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:

cassini_pan_enckegap
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

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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!

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!