10. Saturn: lord of the rings

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Fundamentals

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Majestic Saturn, the sixth planet from the Sun, was the most distant world known to the ancients, and it moved least rapidly around the zodiac. The Greeks identified the planet with Kronus, the father of Zeus, while the Romans named the planet Saturn after their god of sowing. Both the Greeks and the Romans associated Saturn with the ancient god of time, which later became Father Time.

You can see Saturnís oblong, golden disc with a small telescope, girdled by its beautiful rings, unattached to the globe. They set Saturn apart from all the other planets. Even though we now know that all four of the giant planets possess ring systems of some kind, Saturnís rings easily outclass the others.

Saturnís orbital radius is 9.5 times the radius of the Earthís orbit, and it takes 29.458 Earth years for Saturn to complete one revolution around the Sun. Perhaps because of its remote orbit and slow motion, the planetís name has been adopted for the word ďsaturnineĒ, to describe a cool and distant temperament.

The volume of Saturn is great enough to encompass 764 Earth-sized planets. But Saturnís mass is only 95 times greater than the Earthís mass, so the giant planet must be composed of material that is much lighter than rock and iron, the primary ingredients of the Earth.

From Saturnís mass and volume, we calculate its average mass density to be only 688 kilograms per cubic meter, the lowest of any planet and less than that of liquid water. If Saturn were placed in a large enough ocean of water, it could float. It has a low average density because it is mainly composed of the lightest elements, hydrogen and helium, in the gaseous and liquid states.

Physical properties of Saturna

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Mass5.6865 x 1026 kilograms = 95.184 ME
Equatorial radius at one bar6.0268 x 107 meters = 9.46 RE
Polar radius at one bar 5.4364 x 107 meters
Mean mass density688 kilograms per cubic meter
Rotation period10 hours 39 minutes 22.3 seconds = 10.6562 hours
Orbital period29.458 Earth years
Mean distance from Sun1.4294 x 1012 meters = 9.539 AU
Age4.6 x 109 years
Atmosphere97 percent molecular hydrogen, 3 percent helium
Energy balance1.79 Ī 0.10
Effective temperature95.0 degrees kelvin
Temperature at one-bar level134 degrees kelvin
Central temperature13,000 degrees kelvin
Magnetic dipole moment600 DE
Equatorial magnetic field strength0.22 x 10-9 tesla or 0.72 BE
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a The symbols ME, RE, DP, BE denote respectively the mass, radius, magnetic dipole moment and magnetic field strength of the Earth. One bar is equivalent to the atmospheric pressure at sea level on Earth. The energy balance is the ratio of total radiated energy to the total energy absorbed from sunlight, and the effective temperature is the temperature of a black body that would radiate the same amount of energy per unit area.

Saturnís winds and clouds

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The wind speeds of Saturnís equatorial jet streams reach 500 meters per second, almost four times the speed of Jupiterís fastest winds and ten times hurricane force on the Earth. The dominant winds on Saturn blow eastward, in the same direction as the planetary rotation, at almost all latitudes, with the most powerful nearest to the equator. Reversals in wind direction are only found near Saturnís poles, where the clouds counter flow in the eastward and westward direction. They form banded belts and zones similar to those observed almost everywhere on Jupiter.

Despite its raging winds, Saturn lacks the dynamic and colorful storm clouds of Jupiter. Stormy weather on Saturn is apparently masked by an upper deck of dirty, smog-coated particles that give the planet a pastel, butterscotch hue. Jupiter, being warmer than Saturn, has less of this smoggy haze, and its cloud features are more distinct.

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When Voyager 2 passed behind Saturn, its homebound radio signals penetrated the upper atmosphere, and alterations in these transmissions have been used to deduce the pressure and temperature below the clouds. Because there is no solid surface directly below the clouds, altitudes are referred to the level in the atmosphere where the pressure is equal to 0.1 bars, or one-tenth the sea-level pressure on Earth. This is the approximate level where the temperature bottoms out, at about 82 degrees kelvin, and the obscuring veil of haze may be formed.

