Tethys flyby geometry (Hill sphere radius is only RH=~ 2100km):
Click to view attachment
And Mimi lemms electron data from PDS:
Click to view attachment Click to view attachment
The yellow dotted vertical lines correspond to yellow crosses overlayed onto the Cassini trajectory in the geometry plot.
Nice and flat with the wake shadow clearly cut out.

And Dione flyby:
Click to view attachment
Cassini went directly over the south pole, and that is probably the reason why there are absorptions of different width with increasing electron energy:
Click to view attachment
Edit: forgot to write that Hill sphere radius is about RH=~ 3250km

Thanks for the plots. Closest approach was 5737km... wasn't that just inside the Hill sphere?

If that number is altitude then not.

Good point... yep, looks like it was just outside then.

Probably not. But it may be worth mentioning here that, when Uranus's rings were first discovered, the discoverers at first reported a "swarm of satellites" around Uranus.

Although that description doesn't cover any spatial distribution criteria, otherwise it may be about as good a blanket definition of rings as any.

This Rhea business is certainly an odd one. I found it hard to believe from the get-go and see that this has been a common reaction at UMSF. Still, the evidence seems to show *something* peculiar is going on there.

(My apologies if I didn't get the calculations correct, it's still early here)

Looks like orbital periods for the structures are ~821 minutes for the 1615km material, ~931 minutes for the 1800km, and ~1068 minutes for the 2020 km.

Divying out to the 6505 minute period of Rhea about Saturn yields some interesting numbers, ~8, ~7, and ~6. Perhaps dynamical stability might be enhanced in this kind of relationship with structures near resonances? Maybe this material has been there for a very long time ?

If you're right that's very interesting. Maybe the exact resonances would be swept clear, with material in slowly decaying orbits 'piling up' just outside them?

Have I missed an official announcement? has ring material been definitively confirmed at Rhea??

No. What started us up again was this nice CHARM presentation:
http://www.unmannedspaceflight.com/index.php?showtopic=5780

(Seems like a pretty good case to me. Of course I particularly like those equatorial surface scars presented as 'other evidence' near the end.)

I don't get the same numbers as you, but regardless of who is right, this is still an interesting point. If those rings exist, and are narrow "hoop-like" features, something's gotta be confining them. Resonances would be the best bet, and since Rhea's orbit isn't perfectly circular, an orbit-orbit tidal resonance could well be the culprit.

I also just tried checking whether any resonances might show up between the ratios of the (hypothesized) ring particles' orbital periods, and the synodic periods of Titan and Dione, as seen from Rhea. Unfortunately, I didn't find much. For Titan they were in the range 8:1 to 12:1, depending on the distance from Rhea. But the orbital radii of the (hypothesized) rings don't seem to be very well known, which makes it hard to point at any specific resonance with much confidence.

Yeah, finding accurate numbers, and then being able to crunch them precisely is the trick. (I make no claims to accuracy, my 'good' calculator is on the disabled list) If the commensurability with Rhea's orbital period doesn't work out, do your periods still exhibit the ratios between themselves? {6/8, 7/8, 6/7}

I am still marveling we get stuff organized outside the Roche limit, and inside the Hill sphere, at this apparent very low particle density. Crikey, how does the stuff interact upon itself to stay collimated ?

And the 'residue' along the equator is teeny, there never was much stuff in the equatorial plane. Or, the stuff preferentially escapes off the high end (if there ever was much 'stuff') and is lost to the Rhean realm. But then where does it go?

Do we have oblateness data on Rhea yet ?

I didn't go into that much detail. If you look at the estimates of the "ring radii" from Cassini's pass, they are different on the inbound and outbound legs. The differences are large enough that estimating orbital periods is probably impossible at the present time.

I will admit to still finding this whole thing hard to believe; however, that recent photo shoot of the dark spots on Rhea's equator was very interesting.

Audio and transcript of the CHARM have now been posted.

http://saturn.jpl.nasa.gov/video/products/...aProductsCharm/

As someone (Stu ?) already posted, UMSF gets a plug by Geraint when talking about the equatorial stains image:


I think he was referring to the discussion in this thread.

Go, UMSF!

Ugordan--Way to spot something out of the ordinary and propose the potentially correct cause. You should be awarded a Mars Bar for this one. (So what that Rhea goes around Saturn)

I second Floyd's comment Gordan - very creditable interpretation of what the raws are showing as they come down from Cassini. That's a putative equatorial ring-material stain on Rhea, and the G-ring moonlet S/2008/S1 that you've spotted prior to 'official' discussions/confirmations - any more tricks up your sleeve?

Floyd: wow, my first virtual Mars bar!

jasedm: the G ring moonlet was a bit of a no-brainer to notice, as for this it wouldn't have occured to me if Exploitcorporations didn't rekindle interests in that Rhea flyby. I'd probably never really notice it in true color composites, though in retrospect the markings are visible there as well. EC deserves a Mars bar, too!

Rhea stellar occultation to look for the ring? (plus long exposure of course - I assume the arc/ring in the image is an artefact to be calibrated out)
http://saturn.jpl.nasa.gov/photos/raw/rawi...?imageID=190174 (there's more of these with the star moving)
Didn't find anything about this at the cyclops site... Would be cool if they found something.

