To add to that... electron absorption signatures were seen near Enceladus, and reported previously... these are interpreted as being due to Enceladus plume particles absorbing the charged particles.

The roughly symmetrical narrow dips near Rhea are understandably receiving a lot of attention, and those other dips that zvezdan points out are worth looking at in more detail.

As Elias pointed out a couple of posts back though, the broad decrease is being presented as the primary evidence for an obstacle on the scale of the Hill sphere, i.e. the "debris disk". This broad decrease is pretty muted when plotted on a log scale, but around 2/3 of the electrons in the top panel of zvezdan's first plot disappear between the boundary of the Hill sphere and the wake of Rhea itself.

Elias, looking at your description of fields around a planet, and using Earth as an example, makes me want to ask this question - and I apologize to folks for opening a bucket of worms - but: Is there a standard definition for what constitutes a "ring" around a body? I.e., does it have to be a certain density? Does it have to be molecular, or would ions suffice? Does it have to be visible? Does it have to be planar, or is a torus also a "ring"? Does our constellation of geosynchronous satellites in equatorial orbit constitute a ring?

Accurate definitions are always hard to achieve. I dont know any real definition for "rings" - maybe its not fully appropriate for what is inferred to be present at Rhea. Nevertheless it would be exiting to confirm the presence of debris orbiting Rhea, no matter how one would call them.

If I may stick my oar in... I think at a minimum, solid matter is implied when a ring is being talked about; how "flat" that has to be to be defined as a ring rather than a torus, I don't know. If you google Mars and dust ring, there are a few entries that refer to a putative Martian "dust ring/torus".

In the recent Rhea paper, it isn't just a ring that's suggested: an expected spherical cloud of dust was detected, but the suggestion for additional material is in the form of "an equatorial debris disk", and that within this disk "may reside denser, discrete rings or arcs of material".

Thanks Emily for the dates. There are indeed very few close flybys of the icy moons

So I also checked the Tethys flyby. But for that one I needed geometry and had to solve it by myself using Naif Spice.
As a check I reproduced Rhea flyby geometry and it agrees with Jones et al values to within 1 second (good enough for me):


The more recent Rhea flyby reported in their supplement did not actually enter the Hill sphere (for Rhea: RH=~ 5800km) and was over the North:

This is bit contradictory with the paragraph from the supplement where they say that Cassini did briefly enter the Hill sphere, but my numbers are consistent with this link.
To avoid the confusion Tethys goes into the next post.

Tethys flyby geometry (Hill sphere radius is only RH=~ 2100km):

And Mimi lemms electron data from PDS:

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:

Cassini went directly over the south pole, and that is probably the reason why there are absorptions of different width with increasing electron energy:

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.)