Under the assumption that Saturnís gas mixture is in chemical equilibrium, with a uniform composition like that of the Sun, ammonia is expected to condense and form clouds at about 100 kilometers below the reference level, where the pressure has risen to about 1 bar. These clouds of ammonia ice presumably rise to form the bright, white storms that are occasionally seen above the global haze. Water clouds may form much lower in the atmosphere, where the pressure rises to almost 10 bars, but no one has ever seen them.

Beneath the clouds of Saturn

The internal constitution of Saturn

Saturnís low mass density indicates that the lightest element, hydrogen, is the main ingredient inside the planet, just as it is for Jupiter and the Sun. The lightweight material, just 68.8 percent as dense as water, is hurled outward in its equatorial regions by the planetís rapid 10.6562-hour rotation, making Saturn the most oblate planet in the solar system. Its equatorial bulge amounts to about 10 percent of the radius, and is about as big in extent as the Earth. Or, as some view it, the polar regions of Saturn are squashed and flattened by this amount.

The oblong shape of Saturn can be seen with a small telescope, and measured precisely from its satellite orbits and ring positions as well as by the trajectories of the passing Voyager spacecraft. When these measurements are combined with Saturnís known mass, volume and rotation rate, scientists can obtain information about its internal distribution of mass.

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The model the experts come up with is just a scaled down version of Jupiter, with a small, dense core of melted ice and molten rock surrounded by a vast globe of liquid hydrogen and topped by a thin gaseous atmosphere. Like Jupiter, giant Saturn is not a solid world, and is essentially a great big drop of liquid.

Deep down inside, the liquid hydrogen is compressed to such high pressures that it conducts electricity like a metal. But since Saturn is less than three times as massive as Jupiter, and only slightly smaller, the internal pressure at a given depth is less, and the liquid hydrogen turns into a metal further down in the ringed planet. Saturn therefore has a smaller shell of liquid metallic hydrogen.

Interior heat and helium rain

Precise measurements from the Voyager 1 and 2 spacecraft indicate that Saturn is radiating 1.78 times more energy in visible and infrared light than it absorbs from incoming sunlight. This excess energy must be coming from within the planet. It implies that Saturn, like Jupiter, is an incandescent globe with an internal source of heat.

Both Jupiter and Saturn radiate almost twice as much energy as they receive from the Sun, but the dominant source of internal heat is different for the two giant planets. Jupiterís internal heat is primarily primordial heat liberated during the gravitational collapse when it was formed, and Saturn must have also started out hot inside as the result of its similar formation. But being somewhat smaller and less massive than Jupiter, the planet Saturn was not as hot in its beginning and has had time to cool. As a result, Saturn lost most of its primordial heat and there must be another source for most of its internal heat.

Saturnís excess heat is generated by the precipitation of helium into its metallic hydrogen core. The heavier helium separates from the lighter hydrogen and drops toward the center, somewhat like the heavier ingredients of a salad dressing that hasnít been shaken for awhile. Small helium droplets form where it is cool enough, precipitate or rain down, and then dissolve at hotter deeper levels. As the helium at a higher level drizzles down through the surrounding hydrogen, the helium converts some of its energy to heat. In much the same way, raindrops on Earth become slightly warmer when they fall and strike the ground; their energy of motion Ė acquired from gravity Ė is converted to heat.

The remarkable rings of Saturn

Billions of whirling particles of water ice

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The austerely beautiful rings of Saturn are so large and bright that we can see them with a small telescope. And because the glittering rings are tipped with respect to the ecliptic, the plane of the Earthís orbit about the Sun, they change their shape when viewed from the Earth. The rings are successively seen edge on, when they can briefly vanish from site in a small telescope, from below, when they are wide open, edge-on again and then from above. The complete cycle requires 29.458 Earth years, the orbital period of Saturn, so the rings nearly vanish from sight every 15 years or so. The last disappearance took place in 1995.

The three main rings of Saturn have been observed for centuries. There are the outer A ring and the central B ring, separated by the dark Cassini Division, and an inner C, or crepe, ring that is more transparent than the other two. They remain suspended in space, unattached to Saturn, because they move around the planet at speeds that depend on their distance, opposing the pull of gravity.

