SATURN

in astronomy, sixth planet from the Sun. It is named after the Roman god of agriculture. Saturn is the second largest nonstellar object in the solar system, after Jupiter. It is about 95 times as massive as the Earth and has more than 700 times the volume. Saturn has at least 19 icy satellites, nine of which were discovered before 1900. It is also surrounded by an extensive ring system consisting of seven main rings. Spectroscopic research conducted over a large range of the electromagnetic spectrum from a variety of observational platforms indicates that the outer layers of Saturn are gaseous; they are composed principally of hydrogen, with helium depleted by a factor of about two compared with the Sun. The planet emits approximately 80 percent more radiation in the thermal infrared portion of the electromagnetic spectrum than it receives from the Sun. Laboratory and computer modeling experiments performed on hydrogen under pressures of several million atmospheres indicate that it becomes a highly conducting liquid metal. Models of the interior of Saturn thus suggest that 1020 percent of its mass resides in a rocky core that is surrounded by a shallow layer of liquid metallic hydrogen encased by an envelope of molecular hydrogen. The rapid rotation of the planetary liquid metallic core (10 hours 39 minutes 24 seconds) induces a variation in electric-current strength in this region that is manifested as a magnetosphere stretching 24 RS (where RS is Saturn's equatorial radius of 60,268 km [37,449 miles]) on the sunward side, the magnetopause, and 80 RS on the other side, the magnetotail. Charged particles consisting primarily of electrons, protons, and heavy ionscaptured partly from the atmosphere of Saturn's largest satellite, Titan (q.v.)are trapped by the planetary magnetic field into Van Allen radiation belts that enclose 13 of the planet's satellites and the ring system. Low-frequency radio waves are emitted at a characteristic period used to define the planetary rotation rate. Also emitted are sudden electrostatic discharges associated with the formation of narrow radial markings, called spokes, which are mostly confined to the outer region of the B ring. Narrow polar aurora produced by a cascade of charged particles sucked in from the solar wind do not appear to have the same spatial extent as their counterparts on Jupiter. Although it appears that lightning occurs in Saturn's atmosphere, no conclusive evidence has been brought forward as yet. The inclination of Saturn's rotational axis to the ecliptic (i.e., the plane passing through the Sun and Earth) is approximately 27, nine times larger than that of Jupiter. Seasonal climatic changes are therefore likely, and tentative evidence exists for a one-season thermal lag manifested by the contrast in the appearance of the visible cloud tops in the two hemispheres. Spectroscopic evidence for a relatively large amount of photochemically unstable phosphine at the cloud-top level suggests that vigorous vertical mixing processes occur. This circumstance may help to account both for the production of stratospheric smogs and for the consequent apparent lack of distinctive large-scale cloud features at any but polar latitudes. The circulation of the winds in Saturn's atmosphere is not highly correlated with the location of the cloud bands. Red- and brown-coloured ovals resembling Jupiter's Great Red Spot are observed at mid-latitudes, and a cloud feature morphologically similar to a classical fluid vortex street flow has been identified in photographs taken by the U.S. Voyager planetary probes. Saturn's rings were first observed in 1610 by Galileo with the aid of a very primitive telescope in which they looked like handles. By 1659 Christiaan Huygens, using an improved telescope, had deduced that Saturn was encircled by a flat ring structure, though he erroneously believed that the structure was solid and of considerable thickness. The rings average less than a few hundred metres in thickness and lie in the plane of the planet's equator, which has a fixed orientation in space. Because Saturn's equatorial plane is tilted by 27 from its orbital plane, the rings are seen from different angles as Saturn moves through its orbit, producing a variety of aspects that change from edge-on to opened-up about every 7 1/2 years. Because of their thinness, the rings cannot be seen edge-on. The rings are made up of countless separate particles of all sizes, ranging from grains of fine dust to bodies measuring possibly tens of kilometres across. The larger particles, which could even be called moons, are much less numerous. Individual particles have not been observed, but their sizes are inferred from the way they reflect sunlight and radar waves. Water ice has been observed on their surfaces, and it probably constitutes the bulk of the ring material. The particles are all in separate orbits around Saturn, with those closer to the planet moving more rapidly. The main rings of Saturn are the central or B ring (from 1.5 to 2 RS); the outer or A ring (from 2.0 to 2.3 RS), separated from the B ring by a relatively empty region called the Cassini division; and the inner or C ring (from 1.2 to 1.5 RS). Each of these regions exhibits a great variety of fine structure, most likely because of different