|
| |
Saturn: The Ringed Planet
From: Cambridge University Press
| By:
Paul Hodge |
EDITOR'S INTRODUCTION |
 Saturn is considered by most people to be the most spectacular planet in the solar system. With its rings and bright appearance, it certainly makes an impression through the telescope and in photographs. And since it was visited by the first Voyager spacecraft in 1979, what we know about Saturn has greatly improved. In this extract from his book Higher than Everest, Paul Hodge unravels the mystery of Saturn's rings by outlining the history of our knowledge of the planet. |
 f you should ask any assembled group to name its favorite planet, the winner will surely be Saturn. From the first grade class to the nursing home, almost everyone thinks of Saturn as the beautiful one, the one most likely to stir feelings of awe. When observed through a telescope on a good, steady night, Saturn is a wondrous sight, likely to cause a spontaneous reaction of "Oh, wow!" or "Cool!". The planet itself, however, is really pretty bland. Only really sharp eyes can make out any details on its cloudy surface: faint, wispy bands that seem to come and go. It's not the planet that wows the observer, it's the rings. Bright, symmetrical, set at a jaunty angle, Saturn's rings are the Solar System's best. The other three giant planets also have rings, but they are nothing like Saturn's, being barely detectable from the Earth, even from space telescopes. Saturn's are one of the wonders of the planetary world, well worth a special exploratory expedition. |
 | |
| A Voyager image of Saturn and its rings. | |
|
Ears or what?
When Saturn was first observed with a telescope by Galileo, he was mystified by its strange appearance. He and other seventeenth-century astronomers often drew it as a triple planet, a large one attended by two smaller ones on either side. In some cases they were drawn as giant Saturnian ears, turned our way as if trying to hear our exclamations of amazement. As telescopes improved, better clarity showed that the aspect of these strange things at the sides of Saturn changed over time. Sometimes they seemed large and then, months or years later, they almost disappeared. More than 50 years passed before telescopes were good enough to show them as what they are: thin rings that extend out from above Saturn's equator to dizzying heights above the planet. |
The outermost edge of the bright rings lies more than twice as far from Saturn's center as its surface. Because Saturn is a very large planet, with a diameter almost ten times the Earth's, this is a large distance. The ring's edge is about 82,000 miles out. The inside of the main rings is about 55,000 miles from the center of Saturn, meaning that it hangs 20,000 miles above the planet's surface. Although we are not planning this expedition to include a visit to Saturn's surface, it would make a nice detour just for the spectacular view you would have of the sky. The rings would be dazzling sheets of light arcing across the dark Saturnian sky. Their brilliance would be most impressive just before dawn or after dusk, when the Sun would be below the horizon, but the illuminated parts of the rings would be spectacular curtains hanging above you in the star-filled sky. Note, however, that you would not be standing on Saturn's surface to see this view. Like Jupiter's, the surface is a deck of clouds that extends hundreds of miles down into the planet's interior, which is mostly made up of liquid molecular and metallic hydrogen. There is nothing to stand on. |
Gaps of mystery
People speak about the "rings" of Saturn, but why not just the "ring"? Its plural nature was first realized in 1675 by Gian Domenico Cassini, a talented Italian astronomer who had been lured by Louis XIV to France to direct the Paris Observatory. The excellent telescope there and his perceptive eye allowed him to see a thin line that divided the ring into two nearly equal halves. This gap in the ring structure is referred to as "Cassini's Division" and it is a good test of the quality of a small telescope. Modern measurements show that the gap is about 3,000 miles wide. |
 | |
| Backlit view of the rings, like a view that might be had from the surface of the planet. | |
|
A second, smaller gap was detected somewhat later by Johann Franz Encke. Encke's Division is only about a tenth the width of Cassini's and is difficult to see from the Earth except with a moderately large telescope and excellent atmospheric conditions. It lies about 2,000 miles from the outer edge of the outer bright ring. The discovery of gaps led to the necessity of having names assigned to the different rings. Cassini's Division separates what astronomers came to call the "A" ring, the outer portion, and the "B" ring lying inside the gap. In time another ring, much fainter and more transparent, was identified and called the "C" ring. It extends inside the B ring and can be traced down some 8,000 miles towards the planet's surface. The gaps in Saturn's rings remained a mystery for two centuries. No one had any idea what the physical properties of the rings were and no physical principles were applied to them to attempt to understand their amazing appearance. History suggests that astronomers in those days were content to describe an object in the heavens without questioning the "why's" of its properties, probably because physics was a young and slowly growing discipline. It is hard now for us to imagine anyone being happy with a description of a mysterious natural phenomenon without trying to understand why it is like it is. |
But no one could have understood the gaps until there was some clear idea of the physical nature of the rings themselves. Were the rings giant sheets of some solid substance that formed in some remarkable way to hang there like the rim of a hat? For two centuries this question was debated by astronomers; Cassini himself, as well as his son, who succeeded him at the Paris Observatory, claimed that they must be instead made up of swarms of little moons. |
