The Rings of Saturn

Dr. David G. Simpson
NASA Goddard Space Flight Center, Greenbelt, Maryland


I thought you might be interested in doing a poster on the rings of the planet Saturn, including some of the latest information we've learned from the Cassini mission. I can send you some basic information at first, then more details as we go along. First of all, you've probably seen pictures of Saturn like this one, taken by the Hubble Space Telescope:

One of Saturn's most prominent features is the set of rings that encircle the planet. In the past few years, we've discovered that ALL of the major planets (Jupiter, Saturn, Uranus, and Neptune) have ring systems, and they're all different. Saturn has the largest and most spectacular system of rings, though -- the others aren't as easy to see.

From looking at the Hubble picture, you might think that the rings are solid disks circling Saturn (since they look solid), but scientists showed long ago that it would be impossible for them to be solid. In fact, they're made of of lots chunks of water ice or ice-covered rock, mostly ranging in size from an inch or less up to about the size of an automobile or so.

Although the rings are very large and wide, they're also very thin -- their thickness is less than the length of a football field. That's extremely thin, given Saturn's size. For comparison, if Saturn were shrunk down to the size of a basketball, then the thickness of the rings would be about 1/250 the thickness of a human hair.

From the Earth, we see Saturn at different angles, depending on where the Earth is in its orbit compared to where Saturn is in its orbit. Since the orbits of the two planets are tilted with respect to each other, we see Saturn and its rings from different angles at different times. In fact, sometimes the rings seem to disappear, since they're so thin and we see them edge-on.

Some astronomers believe that planetary rings like Saturn's are only temporary, and last for only a few million years (compared to the age of the Solar System, about 4.5 billion years). If that's so, then sometime in the distant future Saturn's rings may disappear, and maybe another planet will have a large set of rings.

If you look at the pictures of Saturn (like the Hubble picture), you'll notice that the rings aren't one uniform disk; the ring system is actually made up of a series of concentric rings, some of which are brighter than others. The individual rings have been named with letters, from A through G.

One nice thing to put in your poster might be a "map" of the rings. That will give you a good idea of where each of the rings is located, and how wide they are. I've attached a plot that shows the way you might make such a map. This plot shows just one quarter of Saturn and its ring system. Distances from Saturn are shown on the X and Y axes, in units of Saturn radii (which is how we usually measure distances on the Cassini project). I've drawn in a quarter-circle with radius 1 to show where Saturn is, and another pair of quarter-circles to show where the A ring is. You might like to use a compass to fill in the locations of all the other rings, or you might like to make a map of your own design. The location of each of the rings is shown in the table below.

 Locations of rings (in Saturn radii). 
 D ring  1.110 - 1.236 
 C ring  1.239 - 1.527 
 B ring  1.527 - 1.951 
 A ring  2.027 - 2.269 
 F ring  2.326 
 G ring  2.82 - 2.90 
 E ring  3 - 8 

That should do for now. I'll follow up with another message later with some more details about Saturn's rings. We have some ideas about where the rings might have come from, and Cassini has shown us a lot of detail in the structure of the rings: spokes, gravity waves, shepherding moons, and so on. More on that later. Meanwhile, if you have any questions about Saturn's rings, please feel free to send them to me.

Above: Beginning of a map of Saturn's rings.


Here's some more information on the rings of Saturn for your poster. I hope it's coming along well.

Last time I covered some basic facts about the rings:

  1. They're not solid, but rather are composed of chunks of water ice, ranging from the size of an automobile or small building down to the an inch or smaller (including some microscopic dust).
  2. The rings are very thin -- maybe just 100 yards or so in thickness.
  3. We see the rings at different angles from the Earth, depending on where the Earth and Saturn are in their orbits. Once in a while we even see the rings edge-on; this is called a "ring plane crossing".
  4. Some astronomers believe that the rings may be only a few million years old, and may eventually disappear. We're not sure about this, though; this topic is still being debated.
  5. Saturn actually has a number of rings; these have been designated by letters, A-G.

