Two Moons, Five Moons, A Trillion Moons

By Phil Plait  - Slate

I have two quite beautiful images for you today, two different views of Saturn’s rings and moons. They’re both fascinating, and have a lot to say about what the neighborhood’s like around this strange and lovely planet.

First up: A quintet of rocky, icy moons:

A quintet of weird moons. Click to encronosenate.

Photo by NASA/JPL-Caltech/Space Science Institute 


I love shots like these; it’s like something you might see in the backdrop of a science fiction movie. But it’s real.

There are five moons visible in this image. The biggest one, on the right, is Rhea, Saturn’s second-largest satellite. It’s mostly ice with some rock, and is about 1530 kilometers (950 miles) across. Just over Rhea’s limb is Mimas, Saturn’s Death Star moon.

In the middle is Enceladus, which is about 500 km (330 miles) across. Notice how much brighter it looks than the other moons? That’s because it’s almost entirely made of ice, covering what may be a liquid ocean under the surface. We know it’s there because there are water geysers erupting from its south pole!

Superposed on the rings is tiny Pandora, just 80 km (50 miles) wide. And finally, all the way to the left, is Janus, a battered chunk of ice 200 km (120 miles) along its widest axis.

An image like this is possible because Saturn’s moons all orbit the planet on almost exactly the same plane. Cassini orbits Saturn in a way that brings it through that plane occasionally, so the moons can line up dutifully.

You can tell just how far off the plane the spacecraft was by looking at the rings, actually. If it were exactly in that plane, the rings would be razor thin. Since we see them arcing across the picture, we know Cassini was off by a bit.

Of course, if Cassini is way above or below the rings, it sees them more face-on. And when it does, it can get spectacular shots like this:

A flock of rings and their shepherd moons.

Photo by NASA/JPL-Caltech/Space Science Institute

What you’re seeing here are two of Saturn’s main rings: the narrow F ring (on the left), and the much broader A ring. There are two gaps in the A ring, the Encke gap on the right (which is about 325 km (200 miles) wide), and the very narrow Keeler gap (42 km (25 miles) wide) which is nearly in the middle of this picture.

There are also two moons in the picture: Prometheus (136 km long) to the upper left, and teeny Pan (34 km), which is in the Encke gap. That’s not a coincidence: The gravity of Pan is what keeps the gap open. Any particle that wanders out of the rings closer in or farther out than Pan gets yanked by the wee moon, flung back into the rings.

Prometheus does the same thing. It orbits just inside the F ring (Pandora, not pictured, orbits just outside) and together their gravity forces the ring particles to stay confined to the very narrow ring. That’s why these kinds of satellites are called shepherd moons.

Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death from the Skies! Follow him on Twitter.

Which brings me to one last note. It was thought at first that Saturn’s rings were solid disks. But it became clear over time they must be made up of countless small particles, or else they’d tear themselves apart, since the inner parts would orbit Saturn faster than the outer parts. Now we know the rings are actually made of gazillions of chunks of ice, each on their own orbit around the planet.
The biggest are probably no bigger than a house, the smallest too small to see with the naked eye even if it were right in front of you.

Hence the title of this post. Saturn has at least 60 largish objects orbiting it that we would call moons outright, but the rings themselves are composed of trillions more, each tiny, but each a satellite of the solar system’s second largest planet.

And that, my friends, is why I love this stuff so much. The Universe is an amazing place, and if there are wonders like this in our own back yard, what else is out there in the vast depths of truly deep space?