Space.com article

If this pans out, there goes Rhea being the most boring moon...

I'm not buying the idea that easily. They make it sound that rings are the only plausible explanation for the observation. Are missing electrons really a tell-tale sign of rings and nothing else?

If indeed there was a ring system around Rhea, would there be surface effects as well?

(f'r instance: Higher small crater density on the equator, pile of rubble deposited along equatorial bands - all of these were discussed by tasp regarding Iapetus in another thread.)

-Mike

The article below goes into more detail:
http://www.newscientist.com/article/dn1342...moon-rings.html

While I am not totally convinced, it is pretty interesting if true. Makes me wonder if Iapetus or some of the outer Uranian moons could have rings too.

Now also on the NASA Cassini-Huygens website:

http://saturn.jpl.nasa.gov/news/press-rele....cfm?newsID=820

I'll have an article later but have to board a plane first.....grrr....check back tonight!

Anybody feel like playing with the June 11 2007 ISS observations of Rhea in which they looked off the edge of the nightside to search for the rings? They've decided they saw nothing, and of course even if there were something the JPEG compression would have killed it, but still it's fun to play

--Emily

N1516373983_1.IMG appears to be by far the longest exposure image of Rhea at 100 seconds (!) available at the PDS, contrast enhanced here:


It doesn't show anything out of the ordinary, apart from a brightening behind Rhea's dark limb which, I suspect, is the E ring background. The bright line running almost diagonal across the sunlit limb is a diffraction pattern from the NAC optics support rods.

A couple of years is a long time to check and re-check your data, run rafts of simulations and design follow-up observations, so I would guess they're almost positive that there is some sort of orbital debris at Rhea.
Perhaps the collision which created the very young white splat crater kicked enough debris into a short-lived orbit to create some partial rings?

I'm reminded of : http://www.the-reel-mccoy.com/movies/2001/...onstersInc1.jpg

Well, thanks to absolutely awful weather in Texas forcing the cancellation of my flight I was able to post this afternoon:
http://www.planetary.org/news/2008/0306_A_...rn_Cassini.html

Thanks for digging that photo out, Gordan. 100 seconds!!! that is one steady spacecraft.

--Emily

Great article, Emily!

Sounds pretty strong circumstantially; I'm relieved to hear that planned future encounters will avoid close approaches to the putative ring plane.

OT here, but let me guess: Were you trying to fly to/from (or through) Houston? I've never gotten in or out of there on time during the storm season!

No, Dallas/Fort Worth. March is actually usually kind of pretty in Texas but it snowed there today -- and it stuck -- which is practically unheard of. I wonder how many Texans looked out their windows today and said "feh. Global warming is a hoax!" Anyway I'd much rather have a flight canceled before I boarded it than be stuck in an airport for hours and hours.

I should hopefully have a bit more on the rings story tomorrow, if Her Majesty permits me a few minutes to read my email and post about it.

--Emily

When the word 'ring' is used in regards to a solar system object, I imagine something that would show up plainly in a 10 second exposure, let alone a 100 second one.

Having said that, there is something interesting here, and it merits further investigation.

But characterizing it as a 'ring' or 'ring system' seems to be pushing it just a tad. Laplace envisioned something with particles interacting and 'collapsing' to a planar form over an oblate bodies equator, and thus, explained Saturn's rings. Subsequent work, (again a plug for the superlative Planetary Rings chapter in the classic New Solar System) has fleshed out the 'bump' process (dynamical ring spreading, as I recall) that tends to dissipate such structures in the absence of constraining resonances.

Here at Rhea, can we look at this structure and perhaps compare to other structures we have discerned, and maybe delve a deeper understanding of both examples?

I am thinking of the IRAS dust bands seen in the asteroid belt, and the zodiacal light seen from earth at sunset and sunrise, a result of sunlight reflecting from a plane of dust in the inner solar system.

Also, I note the symmetry in some of the ring structures (scallops) seen astride some of the outer Saturnian rings, and am pondering diffused E-ring materials wafted and concentrated near Rhea as perhaps explaining the symmetrical electron data.

I am just having a bit of trouble with the nomenclature for something that, to me, seems to be a bit of a stretch.

At an arm-waving speculative level....

