Saturn is the outermost planet of the planets known in ancient times. The earliest known observations of Saturn, by the Babylonians, can be reliably dated to the mid-7th century BC, but it was probably noticed much earlier, since Saturn tends to shine brighter than most stars. To the naked eye it appears yellowish. The Greeks named it after Cronus, the original ruler of Olympus, who in Roman is the god Saturn.
Saturn is the 6th planet in order distance from the sun. It cannot approach the planet Earth closer than 1,190,000,000 kilometers. Its brightness is due to its large size. Saturn’s equatorial diameter is 120,660 kilometers, but its globe is kind of flattened, and the polar diameter is only 108,000 kilometers. The mass of Saturn is 95.17 times that of the Earth, and the escape velocity, which is the velocity which once attained it will enable the object to “coast” away from the planet, is 32.26 kilometers per second, more than three times that of the Earth. Saturn’s outer layers are made up of gas, it is a world quite unlike our own.
Saturn’s ring system is in a class of its own. While Jupiter and Uranus also have rings, those of Saturn are striking, and a telescope of moderate power will show them excellently. There can be no doubt that Saturn is one of the most beautiful objects in the sky.
The first telescopic observations of Saturn have been made by Galileo in July 1610. He saw the disk of the planet clearly, but his telescope gave only a magnification of 32 diameters and that was not good enough to show the ring system in the way we know it nowadays. Galileo thought that Saturn must be a triple planet and wrote that “Saturn is not one alone, but is composed of three, which almost touch one another.” Two years later, he found to his surprise that the “companions” had vanished, so that Saturn appeared as a single object. The ring system was then edge-on to the Earth, and this is why it could not be seen in Galileo’s telescope. The original aspect was seen again in the years following 1613, but Galileo was never able to interpret it correctly. Various strange theories were proposed to explain the planet’s unusual form. Hevelius of Danzig, for example, believed Saturn to be elliptical in shape, with two “appendages” attached to the surface.
The problem was solved by a Dutch astronomer, Christian Huygens, who began his observations in 1655. The telescopes that he used were much more powerful than Galileo’s, and gave a sharper defintion, so in a short time he concluded that “ Saturn is surrounded by a thin, flat ring which nowhere touches the body of the planet.” His theory was not widely excepted, but by 1665 it had been universally accepted, even though the nature of the ring system was not established until much later. Data for Saturn are given in table one on the next page.
Table 1: Planetary Data For Saturn
Distance from the Sun Mean 9.54 a.u. (1,472,000,000 km)
Maximum 10.07 a.u.
Minimum 9.01 a.u.
Sidereal period of revolution 10,759.20 days, or 29.26 years
Mean synodic period 378.1 days
Rotation period(means) 10 hours 39 minutes 24 seconds
Mean orbital velocity 9.6 km/sec.
Axial inclination 26 44’
Orbital inclination to the elliptic 2 29’22”
Orbital eccentricity 0.056
(polar) 120,660 km
Apparent diameter seen from Earth Maximum 21”
Mass (Earth=1) 95.17
Volume (Earth=1) 744
Density (water=1) 0.7
Among planetary orbits, that of Saturn is of fairly low eccentricity (0.056), though the difference in distance between perihelion (closest approach to the Sun) and aphelion (farthest retreat from the Sun) amounts to 160,000,000 kilometers. Because Saturn is so far from the Sun and the Earth Saturn always appears to be full, or nearly so, in the sense of the full moon.
The mean synodic period, the interval between successive oppositions, when the Earth passes approximately between Saturn and the Sun, is 378.1 days, so that Saturn is well placed for observation during several months in each year. Opposition dates up to 1990 are April 8, 1982; April 21, 1983; May 3, 1984; May 15, 1985; May 27, 1986; June 9, 1987; June 20, 1988; July 2, 1989; and July 14, 1990.
Like all the superior planets, that is all outwards from the Earth, Saturn moves for the greater part of each year eastward. Its average rate is about 1 in eight days. As it approaches opposition its motion seems to slow down and to stop all together for about 70 days before the opposition date. For a period that may be as little as 133 days or as great as 141 days, it then seems to move in a retrograde, or westward, direction before reaching another stationary point and resuming its eastward movement. This behavior does not indicate any real alteration in motion. The apparent regression is due to the fact that the Earth moving in a much smaller orbit at a greater velocity, is catching up with Saturn and passing it.