The inner parts of the rings move around Saturn faster than the outer parts, all in accordance with Keplerís third law for small objects revolving about a massive, larger one. They orbit the planet with periods ranging from 5.8 hours for the inner edge of the C ring, to 14.3 hours for the outer edge of the more distant A ring. Since Saturn spins about its axis with a period of 10.6562 hours, the inner parts of the main rings orbit at a faster speed than the planet rotates, and the outer parts at a slower speed.

The difference in orbital motion between the inner and outer parts of the rings means that they are not a solid sheet of matter, for they would be torn apart by the differential motion. The rings are instead made up of vast numbers of particles, each one in its own orbit around Saturn, like a tiny moon. Billions of ring particles revolve about the planet. They have been flattened and spread out to a thin, wide disk as the result of collisions between particles.

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The rings of Saturn are flat, wide and incredibly thin. Measured from edge to edge, the three main rings span a total width of 62.2 thousand kilometers, so they are a little wider than the planetís radius, at 60.3 thousand kilometers. When observed edge on, from on or near the Earth, the rings practically disappear from view. They look about a kilometers thick, but this is an illusion attributed to warping, ripples, embedded satellites and a thin, inclined outer ring. When instruments on Voyager 2 monitored starlight passing through the rings, they found that the ring edges extend only about 10 meters from top to bottom. If a sheet of paper represents the thickness of Saturnís rings, then a scale model would be two kilometers across.

What are the ring particles made out of? At visible wavelengths, the rings are bright and reflective, but at infrared wavelengths they are dark and less reflective. This suggests that the particles are cold and made of ice. In fact, they are composed largely, and almost exclusively, of water ice. The total mass of the prominent A, B and C rings is about equal to that of Saturnís satellite Mimas, which weighs in at 4.5 x 1019 kilograms, and such a mass is consistent with particles composed of water ice.

The ring particles are too small for spacecraft cameras to see individually, but scientists can infer their size from radio measurements. Since the rings are very reflective to ground-based radar transmissions, we know that their particles are comparable to, or larger than, the radar wavelength of about 0.1 meters. The particle size distribution has been determined from the way the rings blocked the radio signals from Voyager 1 and 2 when the spacecraft passed behind the rings. This method showed that there are remarkably few particles larger than 5 to 10 meters in size or smaller than 0.01 meters. Within these bounds, the number of particles in the main rings decreases with increasing size, in proportion to the inverse square of their radius.

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However, four additional rings, designated the D, E, F and G rings, consist of much smaller, microscopic ice crystals. These rings, discovered using ground-based or spacecraft observations, are all very diffuse, tenuous and nearly transparent. The way that their particles scatter light indicates that they are the smallest of all, roughly a micron in size Ė a micron is millionth, or 10-6, meters.

Pioneer 11 discovered the incredibly narrow F ring, that lies just outside the A ring, by its absorption of energetic particles; while images from the Voyager spacecraft showed the F ring in great detail, demonstrating that its width varies from a few thousand to tens of thousands of meters. Moreover, it is not just a single ring, Voyager 1 spotted a contorted tangle of narrow strands that had smoothed out by the time Voyager 2 arrived about 9 months later. Because the F ring particles are brighter when backlit by the Sun, and fainter in reflected sunlight, we know that the particles are also micron-sized, much smaller than snowflakes and comparable in size to the dust in your room.

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But how can this ring retain such narrow boundaries? In the absence of other forces, collisions between ring particles should spread them out, causing the particles to fall inward toward Saturn and expand outward from it, thus creating a broader and more diffuse ring. Two tiny moons, named Pandora and Prometheus, flank the F ring and confine it between them, thereby keeping the particles of the F ring from straying beyond the ringís narrow confines.

Ringlets, waves, gaps and spokes

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From a distance, the principal rings of Saturn look like smooth, continuous structures. Up close, however, from the views provided by the Voyager 1 and 2 spacecraft, the icy material is marshaled into thousands of individual ringlets. Some of the ringlets are perfectly circular, others are oval-shaped and a few seem to spiral in towards the planet like the grooves on an old-fashioned record. In some places, the flat plane of the rings is slightly corrugated, and ringlets are seen at the crests and dips of the corrugations, like ripples running across the surface of a pond.