By the middle of the nineteenth century, astronomers were finally able to settle the question. In 1859 the talented mathematical physicist James Clerk Maxwell showed that the rings could not possibly be solid. He proved mathematically that solid rings would be completely unstable and would break up into pieces that would then orbit the planet like tiny moons. A direct demonstration of the truth of this conclusion came several decades later when James Keeler used the largest telescope in the world at that time, in California, to measure the velocities of the rings. He found that the outer parts of the rings revolved more slowly than the inner parts, obeying exactly Newton's laws for orbiting satellites. He showed that the rings do not revolve like a phonograph record, but behave like thousands (actually, billions) of small orbiting moons, just as predicted by Maxwell's physics. Actually, anyone can prove for himself or herself that the rings are not some solid material, like stainless steel, by observing Saturn when it passes in front of a star. The star remains visible when it is behind the rings. They are transparent, as the particles are loosely packed, allowing light to pass through. |
 | |
| Mimas, an inner satellite of Saturn. | |
So what causes Cassini's gap? This question was answered when astronomers noticed that Cassini's Division was at the exact place in the rings where the moonlets have a period of revolution that is just half that of the moon Mimas. The innermost of Saturn's larger moons, Mimas, was discovered in 1789 by Sir William Herschel. Its period around Saturn is just under one Earth day, meaning that it really barrels along, to cover its 700,000 mile orbit in such a short time (this rapid orbital speed is the result of Saturn's large mass; if Mimas moved more slowly, Saturn's gravity would cause it to plummet down into the planet). The period of ring particles that would be in Cassini's Division is just under half a day. Astronomers of the nineteenth century realized that this would set up what is called a resonance, so that the gravitational pull on a particle by Mimas would build up to the point that the particle would move to a non-resonant orbit. In that way the gap is cleared and maintained. |
Thousands of rings
When the first Voyager spacecraft arrived at Saturn back in 1979, it revealed some additional rings, named (unimaginatively, but following tradition) the D, E, F, and G rings. The D and E rings are very faint and diffuse, barely visible above the sky background. The D ring is really just an inner extension of the C ring, ranging from its inner edge all the way down to the cloudy surface of the planet. From a spaceship orbiting close to the cloud deck, you would probably be able just to make it out as a tenuous sheet of ghostly light, extending up from the equator towards the brilliant rings above you. |
The E ring, on the other hand, is a very different affair. Its faint light was first detected from the Earth, but it was the Voyager images that allowed it to be mapped and named. It lies outside the main system of rings, with its inner diffuse edge about 25,000 miles above the outer edge of the A ring, and it extends 180,000 miles farther up above the planet. Remarkably, this brings it out amongst the inner main moons of Saturn. The moons Mimas, Enceladus, Tethys and Dione all plow right through the E ring as they spin around the planet. When it was noticed that the E ring is brightest right near the orbit of Enceladus, astronomers guessed that it might be derived from that peculiar moon as debris ejected somehow from its surface. Voyager images of Enceladus showed it to be much brighter than the other moons near it, suggesting that it must have a fresh icy surface. Perhaps it suffers volcanic-like eruptions, triggered by tides as in the case of Jupiter's Io, and these eruptions eject pieces of ice into space, where they go into orbit around Saturn, forming the E ring. Enceladus may be the Solar System's champion snowball thrower! |
The F and G rings are faint and narrow. They both lie outside the bright rings, but inside the orbits of the main satellites. When first seen by Voyager they presented a major mystery, as they were so narrow and crisp. Astronomers asked, "what keeps them from dispersing and becoming diffuse like the other rings?" The answer soon came when it was discovered that these rings act like sheep. Two small moons, named Pandora and Prometheus, orbit on either side of the F ring. Called "shepherd satellites," these moons act as gravitational shepherds; when a moonlet in the ring strays outside its narrow confines, the shepherd satellite comes along and interacts gravitationally so that the moonlet goes back into the ring. The inner shepherd, Prometheus, moves faster than the ring's moonlets, so that a wanderer that moves into a smaller orbit is soon encountered by the faster Prometheus. As it approaches, its gravitational pull slows down the moonlet, causing it to fall back out into the ring. Pandora does a similar job to keep the moonlets from wandering beyond the outer edge of the ring. The fainter G ring lies even farther out and seems also to have its shepherds. |
So we have rings A through G. This is only seven rings, but this section is entitled "thousands of rings." How can that be? The answer comes from another remarkable discovery made by the Voyager spacecraft. Just as you can see stars through the rings, astronomers were able to trace the spacecraft when it passed behind the rings. They recorded the signals as it did so and discovered rapid fluctuations, indicating that the ring particles are actually divided into thousands of little rings all stacked together. These are the result of various gravitational forces, caused by Saturn's moons and the moonlets themselves. The best images of the rings from the Voyagers show the most conspicuous of the ringlets that comprise the main rings of Saturn. |
|
| |
