Saturn and its rings


Tom Burns - Stargazing



Saturn is finally visible in the evening sky again and no night out under the stars is complete without a glimpse of those fabulous rings.

Saturn’s ring system is one of those few astronomical objects that even a dime-store telescope will show pretty well, so take a look.

You can start around 11 p.m. Saturn is decently above the horizon by then. Look for the pale orange “star” close to the southeastern horizon in the constellation Ophiuchus, the Serpent Bearer.

Saturn is about 80 million miles away from the sun, around times the earth-sun distance. We can see it so well because the planet is so large. At 74,000 miles wide, it is more than nine times the diameter of the Earth.

Despite its size, the disk of the planet generally doesn’t show much detail in a telescope. The rings, however, are the finest sight in the cosmos.

We take the sheer beauty of the rings for granted, but up to a few hundred years ago, they were invisible to human eyes.

Ancient peoples noticed Saturn, of course. It was the slowest moving of the ”wandering” stars. The Greeks associated it with the King of the Titans, who ruled Olympus before Jupiter. His one great fear was that his children would overthrow him, so as they were born, he ate them!

In 1610, when Galileo first pointed a telescope at the heavens, it didn’t take him long to train his primitive instrument at Saturn, the farthest planet from the sun known at the time. But his telescope was tiny, with a light-gathering lens only 1 inch in diameter.

He couldn’t resolve the rings. Instead, he saw what he thought were two tiny balls touching the surface of the main disk. He concluded that Saturn was a triple planet.

Galileo was so afraid people would think he was crazy that he published his results as an anagram – words with their letters scrambled. He figured that if he were right, he would eventually get credit. If he were wrong, no one would be the wiser.

Two years later, he looked again at Saturn, and the companions had disappeared completely. Remembering the Greek story about Saturn, Galileo suggested that the

Titan had eaten his own children!

As the century passed, telescopes got better. In 1655, Christiaan Huygens directly observed the structure as a flat, thin ring but was so overcome by the weirdness of the experience that he also published it as an anagram.

By 1665, most astronomers had warmed to the ring theory. In 1675, Italian astronomer G.D. Cassini discovered a gap in the ring structure, a dark line all the way around that divided the ring system in two. The number of rings increased slowly as telescopes got better.

In 1787, Pierre-Simon Laplace proposed that the rings were made up of a many solid ringlets. They would have to be spinning rapidly around Saturn or Saturn’s gravity would make them collapse into the planet.

However, in 1859, James Clerk Maxwell proved that solid rings in rapid rotation would simply tear themselves apart. Instead, he suggested that the rings had to be made up of an enormous number small particles, tiny “moons,” all of which were independently orbiting Saturn. In 1895, spectroscopic analysis of the rings proved that Maxwell was right.

We now know that the rings are about 235,000 miles wide around the planet. Saturn and its ring system would just fit between the Earth and its moon.

Yet at their widest, they are no more than half a thick, and in some places they are no thicker than 10 yards or so. They are far thinner than a sheet of paper if you scaled it up to ring size.

They are made of mostly tiny particles of water ice with a trace of rocky material mixed in. In other words, they are like the dirty hunks of ice you might find frozen to your car’s wheel wells.

Only our distance from them makes them look solid in a telescope. Even the Cassini spacecraft in orbit around Saturn cannot resolve the rings into particles. However, when the Voyager I spacecraft did its flyby of Saturn in the early 1980’s, it passed through one of the outer dusty rings. I will never forget hearing the sizzling sound of innumerable tiny particles hitting the hull of the spacecraft as it zipped past Saturn.

You would think that four encounters with robotic spacecraft would have told us more about the origins of the rings. Several theories have been proposed.

The rings show signs of being very old. Perhaps they are moons that never formed because of Saturn’s gravity. Or perhaps they are the result of a moon that was torn apart by gravitational forces early in Saturn’s history.

The rings also show signs of being very young. Saturn’s dustier rings are constantly fed by micrometeoroids striking Saturn’s larger moons. The bright, visible rings might have been formed as recently as a hundred million years ago by the collision of a passing asteroid with one of Saturn’s smaller moons.

But what of Saturn eating his children? It turns out that from our earthly point of view, the rings slowly tilt over a 29-year cycle, one trip by Saturn around the sun.

For two seasons of the 29, the rings are tilted so that we see them edge-on, and they appear to our modern telescopes as a thin line.

No worries, however. Saturn looks great in the Perkins telescope at our public programs on Friday nights this season. The rings are wide open, and all is right with the world.

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Tom Burns

Stargazing

Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.