Two of Saturn's moons coast along the outer edge of the main ring system. The orbits of seven small moons cluster just outside the F ring -- between the orbits of Pan and the co-orbital moons Janus and Epimetheus.
Pan (28 kilometers, or 17 miles across at its widest point) appears as a bright dot within the Encke Gap, right of center. Janus (179 kilometers, 111 miles across at its widest point) lies outside the A and F rings, below center. Credit: NASA/JPL/Space Science Institute
Sept. 23, 2008 - Saturn's rings may be more massive than previously thought, and potentially much older, according to calculations that simulate colliding particles in Saturn's rings and their erosion by meteorites. These results support the possibility that Saturn's rings formed billions of years ago, perhaps at the time when giant impacts excavated the great basins on the Moon. The findings also suggest that giant exoplanets may also commonly have rings.
Dr Larry Esposito, Principal Investigator of Cassini's UVIS instrument, will be presenting the results at the European Planetary Science Congress in Munster on Tuesday 23rd September.
"Both Cassini observations and theoretical calculations can allow the rings of Saturn to be billions of years old. 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," said Dr Esposito.
Esposito's colleagues at the University of Colorado, Glen Stewart and Stuart Robbins, have computed the gravitational attraction and collisions between more than 100,000 particles, representing a sample of those in Saturn's rings. They followed the orbit and history of each individual particle, and calculate the amount of starlight that would pass through the ring. These results have been compared to Cassini observations of starlight blocked by the rings, which has traditionally been used to estimate the total amount of material in the ring system. Esposito used this method in 1983 to estimate that rings of Saturn contain as much material as Saturn's small moon Mimas, which is about 250 miles across. The new simulations show Saturn's ring particles aggregate into clumps, which would lead to the previous estimate being low by a factor of 3 or more.
Calculations by Esposito and his student Joshua Elliott show that meteorites slowly grind and shatter the particles in the ring. Gradually, a layer of dust and fragments builds up and covers each particle. This layer includes both ice (from the particle) and meteoritic dust. As time passes, the ring system is more polluted and darkened by meteoritic dust.
Because the rings appear so clean and bright, it was argued that the rings of Saturn were much younger than Saturn, which is some 4.5 billion years old. It was calculated from Voyager measurements that the rings are only about 100 million years old, approximately as ancient as when dinosaurs inhabited the Earth. The new calculations show that if the rings are more massive, they appear less polluted, and thus could be proportionately older. Recycling of ring material extends their lifetime and reduces the expected darkening.
One problem with this proposal for more massive and ancient rings is that the Pioneer 11 space mission to Saturn in 1979 measured the ring mass 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. However, we now recognize 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," said Dr Esposito.
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