Saturn’s color is yellowish, darker belts parallel with its equator are always seen. These belts are not nearly as attracting as those on Jupiter, nor do they show so much detail. Saturn’s greater distance and smaller size make it less easy to study than Jupiter, the beauty of Saturn’s rings system tends to divert attention from its disk, particularly when the rings are wide open, thereby hiding a considerable part of the globe.
Saturn’s albedo, which is the proportion of incident light it reflects, is 0.61. The planet’s apparent magnitude, which is its brightness as it appears from Earth, depends largely upon the angle at which the ring system is displayed, largely because the rings are more reflective than the disk. When the rings are wide open, the magnitude attains -0.3, so that, of stars, only Sirius and Canopus appear brighter than Saturn. At oppositions when the rings are edge-on, as in 1980, the magnitude is as low as +0.08, though even at these times Saturn is still prominent.
The equatorial zone appears creamy, sometimes almost white. The polar regions are almost always less brilliant. The belts, unlike those of Jupiter, do not show obvious colors.
Well-defined spots on Saturn are rare. The most prominent example sighted during the present century was that discovered on August 3, 1933, by the English amateur W.T. Hay, using a six-inch refractor. The spot took the form of a large white oval patch in the equatorial zone, about one-fifth of the planet’s diameter in length, and with both ends well defined. During the next few weeks it lengthened rapidly, until by mid-September it had spread out so much that it could no longer be called a spot.
Pioneer 11 was the first unmanned space probe to meet with Saturn in September 1979. It had already passed by Jupiter, which had been its main objective, but valuable information about Saturn was obtained. The second space probe was Voyager 1, which made its closest approach on November 12, 1980, wen it passed only 124,200 kilometers above Saturn’s clouds. Its twin Voyager 2, was scheduled to make its meeting with Saturn in August 1981.
Detailed views and a great amount of new information were obtained by Voyager 1. Though Saturn’s disk is blander than that of Jupiter because of a greater amount of overlying “haze,” much detail was shown. A red spot was detected in the southern hemisphere, with brownish ovals in the northern hemisphere. Measurements of the locations of individual features at different times yielded data concerning the speed of the winds, and it seems that Saturn’s circulation is different from that of Jupiter. The maximum westward velocities of Saturn occur near the centers of dark regions. In the polar latitudes, the large-scale light and dark bands break down into small-scale features, giving the impression of waves and eddies. The wind velocity at the cloud tops seems to be about 1,400 kilometers per hour, or twice that of Jupiter.
According to a recent theory, Saturn has an iron and rock core that extends out 13,000 to 14,000 kilometers from its center and is so compressed that it contains 15 to 20 times the mass of the Earth. Surrounding the core is a layer of electrically conductive metallic hydrogen in liquid form, outside of which is an envelope of hydrogen and helium. There is a magnetic field 1,000 times stronger than that of the Earth, though far weaker than that of Jupiter. The magnetic axis is almost coincident with the axis of rotation, and the magnetosphere extends out beyond the orbit of the largest satellite, Titan. There are fairly strong radiation belts, but these, again, are much weaker than those of Jupiter.
The temperature at the cloud tops of Saturn is approximately –180 C. A theory that the planet might be self-luminous has been disproved. Saturn, like Jupiter, releases about twice the amount of energy it receives from the Sun, indicating the existence of an internal source of heat. While Jupiter emits energy from the gravitational contraction that occurred when the planet was formed about 4,600,000,000 years ago, it is unlikely that the heat source of Saturn is similar. Saturn is smaller and less massive and has a lower overall density, therefore, any heat remaining from the gravitational contraction of Saturn would have been dissipated long ago. Instead, the heat source of Saturn may result from the separation of hydrogen and helium in the planet’s outer layers, with the heavier helium sinking through the liquid hydrogen middle layer.
The rings of Saturn are much more prominent than those of Jupiter or Uranus, and they are different in nature. Saturn’s rings are made up of icy or ice-covered particles. Each ring moving around Saturn moves in its own independent orbit. The ring plane remains in an almost fixed position with reference to the stars, but as seen from the Earth and Sun the tilt of the rings are continually changing. Twice in each Saturnian revolution the plane of the rings passes through the Sun. A few months before and after each such occasion, the ring plane passes through the Earth, which is near the sun as viewed from Saturn.
