Saturn

Saturn, sixth planet from the Sun , is the most remote of the planets that were known to premodern astronomers. Saturn is a gas giant with no solid surface; it is 9.45 times wider than Earth and 95 times more massive. It is circled by hundreds of rings consisting of small, ice-covered particles and is also host to at least 30 moons, including Titan, largest moon in the solar system and the only one with an extensive atmosphere.

Basic characteristics

Saturn orbits the Sun at a mean distance of 9.539 astronomical units (AU, where 1 AU is the average distance between the Earth and the Sun). Its slightly eccentric (noncircular) orbit , however, allows the planet to be far from the Sun as 10.069 AU and as close as 9.008 AU. Saturn takes 29.46 Earth years to complete one orbit around the Sun. Saturn has an equatorial diameter of 74,855 mi (120,540 km), making it the second-largest planet in the solar system (Jupiter is about the same size as Saturn but is 3.35 times more massive). Saturn spins on its axis 2.25 times more rapidly than the Earth, that is, every 10 hours 14 minutes. This rapid rotation causes it to be 8,073 mi (13,000 km) wider at the equator than it is from pole to pole.

Saturn has an average density of 0.69 g/cm3, less than that of water (1.0 g/cm3) and lowest of all the planets in the solar system. This low density indicates that the planet must be composed mainly of hydrogen and helium (the most abundant elements in the Universe). Theoretical models suggest that Saturn has a rocky inner core that accounts for only about 26% of its mass . This central core is surrounded by a thick layer of liquid metallic hydrogen, a form of hydrogen that occurs only under extreme pressure . This mantle is surrounded by an atmosphere composed mostly of molecular hydrogen and helium that is liquid at its base and gradually becomes less dense at higher altitudes, finally becoming a gaseous atmosphere at the highest levels.

When the two Voyager spacecraft flew past Saturn in 1980 and 1981, they confirmed that Saturn has a magnetic

TABLE 1. SATURIAN SATELLITES LARGER THAN 200 KM IN DIAMETER ²

Name	Diameter (km)	Density (kg/m 3)	Albedo	Mean distance (10000 km)	Orbital period (day)
2 Distances are given in units of 1000 km. The albedo is a measure of the amount of sunlight reflected by the satellite. An albedo of zero corresponds to no reflection, while an albedo of unity corresponds to complete reflection.
Phoebe	220	—	0.05	12,960	550.46
Hyperion	255	—	0.3	1481	21.276
Mimas	390	1200	0.8	187	0.942
Enceladus	500	1100	1.0	238	1.370
Tethys	1060	1200	0.8	295	1.888
Dione	1120	1400	0.6	378	2.737
Iapetus	1460	1200	0.08 - 0.4	3561	79.331
Rhea	1530	1300	0.6	526	4.517
Titan	5550	1880	0.2	1221	15.945

field. Like Jupiter's magnetic field, Saturn's is probably produced in the planet's metallic-hydrogen mantle. The magnetic field at Saturn's cloud tops, however, is about one tenth that observed on Jupiter. Indeed, Saturn's equatorial magnetic field is only about twothirds as strong as Earth's.

Careful measurements of Saturn's energy budget (balance of energy absorbed versus energy radiated) show that the planet radiates 1.5–2.5 times more energy into space than it receives from the Sun. This radiated energy indicates that the planet must have an internal heat source. Scientists accept that Saturn draws its extra energy from two sources: (1) heat left over from the planet's formation approximately 4.5 billion years ago, still radiating out into space, and (2) the "raining out" of atmospheric helium. Just as water condenses in terrestrial clouds to produce rain, droplets of liquid helium form in Saturn's atmosphere. As these droplets fall through Saturn's atmosphere they acquire kinetic energy. This energy is absorbed into deeper layers where the droplets meet resistance and slow their fall, and the temperature in those regions increases. This thermal energy is eventually circulated by convection back up through the higher layers of the atmosphere and radiated into space. The helium raining out of Saturn's upper layers is left over from the planet's formation; in about two billion more years all of Saturn's helium will have sunk deep into the planet, at which time heating by helium condensation will cease.