Here are a few more details about the rings of Saturn:


Why are Saturn's rings so thin? It has to do with the ring particles colliding with each other. Ring particles that are high above or below the rings are in a highly "inclined" (tilted) orbit, and have more energy than ring particles that are closer to the ring plane. When those particles collide with other particles, some of their energy is lost, so causing them to move to lower-energy orbits closer to the ring plane. Over time, the ring particles (especially the larger ones) tend to flatten themselves into a thin plane, as we see today. This is the general picture, but some of the details are still being worked out.


If you look at a picture of the rings of Saturn such as the one taken by Hubble shown above, which is also available on this Web site:

You notice a large gap between the A and B rings. This is called the "Cassini division". (See attached picture, where I've labeled the A, B, and C rings, along with the Cassini division.) There are other gaps in the rings at other places, but this one is the largest.

The Cassini division is caused by the pull of one of Saturn's moons called Mimas. A ring particle in the Cassini division would go around Saturn twice for every time Mimas goes around once; this is called a "2:1 resonance orbit". What happens is that if a ring particle were in the Cassini division, it would be pulled on by Mimas's gravity at the same place in its orbit every time Mimas passes by; the little gravitional "tugs" add up, just as pushing someone on a swing over and over makes the swing go higher. The gravitational tugs by Mimas eventually would pull the ring particle out of the Cassini division -- and that's why there's a gap there, with no ring particles inside.

I think that should do for now. I can send you some more information later on spokes, density waves, shepherding moons, and some more specific information on some of the more interesting individual rings like F and E. Let me know if you have any questions in the meantime.

Above: Rings A, B, C; the Cassini division (Hubble Space Telescope picture).


Here is some more information on the rings of Saturn for your poster. This time, I thought I'd provide you with a brief history of the discovery of the rings, along with some specific information about each of the individual rings.


Although the planet Saturn has been known since ancient times, the rings were only first observed by the Italian astronomer Galileo Galilei when he saw them in an early telescope in 1610. His telescope wasn't of very good quality, though, and he could only make out "blobs" on either side of Saturn, which he thought were moons. It was the Dutch astronomer Christiaan Huygens who in 1656 first correctly identified the "blobs" as rings around Saturn.


As I mentioned earlier, Saturn has an extensive system rings, consisting of several individual rings named A, B, C, D, E, F, and G (named in the order of their discovery). The main or "classical" rings are A, B, and C; we have known about these rings since the 17th century. The others have been discovered only very recently.


The A and B rings are the brightest rings; they're the ones you'll see when looking at Saturn in a telescope. (The A and B rings are marked on the picture I sent you last time.)

The A and B rings are separated by the "Cassini division", which is a large gap in the rings caused by the gravitational pull of Saturn's moon Mimas. The A ring is the one farther from Saturn, on the "outside" of the Cassini division. It's a bit darker than the B ring.

The A ring is made up of chunks of water ice, from the size of a building down to an inch or smaller.

As I mentioned before, the inner edge if the A ring -- at the Cassini division -- is due to the gravitational pull of Saturn's moon Mimas. The outer edge of the A ring is similarly due to the gravitational pull of two other moons called Janus and Epimetheus. Any particles just outside the outer edge of the A ring will be pulled out of that area by the gravitational pull of these moons.

Within the outer parts of the A ring, we find two narrow gaps in the ring (much narrower than the Cassini division):


The B ring is the other of Saturn's two main bright rings. It's inside the A ring (closer to Saturn), and is the brightest of all the rings. Like the A ring, it is made of chunks of water ice of varying size.

The outer edge of the B ring is at the Cassini division; it is due to the gravitational pull of Saturn's moon Mimas.


The C ring is located inside B ring (closer to Saturn). It is fainter and harder to see than A and B rings. Like the A and B rings, it is also made of particles of water ice, but these are covered with dark material that makes the C ring darker than the A and B rings.


The dark D ring is the closest ring to Saturn (as you can tell from your ring map). Like the C ring, its ice particles are also covered with dark material. The D ring is also very tenuous, and is made of microscopic particles, so it's fairly difficult to see. I don't think it's been seen from Earth yet; it was detected by one of the Voyager spacecraft in the early 1980s.


