The Gas Giants (cont.)

Saturn

To ancient astronomers, Saturn was a wandering light near the edge of the known universe. The planet and its rings have been objects of beauty and wonder ever since Galileo noticed the "cup handles" that seemed attached to a round world.

Saturn is a smaller version of Jupiter, made up of a similar mix of gases, mostly the very light hydrogen and helium. Like Jupiter, all we see of Saturn is the tops of its clouds. With their subdued hues of yellow, gray and brown, they lack the sharp, brightly colored belts of Jupiter. Above the clouds there seems to be a high haze of frozen ammonia crystals. Saturn's rings, its best known feature since Galileo's time, stretch far out around the planet, extending to more than half the distance from the Earth to the Moon. They actually consist of a series of individual rings separated by small gaps and composed of huge numbers of small particles. Saturn has at least 15 moons, some only recently discovered by passing spacecraft. Among those that have long been known is Titan, one of the largest moons in the solar system, massive enough to retain a thick, cold atmosphere that obscures any view of its surface. Organic compounds are present on Titan, and it has occasionally been speculated that life may exist there.

Saturn is far out. A billion miles from Earth, the planet was not reached by any spacecraft until September, 1979, when Pioneer Saturn, after a six-year trip by way of Jupiter, flew past the rings to within 21,000 kilometers (13,000 miles) of Saturn's cloud tops. Voyager 1, with better, more sophisticated instruments (which had explored Jupiter in 1979) reached Saturn slightly more than a year after Pioneer. On November 12, 1980, Voyager 1 ducked under the rings, passed Saturn at 124,000 kilometers (77,000 miles) above the cloud tops, crossed the ring plane again, and headed out of the solar system at 90,000 kilometers per hour (56,000 miles per hour). In a few hectic days, the television cameras and other instruments of Voyager 1 told us more about Saturn, its rings, and its moons than was learned in all the centuries that people have studied this strange and beautiful world.

Saturn comes of age.
Saturn, the most distant planet known to the ancients, was finally reached by spacecraft in 1979 and 1980. These three pictures show how, our ability to see the details of the mysterious ringed planet has grown dramatically in the last five, years as we have moved from Earth-based telescopic observations (1) to pictures returned from spacecraft that actually flew past Saturn, Pioneer Saturn (2) and Voyager 1 (3).
The telescopic view from Earth (1) shows the planet, which is about 115,000 kilometers (71,000 miles) in diameter and is distinctly flattened at the poles. Faint colored bands are visible in the atmosphere, and the multiple nature of the rings can be seen.

The Pioneer Saturn picture (2) was taken from about 2.5 million kilometers (1.6 million miles) away; it shows one of Saturn's moons (Rhea, lower right) as a tiny dot. In this picture Saturn's rings are illuminated by sunlight scattered through them, so that only the thin rings appear bright; the thickest ring (the B ring) appears as a dark band between the two bright ones. The shadow of the rings is a dark band on Saturn's equator.

The Voyager 1 TV picture (3), taken from a distance of 34 million kilometers (21 million miles), shows details in the rings that are invisible to Earth-based telescopes or to Pioneer Saturn. A new gap has appeared in the dark inner C ring, and material is now visible within the surrounding empty Cassini division that separates the two largest rings (the A ring and the B ring). The dark band on the planet's equator is the shadow of the rings. Some of the details of the structure of the rings can also be seen in the shadow. The small dot below Saturn is its moon Dione. (Photo 1 courtesy of Catalina Observatory, Arizona.)

These closeup looks at Saturn revealed several properties that the ringed planet shares with Jupiter, as well as some differences. Like Jupiter, Saturn is radiating away about twice as much energy as it receives from the Sun, but its frigid cloud tops have a temperature of about -183° C. Pioneer Saturn discovered that Saturn has a magnetic field. The field is much weaker than that of Jupiter, but it is probably produced in the same way, by circulating flows of liquid metallic hydrogen deep within the planet that generate electric currents and thus create a magnetic field. Saturn's magnetic field fills a smaller volume of space than does Jupiter's, and its radiation belts are correspondingly weaker. In fact, Saturn's radiation belts are comparable in intensity to the Van Allen Belts of Earth.

As Voyager 1 approached Saturn, the planet's atmosphere began to show structural details within the indistinct, faintly colored bands. The television cameras peered through the frozen ammonia haze, down to the thicker clouds. At this still fairly distant range, Saturn looks more like Jupiter. Turbulent belts and streams of clouds stretch east-west across the planet. Cloudy jet streams were photographed that move four times as fast as the winds of Jupiter, as much as 1500 kilometers per hour (930 miles per hour). A red oval was found in the clouds, a swirling storm like Jupiter's Red Spot, but much smaller (only about the size of the Earth!).

