| in astronomy, 6th planet from the sun. Astronomical and Physical Characteristics of Saturn Saturn's orbit lies between those of Jupiter and Uranus; its mean distance from the sun is c.886 million mi (1.43 billion km), almost twice that of Jupiter, and its period of revolution is about 29 1 / 2 years. Saturn appears in the sky as a yellow, starlike object of the first magnitude. When viewed through a telescope, it is seen as a golden sphere, crossed by a series of lightly colored bands parallel to the equator. Saturn, like the other Jovian planets (Jupiter, Uranus, and Neptune), is covered with a thick atmosphere composed mainly of hydrogen and helium, with some methane and ammonia; its temperature is believed to be about −270 degrees Fahrenheit (−168 degrees Celsius), suggesting that the ammonia is in the form of ice crystals that constitute the clouds. Like Jupiter's interior, Saturn's consists of a rocky core, a liquid metallic hydrogen layer, and a molecular hydrogen layer. Traces of various ices have also been detected. The wind blows at high speeds—reaching velocities of 1,100 mph (1,770 kph)—across Saturn. The strongest winds are found near the equator and blow mostly in an easterly direction. At higher latitudes, the velocity decreases uniformly and the winds counterflow east and west. Because no permanent markings on the planet are visible, the planet's exact period of rotation has not been determined. However, the period of each atmospheric band varies from 10 hr 14 min at the equator to about 10 hr 38 min at higher latitudes. This rapid rotation causes the largest polar flattening among the planets (over 10%). Saturn is the second largest planet in the solar system; its equatorial diameter is c.75,000 mi (120,000 km), and its volume is more than 700 times the volume of the earth. Its mass is about 95 times that of the earth, making Saturn the only planet in the solar system with a density less than that of water. Saturn has been encountered by three space probes: Pioneer 11 (1979), Voyager 1 (1980), and Voyager 2 (1981). The Cassini probe, launched in 1997, is expected to reach Saturn in 2004. Among the discoveries made by the Voyager probes was a magnetosphere (a region of charged particles consisting primarily of electrons, protons, and heavy ions captured partly from the atmosphere of the satellite Titan) that encloses 13 of Saturn's satellites and its ring system. The Ring System Saturn's most remarkable feature is the system of thin, concentric rings lying in the plane of its equator. Although first observed by Galileo in 1610, it was not until 1656 that the rings were correctly interpreted by Christiaan Huygens, who did not reveal his findings about their phases and changes in shape until his treatise Systema Saturnium was published in 1659. Saturn's rings were believed to be unique until 1977, when very faint rings were found around Uranus; shortly thereafter faint rings were also detected around Jupiter and Neptune. Although the ring system is almost 167,770 mi (270,000 km) in diameter, it is only some 330 ft (100 m) thick. From earth, this system appears to consist mainly of two bright outer rings, denoted A and B, separated by a dark rift—discovered by the Italian-French astronomer Gian Domenico Cassini—known as Cassini's division, plus a third, faint inner crepe ring (denoted C). The Encke Division, or Encke Gap, which splits the A ring, is named after the German astronomer Johann Franz Encke, who discovered it in 1837. Pictures from the Voyager probes show four additional rings. The exceedingly faint D ring lies closest to the planet. The faint F Ring is a narrow feature just outside the A Ring. Beyond that are two far fainter rings named G and E. In 1859 the Scottish physicist James Clerk Maxwell showed that the rings must consist of countless tiny particles each orbiting the planet in accordance with the laws of gravitation. When edgewise to the earth the rings appear as a nearly imperceptible ribbon of light across the planet; this occurs twice during the 29 1 / 2-year period of revolution. Twice during each orbit the rings reach a maximum inclination to the line of sight, once when they are visible from above and once when visible from below. The Voyager 1 (1980) and 2 (1981) space probes revealed incredible new detail as they passed within 78,000 mi (126,000 km) and 63,000 mi (101,000 km) of Saturn, respectively. They recorded hundreds of tiny rings that are grouped into the seven major rings. The three brightest rings are lettered from the outermost, A, B, and C. The A, B, and C rings dissolved into more than 1,000 narrow ringlets, 100 of which are in the Cassini division. The outer F ring was found to contain braids, knots, and strands, possibly caused by nearby moons that shepherd it, that is, limit the extent of a planetary ring through gravitational forces. The origin of the rings is unknown, although it is believed that they may have been formed from larger satellites that were shattered by the impact of comets and meteoroids. The Satellite System Saturn has 18 confirmed and named natural satellites: Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Mimas, Enceladus, Tethys, Telesto, Calypso, Dione, Helene, Rhea, Titan, Hyperion, Iapetus, and Phoebe (in order of increasing distance from the planet). Twelve additional satellites, with orbits at least 9.3 million mi (15 million km) from the planet's surface, were reported in 2000 and 2001. If confirmed, the findings will raise the number of Saturn's satellites to 30. More than a dozen others have been reported and given provisional status; nine of these were derived from the analysis of data from the 1981 flyby of the Voyager 2 space probe, and seven were identified from Hubble Space Telescope photographs taken during the 1995 ring plane crossings. It is possible that some of these are actually the same as some of the known satellites or duplicates of one another. The data probably will not be sorted out until the arrival of the Cassini probe, which is designed to do a detailed study of Saturn, its rings, its magnetosphere, its icy satellites, and Titan. All but two of Saturn's moons form a regular system of satellites; that is, their orbits are nearly circular and lie in the equatorial plane of the planet. The exceptions are Iapetus, whose orbit is inclined almost 15°, and Phoebe, whose orbit is inclined 175°. Except for Hyperion, which has a chaotic orbit, and Phoebe, all the satellites are believed to have synchronous orbits; that is, their orbital and rotational periods are the same so that they keep the same face turned toward Saturn, just as the moon keeps the same face turned toward the earth. The largest satellite, Titan, is 3,200 mi (5,150 km) in diameter and has the size and cold temperatures necessary to retain an atmosphere; it is the only natural satellite in the solar system with a substantial atmosphere. Saturn has six major icy satellites that can be easily seen through earth-based telescopes. The most prominent feature of heavily cratered Mimas, the innermost of the six, is a large impact crater about one third the diameter of the satellite. Certain broad regions of Enceladus are uncratered, indicating geological activity that has somehow resurfaced the satellite within the last 100 million years. Tethys also has a very large impact crater, as well as an extensive series of valleys and troughs that stretches three quarters of the way around the satellite. Both Dione and Rhea have bright, heavily cratered leading hemispheres and darker trailing hemispheres with wispy streaks that are thought to be produced by deposits of ice inside surface troughs or cracks. Iapetus, the outermost of the large icy satellites, has a dark leading hemisphere and a bright trailing hemisphere. The remaining eleven satellites, some sharing orbits with others, are smaller. The two largest of these, the dark-surfaced Phoebe and the irregularly shaped Hyperion, orbit far from the planet; the outermost satellite, Phoebe, orbits with retrograde motion, i.e., opposite to that of the planet's rotation. The smallest, ranging from c.12 to 20 mi (20 to 32 km) in diameter, are Pan and Atlas, the satellites closest to the planet, and Telesto, Calypso, and Helene. Prometheus and Pandora, c.55 mi (90 km) in diameter, share an orbit, as do Epimetheus and Janus. ____________________The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved. -42379- | |
