Living at the Edge of Saturn’s Rings F ring The F ring is a dusty ringlet located just beyond the main ring system at a distance of 140,200km from the center of Saturn. At this dynamically peculiar location, the Roche zone, the tendency for ring particles to merge due to gravity is balanced by the tidal effects of the planet acting to tear them apart. The F ring has a complicated, continually changing structure dominated by a bright, ~50km-wide core; the adjacent strands have a spiral structure and are thought to the icy debris from small objects which have impacted the ring. The gravity of the small moon Prometheus (86km wide) produces “channels” across the F ring when it passes and these shear over time. However, new results from the Cassini spacecraft have shown that Prometheus does more than just create pretty patterns and that it has an interesting dynamical history. The F ring itself may be the “signature” of a process by which new moons are formed regularly at the outer edge of Saturn’s ring system. Cassini scientists have shown that Prometheus triggers the gravitational collapse of ring particles in the F ring core. The resulting objects have sufficient mass to perturb the surrounding material and create “fan” structures which reveal their presence. Similar processes on a much larger scale are thought to have operated in the early history of the solar system as planets formed out of clouds of dust and gas. Prometheus “channels” F ring core Additional research has shown that there are checks and balances that determine the lifetimes of the objects formed in the F ring by Prometheus. As more objects form there is an increase in the number and speed of collisions. Over time this ultimately acts to decrease the number of objects, but then they start to grow again. The whole process can be studied using “predator-prey” models from ecology systems theory. Prometheus is just one of many small, icy moons with elongated shapes on the outskirts of Saturn’s rings. Recent work has shown that such moons must have accreted in the outer part of Saturn’s main rings (perhaps agitated by predator-prey cycles) and evolved outwards due to their gravitational interaction with the rings within the last 10 million years, creating the F ring on their path outwards. In many ways Saturn’s rings behave like a miniature protoplanetary disk. sheared “channels” F ring core Janus “fan” Atlas Prometheus Pandora Epimetheus Two images of a 40,000km-long section of Saturn’s F ring taken by Cassini’s cameras on June 1st, 2010. The small moon Prometheus can be seen in the upper image along with several channel structures it has created in the ring. In the lower image, taken 50 minutes later, Prometheus has moved out of the field of view but a “fan” structure in the rings indicates the presence of an object in the ring’s core. 20km A selection of the small, icy, irregularly-shaped moons orbiting just outside the main ring system. Their odd shapes are due to the strong tides of Saturn.