Support for the helium-condensation model was obtained during the Voyager encounters, when it was found that the abundance of helium in Saturn's atmosphere was much lower than that observed in Jupiter's. Upper-atmospheric depletion of helium has not yet occurred on Jupiter because its atmosphere has only recently become cool enough to permit helium condensation; on Saturn, in contrast, helium has been raining out for about two billion years, settling half the available helium toward the core.

Saturn's atmosphere

The intensity of sunlight at Saturn's orbit is about one hundredth that at the Earth's orbit and about one fourth that the orbit of Jupiter. Consequently, and in spite of its internal heat sources, Saturn's surface is cold. When compared at levels with corresponding pressures, Saturn's atmosphere is some 270°F (150°C) cooler than Earth's and about 90°F (50°C) cooler than Jupiter's.

The atmospheres of both Saturn and Jupiter feature distinctive banded structures, horizontal zones of wind flowing in opposite directions at high speeds (e.g., 1,100 miles/hr [1,800 km/hr] at Saturn's equator). Jupiter and Saturn's zonal jets, as these winds are termed, are, according to one theory, the surface manifestations of gigantic, counter-rotating cylindrical shells of fluid in these planets' interiors. Such cylinders form because fluids in a rotating body tend to align their motions with the body's axis of rotation (in this case, the planet's); where the edges of these cylinders contact the approximately spherical outer surface of the planet, matching zonal jets are produced in the northern and southern hemispheres.

Saturnian storms

The outermost regions of Saturn's hydrogen-helium atmosphere support ammonia , ammonium hydrosulfide, and water clouds. Saturn's storm features are not as pronounced or as long-lived as those observed on Jupiter, and Saturn has no weather feature as long-lived as Jupiter's Great Red Spot. However, isolated spots and cloud features are occasionally distinguishable from Earth. English astronomer William Herschel (1738–1822), for example, reported seeing small spots on Saturn's disk in 1780. Since that time, however, very few other features have been reported. The most dramatic recurring feature observed on Saturn is its Great White Spot. This feature was first observed by American astronomer Asaph Hall (1829–1907) on December 7, 1876, and six subsequent displays have been recorded. A Great White Spot was observed by the Hubble Space Telescope in 1990; smaller spots are observed in most Saturnian summer seasons .

All the white spots observed on Saturn are thought to be giant storm systems. When they first appear, the Great White Spots—the largest of these storms—are circular in form and some 12,420 mi (20,000 km) in diameter. Atmospheric winds gradually stretch and distort the spots into wispy bands, which can often be seen for several months. All of the Great White Spots have been observed in Saturn's northern hemisphere, with a recurrence interval equal to one Saturnian year (29.51 years). That the storms tend to repeat every Saturnian year suggests that they are a seasonal effect, with the storms being produced whenever Saturn's northern hemisphere is at maximum tilt toward the Sun. It is likely that storms also occur in Saturn's southern hemisphere when it is tilted toward the Sun, but the angle for viewing such events from the Earth is not favorable. It is assumed that the Great White Spots are produced by an up-welling of warm gas. Indeed, they have been likened to atmospheric "belches." In this manner the spots are similar to the cumulonimbus thunderheads observed in terrestrial storm systems. The prominent white color of the Saturnian storms is due to the freezing-out of ammonia ice crystals. These crystals form as the warm gas pushes outward into the frigid outer layers of the planet's atmosphere.

Saturn's rings

When Italian astronomer Galileo Galilei (1564–1642) first pointed his telescope towards Saturn in 1610, he saw two features protruding from the planet's disk. These puzzling side-lobes were in reality Saturn's ring feature, though Galileo's telescope was too small to resolve their shape and extent. When these side-lobes started vanishing, as the rings began gradually assuming a position edgewise to the Earth, Galileo was not able to explain the nature of his observations. Dutch astronomer Christiaan Huygens (1629–1695) was the first scientist to suggest, in 1659, that Saturn was surrounded by a flattened ring.