Saturn's rings are a landmark of the solar system. They seem solid, but we have long known that they are made up of innumerable particles of water ice and perhaps frozen ammonia, a few centimeters to a few meters in size. Nevertheless, in close-up, the rings were one of the biggest suprises in the history of planetary exploration, far more numerous, complex, baffling, and beautiful than ever was expected.

From Earth, we could clearly see only three rings. PioneerSaturn discovered three more. But as Voyager 1 drew close, the once seemingly uniform rings separated into dozens and then into hundreds of distinct, thin rings separated by narrow gaps. As Voyager 1 flew under Saturn, the rings stretched above the spacecraft like a huge rainbow and the Voyager cameras photographed perhaps a thousand separate rings, looking like the grooves of a cosmic phonograph record. Even in the Cassini division, a dark, seemingly empty band as seen from Earth, Voyager 1 photographed over 50 of the thin rings, floating within what had long been regarded as a permanent empty space in the ring system.

of Saturn's prominent rings and 2 of her moons

A close look at a large planet.
The looming shape ofSaturn stretches across this picture taken by Voyager 1 from 13 million kilometers (8 million miles) away. Nearby and to the left are two of Saturn's moons, Tethys (above) and Dione (below), each about 1000 kilometers (622 miles) in diameter. Saturn's rings stretch from upper left to lower right. The Cassini division, the narrow gap between the large A-ring and the B-ring, is clearly visible, and Saturn's cloud tops can be seen through it. The shadow of the rings is a series of light and dark bands that stretches across the planet. The small black dot at the lower right is the shadow of the moon Tethys.

The rings are far more complicated than anyone had imagined. For some unexplained reason, a few rings are not circular but appear elliptical, as though some force has pulled them a little out of round. The thin, outer F-ring was resolved by Voyager 1 into several distinct strands that apparently are braided about each other. Equally mysterious and unexpected are the so-called "spokes," radial bands cutting right across the rings much like the spokes in a wagon wheel. They may be clouds of fine dust, held in place by electrical forces generated around the rings, but no one really knows.

photo of Saturn's rings with broken section visable

Spokes in Saturn's wheels.
The dark, fingerlike area that cuts across Saturn's bright B-ring is one of the many baffling spokes detected in Voyager 1's close-up pictures. The spokes rotate around Saturn with the rings, and individual spokes may persist for several hours. The existence of such objects that cut radially across the rings is baffling. Inner ring particles move around Saturn faster than outer ones, and the radial structure of the spokes should be quickly destroyed as the particles move past each other. The spokes appear dark in reflected sunlight (as in this picture, taken when Voyager 1 was between Saturn and the Sun), but they appear bright when photographed by forward-scattered sunlight from the other side of Saturn. This behavior suggests that the spokes are actually clouds of fine dust that are carried around Saturn by its magnetic field, but there is still no definite explanation. This Voyager 1 photograph was taken from a distance of 51 million kilometers (32 million miles.)

A rainbow of rings.
The intricate baffling detail of Saturn's rings are spread out in this computer-enhanced view taken by Voyager 1 at a distance of 8 million kilometers (5 million miles). At least 95 separate rings are clearly shown, far to many to be explained by our current theories of simple gravitational forces between Saturn's nearby moons and the ring particles. At the upper left, the thin outer F-ring appears as a narrow curved streak beqond the main body of the rings. The tiny dot nearby is a small moon discovered by Voyager 1, one of a pair that seems to act like shepherds, keeping the thin F-rin in place.

The Voyager 1 pictures demolished all existing theories about Saturn's rings. Previously, the structure of the few known rings and gaps was explained in terms of the gravitational influence of some of Saturn's adjacent moons on the small ring particles. Indeed, Voyager 1 did show how this mechanism may work on a small scale: Two small moons were discovered, one on each side of the thin F-ring. The pair may be "shepherds", their gravity keeping the ring particles from moving in or out of their present orbits. A third small moon, discovered just outside the large A-ring, may have a similar stabilizing effect.

These examples are helpful, but they are not enough. No theory involving only a few moons can explain the presence of a thousand narrow rings, the elliptical rings, the ring braids, or the spokes. Nor can we yet explain how the delicate rings apparently have survived over the 4.5-billion-year age of the solar system. The problems of the rings are now seen more fully and clearly, and unfortunately, they are more complex. New theories are needed to understand the entire fantastic Saturn system.

In its dash past Saturn, Voyager photographed seven of the planet's large moons, previously unexplored worlds that had been only tiny spots of light in earthbound telescopes, despite their fanciful mythological names: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus. Even before Voyager 1 arrived, we thought that these moons would be different.

In this cold, distant region of the solar system, water does not exist as a liquid but as ice, in effect a brittle kind of rock. Saturn's moons are huge icebergs, perhaps with small cores of true rock, and they may be more like huge versions of the nucleus of a comet than like our own rocky Moon.

