How’s that for a title? Well, how’s this for a picture?
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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:
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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.
September 8th, 2008 at 12:44 pm
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.)
September 8th, 2008 at 12:50 pm
Love these posts, Phil. Shows you how much there is to learn just by constantly pushing ourselves out there.
September 8th, 2008 at 12:50 pm
Oh well, I guess I just misread the article:
Cassini detects partial rings with Saturn’s moons
September 8th, 2008 at 1:03 pm
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……
September 8th, 2008 at 1:14 pm
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”).
September 8th, 2008 at 1:16 pm
Did you mean to say “moonlets hoard particles”?
September 8th, 2008 at 1:18 pm
FOR THE HORDE
September 8th, 2008 at 2:50 pm
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?!
September 8th, 2008 at 2:54 pm
You say Anthe is in a resonance, but you neglected to mention what the specific resonance is in this case.
September 8th, 2008 at 2:55 pm
What in the heck is this? Astronomy stuff??
Great change of pace. Carry on.
September 8th, 2008 at 3:30 pm
Another astounding example of why it is more fun to look for reality how really is, than it is to make it for yourself.
Scott
September 8th, 2008 at 3:48 pm
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*
September 8th, 2008 at 3:57 pm
Only one answer for this. goddidit. lol
September 8th, 2008 at 5:10 pm
Andy:
Correct me if I’m wrong Dr. Phil, but I’ve worked it out as 11:10.
Basis of calculation:
Therefore, orbital resonance of Mimas & Anthe is 11:10
*Source: Wikipedia.
September 8th, 2008 at 5:39 pm
Awesomely cool.
September 8th, 2008 at 6:04 pm
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!
September 8th, 2008 at 10:37 pm
Michael Lonergan, you mean these?
(http://whom.co.uk/squelch/racespace.htm)
September 9th, 2008 at 12:29 am
@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!
September 9th, 2008 at 6:31 am
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.
September 9th, 2008 at 3:48 pm
@ 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 …
September 10th, 2008 at 1:10 am
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 
September 10th, 2008 at 4:19 pm
Waythehellc000000000000000l series of pix, but sad too once you get to the bottom……..
Where did we go wrong……………..
September 11th, 2008 at 12:52 pm
Thanks John, I’ll check out E-Bay.
Madlolscientists… where did we go wrong… Indeed.
November 18th, 2008 at 11:27 pm
[...] Ring arcs show moonlets horde particles [...]