The E ring is the ring farthest from Saturn. As you can see from your map of Saturn's rings, the E ring is MUCH wider than any of the other rings. The ring is very tenuous, though, so the E ring is fairly difficult to see. It is possible to see it from Earth when the rings are nearly edge-on.

The E-ring is centered on Saturn's moon Enceladus. (Enceladus is about 4 Saturn radii from Saturn, if you want to locate it on your ring map.) The densest part of the E ring is right at Enceladus' orbit, which suggests that Enceladus is the source of the particles in the E ring. In fact, we've just recently discovered that that's what's happening: there is a geyser at the south pole of Enceladus that is shooting a jet of water particles into space, and it's particles from this geyser that has created the E-ring. We're still not sure how the geyser works, where the water is coming from, and similar details, but we're working on that now.

The water particles that make up the E ring are microscopic in size (about 1/1000 of a millimeter).


The F ring was discovered in September 1979 by the Pioneer 11 spacecraft, so it is a fairly recent discovery.

If you look at your ring map, you will find that the F ring is the narrowest of the rings. It is located just outside the A ring.

Close observation of the F ring has shown that it is made of "strands" of intertwining ring particles, so it has a very interesting structure.

Just inside the F ring is a moon called Prometheus, and just outside the F ring is another (smaller) moon called Pandora. These are "shepherding" moons, whose gravitational pull keeps the F ring to its narrow width.

The F ring is still a bit of a mystery. We don't really understand the details of how the strands are formed, or exactly how the shepherding moons do what they do. That's something we're still working on.


The G ring is the most recently discovered. It is a narrow ring located between F and E rings. It is made of particles that are mostly microscopic, up to maybe 1 millimeter or so in size.

I think that should do for now. Later I'll send along some further information on shepherding moons and some of the detailed features (like spokes) that we see in the rings.


Here is some information I promised earlier on "shepherding moons" of Saturn.


You notice from your map of Saturn's rings that the F ring is very narrow. (Unlike, for example, the E ring, which is very wide.) A narrow ring like that presents a bit of a puzzle, since astronomers have known for a while that rings have a natural tendency to spread out and become wider as the particles in the rings collide with each other. There were also some very narrow rings discovered around the planet Uranus in the late 1970s, which presented the same puzzle.

A theory was proposed in 1979 that the narrow rings might be due to what are called "shepherding moons". It turns out that when you have a small moon orbiting Saturn just outside one of the rings, the gravitational pull of the moon on the ring particles will try to push the ring particles into smaller orbits; it's as if the moon "repels" the ring particles so make them orbit closer to Saturn. Similarly, if there is a small moon just inside one of the rings, the gravitational pull of the moon on the ring particles will try to push the ring particles into larger orbits; again the ring particles act as if they are "repelled" by the moon, but this time they are made to orbit farther from Saturn.

Now imagine that there are TWO small moons: one orbiting just outside a ring, and one orbiting just inside the ring. The outer moon will push the ring particles toward Saturn, and the inner moon will push the ring particles in the opposite direction, away from Saturn. The result is that the ring particles will be "shepherded" into a ring between the two moons, much like a shepherd herding sheep.

We have since discovered that there are indeed two shepherding moons that are responsible for the narrowness of Saturn's F ring. They are called Pandora (the one outside the ring) and Prometheus (the one inside the ring). I've attached pictures of Pandora and Prometheus (near the F ring that they maintain), as seen by the Cassini spacecraft. Notice the several braided "strands" of the F ring (especially noticeable in the Prometheus picture).

Pandora and Prometheus are fairly small moons. They're both irregularly shaped, with Prometheus being a bit larger than Pandora. Prometheus is about 45 miles across, and Pandora is about 35 miles across. That's actually a bit of a puzzle right now - current theories say that to explain the features in the F ring, the larger of the two moons should be on the outside (farther from Saturn), but it's the other way around. Scientists are still trying to explain that.


I mentioned earlier (in Part 3) that the A ring contains a gap called the "Encke gap", and a narrower one called the "Keeler gap". These gaps are maintained by "embedded" moons, i.e. moons that orbit Saturn within the A ring. Remember from the discussion of shepherding moons above that a small moon will tend to push ring particles away from it: "inside" particles (closer to Saturn) will be pushed even closer to Saturn, and "outside" particles (farther from Saturn) will be pushed even farther from Saturn by the moon's gravity. The result is that an embedded moon will clear a path in the ring around it, resulting in a gap.

