground with caverns.