Freaky F Ring
Saturn’s F ring exists on the very boundary separating the realm of rings and moons. Planetary rings exist because the ring particles are so close to the planet they orbit that they are unable to gravitationally stick together to grow into a moon. Being close to the planet means that there is a strong tidal force from the planet that tends to pull accreting particles apart. One way to look at this is to remember that all ring particles orbit their host planet on a distinct orbit, and the speed they travel on that orbit varies with their distance from the planet. Closer particles orbit slightly faster than more distant particles. Two ring particles that bump into each other (and they do so ever so gently, at a fraction of a centimeter per second) are on slightly different orbits around their planet, with the one closer to the planet moving slightly faster. This difference in orbital speed induced by the planet’s gravity is enough to overcome the gravitational attraction between the two ring particles when the particles are close to the planet. The region where this tidal inhibition to accretion is effective is called the Roche zone after French mathematician Eduard Roche, who first formulated this problem. Imagine running around a race track with a friend in two different lanes while holding hands. The strength of your handhold represents the gravity pulling two ring particles together. But you are obligated to run at a certain speed, and if you are on the inner lane, you will gradually pull away from your friend and break that bond. If you are on a giant racetrack (that is, farther from the planet) then the difference between your lane and your friend’s lane might be so small that your handhold can overcome the tendency of one of you to move ahead of the other.
Saturn’s F ring is right at the hairy edge of the Roche zone, about two and a half times the radius of Saturn from the center of the planet. Here, rings no longer have a nice uniform circular appearance, and moons and rings co-exist, though you might say they don’t co-exist very peacefully judging from pictures like the one below.
Image credit: NASA/JPL/Space Science Institute
Rings look circular precisely because of this different lap-speed phenomenon mentioned above. Particles a little closer will eventually lap their more distant neighbors, and a clump of particles like that seen in this image will quickly (years) smear out into a nice uniform ring like we’re used to seeing. The F ring is filled with strange structures due to repeated gravitational stirring from nearby moons Prometheus and Pandora. Originally dubbed “shepherd satellites” for their role in confining the F ring, a new nickname like “cheerleading satellites” might be more appropriate since their most dramatic effect on the F ring is the creation of waves as they lap the ring, just like cheerleaders excite a wave in a stadium (okay, maybe not just like cheerleaders). This cheerleading can cause some of the crowd of particles in the F ring to bump into each other a little more vigorously, producing new clumps of particles. Some clumps in the F ring may manage to stick together longer and form temporary moonlets because they are not so deep in Saturn’s Roche zone. These new moonlets also stir up the F ring producing strange ephemeral structures like the ones seen below.
Image credit: NASA/JPL/Space Science Institute