Rings of Saturn remain great mystery


By Tom Burns - Stargazing



Here’s a good reason for checking out Saturn, which is getting low in the southwest just after dark right now. The first view of Saturn in a telescope can be a life-changing experience.

That I’m still writing this weekly column after 34 years of keyboarding toil is a testament to my slack-jawed wonder when I saw the rings of Saturn for the first time at the tender age of 12.

Over the decades that I hosted school programs at Perkins Observatory, I showed thousands of elementary school kids the rings. Their reactions were invariable: the quick intake of breath and the slow exhalation. “Oh. Wowwww.” That “Oh wow” was the air I breathed and the food that sustained me over long, arduous days and many sleepless nights.

The rings were as much of a revelation to me as they were to Christiaan Huygens when he saw them as a single ring in 1659. Previously, Galileo saw two lumps on either side of the planet and proclaimed that “the farthest planet is triple in form.” Huygens’ more advanced telescope showed that Saturn “is surrounded by a thin, flat ring, nowhere touching.”

For centuries, astronomers could find no other planet so endowed.

The visible portion of that ring is enormous. Saturn is gigantic at 75,000 miles wide. We now know that Huygens’ ring spans 220,000 miles or so. It would just fit between Earth and its moon.

Yet, it appeared exceedingly thin. Its tilt varies from our vantage point as Saturn orbits the sun. Huygens knew that the ring would eventually appear “edge-on” to observers a few years later. When astronomers observed it then, the rings were thin enough to have disappeared entirely for a few days.

Huygens had explained Galileo’s lumps but had raised three more confounding puzzles. Why was Saturn the only planet with a ring encircling it? Of what was it composed? 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 most obvious gap in the ring, which is called Cassini’s Division in the astronomer’s honor. Since then, using technologically superior telescopes and even a planetary orbiter aptly named Cassini, astronomers have discovered that the ring is composed of over 100,000 ringlets.

Even Cassini’s Division is not a real 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 astronomer’s 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. However, spinning, solid rings would tear themselves apart.

According to Maxwell, they must be innumerable, mostly tiny particles, all in independent orbits around the planet.

Such assertions are difficult to prove. To this day, neither telescopes nor orbiters can resolve the rings into their component particles. It took over 200 years to demonstrate, albeit indirectly, that Maxwell was correct.

In effect, Saturn has as many as a billion trillion moons in orbit around it. Most of them are the size of dust or sand. A relative few are the size of mountains on Earth. The bright rings that we can see in amateur telescopes extend from a scant 4,300 miles to about 50,000 miles from Saturn’s surface.

Astronomers estimate that if all the ring particles were lumped together, they would form a Saturnian moon only 30-40 miles wide. Given the ring’s system’s enormous span, they must be exceedingly thin. In some places, they are only a few hundred yards thick.

And yet they shine so brightly that even a casual observer will note that they are brighter than the surface of Saturn. Only ices of various varieties — water ice mainly — explain such brightness.

Astronomers still argue about the origin of the rings. Ring theorists divide roughly into two camps: early ringers, who say that the rings were present soon after Saturn’s formation, and late ringers, who believe that the rings formed within the last few hundred million years.

The starting place for the current controversy is the work of nineteenth-century astronomer Edouard Roche.

He proposed that the rings were once a moon of Saturn that spiraled into the planet. When the moon, which Roche dubbed Veritas, came within 2.5 Saturnian radii, a distance that has come to be called the Roche Limit, it was ripped apart by Saturn’s considerable gravity.

Some early ringers speculate that the rings consist of particles that did not coalesce into Saturnian moons. Its proponents include astronomers Robin Canup and Sebastien Charnoz. They argue that the rings were present soon after Saturn’s formation.

The ring’s rocky materials were massive enough to attract more material to themselves, and in that way, some moons of Saturn formed.

Subsequently, gravitational interactions with Saturn’s other moons expelled them from the rings. Thus, the rings we now see are nothing but icy leftovers.

Early-ring theorists also argue that an early moon of Saturn was struck by a sizeable asteroid-like object early in Saturn’s life. Such an event could have happened at any time in Saturn’s history, of course. However, such impacts were most likely when numerous objects were crashing into planets and their moons.

That period, dubbed the Late Bombardment Period, happened about four billion years ago. We see evidence for the LBP in the many old craters on Earth’s moon.

Current evidence leans toward a late formation, perhaps because Saturn’s gravity stripped one of the moons of its icy outer covering as the moon spiraled into Saturn. Alternatively, the ring-creating object could have been an asteroid or comet that came too close to Saturn’s sway.

The late-ringers have a powerful argument for their theory. The rings’ icy particles appear to be bright and pure. If the rings had formed early, the ice should have darkened as they were battered by rocky micrometeorites and the sun’s radiation, both of which tend to darken ice over time.

Saturn will always have nearly invisible rings as micrometeorites bombard its outer moons and release a thin cloud of dusty, orbiting debris. Such is the case with the Phoebe Ring, which was not discovered until 2009 by NASA’s infrared Spitzer Space Telescope. The ring is a tenuous mixture of dust-sized particles of mostly icy materials.

The ring continues to form just inside the orbit of Phoebe, one of Saturn’s most distant moons. As meteoroids, large and small, batter Phoebe’s already battered surface, the cloud of dusty debris spreads itself into a tenuous ring. As long as Phoebe exists, that ring will survive.

However, if the late-ringer theorists are correct, the bright rings we see today may be a passing phase in Saturn’s long life.

The possibility still exists that they could be ancient and permanent fixtures billions of years old. More likely, they could be very young — a mote in Saturn’s eye that Saturn’s gravity will eventually wash away as the planet draws the larger particles inward.

The ring particles slowly spiral into Saturn because of their relative closeness to a massive and gravitationally powerful planet. Estimates of the rate of absorption by Saturn vary, but the message is clear.

Find a telescope and point it at Saturn and its rings. In 100 million years, give or take, it will be too late to see them.

If that sounds like a long time to you, consider Saturn’s age. Like all the solar-system planets, it formed about 4.5 billion years ago. One hundred million years would be the equivalent of about a year and a half in a 70-year human life.

In that relatively brief time, all that will be left is a thin, nearly invisible ring-shaped cloud of dirty, icy dust, which meteoric impacts will continuously replenish from one or more of Saturn’s outer moons.

Over the centuries, the ring battle has raged, and still, astronomers are uncertain.

During my tenure at Perkins, many of my temporary students were frustrated by the lack of certainty on this and many other astronomical questions.

I would always say something like this:

When you are staring at the rings, you are staring at one of astronomy’s greatest mysteries. Revel in that uncertainty. Perhaps you or someone from your generation will be the one who discovers the definitive answer. Humanity has yet to crown the Lord of the Rings.

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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.