The Universe in the Classroom

www.astrosociety.org/uitc

No. 40 - Fall 1997

© 1997, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112.

Lord of the Rings

Terence Dickinson
SkyNews

It would be the ultimate backpacking trip: an exploration of Saturn's rings. The astronauts squeeze into their space suits, strap on their rocket-powered backpacks, and load their cameras. Their spaceship brings them within a few kilometers of the rings. So long as the ship remains in a circular orbit, its speed is identical to that of the chunks of ice and rock which make up the rings. A collision with any ring material under these conditions is just a gentle nudge.

Stepping outside, the backpackers -- using brief rocket bursts -- slowly glide toward the glittering golden plain. The explorers touch down on a large boulder, and in one smooth push of the foot, propels themselves onto the next big piece. In between this slow-motion ballet toe-step, the explorers feel the blizzard of tiny particles gently bouncing off the front of their space suits.

Floating in the silvery gravel is intoxicating. The partly hidden Sun glinting off the field of ring chunks, the gentle gravitational symphony of collective motion that carries both chunks and explorers safely around the planet, the feeling of being surrounded yet freely floating -- all disguise the fact that everything is whirling around Saturn at tens of thousands of kilometers per hour.

Jewel of the Solar System
Know Your ABCs
Blandness is Skin-Deep
Activity: A Grapefruit Saturn

Jewel of the Solar System

You don't have to travel millions of kilometers to visit Saturn. It comes to you. Of all the celestial sights available to backyard telescopes, only Saturn and the Moon are sure to elicit an exclamation of delight from those who have never looked through a telescope before. And one look is seldom enough. No photograph or description can duplicate the beauty of the colossal ringed planet floating against the black velvet of the night sky.

Saturn
Dark side of the ringed planet. This is most people's favorite picture of Saturn. With the Sun off to the right, Saturn casts a shadow on its rings; its daytime hemisphere looks like a crescent. To see a planet or moon as a crescent, you need to be looking from the side -- a perspective on Saturn which Voyaer 1 achieved in 1980, but which is impossible from Earth. Photo courtesy of NASA JPL.

People have been watching Saturn for millennia, but Galileo Galilei was the first to point a telescope at the planet and see its rings. From his discovery, in 1610, until the 19th century, astronomers debated whether Saturn's rings were a solid disc or a swarm of objects. In a telescope, the rings look solid, yet Saturn's gravity should tear a solid structure apart. American astronomer James Edward Keeler resolved the dilemma in 1895. Using a spectroscope to study sunlight reflected off different parts of the rings, he found that they do not all move at the same speed, as they would if they were solid. Instead, the parts closest to Saturn are moving faster than the parts farther out. Keeler concluded that the rings must consist of individual objects revolving around Saturn just like tiny moonlets.

These objects range from tiny crystals, like those in an ice fog, to flying icebergs. Each has its own orbit about Saturn, but occasionally jostles its neighbors. The gentle collisions gradually grind down the larger particles. Meanwhile, the smallest particles tend to stick to one another and create larger clumps. These competing actions have established an equilibrium of sizes. For every house-sized boulder, there are a million baseball-sized chunks and trillions of sand-sized grains.

In the denser rings, the baseball-sized particles are separated by a meter or so; the house-sized ones are kilometers apart. In fact, most of the rings are empty space. If they could be melted and refrozen as a solid body, they would be a solid disc less than 2 feet thick.

The rings are enormous. From one edge to the other, they span a distance equivalent to two-thirds of the gulf between Earth and the Moon. Yet the ring particles seldom stray more than a few hundred meters from a perfectly flat plane, making the rings the height of a 30-story building. If the rings were the size of a football field, they would be paper-thin.

The reason why the rings are so flat -- rather than a random haze -- has to do with Saturn itself. Saturn is the least dense of the gaseous giant planets, yet a day on Saturn is only 11 hours long. This rapid rotation has bulged the planet at the equator and compressed it at the poles. As a result, there is more material at the equator, so the gravity is stronger there. A body orbiting Saturn feels a greater gravitational pull as it passes over the equator, compared with the poles. Over time, this difference distorts the orbits of the ring particles, causing them to collide and settle into a circular orbit above the equator.

Although astronomers are still trying to determine exactly where the rings came from, they think a collision, either between two of Saturn's moons or between a moon and a comet, blasted debris into orbit around the planet. The debris became the rings; it could not regroup into a moon because, near the planet, gravity rips large objects to shreds. Some scientists think the rings are less than a billion years old -- fairly young by astronomical standards -- while others think they date back to the early days of the solar system.

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