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SYFY WIRE Bad Astronomy

My God. It's full of propellers.

By Phil Plait

Having orbited Saturn now for nearly 13 years, the Cassini spacecraft has sent some pretty amazing images back to Earth. Of course, the planet, itself, the moons, and the rings have provided us with stunning portraits, but there’s been a lot of truly weird stuff as well. Like propellers.

propeller in Saturn's rings

These are weird features in the rings that look very much like what their name says: airplane propellers. They’re semi-permanent, meaning they last for years, and so planetary scientists have given some of the bigger ones nicknames to keep track of them. They name them after famous aviators; that one above is Bleriot, after the first pilot to fly across the English Channel (note: that’s a raw image right off the spacecraft, so it hasn’t been processed to clean off particle ray hits and other defects).

What are propellers? Well, that takes some explanation. Happily, I’ve done this before:

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!

In the shot of Bleriot, we’re looking at the unlit side of the rings; that is, the Sun us shining on the other side of them. The dark parts are caused by regions where there are either no icy ring particles (so you’re seeing through them into space beyond) or where there are so many particles they block the sunlight. Bright spots are where the particle density is intermediate, and sunlight shines (or is scattered) through.

Now, look at Bleriot again. The moonlet shaping it is probably 1500 meters across (just under a mile), and the particles close to it are so dense they block the sunlight. The leading and trailing blades are also where enough particles have been piled up that they too block the light, but just outside the blades they thin out, letting sunlight through.

Propeller model

The ripply waves you can just barely see in the blades are very interesting. I strongly suspect they are due to either the moonlet’s orbit not being perfectly circular, or being tipped just a wee bit to the rings. If that’s the case, then every orbit the moonlet bobs a bit back and forth, so the changing gravity, feeble as it is, felt by the particles nearby would alter their orbits a little, creating the ripples (think of it like creating waves by pushing a ball up and down or back and forth as it floats in water; though the underlying physics is different in this example the effect is similar).

How many propellers are there? There aren’t many big ones like Bleriot, but you don’t need much mass to create them. Even a mini-moonlet a hundred meters across would make them, and there may be lots of chunks that size. In fact, there are probably many thousands of propellers! This image attests to that:

dozens of propellers in Saturn's rings

Holy Bernoulli! Pretty much every white streak you see in this image is a propeller, caused by a moonlet far too small to see. The resolution of this shot is just under 400 meters per pixel, so the moonlets are smaller than that, and clearly many dozens of propellers are visible.

By the way, the stripes going diagonally through the image show a spiral pattern in the rings. Usually, each ring is nearly a perfect circle, but sometimes, gravitational disturbances from the major moons can stir up the particles, creating spiral ripples that wind around. They can also be caused by comet or small meteoroid impacts in the ring, which can create ripples like corrugations in cardboard that fade away with time.

OK, after all this, may I now take a step back and say how freaking weird is this? Saturn’s rings are far more than just gaudy ornaments. They are a perfect laboratory for studying the physics of a collisionally relaxed system —that is, one filled with zillions of particles that settled down a long time ago (like literally waiting for the dust to settle), each in their own separate orbit, and now just reacting to gravitational influences from the moons, the planet, and any small chunk that happens to be there (or slams into them like the aforementioned comet or meteoroid).

Amazingly, to me, propellers were actually predicted to exist before Cassini got to Saturn and ever saw one, which to me is a testament to the imagination of scientists. Someone was thinking about the rings one day, and then said, “Huh, what would happen if we just stuck a slightly bigger ice chunk in the middle; what would happen?” And then we sent this schoolbus–sized robot to Saturn, pointed it at the rings, let it take some snapshots, and there they were. Boom. Propellers.

I get chills thinking about things like this. Science! Some people think scientists are just coldly logical, but I argue scientists are among the most imaginative of humans. They have to see past what we can see in front of us and imagine what things are like in literally alien environments. They then let the rules of science guide that flight of fantasy, turning it into a hypothesis that makes predictions about what actually exists.

And then, sometimes, fantasy becomes reality. Saturn is reality, as fantastic — both literally and metaphorically — as it may seem.