An outside hand is at work sculpting at least some of the intricate ring structures through the force of gravity. The combined gravitational pull of Saturn and the accumulated pull of nearby moons can redistribute the ring particles, concentrating them into many of the observed shapes. Although small nearby moons have only a weak gravitational pull on the particles in the rings, the pull is repeated over and over again at certain resonant locations. Just as we can make a child on a swing arc high above the ground with a gentle, repeated push in the same place of the swing, so the repeated gravitational pull of a small external moon during each orbit can give an unexpectedly large perturbation. The interplay of this effect and Saturnís inward gravitational pull can repel and attract the ring particles, pushing and pulling them into localized concentrations such as ringlets.

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But simple interactions with known moons have not been completely successful in accounting for all of the intricate detail found in Saturnís rings. The apparent gaps in the system are not completely empty. The Cassini Division, for example, contains perhaps 100 ringlets, with particles just as large as those in the neighboring ring. Some gaps do not even occur at known resonant positions or contain detected moons embedded within them. Unseen moons might influence the clumping and removal of material in these locations.

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Perhaps the most bizarre Voyager discovery was the long, dark streaks, dubbed spokes, that stretch radially across the rings, keeping their shape like the spokes of a wheel. These ephemeral features are short-lived, but regenerated frequently. They are found near the densest part of the B ring, that co-rotates with the planet at a period of 10.6562 hours. But the inner and outer parts of Saturnís dark spokes also whirl around the planet with this period, at constant speed in apparent violation of Keplerís third law and Newtonís theory of gravity. If the spokes consisted of dark particles embedded in the rings, the particles would move with speeds that decrease with increasing distance from Saturn, and the spokes would quickly stretch out and disappear.

According to one hypothesis, the small dust particles may become charged, perhaps as the result of collisions with energetic electrons. Electromagnetic forces then raise or levitate the tiny, charged particles off the larger ring bodies, and the spokes are swept around Saturn by its rotating magnetic field. It sounds bizarre, but subtle forces are required to overcome gravity.

Why do planets have rings?

One might expect the particles of a ring to have accumulated long ago into larger satellites. But the interesting feature of rings Ė and a clue to their origin Ė is that they do not coexist with large moons. Planetary rings are always closer to the planets than their large satellites.

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The rings are confined to an inner zone where the planetís tidal forces would stretch a large satellite until it fractured and split, while also preventing small bodies from coalescing to form a larger moon. The outer radius of this zone in which rings are found is called the Roche limit after the French mathematician Eduoard A. Roche (1820-1883), who described it in 1848. For a satellite with no internal strength and whose density is the same as the planet, the Roche limit is 2.456 times the planetary radius, or about 147 thousand kilometers for Saturn.

And where did Saturnís rings come from? There are two possible explanations for their origin. In the first explanation, the rings consist of material left over from Saturnís birth about 4.6 billion years ago. This hypothesis assumes that the rings and moons originated at the same time in a flattened disk of gas and dust with large, new born Saturn at the center. According to the second explanation, a former moon or some other body moved too close to Saturn and was torn into shreds by the giant planetís tidal forces, making the rings. In this case, the rings could have formed after Saturn, its satellites and much of the rest of the solar system.

Astronomers now estimate that Saturnís rings are less than 100 million years old, or less than two percent of Saturnís life span. The dazzling, sparkling brightness of Saturnís rings provides evidence for this youth. They glisten with clean particles of pure water ice, unsullied by the constant pelting by cosmic dust. The rings would look much darker if they were very old, just as new-fallen snow becomes dirty over time. Calculations indicate that in 100 million years Saturnís bright rings will be darkened by the pervasive cosmic debris to the same extent as the older, coal-black rings of Uranus and Neptune.

The gravitational tugs of Saturnís moons on the rings will shorten the lives of the rings, providing another indication of their youth. When setting up density waves in the rings, nearby moons extract momentum from the ring particles, causing them to slowly spiral toward Saturn; to conserve momentum in the overall system, the moons gradually move away from the planet. The A ring will eventually be dragged down into the B ring, and all the rings should collapse as the result of this moon-ring interaction in about 100 million years.