forms of gravitational effects by satellites outside or possibly within the rings. Other fainter rings, two of which (F and G) are very narrow and one (E) very broad, have recently been discovered around Saturn. Saturn's rings may have formed in place by direct condensation of icy material from the cooling cloud of gas and dust that also formed the planet. Because of tidal effects, this material could never collect itself into a satellite. Alternatively, the rings may have resulted from the fragmentation of a satellite or satellites that entered the region of tidal destruction. Collisions with meteoroids may have contributed significantly to the breakup of such moons. in space flight, any of a series of large two- and three-stage vehicles for launching spacecraft, developed by the United States beginning in 1958 in connection with the Apollo Moon-landing program. Saturn I, the first U.S. rocket specifically developed for space flight, was a two-stage vehicle that placed test versions of Apollo spacecraft into orbit, and also launched unmanned spacecraft. The first firing, on Oct. 27, 1961, was followed by nine subsequent launches, all successful. Saturn V, a three-stage vehicle, was used for manned Apollo lunar flights. The first Saturn V was launched in November 1967. In flying to the Moon, the first stage, weighing nearly 5,000,000 pounds, lifted the second and third stages along with the spacecraft to a speed of 5,400 miles per hour and to a point 41 miles above the Earth. The second stage, weighing more than 1,000,000 pounds, then took over, increasing speed to more than 14,000 miles per hour. At a point 120 miles above the Earth, the second stage was jettisoned, and the third stage ignited briefly to accelerate the spacecraft to 17,000 miles per hour, putting the spacecraft into Earth orbit. The astronauts then reignited the single engine of the third stage, which burned for 5 1/2 minutes, cutting off at an altitude of about 190 miles and a speed of 24,300 miles per hour. From that point, the rocket traveled through space until it reached a Moon orbit. See also Apollo program. False-colour image of Saturn. Three of its satellites (Tethys, Dione, and Rhea) are visible as second largest of the planets in mass and size. Its dimensions are almost equal to those of Jupiter, while its mass is about three times smaller; it has the lowest mean density of any object in the solar system. Both Saturn and Jupiter resemble stellar bodies in that their bulk chemical composition is dominated by the light gas hydrogen. Saturn's structure and evolutionary history, however, differ significantly from those of its larger counterpart. Like the other giant planets Jupiter, Uranus, and Neptune, Saturn has an extensive satellite and ring system, which may provide clues to its origin and evolution. Saturn's dense and extended rings, which lie in its equatorial plane, are currently the most impressive in the solar system. View of Saturn from Voyager 1 on Nov. 16, 1980, four days after its closest approach, at a distance Saturn, designated in astronomy, is the sixth planet in order of distance from the Sun, with an orbital semimajor axis of 1.427 billion km. Its closest approach distance from the Earth is never less than about 1.2 billion km, and thus Earth-based observations of Saturn always show a nearly fully illuminated disk, unlike the Voyager 1 image shown here. Latin Saturnus, in Roman religion, the god of sowing or seed. The Romans equated him with the Greek agricultural deity Cronus. The remains of Saturn's temple at Rome, eight columns of the pronaos (porch), still dominate the west end of the Forum at the foot of the Clivus Capitolinus. It served as the treasury (aerarium Saturni) of the Roman state. Saturn's cult partner was the obscure goddess Lua, whose name is connected with lues (plague, or destruction); but he was also associated with Ops, another obscure goddess (perhaps of the earth's fertility), the cult partner of Consus, probably a god of the storage bin. Saturn's great festival, the Saturnalia, became the most popular of Roman festivals, and its influence is still felt in the celebration of Christmas and the Western world's New Year. The Saturnalia was originally celebrated on December 17, but it was later extended to seven days. It was the merriest festival of the year: all work and business were suspended; slaves were given temporary freedom to say and to do what they liked; certain moral restrictions were eased; and presents were freely exchanged. The weekday Saturday (Latin Saturni dies) was named for Saturn. Additional reading David Morrison, Voyages to Saturn (1982), is a nontechnical presentation. More advanced treatments are contained in Tom Gehrels and Mildred Shapley Matthews (eds.), Saturn (1984), a collection of essays. Journal articles include G.F. Lindal, D.N. Sweetnam, and V.R. Eshleman, The Atmosphere of Saturn: An Analysis of the Voyager Radio Occultation Measurements, The Astronomical Journal, 90(6):11361146 (June 1984), specific experimental results on the structure of Saturn's atmosphere; and two issues of Journal of Geophysical Research, pt. A, Space Physics: vol. 85, no. A11 (Nov. 1, 1980), devoted to Pioneer 11 results; and vol. 88, no. A11 (Nov. 1, 1983), devoted to Voyager 1 and 2 results. William B. Hubbard