May 4, 1990
Courtesy of: NASA/JPL
Table of Contents
The two Saturn encounters increased our knowledge and altered our understanding of Saturn. The extended, close-range observations provided high-resolution data far different from the picture assembled during centuries of Earth-based studies.
Here is a summary of scientific findings by the two Voyagers at Saturn.
Saturn's atmosphere is almost entirely hydrogen and helium. Voyager 1 found that about 7 percent of the volume of Saturn's upper atmosphere is helium (compared with 11 percent of Jupiter's atmosphere), while almost all the rest is hydrogen. Since Saturn's internal helium abundance was expected to be the same as Jupiter's and the Sun's, the lower abundance of helium in the upper atmosphere may imply that the heavier helium may be slowly sinking through Saturn's hydrogen; that might explain the excess heat that Saturn radiates over energy it receives from the Sun. (Saturn is the only planet less dense than water. In the unlikely event that a lake could be found large enough, Saturn would float in it.)
Subdued contrasts and color differences on Saturn could be a result of more horizontal mixing or less production of localized colors than in Jupiter's atmosphere. While Voyager 1 saw few markings, Voyager 2's more sensitive cameras saw many: long-lived ovals, tilted features in east-west shear zones, and others similar to, but generally smaller than, on Jupiter.
Winds blow at high speeds in Saturn. Near the equator, the Voyagers measured winds about 500 meters a second (1,100 miles an hour). The wind blows mostly in an easterly direction. Strongest winds are found near the equator, and velocity falls off uniformly at higher latitudes. At latitudes greater than 35°, winds alternate east and west as latitude increases. Marked dominance of eastward jet streams indicates that winds are not confined to the cloud layer, but must extend inward at least 2,000 kilometers (1,200 miles). Furthermore, measurements by Voyager 2 showing a striking north-south symmetry that leads some scientists to suggest the winds may extend from north to south through the interior of the planet.
While Voyager 2 was behind Saturn, its radio beam penetrated the upper atmosphere, and measured temperature and density. Minimum temperatures of 82 Kelvin (-312°F) were found at the 70-millibar level (surface pressure on Earth is 1,000 millibars). The temperature increased to 143 Kelvins (-202 °F) at the deepest levels probed - - about 1,200 millibars. Near the north pole temperatures were about 10°C (18°F) colder at 100 millibars than at mid-latitudes. The difference may be seasonal.
The Voyagers found aurora-like ultraviolet emissions of hydrogen at mid-latitudes in the atmosphere, and auroras at polar latitudes (above 65°). The high-level auroral activity may lead to formation of complex hydrocarbon molecules that are carried toward the equator. The mid-latitude auroras, which occur only in sunlit regions, remain a puzzle, since bombardment by electrons and ions, known to cause auroras on Earth, occurs primarily at high latitudes.
Both Voyagers measured the rotation of Saturn (the length of a day) at 10 hours, 39 minutes, 24 seconds.
Perhaps the greatest surprises and the most perplexing puzzles the two Voyagers found are in the rings.
Voyager 1 found much structure in the classical A-, B- and C-rings. Some scientists suggest that the structure might be unresolved ringlets and gaps. Photos by Voyager 1 were of lower resolution than those of Voyager 2, and scientists at first believed the gaps might be created by tiny satellites orbiting within the rings and sweeping out bands of particles. One such gap was detected at the inner edge of the Cassini Division.
Voyager 2 measurements provided the data scientists need to understand the structure. High-resolution photos of the inner edge of the Cassini Division showed no sign of satellites larger than five to nine kilometers (three to six miles). No systematic searches were conducted in other ring gaps.
Voyager 2's photopolarimeter provided more surprises. The instrument measured changes in starlight from Delta Scorpii as Voyager 2 flew above the rings and the light passed through them. The photopolarimeter could resolve structures smaller than 300 meters (1,000 feet).
The star-occultation experiment showed that few clear gaps exist in the rings. The structure in the B-ring, instead, appears to be variations in density waves or other, stationary, forms of waves. Density waves are formed by the gravitational effects of Saturn's satellites. (The resonant points are places where a particle would orbit Saturn in one-half or one-third the time needed by a satellite, such as Mimas.) For example, at the 2:1 resonant point with Janus (1980S1), a series of outward-propagating density waves has about 60 grams of material per square centimeter of ring area, and the velocity of particles relative to one another is about one millimeter per second. Small-scale structure of the rings may therefore be transitory, although larger-scale features, such as the Cassini and Encke Divisions, appear more permanent.
The edges of the rings where the few gaps exist are so sharp that the ring must be less than about 200 meters (650 feet) thick there, and may be only 10 meters (33 feet) thick.
