Some interesting news broke last week from NASA's Cassini spacecraft, which has been orbiting Saturn since, oh, 2004 or thereabouts, checking out the planet's mysterious rings. The infamous "F" ring, discovered in 1979 by Pioneer 11, is one of the most unpredictable in its appearance: it's not a smooth thin band of icy particles, but has lots of kinks in it, and is flanked by two small "shepherding" moons, Prometheus and Pandora (no relation to the Na'vi).
Here's some handy background information (adapted from an earlier blog post from 2008). Scientists have been fascinated by Saturn's rings ever since Galileo Galilei trained a rudimentary telescope on the planet in 1610 and noticed their blurry outlines. The instrument's resolution wasn't quite sufficient for Galileo to make out what those blurry bits actually were, but by 1655, the technology had improved to the point where Christian Huygens could accurately describe them as a disk surrounding the planet. Ever-better telescopes provided ever-better views of Saturn's rings in the ensuing centuries, but eventually we had to go see for ourselves. Voyagers 1 and 2 sent back loads of images in 1980 and 1981, and as is often the case in science, the images raised as many questions as they answered.
Sure, the rings are pretty and all. But there's some really fascinating physics going on, too. Here's what we do know: The rings are made of lots of small particles that range in size from microns (very tiny) to meters, with the bulk falling in the range of the size of seashells to the size of surfboards, as Burns colorfully described them. Those particles have clumped together as they orbit Saturn, "innumerable small objects orbiting a dominant central mass." They're made of water ice, mostly, with bits of dust and other chemicals creeping in from time to time.
What don't we know? Well, for one thing, scientists aren't sure how old the rings are; Saturn's rings guard that secret as zealously as any aging star in Hollywood. The rings could be a mere 100 million years old, given the relative brightness and purity of the "water ice" that make up the ring particles. Incoming meteoric dust should have darkened the rings substantially by now. They might be ancient, dating back to when the planet first formed. Maybe the particles are the remnants of an old moon broken apart by a collision. Those pieces in turn collided with each other, losing energy in the collisions while still conserving angular momentum, with the end result that the systems flattened out into the thin disks we now see around Saturn.
The most likely circumstances for such an initial "trigger collision" (an old moon with another celestial object) existed during a period known as the Late Heavy Bombardment (would Wikipedia lie to me?) some four billion years ago. But even so, if collisional physics were the sole mechanism for the ring formation, there would be more diffusion, i.e., the edges would be blurry. They're not: they're sharply delineated, although they do show evidence of "sinusoidal scalloping" along the edges: waves that kind of "flow" along the rim. Also, there are areas where the ring's texture becomes ropy in places and strawlike elsewhere, indicating that the simple angular momentum model is insufficient to explain all the rings' features.
There are three main rings, rather unimaginatively dubbed A, B and C, by far the densest of the rings with larger particles. There are also several fainter rings (Dusty Rings) that have been discovered in recent years. The D ring is faintest, and closest to the planet itself. Just outside the A ring, one finds the narrow F ring, and beyond that are two very faint rings known as G and E.
While we tend to think of Saturn's disk as a series of little ringlets, there are very few true "gaps" in the disk. The best known gap is the Cassini Division that separates rings A and B, and has its own Huygens Gap near its inner edge; the inner C Ring contains the Colombo Gap, while the outer C Ring contains the Maxwell Gap; and within the A Ring we have the Encke and Keeler Gaps. Those gaps aren't always empty, either: both the Encke Gap and the narrower Keeler Gap contain embedded moons, for instance. In 2006, Cassini images revealed evidence of four tiny "moonlets" in the A ring, roughly 100 meters in diameter, too small to be directly observed. But they produce propeller-shaped disturbances that span several kilometers. Scientists now believe the A ring could contain thousands of similar objects; 250 "propeller" moonlets have been detected to date.
Why do things like these gaps occur? Perhaps the spreading outward is interrupted by destabilizing orbital resonances -- essentially gravitational relationships -- that arise when the local orbital period of a particle corresponds in a simple ratio to the orbital period of a nearby satellite (a moon or moonlet). For instance, the G ring is basically a faint, rubble-filled arc that orbits Saturn seven times for every six orbits of Mimas, one of Saturn's moons. So that would be, say, a 7:6 ratio orbital resonance. At these resonant locations, a moon's gravity can stir up a ring's mass distribution, giving rise to spiral density or bending waves, which transfer angular momentum between the moons and the rings and ultimately drive them apart." Such a process (a kind of shepherding mechanism) would serve to "truncate rings and pry open gaps." Should Cassini identify changes in any moon's motion, this hypothesis could be verified.
Scientists are interested in studying all these features of Saturn's rings in greater detail because the processes at work are analogous to other systems, including protoplanetary disks (the nurseries where nascent planets are born), our own asteroid belt, the formation of spiral galaxies (such as the whirlpool galaxy), and even the accretion disks around black holes. It's not the exact same processes at work, but there are striking similarities, so the expectation is that studying Saturn's rings can teach us about the mysterious underlying physics of some of these other celestial objects, as well as the origins of our own solar system. How do hordes of orbiting bodies crowd together? How does material accumulate in such systems to form moons or planets? And how do nearby masses disturb -- and are in turn perturbed by -- adjacent disks?
The latest images from Cassini shed some light on what might be going on specifically inside the F ring, giving rise to its uniquely complex structure. The culprit: Prometheus! Apparently the little potato-shaped moon can't leave well enough alone; it's just got to shake things up now and then. Every time Prometheus passes by the F ring, all the tiny icy particles within it get agitated by the moon's gravitational pull, and start engaging in the particle equivalent of slam-dancing.
These rippling perturbations create "streamer channels," and the particles start to bunch up and clump together into much bigger "snowballs" -- and I do mean big. Some measure as much as 12 miles in diameter, which means they are large enough to have "self-gravity" -- that is, they can attract more icy particles and continue to grow.
Prometheus swings by this particular segment in Saturn's F ring roughly every 68 days, and most of the smaller snowball clumps get ripped apart with the next pass. But that's not enough time for the streamer channels to subside, and with each pass, more channels are created and complicate an already complex kinky pattern. It's like a giant churn in space.
"Every time [the snowballs] survive an encounter [with Prometheus], they can grow and become more and more stable," said Cassini imaging scientist Carl Murray of Queen Mary, University of London. While it's likely that even the largest snowballs have a lifetime of just a few months, the new data could improve our current understanding of how space debris gets transformed into denser objects.
You can see this whole process in action in the animation below. Cassini scientists believe this could be why the F ring's structure is so surprisingly complex. And it just might help explain another mysterious objects first detected by Cassini scientists back in 2004: it's called S/2004 S 6 -- being too new to warrant a more formal moniker -- measures between 3 to 6 miles in diameter, and every now and then it bumps into the F ring and produces a spray of debris. Maybe it's one of those giant surviving snowballs. Maybe it's something else entirely. We'll have to wait to see what else Cassini tells us.