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SYFY WIRE Bad Astronomy

Take a deep dive into the rings of Saturn

By Phil Plait

[Artwork credit: NASA/JPL/Caltech (Illustration by David Seal)]

This is my first science post at Syfy Wire (yesterday’s article was just me saying “hi”), and I’ll admit I fretted over what it should be about. I searched through a pile of ideas, and while I was looking, the Cassini spacecraft — a gift rather literally from heaven — sent back images that perfectly fit what I needed. Saturn is a ridiculously beautiful bauble in the sky, and Cassini never fails to return amazing scenes.

Cassini image of Saturn's rings

Like the one pictured above. What you’re seeing is a phenomenal close-up of Saturn’s rings, one of the highest-resolution images of the rings we’ve had since Cassini entered the planet’s orbit in 2004. This particular shot shows the A ring, a fainter-than-average but broad outer ring. Cassini was 54,000 kilometers (33,000 miles) from this spot in the rings when it took the shot; for comparison, the Earth is 13,000 km across.

The rings are not a solid disk, but instead are composed of trillions of small chunks of nearly pure water ice. If that were all there was to it, each chunk would orbit the planet independently, like a bazillion little moons.

But there is more to it. Saturn has lots of actual moons, some of which are hundreds of kilometers across and orbit near, or even inside, the rings. The gravity of these passing moons disturbs the ice particles, causing them to gather in collections of rings. The bright diagonal lines on the left of the image are one example of this; those are “density waves,” a spiral-shaped pattern in the rings caused by the moon Prometheus.

Unlike most rings, which really are circular bands, that wave is a spiral that winds all the way around Saturn several times. It’s like a traffic jam in which the gravity of the moon causes particles to move in and out of the spiral like cars entering and leaving a backed up, and the jam persists even if the individual cars are in it for a short time (this is similar to the spiral arms in galaxies). Prometheus orbits Saturn outside the A ring, but its gravitational influence can literally be seen here.

But there’s more. The image at the top has been processed to remove cosmic rays, which are subatomic particles that slam into Cassini’s digital cameras, leaving annoying streaks everywhere. Cleaning the image smooths it out a bit, and fainter features can get smeared almost to invisibility. To help you see some of these fainter things, I’ve created an animation that blinks between a “raw” (unprocessed) image and a cleaned one.

Propellers animation

The cosmic rays are obvious, but note the circles: They mark the locations of faint linear features that seem to be aligned along the rings in the direction of orbital motion.

Those features are called “propellers” because of their shape (in close-ups, they look like airplane propellers, and individual examples have been nicknamed after important aviators), and they are truly weird. They mark the locations of very small moonlets, probably only a hundred meters across or less, and too small to see in this image. The moonlets are in the middle of the propellers, and shape them with their gravity. This is cool, so bear with me.

Imagine a very small moonlet orbiting in the rings. The moonlet has very weak gravity — you could easily jump off one and, in fact, rolling over in your sleep would be enough to launch you into space from one — but it’s there.

Now, picture a small piece of ice in the same orbit, but just ahead of it, moving in the same direction. The ice chunk is pulled a bit toward the moonlet, backwards in its orbit. Relative to Saturn, it loses a tiny amount of orbital energy, and falls toward the planet just a wee bit - maybe a few dozen meters. A smaller orbit is a faster one, so the particle speeds up a little, moving ahead of the moonlet.

If you take a whole bunch of particles like that in front of the moonlet, what you get is a pile-up of them ahead and slightly inside the moonlet’s orbit. That’s the leading propeller blade.

Now, let’s put a particle behind the moonlet, trailing it in its orbit around Saturn. It also gets pulled toward the moonlet but, this time, it’s pulled ahead in its orbit, so it gains energy. That moves it slightly farther out from the moonlet, and it slows down, trailing behind. That means particles pile up outside and behind the moonlet: The other blade of the propeller!

Propeller model

The diagram above is a physical model of a propeller; the moonlet location is marked, and the white fan blades are where ice particles in the rings have been moved out by the moonlet's gravity (note the black ridges below the leading blade and above the trialing one, where particles have piled up). In this diagram, Saturn is toward the bottom, and orbital motion is to the right.

Even though those moonlets are too small to see, the gravitational wakes they create ahead and behind them literally point right at them! The propeller blades are only a few kilometers long; you could walk from one end to the other in an hour or so (assuming you had a spacesuit and something to walk on).

Propellers had been predicted in theory before Cassini reached Saturn, but were first seen in the weeks after the spacecraft reached its destination, when it first approached the rings closely so that it could enter orbit around the giant planet. Since then, though, it’s kept well away from them; even a small chunk of ice could fatally damage Cassini given the spacecraft’s rapid speed around Saturn.

The mission, however, is winding down, and engineers are willing to take more risks. Cassini's been put into a more polar orbit, passing over Saturn’s north pole and dipping down just outside the main rings. This is riskier, but allows closer views of the rings. We’re seeing details now we’ve never seen before — like these small propellers (bigger ones have been seen with blades hundreds or even thousands of kilometers long). I’m really hoping that we get lucky and see one of the moonlets causing a propeller. It would be tiny, and maybe only a few pixels across at most, but actually seeing one would be yet another triumph in this mission’s long series of them.

And if it happens, it had better happen soon. To prevent accidental contamination of any of Saturn’s moons, Cassini will be purposely dropped into the planet’s atmosphere on September 30, 2017. It will take data as long as it can, but the immense pressure and heat won’t be denied for long. Cassini will be smashed and melted, its mission ending in a climax written for epic poetry: It will become part of Saturn, itself.

Fitting. Cassini has spent 12 long years orbiting the ringed planet, sending back images of one wonder after another, and it will continue to do so with its dying breath. That will be a bittersweet moment, but what a legacy the spacecraft has! It has opened up our minds to an entirely alien and spectacular world, and shown us things we had only dreamed of in our wildest physics and mathematical imaginations. We should all do so well in our own lives.