Soon after Huygens had suggested that a ring existed around Saturn, the Italian-born French astronomer Jean-Dominique Cassini (1625–1712) discovered, in 1675, that there were in fact several distinct rings about the planet. Several divisions in Saturn's rings are now recognized, the dark band between the A and B rings being known as the Cassini Division. The A ring is further subdivided by a dark band termed the Encke Division after German astronomer Johann F. Encke (1791–1865), who first observed this feature in 1838.

Saturn's rings are best seen when the planet is near opposition, that is, at its closest approach to the Earth. At this time the rings are seen at the greatest angle. The rings are aligned with Saturn's equator, thus tilted at 26.7 degrees to the ecliptic (i.e., the plane of the planets' orbits about the Sun). During the course of one Saturnian year the rings, as seen from Earth, are alternately viewed from above and below. Twice each Saturnian year (i.e., once every 15 years) the rings are seen edge-on, an event termed a ring-plane crossing. That the rings nearly disappear from when seen edge-on indicates that they must be thin. Recent measurements suggest that the rings are no more than 1.24 mi (2 km) thick.

That the rings of Saturn cannot be solid was first proved mathematically by Scottish physicist James Clerk Maxwell (1831–1879) in 1857. Maxwell showed that a solid planetary disk would literally tear itself apart, and concluded that the rings must be composed of orbiting particles. Subsequent observations have confirmed Maxwell's deductions, and it is now known that the rings are made of pieces of ice and ice-coated rock varying in size from dust particles to chunks on the order of 10 yards across.

Images obtained by the Voyager and Pioneer space probes have shown that Saturn's rings are really composed of numerous ringlets. The apparently empty regions between ringlets are thought to be caused both by gravitational resonance with Saturnian moons (which boost particles out of those regions by delivering periodic pushes or pulls) and by a mechanism called shepherding. Just as gaps (Kirkwood gaps) have been produced in the asteroid belt through gravitational resonance with Jupiter, so gaps have been formed in Saturn's rings due to resonance with its major satellites. The Cassini Division, for example, is the result of a 2-to-1 resonance with the moon Mimas. Some of the narrower rings, on the other hand, are believed to be maintained as distinct objects by shepherding satellites. Orbiting nearby on either side of a ring, the shepherding satellites prevent the ring particles from dispersing into higher or lower orbits. The faint F ring, 62 mi (100 km) wide, for example, is maintained by two small satellites, Prometheus (62 mi [100 km] in diameter) and Pandora (56 mi [90 km] in diameter). Indeed, the F ring, which was discovered by the Pioneer 11space probe in 1979, shows some remarkably complex structures, with the ring being made of several interlaced and (apparently) braided particle strands.

In 1995 and 1996, ring-plane crossings occurred three times with respect to Earth, as well as once with respect to the Sun. This provided a unique opportunity for observing the rings and satellites of Saturn, since glare from the rings is greatly reduced during ring plane crossing, enabling astronomers to observe faint objects. On the ring-plane crossing of May 22, 1995, the Hubble Space Telescope discovered four new Saturn moons, including a third shepherd satellite which may account for the braiding of the F ring. The Hubble also discovered that the orbit of Prometheus had shifted, perhaps due to a collision with the F ring. During the August 10, 1995, ring plane crossing, the Hubble detected clouds of debris near the outer edge of the ring system. These may be the remains of small satellites shattered in collisions. These clouds may be the source of material for Saturn's rings.

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.

See also Kepler's laws; Planetary ring systems.

Resources

books

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.

periodicals

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).

other

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>.

Martin Beech

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Albedo

—The fraction of sunlight that a surface reflects. An albedo of zero indicates complete absorption, while an albedo of unity indicates total reflection.

Oblateness

—A measure of polar to equatorial flattening. A sphere has zero oblateness.

Shepherding satellite

—A satellite that restricts the motion of ringlet particles, preventing them from dispersing.