The wider Encke gap is caused by the gravity from a moon called Pan. Pan orbits Saturn within the Encke gap; its gravity causes ring "inside" particles (closer to Saturn) to be pushed into smaller orbits, and "outside particles (farther from Saturn) to be pushed into larger orbits, resulting in the Encke gap. I've attached a picture taken by Cassini that shows the Encke gap (the larger of the two gaps, on the right). You can see the moon Pan inside the gap.

To the left in the same picture, you can also see the narrower Keeler gap. Just barely visible inside the Keeler gap, you can see the moon Daphnis, whose gravity is responsible for clearing the Keeler gap.

Pan and Daphnis are also small moons. Pan is about 16 miles across; Daphnis is even smaller, only about 4 miles across.

That should do for now. I'll send you some more information on ring structures later. Let me know if you have any questions in the meantime.

Above: Saturn's shepherding moon Pandora and the F ring.

Above: Saturn's shepherding moon Prometheus and the F ring. Note the strands in the F ring.

Above: Saturn's embedded moon Pan inside the Encke gap in the A ring (right). The moon Daphnis is barely visible inside the Keeler gap (left).


Here is some more information on the rings of Saturn. Here you'll find a picture in which you can see all the rings; I'm also including some information about the so-called "spokes" in the rings.


Below is a very interesting recent picture of Saturn taken, in September 2006. It shows a "backlit" Saturn; here the camera is looking toward the Sun, so you can see Saturn illuminated from behind -- a view we never see from Earth. The other interesting thing about this picture is that you can see all the different rings, including some that are ordinarily not very easy to see. Shown below are two versions of the picture: one original, and one with captions that shows the different rings labeled. Notice that the Earth is visible through the rings, so you're in this picture too.

Above: Backlit picture of Saturn, as seen from the Cassini spacecraft.

Above: Backlit picture of Saturn, with captions.


When the Voyager spacecraft visited Saturn years ago, they found unusual markings going across the rings of Saturn, a little like spokes on a bicycle wheel. They called these features "spokes", and they're still a bit of a mystery. A picture of spokes in Saturn's rings (as seen by Voyager 2 in August of 1981) is shown below.

When the Cassini spacecraft arrived at Saturn in 2004, the spokes were nowhere to be found. None of the early pictures returned by Cassini showed these spoke features, so they seem to have disappeared since Voyager 2 visited Saturn. We've since seen them re-appear, though.

There's still a lot we don't understand about the spokes. What we have learned is that they are made of very small dust particles, less than one micron (1/1000 millimeter) across. It's believed that the spoke particles have a negative electric charge, which makes them hover 50 miles or so above the rings. Direct sunlight on the rings seems to inhibit the formation of spokes (for reasons that are not entirely understood yet), so they tend to form when the rings are nearly edge-on to the Sun. That's actually why Cassini didn't see them at first -- the Sun was shining directly on the rings at that time. Cassini has since seen spokes in Saturn's rings, as Saturn has moved in its orbit and the rings are more edge-on to the Sun.

The spokes seem to follow the magnetic field lines of Saturn, but exactly why that is is still being investigated.

Spokes are very short-lived features. They form in a matter of minutes, and last only for a few hours before disappearing.

The origin of the spokes is something else that scientists are still investigating. Some believe that meteoroid impacts in the rings produce a cloud of dust that causes the spokes; but there are other ideas too, so nobody is really sure yet.

As you can see, there's a lot about spokes that we don't understand yet. We've seen them, and we know they're made up of fine dust, but there's not much else that we're sure of.

I hope the poster is coming along well. As always, feel free to write to me if you have any questions.

Above: Spokes in Saturn's rings, as seen by Voyager 2 (August 1981). (The spokes are the dark splotches on the rings.)



Thickness: It appears that the rings are very thin. The main rings (A, B, C) are only something like 100 yards thick, which is about the length of a football field.

