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.
The Paparazzi Effect
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.
Occasionally a clump may persist and continue growing, leading to the creation of a moonlet. Such a moonlet can be the seed for further growth or it may be destroyed by something colliding with it from outside of Saturn’s rings.
“We may just be catching the rings at a particular moment in time,” Esposito notes. “The rings may never achieve equilibrium between accretion and fragmentation and go through a perpetual boom-bust cycle.”
Ring Particle Rotisserie
A closer portrait of individual ring particles is painted by computer modeling and measurements made by Cassini’s composite infrared spectrometer. Researcher Ryuji Morishima, of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., and his team found sizes, spin rates, and the quantity of particles that matched the temperature measurements in four zones of Saturn’s extensive ring system (the A, B and C rings and the Cassini division between the A and B rings).
The model developed by Morishima and colleagues replicates the way individual particles respond to changes in temperature that occur with shifting angles of sunlight. Cassini has orbited Saturn long enough that it has witnessed the changing of the seasons, with direct sunlight warming the rings in the early part of the mission. The rings cooled around August 2009 when the angle of sunlight changed to hit just the edge of the rings during equinox. The model also took into account when ring particles moved in and out of Saturn’s shadow.
Scientists varied the sizes, layering, density and porosity of particles in their model. The model also took into account the way particles spin as they orbit Saturn and how well these spinning particles hold heat. Bigger particles tend to spin slower so they tend to be warm on the sun-facing side and cool on the other. Particles smaller than a meter are more likely to rotate quickly. Because they are on a fast rotisserie, temperatures tend to be more evenly distributed.
The new model, which also incorporates earlier radio science measurements, indicates there are particles that are fast rotators with greater heat retention than that of slow rotators in all of the rings. The fast rotators appear to have thinner, slicker surface layers because the centrifugal force of the particle’s rotation throws the more fluffy materials off the body. Small particles may not have a surface layer at all. Coincidentally, the same effect is observed in asteroids, though the mechanisms are probably different.
The researchers found that a large number of small particles are born in the collisions that occur when ring particles gravitate toward a passing moon. The A ring’s busy outer edge, for example, is dominated by small particles. Their temperatures indicate that many are small, fast rotators – the spinning detritus of gravity-driven collisions between bigger particles. Scientists think so many small, fast rotators appear in the A ring because of gravitational wakes from nearby moons that can enhance the spin and collision velocities of small particles. The low level of heat retention in the Cassini division is attributed to a fluffy coating of dust on its particles.
“We’re starting to understand the behavior of individual particles in response to changes in sunlight and group behavior in the rings,” said Linda Spilker, a co-author on the study and Cassini project scientist at NASA’s Jet Propulsion Laboratory. “We look forward to watching how this behavior changes through the rest of the Solstice mission.”
This Cassini Science League entry is an overview of science papers authored, or co-authored, by at least one Cassini scientist. The information above was derived from or informed by the following publications:
1) "A predator–prey model for moon-triggered clumping in Saturn’s rings," Larry W. Esposito, Nicole Albers, Bonnie K. Meinke, Miodrag Srem?evi?,Prasanna Madhusudhanan, Joshua E. Colwell, Richard G. Jerousek, Icarus 217 (2012) 103–114
2) "A multilayer model for thermal infrared emission of Saturn’s rings. III: Thermal inertia inferred from Cassini CIRS," Ryuji Morishima, Linda Spilker, Keiji Ohtsuki, Icarus 215 (2011) 107–127
-- Stephen J. Edberg, Cassini science communication coordinator