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
Prometheus
sufficient mass to perturb the surrounding material and create “fan”
“channels”
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.
Additional research has shown that there are checks and balances that
determine the lifetimes of the objects formed in the F ring by Prometheus.
F ring core
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
sheared “channels”
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.
“fan”
Janus
F ring core
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.
Atlas Prometheus Pandora
Epimetheus
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.