Saturn’s rings older

and bigger
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 24, 2008

New calculations presented at the European Planetary Science Congress (EPSC) this week suggest that Saturn's rings may be more massive and much older than previously thought.

Speaking at EPSC yesterday, Dr Larry Esposito, Principal Investigator of Cassini's UVIS instrument, describes calculations that simulate colliding particles in Saturn's rings and their subsequent erosion by meteorite impact. The results suggest that the giant planet’s rings could have formed billions of years ago, compared with previous measurements from the Voyager spacecraft that put their age at just 100 million years old.

"Both Cassini observations and theoretical calculations can allow the rings of Saturn to be billions of years old,” says Esposito. “This means we humans are not just lucky to see rings around Saturn. This would lead us to expect massive rings also to surround giant planets circling other stars.”

Artist concept of Saturn's clumpy ring particles. Image: NASA/JPL/University of Colorado.

Esposito and his research team computed the gravitational attraction and collisions between more than 100,000 particles, representing a sample of those in Saturn's rings, and tracked the orbit and history of each individual particle. They also calculated the amount of starlight that would pass through the ring to compare to Cassini observations of starlight blocked by the rings, a method traditionally used to estimate the total amount of material in the ring system. Esposito had previously used this technique to estimate that the rings of Saturn contain as much material as Saturn's small moon Mimas, which is about 400 kilometres across. But the revised simulations show Saturn's ring particles aggregate into clumps, which would lead to the previous estimate being low by a factor of three or more.

“Earlier, it was assumed the rings are uniform,” says Esposito. “For clumpy rings, most starlight passes between the clumps, thus the same observation gives a higher mass.”

The simulations show that over time, meteorite impact with the ring particles slowly grind and shatter the particles, building up a layer of dust, ice and fragments that covers each ‘clump’. As time passes, the ring system becomes more polluted and darkened by meteoritic dust.

Because Saturn’s rings appear so clean and bright, it was argued that they were much younger than the 4.6 billion year old planet. But the new calculations show that if the rings are more massive, they appear less polluted, and thus could be proportionately older. In effect, the recycling of ring material extends their lifetime and reduces the expected darkening.

But there’s one problem. Observations by Pioneer 11 in 1979 agree with the results from Voyager. Pioneer 11 measured the mass of Saturn’s ring indirectly by observing charged particles created by cosmic rays bombarding the rings. "Those mass estimates were similar to the ones from Voyager star occultations, apparently confirming the previous low mass value,” describes Exposito. “However, we now recognise that the charged particles are double-valued. That means they could arise from either a small or large mass. We now see that the larger mass value could be consistent with the underestimates due to ring clumpiness.

The team proposes that the same processes act on all ring systems and that Saturn's rings are largest of the Solar System’s planets because they started out with more mass to begin with, likely derived from large asteroids or satellites that strayed too close and were broken up by the giant planet’s strong gravitational field.