Saturn’s rings are sight to behold


By Tom Burns - Stargazing



Here’s a good reason for checking out Saturn, which is high in the south, right above the lid of the teapot-shaped constellation Sagittarius right now: The first view of Saturn in a telescope can be a life-changing experience. That I’m still writing this column after 30 years is a testament to my slack-jawed wonder when I saw the rings of Saturn for the first time when I was 14 years old.

The rings were as much of a revelation to me as they were to Christiaan Huygens when he saw them as a ring in 1659. Previously, Galileo had seen two lumps on either side of the planet and proclaimed that “the farthest planet is triple in form.” Later, he saw the lumps mysteriously disappear for a few days. His only explanation was a mythological reference: “Saturn has eaten his children.”

It took Huygens’ far better telescope to see that Saturn “is surrounded by a thin, flat ring, nowhere touching.“

The ring turned out to be an odd bird, to say the least. Even the visible portion of the ring is enormous. Saturn is gigantic at 75,000 miles wide. We now know that Huygens’ ring spans a quarter of a million miles. It would just fit between Earth and its moon.

Yet, it appeared to be exceedingly thin, and its tilt varies from our vantage point as Saturn orbits the sun. Huygens thus suspected that the ring would periodically appear “edge on” to observers. For a few days every 15 years, the rings are thin enough to disappear completely. On those rare nights, there’s nothing in the world quite like hosting a bunch of cub scouts who have come to see the glory of Saturn’s rings. “Jip!” one of them yelled.

Huygens explained Galileo’s lumps and their disappearance, but he raised a far greater conundrum. Why was Saturn apparently the only planet to have a ring girdling it? What in heaven’s name was it? How did it get there?

The mystery compounded when in 1675, Giovanni Cassini pointed his much-improved telescope at Saturn and saw that the ring was, in fact, rings.

These days, it doesn’t take much of an amateur telescope to see the major gap in the ring, which is called Cassini’s Division in honor of its discoverer. In the time since, using larger and better telescopes and even a planetary orbiter, also named after Cassini, astronomers have discovered that the ring is really composed of more than 100,000 or more ringlets. Even Cassini’s Division is not really a gap at all. Within it are several faint subdivisions, including a ringlet named after the great Huygens himself.

As the number of rings increased, so did astronomers’ curiosity about their composition. In 1659, James Clerk Maxwell proved that the rings could not be solid. They had to be rotating around Saturn. Otherwise, the planet’s enormous gravity would haul them in. But rotating solid rings would simply tear themselves apart.

According to Maxwell, they must consist of innumerable, small particles, all of which are in independent orbit around the planet.

Today, astronomers estimate that Saturn has around a billion trillion (that’s a one with 21 zeros after it, Earth people) mostly tiny moons circling it. We now know that if those ring particles were lumped together, they would form a Saturnian moon only 30 or 40 miles wide. That tiny amount of material is spread pretty thinly. In some places, the rings are only a few hundred yards thick.

How can such a whole lot of nothing shine so brightly? The most casual of observers can see that they are brighter than the surface of Saturn. Only ice explains such brightness, and most astronomers agree that the rings are composed mostly of plain old water ice.

Astronomers still argue about the origin of the rings. Were they formed with the planet 4.5 billion years ago? Were they the result of early impacts between Saturnian moons? Are they rings of debris that never formed into moons? Are they the result of a moon or moons that came to close to the planet and were torn apart by Saturn’s gravity? Or are they a much more recent collision between a passing asteroid and one of Saturn’s moons?

Despite all the evidence collected from the Cassini spacecraft orbiting Saturn, the battle rages on. Two theories dominate the discussion. Some astronomers argue that the rings are very old, perhaps as old as Saturn itself. They consist of orbiting rings of debris that would normally have formed into a moon (or moons). They were prevented from doing so by Saturn’s enormous gravity.

That supposition has a serious problem. The rings are icy and very bright. Over time, the infall of tiny micrometeorites, meteoric dust really, should have darkened the ring particles.

In fact, the icy brightness of the rings is cited as evidence for the young-ring hypothesis. As early as the 19th century, Édouard Roche argued that the rings might have been formed when one of Saturn’s moons spiraled toward Saturn and was ripped apart by Saturn’s gravity.

More recently, R. M. Canup has refined Roche’s conjecture. As one of Saturn’s moon was drawn closer to the planet by Saturn’s considerable gravity, its icy outer layer was stripped off, which then distributed itself around the planet as ring particles.

In another young-rings scenario, the rings are the result of a massive collision between a passing comet or asteroid and one of Saturn’s icier moons. The impact would have shattered the moon liberating the bright, uncontaminated ice underneath.

Such “disrupted moon” ideas clearly explain the predominance of bright, young, icy particles in the rings. However, proponents of the old-ring theory respond that the brightness of the rings could have been caused by other factors.

Some proponents of the old-rings theory suggest that the rings are being protected from contamination by the big, middle ring, which is just massive enough to absorb the micrometeorites without darkening very significantly.

Others argue a more complex scenario. If the material in the rings is dense enough in spots, that material might start to form into moon-like clumps, which would protect the inner material from contamination by dark micrometeorites. Subsequent impacts by other large ring material might break up the clumps and redistribute their shiny interior pieces back into the rings.

Saturn will probably always have nearly invisible, dusty rings as micrometeorites bombard its icy moons and release a thin cloud of icy, orbiting debris. But the bright rings we see today may be a passing phase in Saturn’s long life as Saturn’s gravity draws them inexorably inward toward absorption by the planet.

Depending on their origin, they could be very old and permanent fixtures. Or they could be very young, a mote in Saturn’s eye that will eventually be washed away as Saturn’s gravity draws the larger particles in.

Over the centuries, the battle has raged, and astronomers are still uncertain. When you are staring at the rings, you are staring in the face of one of science’s greatest mysteries, and one of the solar system’s most beautiful oddities. Go find yourself a telescope and check it out.

Check out Saturn at one of the upcoming Friday night programs at Perkins Observatory. For more information, call 740-363-1257.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.