Part 2

The Rings

The rings of Saturn have amazed and intrigued astronomers for over 300 years. Now that we have seen them up close, they are even more astonishing. Although they stretch over 65,000 kilometers (40,000 miles), they may be only a few kilometers thick. The ring particles—from a few microns to a meter (three feet) in size—have been described as icy snowballs or ice-covered rock. Voyager scientists continue to pore over their data, searching for answers to the puzzles of the rings. The rings were named in order of their discovery, so the labels do not indicate their relative positions. From the planet outward, they are known as D, C, B, A, F, and E.]

Extraordinarily complex structure is seen across the entire span of Saturn’s ring system. The sequence (taken approximately every 15 minutes as Voyager 1 approached Saturn) proceeds from top to bottom in each column and shows radial “spokes” rotating within the B-Ring. The spokes may be caused by a combination of magnetic and electrostatic forces.]

Over 95 individual concentric features can he counted; the final count in higher resolution images may be anywhere from 500 to 1000 separate rings. A few of the ringlets shown in this computer-assembled mosaic are not concentric circles but are instead elliptical. Ring particles are probably ice or ice covered rock.]

D-RING C-RING B-RING “SPOKE” CASSINI DIVISION ENCKE DIVISION A-RING F-RING

The Cassini Division is filled with numerous ringlets. Discovered by Cassini in 1675, this area between the A- and B-Rings had long been thought devoid of material. The Voyager observation of well-defined rings within the Cassini Division was an unexpected discovery.]

Saturn’s ring system, viewed from below, appears dramatically different from its appearance on the sunlit side. This computer-processed image shows the F-Ring circling outside the A-Ring, the A-Ring with its Encke Division, the multiple ringlets in the Cassini Division, and the optically thick B-Ring, seen here in magenta hues (the coloration is an artifact of processing and is not real). The B-Ring appears dark from below the ring plane because it is dense enough to reflect most of the sunlight, causing it to appear very bright when seen from the sunward side. The opaline brightness of the Cassini Division here indicates a great deal of sunlight being scattered through this region. The Encke Division may really be empty, since it appears dark from both above and below.]

Outbound and above the ring plane, Voyager 1 gave us this view of Saturn’s rings eight hours after its closest approach to the planet. The unique lighting accentuates the many hundreds of bright and dark ringlets comprising the ring system. The C-Ring (dark gray area) seems to blend into the brighter B-Ring as the concentric features radiate out from the planet. The dark spoke-like features seen in images taken during the approach to Saturn now appear as bright streaks, indicating that they may be composed of small particles.]

Two narrow, braided rings in the F-Ring are evident in this view, as well as a broader, very diffuse component about 35 kilometers (20 miles) across. A totally unexpected discovery, the braided rings trace distinctly separate orbits intertwining each other. The “knots” may be local clumps of ring material or tiny moons. It is difficult to explain this complicated structure using only the gravitational forces known to be affecting the particles of this ring. It is possible that additional, electrostatic forces may also influence these particles.]

Brightness variations in the F-Ring may be due to clumping in the ring material. The features are seen at the top and again near the left edge of the ring in this image. The “gap” in the ring (left center) is not real but is the location of a reseau mark on the camera’s vidicon tube. These bright features in the F-Ring appear to move at the orbital rate of the ring particles and may be larger bodies or thicknesses in the rings. Saturn’s thirteenth and fourteenth satellites, which orbit on either side of the F-Ring, may act like “sheepdogs,” herding the F-Ring particles between them. Less than 100 kilometers (60 miles) wide, the F-Ring is located outside of the A-Ring. Satellite 14, discovered by Voyager 1, is seen just inside the F-Ring.]

The Satellites

The surface of giant Titan, now dethroned from its seat as the solar system’s largest satellite (Jupiter’s Ganymede is larger), remains an enigma, shrouded beneath thick layers of haze.]

Tiny moons—three new ones and three confirmed from previous sightings—may tell us much about ring dynamics since gravitational forces from satellites probably influence the ring structure. Two of these tiny moons are on the verge of collision in the same orbit, while several others appear to bound the A- and F-Rings. Iapetus, whose two hemispheres differ dramatically in brightness, was photographed in its orbit, almost 3.6 million kilometers (2.2 million miles) from the planet.]

Mimas, Saturn’s innermost large satellite, has an impact crater covering more than one quarter the diameter of the entire moon. Nowhere else in the solar system has such a disproportionately large feature been seen. In fact, it is believed that any impact larger than this would probably have shattered Mimas into two or more fragments. The crater has a raised rim and central peak, typical of large impact structures on terrestrial planets. Additional smaller craters, 15 to 45 kilometers (10 to 30 miles) in diameter, can be seen scattered across the surface, particularly along the terminator. Mimas is one of the small, low density Saturnian satellites implying that it is composed primarily of ice.]