Two of the three main rings, termed A (outer) and B, are bright. In 1675 G.D. Cassini, at the Paris Observatory, discovered the gap between them, known as Cassini’s Division. The “ring” seen by Huygens was a combination of the two, and his telescope was not good enough to show the division. Ring B is much brighter of the two. In 1837 J.F. Encke, at Berlin, discovered a less prominent division in Ring A, known as Encke’s Division. In 1850 a dusky inner ring, ring C, was discovered by W.C. Bond and G.P. Bond. This is known as the Cr?pe Ring.
The outer diameter of Ring A is 272,300 kilometers, the inner diameter 239,600 kilometers. For ring B, the outer and inner diameters are 234,200 kilometers and 181.1000 kilometers. The inner diameter of ring C is 149,300 kilometers, so that it extends 17,000 kilometers above the cloud tops.
New rings have been discovered since then. A dusky ring, closer in than ring C was reported by telescopic observers. Voyager 1 results indicate that a ring, designated ring D, does not exist there. Outside ring A is the clumpy and braided Ring F. Rings E and G were also shown on Voyager 1 images.
Voyager 1 showed that the rings are much more complex than had been thought. Each contains hundreds of components, and there are several distinct narrow rings inside the Cassini Division. It was once thought that the main divisions in the ring system were due to the cumulative perturbation effects of Saturn’s satellites, but this explanation is clearly inadequate, and other forces must be involved.
Saturn has 15 known satellites, details of which are given in table 2 . An additional satellite named Thames, was reported by W.H. Pickering in the early 20th century, but it has not been recovered and probably does not exist. In 1966 A. Dollfus announced the discovery of an inner satellite, named Janus, moving at 169,000 kilometers from Saturn in a period of 0.815 day. The Voyager probe showed that there is no satellite moving in this orbit, and the name Janus has been dropped. It is probable that Dollfus’s observations actually related to some of the small inner satellites discovered by the Pioneer 11 and Voyager probes.
The unnamed satellites S.14 and S.13 move in orbits close to the inner and outer edges of Ring F and probably keep this ring stable. S.10 and S.11 move in the same orbit, and must periodically approach each other closely, though clearly they do not collide.
S.12 moves in the same orbit as Dione.
Phoebe, the outermost satellite, was not within the range of Voyager 1 probe, and little is known about it. It may be a captured asteroid. Iapetus is unique in having one hemisphere of high albedo and the other of low albedo, this explains why when Saturn is observed from Earth the west side appears much more brighter than the east side. As with all the other satellites, Iapetus has a captured rotation, that is its axial rotation period is equal to the time taken to complete one orbit.
Table 2, Satellites of Saturn
Satellite Distance from center of Saturn (km) Period
(days) Orbital eccentricity Orbital inclination
S.15 138,200 0.60 ? ? 80
S.14 138,600 0.613 ? ? 500
S.13 141,000 0.629 0.007 ? 600
S.10 149,400 0.695 0.013 ? 700
S.11 149,400 0.695 0.013 ? 135×70
Mimas 185.400 0.942 0.020 1.5 350
Enceladus 238,200 1.370 0.004 0.0 520
Tethys 294,600 1.888 0.000 1.1 1,020
Dione 377,400 2.737 0.002 0.0 1,120
S.12 377,400 2.737 0.0? 0.5? 80
Rhea 526,800 4.518 0.001 0.4 1,530
Titan 1,200,000 15.95 0.029 0.3 5,100
Hyperion 1,482,000 21.28 0.104 0.4 440
Iapetus 3,558,000 79.33 0.028 14.7 1,440
Phoebe * 12,960,000 550.4 0.163 150 80
* Phoebe moves around Saturn in a retrograde direction.
Rhea, Dione, Tethys, and Mimas have icy, cratered surfaces. One huge crater on Mimas has more than one-third the diameter of the satellite itself, so that, if the crater were formed by impact, it seems that Mimas would have been in danger of breaking up. Enceladus was not well shown from Voyager 1, but it may be comparatively smooth.
Titan, by far the largest of Saturn’s satellites, is in a class of its own because of its dense atmosphere. Data obtained from Voyager 1 indicate that the atmospheric pressure at Titan’s surface is 1.5 to two times that of the Earth at sea level. The actual surface of the satellite has an orange-colored layer of what may be called “photochemical smog.” Titan’s atmosphere is made up almost entirely of nitrogen with little amounts of methane and cyanide. It is possible that the intensely cold surface is covered, at least in part, by oceans of liquid nitrogen.