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Saturn's Raviolo Moon Is A Weird, Tiny Little Ring Shepherd

Mmmm, moon.

James Felton

James Felton

James Felton

James Felton

Senior Staff Writer

James is a published author with four pop-history and science books to his name. He specializes in history, strange science, and anything out of the ordinary.

Senior Staff Writer

A moon which looks a bit like ravioli.

It has a somewhat familiar shape.

Image credit: NASA/JPL-Caltech/Space Science Institute.

In 1984, a team of astronomers poring through images from NASA's Voyager spacecraft noticed small undulations in Saturn's rings.

The movement was observed on either side of the Encke Gap, a 325-kilometer (200-mile) wide gap in Saturn's A ring. To the team who saw the undulations, they implied a moonlet 10 kilometers (6 miles) in radius orbiting within the gap, tidying it up as it went.


"The locations and wavelengths of the observed waves suggest that a single moonlet dominates the gap, but some smaller objects are also probably present with sizes still far larger than typical 1-100 cm [0.4-40 inch] sized ring particles," the team wrote in their paper.

"We believe that the morphology of these wavy edges and the relationship of edge waves to gravitational torque illustrate clearly for the first time the basic working of the shepherding process; however, many open questions remain as to the details of the observed structure and the mechanism responsible."

Looking at the "waves" caused by the undiscovered object, another team was able to predict where it should be, publishing their paper in 1986. This gave astronomers somewhere to look. Thankfully, we didn't have to wait too long, as the moon had been captured in photographs by Voyager 1 and 2. Carefully going back through photographs of where the moon should appear, they found it very close to its predicted orbit, photographic proof of its existence.

A tiny planet, in a gap between Saturn's A ring.
The moon, right where it was predicted to be.
Image credit: NASA/JPL/Space Science Institute.

The moon has been noted to look like a raviolo (a single filled pasta found in a bowl of ravioli), or a tiny version of Saturn, complete with its own (significantly less elegant) ring. The unusual shape of the moon is due to the material it acquired from the rings as it orbits. In turn, the moon – named Pan after the god of nature and shepherds – clears a gap in Saturn's ring and shapes it as it orbits the planet every 13.8 hours.


"Pan creates stripes, called 'wakes,' in the ring material on either side of it. Since ring particles closer to Saturn than Pan move faster in their orbits, these particles pass the moon and receive a gravitational 'kick' from Pan as they do," NASA explains. "This kick causes waves to develop in the gap and also throughout the ring, extending hundreds of miles into the rings. These waves intersect downstream to create the wakes, places where ring material has bunched up in an orderly manner thanks to Pan's gravitational kick."

Pan is the innermost of Saturn's 117 officially recognized moons (so far). Though others, like Enceladus, may be far more interesting to scientists, when you are hungry accept no substitute for Pan, the tiny moon hurtling through Saturn's rings, unaware of its delicious raviolo shape.


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