The tiny moon with the long reach

By Phil Plait | August 2, 2010 7:13 am

When I was a kid, Saturn had one big, flat ring system divided up into maybe three or four broad sections separated by gaps, and that was it.

Turns out, we were wrong. Saturn has thousands of rings made up of billions upon billions of tiny ice particles. There aren’t just a handful of gaps, there are thousands of them, too, and there are moonlets in those gaps. Those tiny moons tug and pull on the rings, distorting them into weird and fantastic shapes. And "flat"? Not quite. The Cassini mission apparently delights in showing us just how wrong we were:

cassini_ringwaves

This image from Cassini shows Saturn’s broad A ring, the one you can see in small telescopes from Earth. On the right is the Encke Gap, a space carved out by the tiny moon Pan. On the left is the narrower Keeler Gap, where the even tinier moon Daphnis orbits Saturn. Daphnis is a lump, only about 9 kilometers (5.5 miles) across. But it has gravity, however feeble, and it’s enough to affect the rings. The waves you see just inside and outside the Keeler Gap are from Daphnis poking and prodding at the ring material. Stuff closer to Saturn (to the right) orbits faster than Daphnis, and stuff farther out (to the left) moves slower.

In the picture above, the ring particles move roughly from the bottom of the picture to the top. Remember, everything is in motion here. As the particles closer to Saturn pass the moon, they get tugged, and as the moon passes particles farther out, they get tugged too. This causes the ripples you can see in the rings.

Here is the same image, rotated and zoomed a bit for clarity:

cassini_ringwaves2

Man, that’s bizarre.

But it gets even weirder. The orbit of Daphnis is not exactly circular, nor is it exactly in the plane of the rings. It bobs up and down by a few kilometers (very roughly its own diameter) every orbit. This causes it to pull the ring particles out of the ring plane, and sometimes it pulls harder than other times. This motion and its effects are extremely complicated (as this technical paper outlines), but the cool thing is, Cassini shows us what happens.

This picture, taken when the Sun was shining straight along the edge of Saturn’s rings in 2009, shows Daphnis slightly out-of-plane of the rings, casting a long shadow on them. You can also see that the ring particles are also being pulled out of the plane; the waves cast shadows too!

What this and other pictures from Cassini are showing us is that the Universe isn’t all that simple. When we first look at something, we may get low-resolution, fuzzy pictures, and that means our understanding may be equally fuzzy. The closer we get, the harder we scrutinize, the more we learn… and in turn we find out that the Universe is more complicated, more interesting, and more beautiful than we first thought.

Tip o’ the Whipple Shield to Carolyn Porco (and for the link to the tech paper, too). Image credits: NASA/JPL/Space Science Institute


Related posts:

Saturn’s rings do the wave
Like the fist of an angry god
More Saturn ring awesomeness
Ring shadowplay on a Saturn moon
Ringless


CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Daphnis, Saturn

Comments (30)

  1. XPT

    What I would do to see even higher resolution pictures from Cassini!!! I know it won’t be able to resolve the objecs forming the rings, but still you gotta dream about it.

  2. Brian Schlosser

    Jupiter may be bigger, but clearly the gem of the outer solar system is Saturn. Its such an endlessly fascinating place!

  3. Sam

    Look at the inner rings; there are ripples there too! Any comment on whats causing that?

  4. Captn Tommy

    Great picture… It leads me to ask, if the inner wake spirals inward toward Saturn as it appears, does the outer forward wake spiral out away from Saturn?

    Would this be one of the causes of the large interferance waves?

    The world wonders.

    Irregardless
    Captn Tommy

  5. Another example of why science doesn’t work. Pick a story and stick with it! Sheesh!

  6. Pete Jackson

    I wonder what it would sound like if you transcribed the ring pattern to the surface of an old 33-rpm record and played it!

