2 sets of measurements made by NASA's Cassini spacecraft in the ultraviolet and infrared ranges of radiation have provided new insights into the behavior and make-up of Saturn’s ring particles.

Researchers using Cassini’s ultraviolet imaging spectrograph have shown that the processes that form temporary clumps of particles and then destroy them are driven by the gravity of some of Saturn’s moons. The numbers of large and small clumps appear to follow what in biology is called a “predator-prey relationship” that governs, for example, the numbers of foxes and hares in an area. Researchers using Cassini’s composite infrared spectrometer and theoretical models have also characterized the sizes, speeds of rotation and the "surface densities" of particles in different zones of the rings.

For centuries, astronomers have puzzled over how Saturn’s glittering rings formed and what they are made of. The impossibility of them being hard disks, like washers, was only demonstrated in 1859. And despite what wags said during flybys by NASA’s two Voyager spacecraft in the early 1980s, the rings are not composed of lost airline luggage. These Voyager flybys provided some answers, showing the rings to be composed of icy particles, large and very small, from several micrometers, or the size of cigarette smoke particles, to tens of meters. NASA’s Cassini spacecraft, which arrived at Saturn in 2004, subsequently revealed ring particles, or at least their outer coatings, to be made of crystalline ice.

But how do ring particles end up big or small? Are these icy surfaces fluffy or slick? Two papers led by Cassini scientists fill in some of these blanks, which will help scientists understand how similar the Saturn ring system is to rings around other planets and how circling discs of debris – like that around our own sun about 4.5 billion years ago -- eventually evolve into systems of planets.

In one recent paper by Cassini researchers using ultraviolet measurements, ring particle clumping appears to be governed by the gravitational effects of some of Saturn’s icy satellites. The clumping, in fact, follows a pattern with tipping points, much like those found in predator-prey relationships in ecology.

Larry Esposito, the ultraviolet imaging spectrograph team lead based at the University of Colorado, Boulder, and colleagues used measurements of events called occultations to look at the way particles are distributed in Saturn’s rings. (An occultation occurs when a larger object passes in front of a smaller object; say Saturn’s rings and a background star.) By making measurements every few thousandths of a second, the instrument can record the boundary of the rings and their thickness as the transmission of ultraviolet light from the background star through the rings changes. Measurements made during many opportunities allow something like a medical CT scan of the rings to be constructed.

Through this method, the researchers found both opaque clumps that may be solid and others that may be temporary aggregates in Saturn’s F ring, one of the outer main rings, and the B ring, one of the main rings closer to Saturn. The positions of F ring objects changed in-step with the predicted motion of the small moon Prometheus relative to the core of the F ring, somewhat like paparazzi crowding around a celebrity. The B ring was a little more complicated. The gravitational influence of the moon Mimas and its orbital path around Saturn appeared to create streamlines in the B ring that drew chunks together about 90 degrees in front of the moon’s orbit and disrupted them a few hours later, 90 degrees behind the moon’s orbit.

Mathematical analysis of the behavior of rings under the influence of outside dense bodies matches the conclusions drawn from the occultation observations. The gravitational influence of a moon (or of a wave called a density wave that perturbs the density of particles in a disk) can drive ring particles to crowd together by pushing them out of circular orbits. The crowding, in turn, slows down the relative velocities of the particles in that area, permitting clumping. But as the clumping occurs, the way particles bump into each other can increase some particles’ velocities to the point that they escape the clump, which slowly fragments.

The pattern matches the population fluctuations of hares and foxes. A good year for hares increases their population, which in turn allows foxes to increase their population. As the increased fox population reduces the hare population, the foxes eventually don’t have enough food and their population dwindles. The hare population can then increase until the fox population grows again, and so on. This is a predator-prey or boom-bust pattern that occurs with particle clumps, driven by gravity in the case of the rings. A moon influences particles to clump, but as the clump grows it ejects more and more of its component particles until it is dispersed. The passage of the moon starts the cycle all over again.

Source: Cassinin homepage