10. Saturn: lord of the rings
The remarkable rings of Saturn
Billions of whirling particles of water ice
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.
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.
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.
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
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.
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.
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.
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.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University