Ring arcs show moonlets horde particles

By Phil Plait | September 8, 2008 12:34 pm

How’s that for a title? Well, how’s this for a picture?

Arc of particles orbiting Saturn near the moon Anthe

This image, taken by Cassini, shows an arc of particles orbiting Saturn. The bright spot is the tiny moon Anthe (pronounced ANN-thee), and it appears to be embedded in the arc. What’s going on here?

Well, several things. The arc is probably caused by meteorite impacts on the surface of Anthe. Being just a couple of kilometers across, it doesn’t have enough gravity to hold on to the ejected material, so the ejecta splash up and out, into space. Normally, that would wind up forming a complete ring around Saturn. So why do we see an arc and not a ring?

Anthe orbits Saturn in a resonance with Mimas, another, larger moon. A resonance is when two objects have orbital periods that are simple fractions of each other, like one takes exactly twice as long to orbit as another (a resonance of 2:1). When this happens, the smaller object gets tugged periodically by the larger object. In the case of Saturn’s rings, this clears a gap in them.

In the case of Anthe, it shepherds the moon, making it perform a strange dance indeed; over the course of its orbit it slows down and speeds up due to the influence of Mimas, forming an arc-like pattern superimposed on its larger orbit. The particles feel this same tug, and perform the same arcing loop. This keeps the particles from completely circling Saturn, so we get an ringlet of material. The particles never get too far from their parent moon, as if its keeping them close to itself.

Another moon, Methone (meth-OH-nee), shows the same thing, and Cassini captured both arcs simultaneously:

Cassini image of Anthe and Methone rings

The bright spots are the moons, and the arcs are pretty obvious. But don’t be fooled! This is a long time exposure by Cassini; the arcs are intrinsically incredibly faint, and can only be seen by sending an amazing machine like Cassini to Saturn. We’d never see these from Earth.

Saturn is an astonishingly complex interacting system. And as we see over and over again in nature, whenever you get complexity like this, beauty is sure to follow. Saturn never disappoints.

CATEGORIZED UNDER: Astronomy, NASA, Pretty pictures, Science

Comments (24)

  1. Does this mean that my local paper got it wrong the other day? They had a sidebar article that said that the moons themselves had partial rings around them, not they they were within rings/arcs. Or did I just misread it? (I have to see if we haven’t thrown it out yet.)

  2. Kirk

    Love these posts, Phil. Shows you how much there is to learn just by constantly pushing ourselves out there.

  3. Oh well, I guess I just misread the article:

    Cassini detects partial rings with Saturn’s moons

    PASADENA, Calif. (AP) — The latest images from NASA’s Cassini spacecraft show faint, partial rings orbiting with two of Saturn’s small inner moons, scientists said Friday.

  4. madge

    A few years ago now I had to do a presntation on Cassini for an Astronomy exam. I have always had a soft spot for it( I have a model of the craft suspended over my head at this moment) and it is still my favourite mission. Awesome i-madge. Cassini does it again and again and again……

  5. kuhnigget

    How could anyone (robots included) take a picture in the Saturn system and not capture an amazing sight? Saturn is still the one astronomical object that can generate a “whoa!” from even the most jaded nincompoop who looks through my telescope (usually after asking if I can “aim it at an airplane”).

  6. How’s that for a title?

    Did you mean to say “moonlets hoard particles”?

  7. MH
  8. JeffS

    Great post. The part about resonance stood out for me. After reading the post, just sitting here (my day off work) with nothing to do, I picked up my monthly copy of Sky & Telescope (September 2008). Picking up where I left off on page 26 and in the first section of that article (Planetary Peculiarities), it talks about a possible medium-small “super-earth” in a 2:1 orbital resonance with a known Neptune-like planet orbiting Gliese 436.

    Anyway, I thought it was a neat coincidence. Or, was it something more? Where’s Randi?! 

  9. andy

    You say Anthe is in a resonance, but you neglected to mention what the specific resonance is in this case.

  10. Dave Hall

    What in the heck is this? Astronomy stuff??
    Great change of pace. Carry on.

  11. scotth

    Another astounding example of why it is more fun to look for reality how really is, than it is to make it for yourself.


  12. Davidlpf

    The universe can be really a beautiful and awesome place.
    The problem with posting something science on the blog as far a can tell is someone will come and try to take over the discussion to promote their own pet theory. *cough* electric universe *cough*

  13. Ignorant Atheist

    Only one answer for this. goddidit. lol

  14. IVAN3MAN


    You say Anthe is in a resonance, but you neglected to mention what the specific resonance is in this case.

    Correct me if I’m wrong Dr. Phil, but I’ve worked it out as 11:10.

    Basis of calculation:

    Orbital period of Anthe* — 1.0365 d (A)

    Orbital period of Mimas* — 0.9424218 d (B)

    (A x B) / (A – B) = S (synodic)

    S / A = 10.01743018 ; S / B = 11.01743018

    Therefore, orbital resonance of Mimas & Anthe is 11:10

    *Source: Wikipedia.

  15. Michael Campbell
  16. Madge,

    Being British, do you remember a series of picture cards that came in boxes of Tea, I believe, back in the 1970’s depicting NASA spacecraft and missions? I had those cards at one time, and when we moved to Canada in ’73, they were left behind. I’d love it if someone could scan them and post them on-line. They really got me interested in the Space Program at such an early age. (I was 6 or 7 at the time.)

    Anyway, have a great day and some dippy toast soldiers!

  17. John Phillips, FCD
  18. @John Phillips FCD:

    YES!!!!! Thank you! Wow! Do those ever bring back memories! I didn’t think anyone else even knew those things existed! Thank you, you have made me very happy!

  19. We’d never see these from Earth… But could we see it from Saturn? I mean, being there and using our own eyes? Or are they too faint that they only appear in long exposures?

    I sometimes like to imagine what would be being there… looking at those wonderful images directly.

  20. @ Ken B –

    I’m not sure what your local paper was referring to, but there was some mention a few months ago about possible rings orbiting Rhea

  21. John Phillips, FCD

    Michael Lonergan: You’re welcome and yes, I used to collect the cards as a kid, though by the time the space race set came out I was busy enjoying my last year as a teenager and had far more important things on my mind, like that mysterious species, the human female :) If you want a set I believe they are available on e-bay, but then again, what isn’t :)

  22. themadlolscientist

    Waythehellc000000000000000l series of pix, but sad too once you get to the bottom……..

    America’s plan for a manned expedition to Mars involves two nuclear-powered spaceships, each carrying six astronauts, launched (according to one plan), on Nov. 12, 1981. Reaching Mars on Aug. 9, 1982, each vehicle would orbit the planet for 80 days while unmanned probes, followed by three men from each ship, would descend to carry out scientific research and collect samples. During the return to Earth (landing on Aug. 141 1983) the expedition would fly past Venus to observe the planet and use its gravity to reduce speed.

    Where did we go wrong…………….. :-(

  23. Thanks John, I’ll check out E-Bay.

    Madlolscientists… where did we go wrong… Indeed.


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