  7. @ bare rodent with cracking rawhide device:

    I’ve had that “argument” used against me when debating a woo-woo crystals will solve all our problems type. With a straight face she said, “I’ll never believe in science because it’s always changing.”

    It’s tough to respond when you’re picking your jaw off the floor.

  8. Messier Tidy Upper

    Awesome photos – thankyou Cassini team & Bad Astronomer. :-)

    @ 1. XPT :

    I know it won’t be able to resolve the objecs forming the rings, but still you gotta dream about it.

    Well Saturn’s gravity is responsible for forming the rings and I’m pretty sure we can see that – but I guess that’s not what you’re meaning there right?! ;-)

    Actually, I guess we could term Daphnis as a ring particle of sorts just about couldn’t we? How large does a ring particle have to get before it becomes a moonlet or moon?

  9. oldebabe

    Is the ring material essentially spiralling, or being pulled into Saturn? i.e. over time this ring materials will disappear and only the moons will be left?

  10. LunaJune

    Wicked… everyday we learn more… awesome.
    Will now be looking at it differently
    thanks

  11. @5. Pete Jackson:

    Rock music.

  12. Pete Jackson:

    I wonder what it would sound like if you transcribed the ring pattern to the surface of an old 33-rpm record and played it!

    fluffy:

    Rock music.

    ITYM “That Old Black Magic”. Here’s a sampling of some of the lines:

    “Those icy fingers up and down my spine”
    “Down and down I go, round and round I go”
    “In a spin, loving the spin I’m in”

    Too bad you can’t commit enough Cassini resources to make a video of this.

  13. Gary Ansorge

    Ah, the universe as we see it, always changing, always in motion.

    My kids are like that,,,

    Monty Python has something to say about that;

    http://dingo.care2.com/cards/flash/5409/galaxy.swf

    From Australia,,,(photos by NASA)

    Gary 7

  14. =mew=

    I’m no astronomer but this blog rocks the universe. Thanks for explaining complicated (to me) phenomena in ways I can understand.

  15. NerdBusters

    [sarcasm] Darn this Science stuff! Always revising theories because of new observations! Stop looking so hard and discovering new stuff like this. It makes my brain hurt!![/sarcasm]

    BTW I’d love to see the tone arm and stylus that would be able to “play” Saturn’s rings. Not to mention the size of the turntable…

  16. Jon Hanford

    8. oldebabe Says:

    “Is the ring material essentially spiralling, or being pulled into Saturn? i.e. over time this ring materials will disappear and only the moons will be left?”

    IIRC, the small moonlets in the ring constantly replenish material that makes up the rings. A mechanism like this was suspected before the Cassini mission, as without replenishment from some source, the rings would indeed be a transient phenomena due to loss of material to the gravity of Saturn.

  17. Michael Bowers

    Are the patterns on the inner rings in the top picture also caused by moons or is it a moire artifact from the resolution of the photo? My guess is it’s the latter since the patterns are so wide.

  18. Sarah

    Wonderful. I wonder – what it would look like from Daphnis’ “surface” itself? Being so close to the rings, would you only see a hint of the icy ring structures?

  19. Tribeca Mike

    Fascinating stuff. And coincidentally, it looks just like my old copy of The Who’s “Tommy.”

  20. Jon Hanford: I don’t know of any moonlets that are big enough to be resupplying the main rings. Daphnis and Pan, for example, are much too small to supply the A ring. By my math, Daphnis would only be able to supply enough material for an annulus of ring less than a kilometer wide. Pan could supply more, but perhaps a few kilometers of ring.

    Perhaps you’re thinking of the F ring and G ring arcs which probably/definitely have moonlets in them that supply some of the dusty material for those rings?