This brings us back to the second explanation for Saturnís rings, in which a pre-existing body strayed too close to Saturn and was torn apart by tidal forces. It might have been one of Saturnís moons, or an interloper from another region of the solar system. A satellite could form outside the Roche limit and move inward due to the pull of tidal forces that would eventually rip the satellite to pieces. As previously mentioned, the total mass of all the ring particles is similar to the mass of Saturnís relatively small satellite, Mimas, so it seems reasonable that the rings could have formed from such a moon, or from a few smaller ones. After all, the Martian moon Phobos is now being drawn inexorably toward the red planet by its tidal forces, and Neptuneís largest satellite Triton is also headed on a collision course toward its planet.

The moons of Saturn

Titan - moon of mystery

Titan is the largest of Saturnís satellites, much larger than the planetís other moons. It is the second largest satellite in the solar system, and the only satellite possessing an extensive, dense atmosphere with a surface pressure comparable to that of the Earthís air.

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Visible light cannot penetrate the veil of orange smog that coverís Titanís surface. In the satelliteís dry, cold atmosphere, the smog builds up to an impenetrable haze that extends to altitudes as high as 300 kilometers. On Earth, smog similarly forms by the action of sunlight on hydrocarbon molecules in the air. The urban smog usually forms within a kilometer of the Earthís surface. Titanís atmosphere extends far above its surface because of the high atmospheric pressure and the relatively low mass and gravitational pull of Titan.

Instruments aboard the Voyager spacecraft determined the composition of Titanís atmosphere. The dominant gas surrounding the satellite is molecular nitrogen, between 82 and 99 percent. Methane, the one gas identified with certainty before the Voyagers arrived, turned out to be a minor constituent, with an abundance of 1 to 6 percent. So, nitrogen molecules account for the bulk of Titanís atmosphere as they do on Earth Ė 77 percent of our air is molecular nitrogen.

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High above Titanís surface, abundant nitrogen and methane molecules are being broken apart continuously by the Sunís energetic ultraviolet light and by the bombardment of electrons trapped in Saturnís magnetic environment. Some of the fragments then recombine to form more complex molecules that have been detected in small amounts by Voyagerís infrared spectrometers. In addition to methane, CH4, the list includes hydrocarbons like ethane, C2H6, acetylene, C2H2, and propane, C3H8, and nitrogen compounds such as hydrogen cyanide, HCN. Many of these molecules can join together in chainlike, polymer structures that contribute to Titanís dark, smoggy haze.

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Although you cannot see beneath the smog-covered globe, Voyagerís radio signals have been used to infer the pressure and temperature down to the surface. The surface pressure is an ear popping 1.5 bars. That is one and half times the 1-bar air pressure at sea level on Earth, and equivalent to the pressure experienced by a deep-sea diver at about 6 meters under the oceanís surface.

Although liquid water cannot now lap the shores of Titan, it might contain shallow hydrocarbon seas. In fact, ethane or methane could play the role of water on Earth. The methane can condense in Titanís cold atmosphere to produce thick clouds that lie beneath the haze. Infrared observations that penetrate the smog suggest the presence of short-lived methane clouds in Titanís lower atmosphere, which form briefly and irregularly. Since the atmosphere is not fully saturated with methane, there cannot be extensive oceans of pure methane on the surface, but both ethane and methane can rain out of the atmosphere. They can exist as a liquid rather than a solid at the surface temperature of 94 degrees kelvin. Evaporation of the liquid seas can resupply the hydrocarbons to the atmosphere, completing the cycle.

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We now know that Titan is not completely covered by a global hydrocarbon sea. Radar signals that penetrate the haze indicate that different regions of the surface reflect radar by varying amounts, so any liquid would have to be pooled in lakes or small seas rather than in a homogeneous covering. Titanís thick orange smog is also transparent enough at infrared wavelengths to allow mapping of the surface. Images obtained with the Hubble Space Telescope indicate an inhomogeneous landscape with bright and dark features that reflect infrared radiation by different amounts. They could be continents and oceans, but no one knows for sure. We might find out when the Cassini spacecraft arrives at Saturn in July 2004, and parachutes the Huygens Probe down to Titanís surface.