Iceworld.
The ancient, cratered surface of Saturn's moon Dione may record an intense bombardment by smaller objects (planetesimals) when Saturn and its moonsformed. Dione, which is about 1100 kilometers (684 miles) in diameter, is composed largely of ice, though it may have a small amount of denser rocky material scattered through it or forming a small core. The largest crater in the picture is about 100 kilometers (62 miles) in diameter and shows a prominent central peak. White streaks on the left side may be rays of material thrown out of a crater on the other side of Dione. Similar cratered landscapes were photographed on other moons of Saturn - Mimas, Tethys, and Rhea. Dione's surface shows evidence of internal forces as well. A long crack at the lower right near the shadow may have been produced by spreading of the moon's icy crust.

Just like the moons of Jupiter, Saturn's moons displays a variety of weird landscapes. Mimas, Dione, Tethys, and Rhea all have heavily cratered terrain, presumably the result of bombardment by smaller objects shortly after the moons formed about 4.5 billion years ago. On Mimas there is one large crater that makes the moon resemble a huge, staring eyeball The crater is so big that the impact that formed it must almost have shattered Mimas into pieces. The landscape of Rhea, photographed close up as Voyager 1 skimmed past only 72,000 kilometers (45,000 miles) above its north pole, resembles the cratered lunar highlands and the battered surface of Mercury, but Rhea's craters have been cut in ice, not in rock.

The craters are caused by an external force, the cosmic bombardment of Saturn's moons, but some of the moons have clearly also been modified by internal forces. Tethys has a huge trench, 60 kilometers (37 miles) across and about 750 kilometers (470 miles) long, perhaps a great crack in a spreading crust. Rhea and Dione display strange streaks and swirls, side-by-side with heavily cratered regions, as though the icy crusts shifted and deformed after the craters were made.

The moon Enceladus was a surprise of a different kind; its surface is smooth and crater-free, even though it orbits Saturn between the heavily cratered moons Mimas and Dione. Perhaps Enceladus is heated by tidal forces imposed by nearby Dione. (Similar tidal heating of Jupiter's moon Io is strong enough to power Io's constantly erupting volcanoes.) Such heating could warm Enceladus' icy crust, deforming it and removing the traces of once-present craters. Perhaps the heating is so strong that icy volcanoes erupt from time to time and floods of molten "lava" - in this case just water - pour briefly across the surface of Enceladus.

Saturn's distant moon Iapetus, photographed at long range by Voyager 1, is even more of a mystery. Telescopic studies from Earth show that Iapetus has two different sides, one dark and one bright. Voyager 1 pictures showed the two regions and their sharp boundary but as yet provide no explanation of the mystery.

Titan is a unique, giant moon, a world about the size of the planet Mercury. It is the only moon in the solar system known to have an atmosphere. Already from Earth we had detected methane and other organic compounds, so Voyager 1 was carefully programmed to take an especially close look at Titan. It flew past Titan only 4000 kilometers (2500 miles) above the atmosphere, the closest flyby past any cosmic object in the history of space exploration. The accuracy involved is comparable to shooting a billiard ball 3200 kilometers (2000 miles) across a giant table and sinking it in the right pocket.

Titan's atmosphere is so thick, perhaps more than 200 kilometers (125 miles) deep, that the Voyager 1 cameras could not see to the surface. Surprisingly, the atmosphere is not rich in the methane detected from Earth. Instead, the most common gas is nitrogen, just as on the Earth itself. Methane makes up only about one percent of the air of Titan. More complex organic molecules, formed by the action of sunlight on the atmospheric gases, produce a natural smog not too different from the choking haze found over many of our cities. At Titan's surface, the atmospheric pressure is two or three times that on Earth, but the temperatures are as low as -150° to -200° C.

The unseen surface of Titan may be a bizarre, frozen swamp, with murky streams and lakes of liquid nitrogen and a cold rain of nitrogen and organic chemicals sleeting down. There may be no life in that incredible cold after all, yet Titan may preserve in its deep freeze a sample of what other primitive atmospheres (including Earth's) resembled before they were changed by phenomena that probably have never affected Titan: volcanoes, oceans, and life. Saturn and its moons still have much to tell us and doubtless more surprises in store.

photo of a large orange Saturn orbited by 5 smaller mmons

The Lord of the Rings.
In this montage of individual Voyager 1 pictures, Saturn is surrounded by the newly revealed faces of six of its moons. Below Saturn at the right is Tethys, a heavily cratered moon with a long mysterious trench running along its surface. To the left of Tethys, just below Saturn's rings, is Mimas, a tiny, pockmarked moon on which one huge crater can be seen. Enceladus is just below the left-hand edge of Saturn's rings; this moon displays a puzzling surface, smooth and apparently uncratered. In the lower left corner is Dione, a brownish moon with a heavily cratered surface. Rhea, at the far left near Saturn's rings, shows a faint bluish tinge and strange patterns of white, wispy swirls on its surface. Titan, Saturn's largest moon, is shown in its distant orbit at the upper left. Titan's thick orange-brown atmosphere completely conceals its surface.