    Messier Tidy Upper:
    “Actually, I guess we could term Daphnis as a ring particle of sorts just about couldn’t we? How large does a ring particle have to get before it becomes a moonlet or moon?”
    Daphnis is, I think, much too large for that. The fact that it has cleared a gap is a show-stopper. Also, the largest particles in the A ring are probably not more than 3 m in radius, so Daphnis is a thousand times larger. I think that that sort of gap in the size distribution merits a distinction between ring particle and moonlet. Still, they do share a lot of similar characteristics, it seems like. (Low densities, for a start.)

  21. Joey Joe Joe

    Is it possible that Daphnis is a growing moon? Will she one day eat up all of the ring material?

  22. Joey Joe Joe:

    “Is it possible that Daphnis is a growing moon? Will she one day eat up all of the ring material?”

    No, probably not. Daphnis (and Pan, Prometheus, and Pandora) has reached a point where the gravity of the moon at the surface exactly balances the forces of the planet. Any material you try to add will feel more pull away than toward and pop back off.

    (See Porco et al, 2007, in Science)

  23. IVAN3MAN_AT_LARGE

    @ John Weiss (#22),

    The Porco, et al., 2007, paper that you referred to, is this — Saturn’s Small Inner Satellites:
    Clues to Their Origins (PDF)
    — it?

  24. Messier Tidy Upper

    @20. John Weiss & 23. IVAN3MAN_AT_LARGE : Thanks. :-)

    This discussion reminds me of a comment by Isaac Asimov (forgetten exactly which book, but in one of his science essays) when talking about moons and which planet has the most that you could (technically / arguably?) count every ring particle orbiting Saturn as a natural satellite or moon of that butterscotch planet! :-)

    Wonder how many “ring particles” Saturn has in total – millions? Hundreds of millions? Billions? Trillions? More? Anybody know and care to enlighten us all?

    Guess we have to draw the line between ring particles and dust particles at some point here too …

  25. When we first look at something, we may get low-resolution, fuzzy pictures, and that means our understanding may be equally fuzzy.

    As someone once quipped to Hoyle: You’d look pretty simple, too, Fred, at a distance of 10 parsecs.

  26. IVAN3MAN_AT_LARGE: Yep, that’s our baby. Some of the most fun I’ve had doing research ever.

    Messier Tidy Upper: doing some quick math on the size distribution, for the A and B rings, I’d estimate the number of particles is in the quadrillions.

  27. Robert Carnegie

    24. Messier Tidy Upper : yeah, is it “moons” and “moonlets” all the way down?

    And will somebody be ticked off at it if or when a rule of taxonomy is imposed?

  28. Alexander

    Saturn’s ring system environment reminds me of those iron filings that one uses to visualize magnetic fields. Except Saturn gives us a perfect platform made of millions of free-falling particles and large masses to trace out the work of the gravitational force.

    I envy the photons streaming out from the sun, able to reach out to touch objects far beyond our reach.

  29. Ken

    Ah, memories. Way back in the 1970s, when the first pictures of Saturn’s rings with any detail became available, some of the larger “braiding” patterns were discovered. Phyllis Schlafly was then a syndicated columnist, appearing in one of my hometown newspapers. Her reaction was something along the lines of “braiding proves a Braider, so God exists”. I don’t think she ever followed up with “oh, it was moons and perfectly ordinary gravity, guess I look pretty stupid,” but she never was one to let reality interfere with her life.

    Oh, and Robert @27: Of course someone will get ticked off. Remember Pluto? A standard will mean a cutoff point, and someone’s favorite moon will get demoted to a moonlet.

  30. Messier Tidy Upper

    @26. John Weiss Says:

    Messier Tidy Upper: doing some quick math on the size distribution, for the A and B rings, I’d estimate the number of particles is in the quadrillions.

    Thanks. Belated but sincere. :-)

    @29. Ken : There are many good and good logical reasons why I think the IAU got their “planet” definition badly wrong. Pluto – along with Eris, Sedna and, yes, even Ceres should, I think, count as planets. But that’s a story for another thread &, even if it wasn’t this is wa-ay too late in this thread now anyhow.

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