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A picture taken by the high-resolution camera on the Cassini orbiter shows Pan’s weird-looking equatorial ridge. (NASA / JPL-Caltech / SSI)

Astronomers have long known that Pan, one of the “shepherd moons” in Saturn’s rings, had a weird shape. But it took this week’s high-resolution images from the Cassini orbiter to show them just how weird.

Cassini got its closest look ever at Pan on Tuesday, when it came within a little more than 15,000 miles of the 20-mile-wide moon. In the close-ups released on Thursday, the thing looks like a flattened flying saucer, complete with a bulging ridge around the edge.

Overnight, the views sparked rounds of hilarity and awe on Twitter. Was it a cosmic turtle shell? Walnut? Ravioli? And what’s behind that strange, strange shape?

Carolyn Porco, who leads the imaging team for the soon-to-be-concluded Cassini mission, pointed to a couple of papers published nearly a decade ago that explain why Pan (named after the pipe-playing shepherd god of Greek mythology) looks the way it does.

Like Saturn’s other shepherd moons, Pan plies its way between sections of the giant planet’s rings, clearing out a gap as it goes. In Pan’s case, that’s the Encke Gap of the A ring.

Pan’s gravitational field pulls in icy particles from the edge of the ring as it makes its rounds. That’s how it helps keep the rings in shape, by “shepherding” stray ring particles. Some of that fine-grained material piles up around the saucer moon’s middle to create a smooth, protruding ridge.

For what it’s worth, Pan is by no means the only weirdo in the Saturnian system. There’s Iapetus, a two-toned moon that has its own walnut-type ridge; Hyperion, a moon that looks like a giant sponge; and Mimas, the “Death Star” moon.

More planetary wonders from this week:

  • Get out your red-blue glasses and check out the 3-D view of Ceres’ white spots, which scientists now suspect are composed of carbonate salt deposits that accumulated over the course of millions of years.
  • NASA says it has used ground-based radar to locate India’s Chandrayaan-1 moon-orbiting spacecraft, which has been out of contact since 2009.
  • NASA’s mission to study Europa, an ice-covered moon of Jupiter, now has an official name: Europa Clipper, a moniker that has been used unofficially in the past. The Europa Clipper mission will send a spacecraft to make repeated flybys of the moon in the 2020s, looking for further evidence of the potentially habitable ocean that’s thought to lie beneath the ice.
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