Diameter (distance across a ring): We typically measure distances in Saturn radii, where 1 Saturn radius is 60,268 kilometers = 37,449 miles. (For comparison, the radius of the Earth is about 6378 kilometers = 3963 miles.) In the first set of notes I sent you, I gave you the inner and outer radii of each ring, in Saturn radii. If you multiply the outer radius by 2, that will tell you the diameter of each ring, in Saturn radii. Multiply that number by 60,268 to get the ring diameter in kilometers, or multiply by 37,449 to get the ring diameter in miles.

Sometimes it's easier to visualize these distances if they're measured in units of Earth diameters. Since the radius of the Earth is 6378 km, the diameter of the Earth is then 2 × 6378 = 12,756 km. So if you divide all distances in kilometers by 12,756, you'll get distances in Earth diameters.

I've done this in the table below. You might like to try doing a few of these calculations yourself to make sure you understand how to do them.

 Diameters of the Rings. 
 Ring  Saturn radii  Kilometers  Miles  Earth diameters 
 D ring  2.472  148,983  92,573  11.679 
 C ring  3.054  184,059  114,369  14.429 
 B ring  3.902  235,166  146,125  18.435 
 A ring  4.538  273,496  169,943  21.440 
 F ring  4.652  280,367  174,212  21.979 
 G ring  5.80   349,554  217,203  27.403 
 E ring  16  964,288  599,181  75.593 

Width: You can do something similar to calculate the width of each ring. In this case, the "width" of a ring is its outer radius minus its inner radius. Again using the numbers in the table I sent you in the first set of notes, I get these results:

 Widths of the Rings. 
 Ring  Saturn radii  Kilometers  Miles  Earth diameters 
 D ring  0.126  7,594  4,719  0.595 
 C ring  0.288  17,357  10,785  1.361 
 B ring  0.424  25,554  15,878  2.003 
 A ring  0.242  14,585  9,063  1.143 
 F ring  -  -  -  - 
 G ring  0.08  4,821  2,996  0.378 
 E ring  5  301,340  187,244  23.623 

So, for example, we find that the main rings (A,B,C) are about 21 Earth diameters across (the diameter of the A ring), and about 4.5 Earth diameters wide (the sum of the widths of the A, B, and C rings).

To help visualize this, shown below is a picture showing the dimensions of the main rings, along with a picture of the Earth for scale.

Above: Size of the main rings, with Earth shown for scale.



The gravitational pull of Saturn and its moons on the ring particles causes some interesting patterns to form in the rings. Among these is the formation of so-called "waves" in the rings, which are really tightly-wound spirals. A description of the exact cause of the waves is a little complicated, but we can look at some basic properties here.

There are two types of waves: "spiral density waves", and "spiral bending waves"

Spiral Density Waves

In spiral density waves, the gravitional pull of Saturn and its moons causes the ring particles to bunch together or spread apart in a tightly-wound spiral pattern. The result is that we can see a series of "bands" in certain parts of the rings. You can tell density waves from the other type (bending waves) by the way the distance between the bands ("wavelength") changes. In density waves, the wavelength of the bands DECREASES as you get farther from Saturn. In other words, the bands get squeezed closer together as you get farther from Saturn.

Spiral Bending Waves

Spiral bending waves are a kind of slight "warping" of the rings. Here the ring particles make a kind of corregated pattern, like you would see in corregated cardboard. In bending waves, the wavelength INCREASES as you get farther from Saturn. In other words, the bands get spread farther apart as you get farther from Saturn.

Both types of waves are visible in the Cassini image below. In this image (a close-up of the A ring), Saturn is out of the picture, off to the lower left, so we get farther from Saturn as we look toward the upper right. Notice the pattern of the bands in the lower-left corner: you see that they get squeezed closer together as we go toward the upper right (farther from Saturn). That means that the wavelength is decreasing with increasing distance, so those are density waves. You notice the opposite pattern in the bands in the upper right: those are bending waves.

Above: Waves in Saturn's A ring. The bands in the lower left are spiral density waves, and the bands in the upper right are bending waves. This image was taken by the Cassini spacecraft in 2004. (Saturn is off to the lower left.)