New theory seeks to explain Saturn's unique and constantly changing rings

A new theory tries to explain the mystery of Saturn's rings.
A new theory tries to explain the mystery of Saturn's rings. (Nasa)
By Ben Harder
Tuesday, January 11, 2011; 12:30 AM

When Larry Esposito says, "Rings are forever," he's not talking about ones made with diamonds. He's referring to rings composed of ice crystals and dust - rings that encircle worlds, not fingers. And he thinks those rings surrounding Saturn and other planets have been around as long as the solar system itself.

As constant as planetary rings may be, however, they're also constantly changing. So when Esposito, an astronomer, adds, "All rings are transient," he's not contradicting himself.

Esposito has witnessed Saturn's rings change literally before his eyes. His career has spanned two major space-probe missions to the planet, the Voyager mission in the 1980s and the Cassini mission, which is ongoing. Up close, the rings look strikingly different today than they did 25 years earlier. Like clouds on a windy day, shapes that Voyager's cameras discerned in the drifting chunks of material have dispersed, replaced by other forms.

In planetary time, a quarter-century is a mere instant. So it's difficult even for pros such as Esposito, who works at the Laboratory for Atmospheric and Space Physics in Boulder, Colo., to fathom how much the rings must have evolved since the solar system was born 4.5 billion years ago.

Scientists have been theorizing about how and when planets got their rings since shortly after 1610, when the astronomer Galileo peered up at Saturn through the just-invented telescope and spied a bright white arc around the planet. Experts have since developed two basic theories: Some rings may have formed in the early days of the solar system, from material left over when the planets and their moons took shape. Others developed later, from debris generated by destructive events such as collisions between moons and meteorites. Gravity and momentum then sculpted this debris into rings.

From the moment it forms, however, a ring is doomed. In addition to spreading out along its circular orbital path, the ring's material also spreads in every other direction: toward the planet, away from the planet, etc. Some of the material may clump together into small moons. Over time, the rest of the ring will thin out until it vanishes.

Unless, that is, the ring has guardians. Some rings are sandwiched between moons whose orbits lie along the inner and outer edges of a ring's path, as if bookending the swath of space the ring occupies. The gravity of those moons, called shepherd moons, helps keep the ring in line. The presence of shepherd moons is thought to significantly slow a ring's gradual but inexorable dissipation. And if meteorites periodically knock material off the moons and into the rings, the rings may last forever.

In the four centuries since Galileo's discovery, astronomers have determined that Saturn is encircled not by a single band but by more than a dozen distinct, concentric rings and gaps, with many of the rings bracketed by shepherd moons. Each ring is made up of a unique mix of particles of varying sizes and compositions, yet the rings share a noteworthy commonality: they're more than 90 to 95 percent pure ice. That sets Saturn's rings apart from other planetary rings.

Rings encircle at least three other planets: Jupiter, Uranus and Neptune. But while those rings contain proportionally more rocky material than Saturn's, they are comparative lightweights, perhaps one one-millionth the mass of the Saturnian rings. If you could pull together all of the material in Saturn's rings, you'd have enough to create an icy moon more than a hundred miles in diameter; the other planets' halos may contain less mass than a modest mountain. Earth's moon, by comparison, is more than 2,000 miles in diameter.

It's relatively easy for scientists to envision how, say, Jupiter's small, rocky rings could have been created by meteorites crashing into the planet's moons. But it's much tougher for them to explain the origins of Saturn's vast and much icier rings. According to a new theory, the unique attributes of Saturn's rings may be evidence of an awesomely violent birth.

In a paper last month in the journal Nature, astrophysicist Robin Canup of the Southwest Research Institute suggested that Saturn's rings are vestiges of a moon - larger than Earth's moon - that got overwhelmed by Saturn's gravitational pull during the planet's formation. Her computer simulations suggest that such a moon, made of frozen water surrounding a rocky core, would have shed the water as its core fell into the planet. Its remains would have formed a massive ring around Saturn, only parts of which survive. The rest of the icy material either coalesced into some of the smaller moons that now orbit Saturn or drifted off into space.

Whether Canup is right or not, other ringed planets may be out there, waiting to be found. Astronomers looking through a much, much more powerful telescope than Galileo's recently spotted what would be the first ringed planet discovered in another solar system. The researchers think starlight reflected off the rings enabled them to pick out a planet that by itself would have been far too small to see, and they concluded that its rings must be vast in comparison with Saturn's. In fact, they may resemble what Saturn's rings looked like in the early days of the solar system, just after Canup's vanished moon plunged to its death.

Harder is general manager of health and science at U.S. News & World Report.

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