In almost every case where clear gaps do appear in the rings, eccentric ringlets are found. All show variations in brightness. Some differences are due to the existence of clumps or kinks, and others to nearly complete absence of material. Some scientists believe the only plausible explanation for the clear regions and kinky ringlets is the presence of nearby undetected satellites.
Two separate, discontinuous ringlets were found in the A-ring gap, known as Encke's Gap, about 73,000 kilometers (45,000 miles) from Saturn's cloud tops. At high resolution, at least one of the ringlets has multiple strands.
Saturn's F-ring was discovered by Pioneer 11 in 1979. Photos of the F-ring taken by Voyager 1 showed three separate strands that appear twisted or braided. At higher resolution, Voyager 2 found five separate strands in a region that had no apparent braiding, and surprisingly revealed only one small region where the F-ring appeared twisted. The photopolarimeter found the brightest of the F-ring strands was subdivided into at least 10 strands. The twists are believed to originate in gravitational perturbations caused by one of two shepherding satellites, Prometheus (1980S27). Clumps in the F-ring appear uniformly distributed around the ring every 9,000 kilometers (5,600 miles), a spacing that very nearly coincides with the relative motion of F-ring particles and the interior shepherding satellite in one orbital period. By analogy, similar mechanisms might be operating for the kinky ringlets that exist in the Encke Gap.
The spokes found in the B-ring appear only at radial distances between 43,000 kilometers (27,000 miles) and 57,000 kilometers (35,000 miles) above Saturn's clouds. Some spokes, those thought to be most recently formed, are narrow and have a radial alignment, and appear to corotate with Saturn's magnetic field in 10 hours, 39.4 minutes. The broader, less radial spokes appear to have formed earlier than the narrow examples and seem to follow Keplerian orbits. Individual areas corotate at speeds governed by distances from the center of the planet. In some cases, scientists believe they see evidence that new spokes are reprinted over older ones. Their formation is not restricted to regions near the planet's shadow, but seems to favor a particular Saturnian longitude. As both spacecraft approached Saturn, the spokes appeared dark against a bright ring background. As the Voyagers departed, the spokes appeared brighter than the surrounding ring areas, indicating that the material scatters reflected sunlight more efficiently in a forward direction, a quality that is characteristic of fine, dust-sized particles. Spokes are also visible at high phase angles in light reflected from Saturn on the un-illuminated underside of the rings.
Another challenge scientists face in understanding the rings is that even general dimensions do not seem to remain true at all positions around Saturn: The distance of the B-ring's outer edge, near a 2:1 resonance with Mimas, varies by at least 140 kilometers (90 miles) and probably by as much as 200 kilometers (120 miles). Furthermore, the elliptical shape of the outer edge does not follow a Keplerian orbit, since Saturn is at the center of the ellipse, rather than at one focus. The gravitational effects of Mimas are most likely responsible for the elliptical shape, as well as for the variable width of the Huygens Gap between the B-ring and the Cassini Division.
Titan is the largest of Saturn's satellites. It is the second largest satellite in the solar system, and the only one know to have a dense atmosphere.
It may be the most interesting body, from a terrestrial perspective, in the solar system. For almost two decades, space scientists have searched for clues to the primeval Earth. The chemistry in Titan's atmosphere may be similar to what occurred in Earth's atmosphere several billion years ago.
Because of its thick, opaque atmosphere, astronomers believed Titan was the largest satellite in the solar system. Their measurements were necessarily limited to the cloud tops. Voyager 1's close approach and diametric radio occultation show Titan's surface diameter is only 5,150 kilometers (3,200 miles) - - slightly smaller than Ganymede, Jupiter's largest satellite. Both are larger than Mercury. Titan's density appears to be about twice that of water ice; it may be composed of nearly equal amounts of rock and ice.
Titan's surface cannot be seen in any Voyager photos; it is hidden by a dense, photochemical haze whose main layer is about 300 kilometers (200 miles) above Titan's surface. Several distinct, detached haze layers can be seen above the opaque haze layer. The haze layers merge with the main layer over the north pole of Titan, forming what scientists first thought was a dark hood. The hood was found, under the better viewing conditions of Voyager 2, to be a dark ring around the pole. The southern hemisphere is slightly brighter than the northern, possibly the result of seasonal effects. When the Voyagers flew past, the season on Titan was the equivalent of mid-April and early May on Earth, or early spring in the northern hemisphere and early fall in the south.
Atmospheric pressure near Titan's surface is about 1.6 bars, 60 percent greater than Earth's. The atmosphere is mostly nitrogen, also the major constituent of Earth's atmosphere.
