Saturn's moon Pan: Cassini image of the rings and moon.

A Tiny Moon of Saturn Carves a Big Gap in the Rings

A Tiny Moon of Saturn Carves a Big Gap in the Rings

Bad Astronomy
The entire universe in blog form
May 17 2014 7:30 AM

Saturn and Satyr

Saturn is a gift. It’s not like our solar system would be boring without it, but with it we do get some amazing stuff.

Like, for example, this:

Saturn's rings and the tiny moon Pan, barely visible in this shot (but easily seen if you click to encronosentate).

Photo by NASA/JPL-Caltech/SSI


This picture, taken by the Cassini spacecraft on Christmas Day 2013, shows the magnificent ring system. Composed of countless trillions of small particles of ice, the rings form a flat plane that is, to scale, far thinner than a sheet of paper. And there’s not just one ring; there are thousands, sculpted by the gravity of the many moons sharing their space.

One of those moons is Pan, a walnut-shaped lump just about 30 kilometers across. You can see it perfectly centered in the picture. It’s right smack dab in the middle of a gap in the rings called the Encke gap, which is about 325 kilometers (200 miles) wide. That’s no coincidence! Pan is what keeps that gap open. If a particle of ice gets close to Pan, the moon’s gravity gives it a kick, speeding the particle up or slowing it down. That moves it to a new orbit outside the gap; in this way Pan keeps the gap clear.

Well, almost clear. It turns out there are very thin and faint ringlets in the Encke gap. The particles in these ringlets play a game of gravity with Pan, creating what are called horseshoe orbits. They take slightly elliptical paths around Saturn, and when they get near Pan it gives them a small nudge that’s not enough to eject them from the gap, but enough to slow them down or speed them up just a wee bit. That keeps them in the gap, and confined to the narrow ringlets. Weirdly, relative to Pan, their orbits are kidney-bean or horseshoe shaped. The physics is a bit hairy, but Wikipedia has a decent article about these peculiar orbits.

Since Pan moves ring particles around, it’s called a “shepherd moon,” naturally enough. And guess what Pan was the god of? That’s no coincidence either; it was named by astronomers after they figured out what it did. Pan, I’ll note, was a satyr, hence the title of this article.

One more thing: Pan was not discovered by Cassini; it was actually predicted to exist when astronomers examined data taken by the Voyager 2 probe back in the 1980s. They saw scallop-shaped waves in Saturn’s rings on either side of the Encke gap, and predicted this was due to a moon in the gap. They were able to figure out mathematically where it was, and when they did, they found the moon in pictures already taken by the spacecraft!