Saturn's Icy Moons
Saturn has many satellites. The known Saturnian moons range in size from a few tens of kilometers up to several thousand kilometers in diameter. In all, 30 Saturnian moons have been discovered and 18 have received officially sanctioned names from the International Astronomical Union. Titan, Saturn's largest moon and the first to be discovered, was first observed by Huygens in 1655. The satellites discovered by Cassini were Iapetus (1671), Rhea (1672), Dione (1684), and Tethys (1684). Herschel discovered Mimas and Enceladus in 1789. The latest of Saturn's moons to be named was the 12.4 mi-sized (20 km) Pan, discovered by U.S. astronomer M. Showalter in 1990.
The densities derived for the larger Saturnian moons are all about 1 g/cm3; consequently their interiors must be composed mainly of ice. All of the larger Saturnian satellites except Phoebe were photographed during the Voyager flybys, and while the images obtained showed, as expected, extensive impact cratering, they also revealed many unexpected features indicating that several of the satellites had undergone extensive surface modification. This observation supports an ice composition for these satellites, as ice—even the extremely cold, extremely rigid ice of the moons of the outer solar system—is easier to melt or deform than rock.
The Voyager images showed that Rhea and Mimas have old, heavily cratered surfaces, just as one would expect for small, geologically inactive bodies. Images of Mimas revealed a remarkably large impact crater, subsequently named Herschel, that was nearly one-third the size of the satellite itself. If the body that struck Mimas to produce Herschel had been slightly larger it probably would have shattered the moon to pieces.
In contrast to Rhea and Mimas, the surfaces of Dione and Tethys, while still heavily cratered, show evidence for substantial resurfacing and internal activity. Both moons were found to support smooth, planar regions suggesting that icy material has oozed from the interior to the surface. The Saturnian moon that shows the greatest evidence for resurfacing and internal activity is Enceladus. The surface of this moon is covered by a patchwork of smooth, icy surfaces, so shiny that they reflect nearly 100% of the light that strikes them. Even the most heavily cratered regions on Enceladus show fewer craters than the other Saturnian satellites. Enceladus also shows many surface cracks and ridges. Planetary geologists believe that the smooth regions on the surface of Enceladus may be no older than 100 million years. Since bodies as small as Enceladus, which is some 310 mi (500 km) in diameter, should have cooled off very rapidly after their formation, it is still unclear how such recent resurfacing could have taken place. Orbital resonance with Dione may supply the heat needed to keep the interior of Enceladus liquid.
Voyager images of Iapetus revealed a remarkable brightness difference between the moon's leading and trailing hemispheres. Iapetus, just like the other Saturnian moons, circles Saturn in a synchronous fashion, that is, it keeps the same hemisphere directed toward Saturn at all times (just as our Moon orbits the Earth). The images recorded by Voyager showed that the leading hemisphere, the one that points in the direction in which Iapetus is moving about Saturn, is much darker than the trailing hemisphere. Indeed, while the trailing hemisphere reflects about 40% of the light that falls on it, the leading hemisphere reflects only about eight percent. The leading hemisphere is so dark, in fact, that no impact craters are visible. The most probable explanation for the dark coloration on Iapetus is that the moon has swept up a thick frontal layer of dark, dusty material as it orbits around Saturn.
Titan is the most remarkable of Saturn's moons. With a diameter in excess of 3,100 mi (5,000 km), Titan is larger than the planet Mercury. The suggestion that Titan might have an atmosphere appears to have been first made by the Spanish astronomer Jose Comas Sola (1868–1937), who noted in 1903 that the central regions of the moon's disk were brighter than its limb (outer portions of its disk). Convincing spectroscopic evidence for the existence of a Titanian atmosphere was obtained in 1944 by American astronomer Gerard P. Kuiper (1905–1973).
Initial Earth-based observations revealed that Titan had an atmosphere containing methane and ethane. The Voyager 1 space probe, however, showed that Titan's atmosphere is mostly nitrogen, with traces of propane, acetylene, and ethylene. The atmospheric pressure at Titan's surface is nearly that at sea-level on Earth.
Titan's aerosol-hazy atmosphere is estimated to be about 250 mi (400 km) thick, with the main body of the satellite being about 3,200 mi (5,150 km) in diameter. The escape velocity from Titan is a mere 1.5 mi/sec (2.5 km/sec), which should have made escape of atmospheric gasses easy; the most likely reason that Titan has maintained its atmosphere is that the Saturnian system itself originally formed at a low temperature. Titan's present-day surface temperature is about -200°F (90K).
Titan's atmosphere is a distinctive dull orange color. Telescopic measurements at optical wavelengths have not been able to probe the surface of Titan; the atmospheric haze that surrounds the moon is too thick. Recently, however, observations made at infrared wavelengths have been able to observe surface features, and Mark Lemmon and co-workers at the University of Arizona reported in early 1995 that Titan, as might well be expected, is in synchronous rotation about Saturn. The world's largest telescope, the Keck telescope on Mauna Kea in Hawaii, detected dark areas on Titan in 1996 that scientists consider may be liquid seas of hydrocarbons formed in Titan's atmosphere by the action solar radiation and rained onto the surface.
One of the many interesting features revealed by the Voyager space probes was that Titan's atmosphere exhibits a distinct hemispherical asymmetry at visual wavelengths. The asymmetry observed on Titan is different from that seen on Iapetus, in the sense that the division on Titan is between the north and south hemispheres, rather than the leading and trailing hemispheres. When the Voyager probes imaged Titan, the northern hemisphere was slightly darker than the southern hemisphere. Follow-up observations of Titan made with the Hubble Space Telescope found that the hemispherical color asymmetry had switched during the ten years since the Voyager encounters, with the southern hemisphere being the darker one in 1990. Scientists believe that the color variation and hemisphere switching is a seasonal heating effect driven by periodic changes in Saturn's distance from the Sun.
The spacecraft Cassini, launched in 1997, will reach Saturn in 2004. It will go into orbit around Saturn and release a separate device, the Huygens probe, that will parachute through the atmosphere of Titan to its surface. If all goes well, Cassini/Huygens may resolve some of the outstanding mysteries about Saturn and Titan.
Lorenz, Ralph, and Jacqueline Mitton. Lifting Titan's Veil: Exploring the Giant Moon of Saturn. Cambridge: Cambridge University Press, 2002.
Morton, Oliver. Mapping Mars. New York: Picador, 2002.
Gladman, Brett, et al. "Discovery of 12 Satellites of Saturn Exhibiting Orbital Clustering." Nature 412 (July 12, 2001): 163–166.
Hamilton, Douglas P. "Saturn Saturated With Satellites." Nature 412 (July 12, 2001): 132–133.
Nicholson, Philip, D. "Saturn's Rings Turn Edge On." Sky & Telescope (May 1995).
Rothery, David. "Icy Moons of the Solar System." New Scientist (28 March 1992).
Jet Propulsion Laboratory, California Institute of Technology. "Cassini-Huygens: Mission to Saturn and Titan" January 17, 2003 [cited January, 20, 2003]. <http://saturn.jpl. nasa.gov/index.cfm>.