The surface temperature appears to be about 95 Kelvins (-289°F), only 4 Kelvins above the triple-point temperature of methane. Methane, however, appears to be below its saturation pressure near Titan's surface; rivers and lakes of methane probably don't exist, in spite of the tantalizing analogy to water on Earth. On the other hand, scientists believe lakes of ethane exist, and methane is probably dissolved in the ethane. Titan's methane, through continuing photochemistry, is converted to ethane, acetylene, ethylene, and (when combined with nitrogen) hydrogen cyanide. The last is an especially important molecule; it is a building block of amino acids. Titan's low temperature undoubtedly inhibits more complex organic chemistry.
Titan has no intrinsic magnetic field; therefore it has no electrically conducting and convecting liquid core. Its interaction with Saturn's magnetosphere creates a magnetic wake behind Titan. The big satellite also serves as a source for both neutral and charged hydrogen atoms in Saturn's magnetosphere.
Before the first Voyager encounter, astronomers believed Saturn had 11 satellites. Now they know it has at least 17 and possibly more. Three of the 17 were discovered by Voyager 1. Three additional possible satellites have been identified in imaging data since the Voyager 2 encounter. (Three others were discovered in ground-based observations.)
The innermost satellite, Atlas, orbits near the outer edge of the A-ring and is about 40 by 20 kilometers (25 by 15 miles) in size. It was discovered in Voyager 1 images.
The next satellite outward, Prometheus, shepherds the inner edge of the F-ring and is about 140 by 100 by 80 kilometers (90 by 60 by 50 miles) in size. Next is Pandora, the outer shepherd of the F-ring and is 110 by 90 by 80 kilometers (70 by 55 by 50 miles) in size. Both shepherds were found by Voyager 1.
Next are Epimetheus and Janus, which share about the same orbit -- 91,000 kilometers (56,600 miles) above the clouds. As they near each other, the satellites trade orbits (the outer is about 50 kilometers, or 30 miles, farther from Saturn than the inner). Janus is 220 by 200 by 160 kilometers (140 by 125 by 100 miles), and Epimetheus is 140 by 120 by 100 kilometers (90 by 70 by 50 miles) in size. Both were discovered by ground-based observers.
One new satellite, Helene, shares the orbit of Dione, about 60° ahead of its larger companion, and is called the Dione Trojan. It is about 36 by 32 by 30 kilometers (22 by 20 by 19 miles). Helene was discovered in ground-based photographs.
Two more satellites are called the Tethys Trojans because they circle Saturn in the same orbit as Tethys, about 60° ahead of and behind that body. They are Telesto (the leading Trojan) and Calypso (the trailing Trojan). Both were found in 1981 among ground-based observations made in 1980. Telesto is 34 by 28 by 26 kilometers (21 by 17 by 16 miles) and Calypso is 34 by 22 by 22 kilometers (21 by 14 by 14 miles).
There are three unconfirmed satellites. One circles Saturn in the orbit of Dione, a second is located between the orbits of Tethys and Dione, and the third, between Dione and Rhea. All three were found in Voyager photographs, but were not confirmed by more than one sighting.
Mimas, Enceladus, Tethys, Dione, and Rhea are approximately spherical in shape and appear to be composed mostly of water ice. Enceladus reflects almost 100 percent of the sunlight that strikes it. All five satellites represent a size range that had not been explored before.
Mimas, Tethys, Dione, and Rhea are all cratered; Enceladus appears to have by far the most active surface of any satellite in the system (with the possible exception of Titan, whose surface was not photographed). At least five types of terrain have been identified on Enceladus. Although craters can be seen across portions of its surface, the lack of craters in other areas implies an age less than a few hundred million years for the youngest regions. It seems likely that parts of the surface are still undergoing change, since some areas are covered by ridged plains with no evidence of cratering down to the limit of resolution of Voyager 2's cameras (2 kilometers or 1.2 miles). A pattern of linear faults crisscrosses other areas. It is not likely that a satellite as small as Enceladus could have enough radioactive material to produce the modification. A more likely source of heating appears to be tidal interaction with Saturn, caused by perturbations in Enceladus' orbit by Dione (like Jupiter's satellite Io). Theories of tidal heating do not predict generation of enough energy to explain all the heating that must have occurred. Because it reflects so much sunlight, Enceladus' current surface temperature is only 72 Kelvins (-330°F).
Photos of Mimas show a huge impact crater. The crater, named Herschel, is 130 kilometers (80 miles) wide, one-third the diameter of Mimas. Herschel is 10 kilometers (6 miles) deep, with a central mountain almost as high as Mount Everest on Earth.
