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SYFY WIRE Bad Astronomy

No, aliens haven't invaded Saturn: It's Pan, the flying saucer moon!

By Phil Plait

[Credit: NASA/JPL-Caltech/Space Science Institute]

The Cassini spacecraft never fails to amaze. Even after 13 years of orbiting Saturn, it can still send back images to Earth that stun. Want proof? Check out this astonishing picture of Saturn’s moon Pan:

Saturn's moon Pan

What the what? We knew it was weirdly shaped from earlier images — maybe like a walnut or a flying saucer — but these new ones are still shocking. Look at it: It has a ridge along its equator that’s several kilometers high! Heck, it’s not a ridge, it’s a brim.

Except it’s not. It’s more like a sand dune. Made of ice. An ice dune. An ice dune that circles the moon.

OK, enough teasing. Here’s what’s what:

Pan is a wee little thing as moons go, only about 35 kilometers across its long axis. It actually is rather walnut-shaped, and it orbits Saturn inside the A ring, the brightest of the planet’s main rings. There’s a gap in the ring, called the Encke Gap, and Pan orbits right in the center of the gap. That’s no coincidence: The gravity of the moon, though feeble, is enough to clear out ice particles in the ring, carving the 325-km-wide gap.

In 1985, astronomers predicted there was a moon in the Encke Gap because the edges of the gap were seen to be wavy in Voyager probe images. A moon on a very slightly eccentric (non-circular) or tilted orbit could cause ripples like that, and sure enough, Pan was discovered in 1990 by an astronomer looking at old Voyager 1 images from 1981. Pan’s orbit is not tilted, but it is very slightly elliptical. As it moves closer to one edge of the gap or the other, it draws up waves in the particles (this phenomenon was recently seen in jaw-dropping detail when Cassini looked at the tiny moon Daphnis).

More recent Cassini images taken from a long distance showed Pan to be weird. These new, much-higher-resolution images we see now are thanks to Cassini’s new orbit, which takes it over the poles of Saturn and then diving down to just outside the main rings. This brings the spacecraft closer to the moons embedded in the rings, allowing us to get far more detailed shots. This is the closest Cassini has ever been to Pan, and these are the best images humans have ever seen of it.


Because Pan orbits inside the rings, it can gravitationally attract the tiny ice particles that make up the rings, and pull them onto its surface. Now, despite being hundreds of thousands of kilometers wide, the rings of Saturn are incredibly thin, in places only just 10 meters thick! That’s the height of a two-story house*, for some perspective.

Because Pan orbits exactly in the plane of the rings, when it pulls in the ice the particles land on its equator, all around the moon. This stuff piles up. On Earth, this would make on low, long ridge, because Earth’s gravity is strong, and the particles would slump if they piled up past a certain height.

But Pan’s gravity is very weak, just one-ten-thousandth as strong as Earth’s. You could easily throw a baseball off the surface and have it leave the moon forever. Because of that teeny force, the ring particles can pile up to tremendous height without slumping. The result is a slightly triangular wall that reaches more than seven kilometers off the surface in some places! That’s higher than any Rocky Mountain, and yet it presents a nearly vertical face. It’s a continuous, sinuous cliff as tall as a mountain ringing (haha) the moon.

Interestingly, you can see a few craters in the ridge. That surprised me; I’d expect it to be very fragile. Perhaps very low-speed collisions with bigger objects carved out those impact craters. I expect scientists will be able to use these craters to estimate how compacted the ridge is, and how well the ring particles adhere to each other. At the top left, you can see a bright streak along the top of the ridge; I’m guessing that’s from the particles slipping, causing a minor slide of material.

The moon, itself, shows some of that, too. I’m no planetary scientist, but there are some obvious conclusions to draw with an experienced eye. There are very few craters, indicating the surface is young — if it were old, we’d see lots of craters from impacts over time. This strongly implies that the entire surface of the moon has a layer of ice or dust on it, falling slowly from Saturn’s environment. Those long steps you can see criss-crossing it are probably where the material has slipped: moonslides.

What a sight that would be to see in action! In that low gravity, such a slide would take a long time to play out, like watching an avalanche in slow motion. [Update (March 10, 2017 at 16:30 UTC): It just occured to me that the moon may be covered in smaller particles due to material on the ridge sliding down. It seems obvious in retrospect, and my apologies for not thinking of it when I inititally wrote this post!]


By the way, Pan isn’t alone. Another moonlet, Atlas, shows the same sort of very tall ridge, making them both look like Klaatu’s spaceship. In that image, you can actually see that Atlas appears to be buried under ring particles even more than Pan is.

If you’re wondering why we’re only getting these close-ups of Pan now, after 13 years of Saturn-gazing, it’s because the Cassini mission is coming to an end. It’s running out of fuel to make maneuvers, and the engineers don’t want it to accidentally impact a moon and contaminate it. So in September they will command it to drop into the atmosphere of Saturn itself, sending back data as it plunges to its death.

That’s bittersweet, to be sure, but it also allows the Cassini team to take more risks, including setting up these ring-diving orbits. Over the next few months we’ll be seeing more spectacular images like these, and I have to admit it brings me some cheer. After all these years of sending back wonders from a billion kilometers away, Cassini is going out in an orgy of observations, and we’ll reap the benefits of its demise.

Correction (March 10, 2017 at 16:50 UTC): I originally wrote that 10 meters is a one-story house, but it's more like two if you include a peaked roof.