If it were a stellar occultation, they'd want to keep the s/c fixed on a star instead of tracking the moon. Still, it's probably got to do with ring fragments search, but the old-fashioned way of trying to spot them directly. It's proving to be hard with the all the scattered light off of a fully lit Rhea.

good point. But who knows, that star might turn out to be useful

I'm wondering what would be the best way of imaging the rings directly??
There is a sub 50,000km flyby coming up later in the year (13th October) where Cassini approaches from the night side of the moon. Might the ring material show up with long camera exposures centred to the 'left' of the dark limb (see white box marked on SSS simulation below) This would possibly show up some material in the ring ansa (if there is something as coherent as an ansa) on that side. This approach would mean that reflected sunlight off Rhea would be at a minimum as phase angle is near 150 degrees.

Flaws as I see them would be:
a. We're right in the plane of the rings at this point
b. Distance to target at this phase angle is ~ 100,000km.

I'd be interested in any thoughts.

Unfortunately, if the rings are comprised of boulders as theorized and not microscopic dust/ice particles, high phase angle will also mean a darker ring appearance. You'd essentially be looking at numerous rocky crescents.

I'd say low phase imaging would still be your best bet for detection of the rings if you could somehow block Rhea's light. I can't think of a good mechanism to do that other than looking at the moon just before it enters/leaves an eclipse and while the potential ring is in direct sunlight, but that requires good timing.

IIRC, as an object darkens in visible light, it brightens in the IR, and the Cassini ISS does have some pretty deep IR filters. On the off chance this is correct, unthought of, and useful, maybe a pic from further out might show something off the Rhean limb to whet our appetites.

If the expected 'boulder density' times IR emissivity does not seem to be generating a practical exposure time for the camera, never mind.

Actually, the best time to do that should be right around the time of the equinox. When Saturn's rings are edge-on, they won't be able to reflect much sunlight onto moons in Saturn's shadow, so Rhea should be darker than would normally be the case when it enters into eclipse. That would be an ideal for spotting any ring material orbiting Rhea.

Of course, there would still be some "after-sunset" sunlight transmitted through Saturn's atmosphere.

Coincidentally, Anne Verbiscer wrote about the eclipse idea, but to observe Enceladus' plumes at a low phase in a TPS blog entry.

Regarding the eclipse method, I actually had in mind mutual events not Saturn's shadow as that would potentially be a too diffuse shadow and one would like a sharp cutoff so the rings are completely illuminated and Rhea completely in darkness. Of course, we arrive at a different problem now - Rhea is the largest icy moon and except for Titan no other moon can totally eclipse it. Titan's eclipses could be pretty bright as well with the extended atmosphere.

All in all, a challenging endeavour.

Thanks for the responses - it does appear that remote sensing of the rings is something that pushes Cassini's capabilities to the limit.
The MIMI results may be all we ever have to work with - unless as suggested, there is a fortuitous eclipse that can be utilised.
Cassini has a very close encounter with Rhea in March next year (~100km) when the spacecraft's orbital inclination is near zero with respect to Saturn and Rhea - I'm sure ring search sequences will be planned for this - hopefully there won't be any 'direct sampling' of the rings such as a 10metre boulder impacting the spacecraft at 8km/s

http://news.sciencemag.org/sciencenow/2010...never-were.html

Cassini saw no material in the visible and constrained any "ring" material down to several orders of magnitude. But, the electron drops are still present, so something is going on there!

I know it sounds silly, but could a Rhean magnetic field be responsible?

Doubt it. My bet is that there's a low-yield gaseous emission of some sort (water vapor?) escaping from the surface that's getting dissociated/ionized & hanging around the moon.

No magnetic field, but maybe an ionosphere.

Would that explain the observed symmetrical structure around the moon?

Hmm. I don't know. Why would a magnetic field be required to produce a symmetrical gas distribution around any random body of sufficient mass? This is not a perfect analogy by any means, but Triton has an exceedingly thin atmosphere & (IIRC) no field.

I don't really think a magnetic field sufficiently describes the observations either.

I just don't have any better ideas

Me neither!

Looking forward to what the pros think; should be very interesting indeed.

{not claiming to be a pro, just a coincidence this post is next}


Following up on the ionosphere inquiry above, how about materials constrained in altitudes where absorptions occur, but not equatorially constrained? We have an actual upper limit for materials in 'equatorial rings' at these altitudes that is insufficient for the observed absorption, but if sufficient materials were in a continuum of inclinations, say + and - 60 to 75 degrees, we might have enough material, but spread thinly enough, to account for the Cassini density measurements, and yet still have enough mass to cause the observed depletion of the electrons. In other words, Cassini didn't see enough material in a 'ring' format to do the job, but what of a spherical arrangement, enough material, just spread out much more thinly?

I hasten to add, this is not a stable situation. The materials will collide. But the collision rate is dependent on density, and we seem to have a system that will not reach an equatorial equilibrium for considerable time.

Perhaps as the materials orbits decay (due to Poynting Robertson drag, and other factors), as they creep below the Roche limit for Rhea, these materials are at last cranked down to low inclination paths that we see the final effects as the Rhean equator staining.



BTW, do all plausible materials deplete electrons equally? Would powdered water ice (non electrically conducting) work as well as (electrically conducting) carbon grains? Or do the size of the particles (and the incredibly low density) render their conductivity unimportant?

young fracture on Rhea
There's no visible craters cutting through this.

Could Rhea's cryovolcanism have stopped much more recently than thought?

What if these particles were violently ejected into orbit around Rhea and the "lucky ones" have been able to stick around Rhea all this time and that these particles were sorted by size or density or overall mass overtime thus explaining them being in three belts like how Saturns rings are divided by type of material.

No field was detected, but the Voyager 2 trajectory was very poor for detecting such a field if it exists, so a weak field is not out of the question.

Ah- didn't know that. Thanks, Ted! It was a painfully strained analogy anyhow.