Saturnís medium-sized icy moons

Saturn has only one large satellite, Titan, comparable in size to Jupiterís four Galilean satellites, but it has an extensive family of smaller moons, including six mid-sized icy bodies that range from 196 to 765 kilometers in radius. The mean mass densities of these moons are low, between 1100 and 1400 kilograms per cubic meter, which suggests that they are mainly composed of pure water ice. This is consistent with their highly reflective surfaces. With the exception of Iapetus, they all reflect more than 50 percent of the incident sunlight, and one of them, Enceladus, reflects almost 100 percent of the sunlight that strikes it. Water ice was also identified by infrared spectroscopy in the years prior to the Voyager missions.

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Saturnís innermost, medium-sized moon, Mimas, has a surface that is saturated with overlapping impact craters, including one crater that is about one-third the diameter of the moon itself. The impact that made this crater was nearly powerful enough to completely shatter Mimas.

Though of comparable size, Enceladus is a very different world from Mimas. Parts of the smooth, nearly crater-free surface of Enceladus have been coated with fresh icy material that rose from the warm interior of the satellite. Other parts of the surface contain cracks and grooves, suggesting that internal stresses may have discharged liquid water that froze into smooth ice. As the satellite moves around its eccentric orbit, produced by Dioneís gravitational tugs, tidal flexing by Saturn probably heats the interior of Enceladus, melting the water ice and permitting its eruption. Active ice volcanoes may be even now be erupting on Enceladus.

Tethys is about twice the size of Enceladus with nearly the same mean mass density, but Tethys is more akin to Mimas, with a large number of impact craters and one enormous impact that nearly broke the moon apart. A gigantic fracture covers three-fourths of the moonís circumference, suggesting internal activity early in its history. But the satellite shows no evidence for current activity.

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Dione is nearly the same size as Tethys but denser, and shows a wide variety of surface features. Next to Enceladus among Saturnís moons, it has the most extensive evidence for an active interior. It has enough rocky material in its makeup to produce internal heat from natural radioactivity. Most of the surface is heavily cratered, but differences in the number of craters within various regions indicate that several periods of resurfacing occurred during the first billion years of Dioneís existence. Bright, wispy streaks, which stand out against an already-bright surface, are believed to be the result of internal heat and subsequent flows of erupting material.

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The surface of Rhea is completely saturated with impact craters. It appears to be a dead world, geologically inert and without signs of internal heat. Yet, it is the largest of Saturnís icy moons. Compression resulting from its greater size and mass may have closed any volcanic vents, shutting off the outward flow of warm material.

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Curiously, early astronomers could only observe the icy moon, Iapetus, on one side of Saturn. The satellite seemed to disappear when its orbit carried it to the other side of the planet. The reason for this strange behavior is that Iapetus is a divided world; half its surface is as bright as ice, and the other is as dark as asphalt or coal and is thought to contain complex organic compounds. Like the Earthís Moon, the satellite Iapetus keeps one side toward its planet, and as it revolves around Saturn the bright and dark parts are successively turned toward the Earth. When the dark half is pointed at the Earth, the moon becomes very difficult to observe.

Small, irregularly shaped satellites of Saturn

Instruments on the Voyager 1 and 2 spacecraft have discovered a host of small, irregularly shaped satellites that reside within the inner parts of Saturnís satellite system. They are all bright objects, probably composed of ice, and many of them have orbits that are remarkable in one way or another. Six of these tiny moons are associated with the rings: Pan, Atlas, Pandora, Prometheus, Janus and Epimetheus. Pan disturbs particles in the A ring to form the Encke division. Pandora and Prometheus shepherd the F ring; Atlas shepherds the outer margin of the A ring. Saturnís two co-orbital satellites, Janus and Epimetheus, are even more bizarre. Janus and Epimetheus move in almost identical orbits. The satellite on the inner orbit that is closest to Saturn moves slightly faster, overtaking the outer satellite every four years. But the bodiesí diameters are greater than the distance between their orbital paths, so they cannot pass without some fancy pirouetting. They avoid a collision at the last moment by gravitationally exchanging energy and switching orbits. The inner one is pulled by the outer one and raised into the outer orbit, and vice versa. They then move apart, only to repeat this pas-de-deux four years later, and exchange again. Three so-called Lagrangian satellites move along the orbits of Saturnís larger satellites Tethys and Dione.

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This concludes our survey of Saturn, the most distant planet known to the ancient. We will now travel out beyond this enchanting world to the next wanderer, Uranus.

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(Summary Diagram.)

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