Saturn's rings may not be as young as they look

Planet-hugging circles of particles might be more massive, and thus much older, than previously thought

6:10pm, September 22, 2008
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There’s something poetic about the notion that Saturn’s shimmering, icy rings are ephemeral, lasting for a mere astronomical blink — only 10 million to 100 million years.

But new simulations suggest that the rings are much heavier and have adorned the planet for much longer — nearly 4 billion years — than previously thought. They might also be a common feature among giant planets beyond the solar system.

Larry Esposito of the University of Colorado at Boulder and Joshua Elliott, now at ITT Visual Information Solutions in Boulder, base their findings partly on recent simulations of ring particles done by other colleagues. Esposito and Elliot combine this model with observations of the rings by the Cassini spacecraft, which has toured the Saturnian system since 2004. Esposito is set to present the results September 23 in Muenster, Germany, during the annual European Planetary Science Congress.

Two University of Colorado colleagues, Glen Stewart and Stuart Robbins, recently computed the gravitational attraction and collisions between some 100,000 particles representing a sample of those in Saturn’s rings. The simulation enabled them to track the history and orbit of each particle and calculate the amount of starlight that the ring particles, collectively, would block. The researchers then compared their results to actual Cassini observations of how much starlight the rings block, which scientists routinely use to estimate the amount of mass in the rings. The comparison, says Esposito, indicates that he and other researchers had previously underestimated the rings’ mass by about one-third.

The simulations show that the particles in the rings clump, rather than being uniformly distributed. When starlight easily passes through tenuous parts of the ring, the measurements can fool researchers into thinking that the rings contain much less mass than they actually do, Esposito says. And more mass could translate into rings that have endured for much longer than he and other researchers had estimated, Esposito adds.

A larger mass indicates that a fundamental competition between two processes — clumping due to the gravity of the particles and collisions by micrometeorites that shatter and disperse these clumps — could continue for several billions of years before the rings would dissipate, Esposito says.

In their own simulations, he and Elliot showed that collisions with micrometeorites both fragment the ring particles and coat them with dust. Over time, the icy particles become polluted with the dust and darken. In fact, the freshness of the ring particles, which appear to be clean and bright despite the collisions, has been a key piece of evidence indicating their youth.

But if the rings are more massive and clumpy, micrometeorite-darkened material that was originally on the surface of the ice particles could be incorporated into the interior of the clumps after they shatter and recombine, Esposito and Elliott suggest. With the darkened material hidden inside the clumps, the rings could still appear fresh and young despite their age, Esposito says.

He emphasizes that the scenario doesn’t require that the particles are billions of years old, just that they could be. One possibility is that the rings formed some 3.8 billions years ago when collisions between giant impactors in the solar system were more common. That’s the era when space debris gouged basins on Earth’s moon and when a stray projectile could have smashed an icy moon of Saturn into fragments that created the rings.

The work by Esposito and Elliott “does point to a significant uncertainty in the mass of the Saturnian rings,” says Mark Lewis of Trinity University in San Antonio, who wrote the simulation code used by Stewart and Robbins but did not participate in the research. “Since the mass is so significant in determining the age, that uncertainty has far-reaching consequences.”

But Lewis notes researchers must gather much more information on the composition and nature or ring particles in order to understand exactly how they clump, which is a central part of Esposito’s argument. “It isn't as straightforward as saying that high-density particles would lead to more clumping,” Lewis cautions.

The origin and age of Saturn’s rings has been a riddle for decades, notes Jeff Cuzzi of NASAAmesResearchCenter in Mountain View, Calif. He notes that the gravitational interactions between particles in Saturn’s A ring and adjacent moons would transfer momentum from the rings to the moons, pushing the moons outward and slowing down the ring particles. If the rings were really as old as Esposito suggests, then the moons would be much farther away than they are and the A ring would have fallen into the B ring, he says.

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