Photos of Tethys taken by Voyager 2 show an even larger impact crater, named Odysseus, nearly one-third the diameter of Tethys and larger than Mimas. In contrast to Mimas' Herschel, the floor of Odysseus returned to about the original shape of the surface, most likely a result of Tethys' larger gravity and the relative fluidity of water ice. A gigantic fracture covers three-fourths of Tethys' circumference. The fissure is about the size scientists would predict if Tethys were once fluid and its crust hardened before the interior, although the expansion of the interior due to freezing would not be expected to cause only one large crack. The canyon has been named Ithaca Chasma. Tethys' surface temperature is 86 Kelvins (-305°F).
Hyperion shows no evidence of internal activity. Its irregular shape causes an unusual phenomenon: each time Hyperion passes Titan, the larger satellite's gravity gives Hyperion a tug and it tumbles erratically, changing orientation. The irregular shape of Hyperion and evidence of bombardment by meteors make it appear to be the oldest surface in the Saturn system.
Iapetus has long been known to have large differences in surface brightness. Brightness of the surface material on the trailing side has been measured at 50 percent, while material on the leading side reflects only 5 percent of the sunlight. Most dark material is distributed in a pattern directly centered on the leading surface, causing conjecture that dark material in orbit around Saturn was swept up by Iapetus. The trailing face of Iapetus, however, has craters with dark floors. That implies that the dark material originated in the satellite's interior. It is possible that the dark material on the leading hemisphere was exposed by ablation (erosion) of a thin, overlying, bright surface covering.
Voyager 2 photographed Phoebe after passing Saturn. Phoebe orbits Saturn in a retrograde direction (opposite to the direction of the other satellites' orbits) in a plane much closer to the ecliptic than to Saturn's equatorial plane. Voyager 2 found that Phoebe has a roughly circular shape, and reflects about 6 percent of the sunlight. It also is quite red. Phoebe rotates on its axis about once in nine hours. Thus, unlike the other Saturnian satellites (except Hyperion), it does not always show the same face to the planet. If, as scientists believe, Phoebe is a captured asteroid with its composition unmodified since its formation in the outer solar system, it is the first such object that has been photographed at close enough range to show shape and surface brightness.
Both Dione and Rhea have bright, wispy streaks that stand out against an already-bright surface. The streaks are probably the results of ice that evolved from the interior along fractures in the crust.
The size of Saturn's magnetosphere is determined by external pressure of the solar wind. When Voyager 2 entered the magnetosphere, the solar-wind pressure was high and the magneto- sphere extended only 19 Saturn radii (1.1 million kilometers or 712,000 miles) in the Sun's direction. Several hours later, however, the solar-wind pressure dropped and Saturn's magneto- sphere ballooned outward over a six-hour period. It apparently remained inflated for at least three days, since it was 70 percent larger when Voyager 2 crossed the magnetic boundary on the outbound leg.
Unlike all the other planets whose magnetic fields have been measured, Saturn's field is tipped less than one degree relative to the rotation poles. That rare alignment was first measured by Pioneer 11 in 1979 and was later confirmed by Voyagers 1 and 2.
Several distinct regions have been identified within Saturn's magnetosphere. Inside about 400,000 kilometers (250,000 miles) there is a torus of H+ and O+ ions, probably originating from water ice sputtered from the surfaces of Dione and Tethys. (The ions are positively charged atoms of hydrogen and oxygen that have lost one electron.) Strong plasma-wave emissions appear to be associated with the inner torus.
At the outer regions of the inner torus some ions have been accelerated to high velocities. In terms of temperatures, such velocities correspond to 400 million to 500 million Kelvins (700 to 900 million degrees F).
Outside the inner torus is a thick sheet of plasma that extends out to about 1 million kilometers (620,000 miles). The source for material in the outer plasma sheet is probably Saturn's ionosphere, Titan's atmosphere, and the neutral hydrogen torus that surrounds Titan between 500,000 kilometers (300,000 miles) and 1.5 million kilometers (1 million miles).
Radio emissions from Saturn had changed between the encounters of Voyager 1 and 2. Voyager 2 detected Jupiter's magnetotail as the spacecraft approached Saturn in the winter and early spring of 1981. Soon afterward, when Saturn was believed to be bathed in the Jovian magnetotail, the ringed planet's kilometric radio emissions were undetectable.
During portions of Voyager 2's Saturn encounter, kilometric radio emissions again were not detected. The observations are consistent with Saturn's being immersed in Jupiter's magnetotail, as was also the apparent reduction in solar-wind pressure mentioned earlier, although Voyager scientists say they have no direct evidence that those effects were caused by Jupiter's magnetotail.