All of the jovian planets have a system of rings. Jupiter has four faint rings: a flattened main ring, a puffier inner ring, and two wispy outer rings that are inside the orbit of Io. The rings are made of very small, dark particles the size of smoke particles. They are produced by dust kicked up from the tiny innermost moons of Jupiter by impacts on the moons.
Saturn's rings were discovered by Christian Huygens in 1659. Galileo's telescope was too small to make them look like more than just a couple of bumps on either side of the planet. In 1675 Giovanni Cassini discovered a gap between the two large (A & B) rings, now called the Cassini division in his honor. With improved telescopes, astronomers were able to see that one of the large rings was in fact, two rings (B & C) and there is a gap in the A ring (the Encke division). There is also a hint of another ring closer to the planet than the C ring (the D ring). When the Pioneer and Voyager spacecraft flew by, astronomers found more rings and complex structure in the rings.
The rings that are visible in even low-power telescopes on the Earth (A, B, and C) extend from about 74,000 kilometers to about 137,000 kilometers from Saturn's center (or 1.23 to 2.28 Saturn radii). The rings are very thin, less than a few hundred meters thick. A scale model of the rings with the width equal to a single piece of regular paper would be 70 to 100 meters across! In 1859 James C. Maxwell (of electromagnetism fame), showed that the rings could not be solid, but, rather a swarm of particles. A solid ring the width of Saturn's ring system would become unstable and break up. James Keeler proved Maxwell correct in 1895 when he measured the doppler shifts of different parts of the rings and found that the outer parts of the ring system orbited at a slower speed than the inner parts. The rings obeyed Kepler's third law and, therefore, must be made of millions of tiny bodies each orbiting Saturn as a tiny mini-moon.
More recently, astronomers bouncing radar off the rings and analyzing the reflected signal found that ring particles must be from a few centimeters to a few meters across. When the Voyager spacecraft went behind the rings with respect to the Earth, astronomers could measure the particles sizes from how Voyager's radio signal scattered off the particles and from how sunlight scattered through the rings. The ring particles range in size from the size of a small grain of sand to the size of a large house, but on average, they are about the size of your clenched fist. Spectroscopy of the rings shows that the particles are made of frozen water. Collisions between the ring particles keeps the ring system very flat and all of the particle orbits circular.
The Voyager and Pioneer spacecraft confirmed the presence of the inner D ring and discovered two other rings beyond the outer A ring: a narrow F ring just outside the A ring and a broad, but faint E ring. The broad rings we see from the Earth are actually systems of thousands of tiny ringlets each just a few kilometers wide, so the rings look like grooves in a phonograph record (youngsters only familiar with CD's will have to ask their parents or grandparents about them). Voyager also found some unusual things in Saturn's ring system: rings that change shape, eccentric ring shapes (some even twist around each other to make a braid), and dark features that look like spokes extending radially outward across the rings.
The grooved pattern of Saturn's rings are probably the result of spiral density waves forming from the mutual gravitational attraction of the ring particles. Narrow gaps in the rings are most likely swept clean of particles by small moonlets embedded in the rings. The tiny moons can also act as shepherd satellites. Two shepherd satellites with one orbiting slightly outside the other satellite can constrain or shepherd the ring particles to stay between the moonlet orbits. The narrow F ring is the result of two shepherd satellites about 50 kilometers across with orbits about 1000 kilometers on either side of the F ring. The shepherd satellites are probably also responsible for the braids and kinks in the F ring, but how they do that is unclear.
Bigger gaps in the rings (such as the Cassini division) are the result of gravitational resonances with the moons of Saturn. A resonance happens when one object has an orbital period that is a small-integer fraction of another body's orbital period, e.g., 1/2, 2/3, etc. The object gets a periodic gravitational tug at the same point in its orbit. Just as you can get a swing to increase the size of its oscillation by pushing it at the same point in its swing arc, a resonance can ``pump up'' the orbital motion of an object. Particles at the inner edge of the Cassini division are in a one-two resonance with the moon called Mimas---they orbit twice for every one orbit of Mimas. The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap. Other resonances with Mimas are responsible for other features in Saturn's rings: the boundary between the C and B ring is at the 1:3 resonance and the outer edge of the A ring is at the 2:3 resonance. It is amazing what a simple inverse square law force can do!
The dark spokes in Saturn's B ring were a surprise. The different orbital speeds of the ring particles should quickly shear apart any radial structure in the rings, but the spokes clearly survived the shearing! The spokes are probably caused by very tiny dust particles hovering just above the rings by their interaction with Saturn's magnetic field or by electrostatic forces created from ring particle collisions.
Where did the rings of Saturn come from? Studies of the various forces on the ring particles show that the rings are transient---they did not form with Saturn as part of the formation of the main planet, nor will they always be there. The rings of Saturn are within the distance at which a moon would experience extreme enough tidal stretching to be torn apart. This distance is called the Roche limit, after M.E. Roche who developed the theory of tidal break up in the 1849. The exact distance of the Roche limit depends on the densities of the planet and close-approaching moon and how strongly the material of the moon is held together. The classical Roche limit considers a moon held together only by its internal gravity. Such a moon would break up at a distance of about 2.44 planetary radii from the center of the planet.
Saturn's rings lie within Saturn's Roche limit so it is likely that they were either formed by the breakup of a moon that came too close or particles too close to Saturn to ever form a large moon. Another possibility is that large collisions on the large moons outside the Roche limit spewed material into the region inside the Roche limit. Saturn's E ring lies outside Saturn's Roche limit and is most concentrated at the orbit of the icy moon, Enceladus. Eruptions of water vapor from Enceladus probably are the source of the E ring material.
The rings of Uranus and Neptune are much darker than Saturn's rings, reflecting only a few percent of what little sunlight reaches them (they are darker than pieces of black, burned wood) while Saturn's rings reflect over 70% of the Sun's light. The rings are also much narrower than Saturn's rings. Uranus' outermost and most massive ring, called the Epsilon Ring, is only about 100 kilometers wide and probably less than 100 meters thick. The other ten dark and narrow rings have a combined mass less than the Epsilon Ring. The six rings of Neptune are less significant than Uranus' and the ring particles are not uniformily distributed in the rings. Like Saturn's F ring, the rings of Uranus and Neptune are kept narrow by shepherd satellites. The narrowness and even clumpiness of the rings means that the rings can last for only a short time---a million years or so, unless the rings are replenished by material ejected off the moons in large collisions.
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last updated: June 2, 2007