Mimas’ other side shows a uniformly and heavily cratered surface—a record of the bombardment that occurred throughout the solar system in its early history 4.5 billion years ago. A long, narrow trough about 5 kilometers (3 miles) wide crosses from northeast to southwest. Mimas’ surface is very reflective (about 60 percent), indicating that it consists largely of ice, which has been chipped and pulverized by aeons of meteoritic bombardment. Such a surface on a small, low mass moon would probably resemble light, powdery snow. Features as small as 3 kilometers (2 miles) across are visible.]

Enceladus appears to be largely devoid of craters or other major surface relief, suggesting that perhaps internal processes may have erased such structures. This satellite will be seen better by Voyager 2 when it flies past Saturn in August 1981.]

This heavily cratered surface of Tethys faces toward Saturn and includes a large valley about 750 kilometers (500 miles) long and 60 kilometers (40 miles) wide. The craters are the result of impacts, and the valley appears to be a large fracture of unknown origin. Tethys has a diameter of 1050 kilometers (650 miles), about one-third that of Earth’s Moon. The smallest features visible in this picture are about 24 kilometers (15 miles) across.]

Dione reveals two distinctly different hemispheres. The photograph shows Dione’s trailing side. Bright radiating patterns are probably rays of debris thrown out of impact craters; other bright areas may be topographic ridges and valleys.]

Dione’s other hemisphere (mosaic) also has many impact craters—the record of cosmic collisions. The largest crater is less than 100 kilometers (60 miles) in diameter and includes a well-developed central peak. Sinuous valleys (seen near each pole) are probably the result of crustal fracturing in the moon’s icy crust. Dione’s diameter is only 1100 kilometers (700 miles), much smaller than any of Jupiter’s icy moons.]

Craters stand shoulder-to-shoulder on the surface of Saturn’s satellite Rhea, seen in this mosaic of the highest-resolution pictures of the north polar region. Rhea is 1500 kilometers (950 miles) in diameter and is the most heavily cratered Saturn moon. The largest crater, made by the impact of cosmic debris, is about 300 kilometers (190 miles) in diameter.]

Impact craters on the ancient surface of Rhea closely resemble those on Mercury and Earth’s Moon. Many of the craters have central peaks formed by rebound of the floor during the explosive formation of the crater. Some craters are old and degraded by later impacts. Many have sharp rims and appear relatively fresh, while others are very shallow and have subdued rims, indicative of their antiquity. White areas on the edges of several of the craters are probably fresh ice exposed on steep slopes or possibly deposited by volatiles leaking from fractured regions. Surface features as small as 2.5 kilometers (1.5 miles) in diameter are visible.]

Titan is a large, bizarre satellite. It is larger (almost 5120 kilometers or 3180 miles in diameter) than the planet Mercury and possesses a dense atmosphere of unique composition. Voyager 1’s cameras show Titan’s surface to be totally obscured by a thick layer of atmospheric haze. In the full-disk photograph, only two features are visible: a faint boundary between the southern and darker northern hemispheres and a dark “hood” overlying Titan’s north polar region.]

This hood and greater detail in the haze layers are shown in the higher resolution photograph.]

Little detail can be seen in this distant view of Hyperion, the satellite which orbits just beyond Titan. Voyager 2 will observe Hyperion at a closer range.]

Saturn’s satellite Iapetus displays a large, circular feature about 200 kilometers (120 miles) across with a dark spot in its center. The circular feature is probably a large impact structure outlined by dark material, possibly thrown out by the impact. The satellite’s leading hemisphere is to the left, and the trailing hemisphere, which is four to five times brighter, is to the right. Iapetus’ diameter is 1450 kilometers (900 miles).]

Two satellites (Saturn’s tenth and eleventh) revolve in nearly identical orbits 151,000 kilometers (94,000 miles) from Saturn’s center. The satellites are each 100 to 200 kilometers in diameter, larger than the distance separating their orbits, and they are currently approaching one another at a rate which promises collision in about two years. Such a collision, however, will probably be averted by orbital changes induced by the satellites’ mutual gravitational interactions as they near one another. The trailing co-orbital satellite, seen in this photograph, has a very irregular outline (the Sun is shining from the left). This color composite was produced from three exposures taken over a period of more than six minutes. During this period, a thin shadow, cast by a previously unknown ring, moved across the satellite causing the “rainbow” pattern shown here.]

Two smaller satellites—Saturn’s thirteenth and fourteenth moons—were discovered on October 25, 1980, in images taken to study the dark “spokes” within Saturn’s B-Ring. The smaller, inner satellite has a diameter of about 500 kilometers (300 miles) and is visible just outside the A-Ring, near the bottom of the picture. It travels in an orbit between the A-Ring and the F-Ring (not visible in this photograph). The second satellite, seen to the left, travels just outside the F-Ring and is about 600 kilometers (400 miles) in diameter. Scientists believe the dimensions of the narrow F-Ring may be determined by these two satellites, which orbit on either edge of the ring.]

A Glimpse Back