As previously suggested, the very unusually fresh appearing bright ray crater could be a source of debris in orbit around Rhea. I'd like to see the dynamics of ejecta in models and long term evolution of orbits of ejecta that didn't escape, but also didn't re-impact on the first periapsis. (Get high enough in the Hill sphere and peturbations could raise periapsis, but peturbations would continue, causing escape or re-impact eventually for most particles.) I suspect that plasmasphere dynamics would fairly rapidly sweep dust size particles out of orbits around any moon within the magnetosphere. leaving a far harder (including at high phase angle) fis-tank-gravel and larger pebble size population of debris in longer life orbits.

Perhaps, to explain the 3 dips in the fields-and-particles data, there were substantial boulders in the ejecta that formed micro-moons, since shattered, each forming micro-rings of gravel without dust.

My arms are getting tired from all the waving and gesticulating. Have a beer.

Me too!

Common sense would suggest that all trajectories taken by ejecta will either escape (if they're going faster than ~630 m/s) or impact the surface within an orbit (with a maximum duration of ~ 26 hours).

But there's a small range of velocities and angles which will take ejecta close to the edge of the Hill Sphere, 7.6 Rhea-radii away. A particle leaving Rhea nearly tangental to the ground, and travelling just shy of 600m/s, will reach apoapsis at this boundary. In the 13 hours it takes to get there, Rhea's "done" 12% of a orbit of Saturn and the other satellites (Titan by far the most influential, Dione less so) will have had a say. I can see "some" material tweaked into stable(ish) near-circular orbits - but the amount of material must be tiny.

Andy

How about a double or multiple impact by a disrupted comet?. There could then be interactions at height between the different ejecta clouds resulting in some material achieving the right velocity to stay in orbit.

... OR how about magical tooth fairies?

Sorry, couldn't resist

Magical tooth fairies impacting Rhea and casting off debris at a velocity low enough to stay in orbit? Hmm...

...would explain why the Magical tooth fairy doesn't visit me anymore.

...Gordan, you're killin' me!

Almost seems as if this might be a 'just right' situation, though: the impact angle has to be just right, the impact velocity has to be just right, the debris composition/size distribution has to be just right...and even then, it ain't gonna last. How long could putative rings endure?

Good question. If the answer is 'not long' and IF we're really seeing one then they could be fairly frequent events though. More so in the distant past too. Major implications all round.

Where would metaphorical Lagrangian points be for Rhea? Is it possible that they are at such a range that a modest impact would chuck material out at a velocity to reach, and then disperse around them?

Doug

Are there any ideas on the total mass in the putative rings? The non-detection by ISS would put a lower limit on particle sizes. If the particles are indeed appreciably large (several mm and upwards) to not effectively forward-scatter light, actually imaging them might pose a problem. They'd show up better in low phase imagery, but then you'd have Rhea's disc light-polluting your instrument as well.

What are the chances the ring material would be detectable in deep IR (thermal?) where ice is weakly reflective? Alternatively, how about a radio science occultation?

EDIT: I get L1/L2 point distances of about 5800 km from Rhea's center, using the formula r=R*(Mrhea/(3*Msat))^1/3

For Rhea, L1, I get 5,810 km from the center of Rhea, towards Saturn.
For Rhea L2, I get 5,850 from the center of Rhea, away from Saturn.

I get these numbers by approximating roots of the 5th-degree polynomial, so they should be as accurate as the Wikipedia data I put into them. And I see UGordon gets essentially the same answer with a LOT less work! :-)

--Greg

ISS non-detection results in mean particle size of ~35 cm and particle concentration ~10^-12 m^-3, see original paper and supporting online material for details.

I can't access that link, unfortunately. 1 particle per 10 cubic km? That's stretching the term "rings" indeed. The total amount of particles (ISS upper limit) would then be on the order of a few hundred thousand for reasonably confined rings (say withing 10 km horizontal and vertical extent per ring).

I can't imagine any optical instrument picking that up. Is radar sensitive enough to detect this if one were to perform a radio occultation as in the case of Saturn's rings?

To put it in perspective, if the ISS was broken up into 35 cm pieces and smeared along its orbital path, that would yield a particle density of ~10^-4 m^-3.


[Living volume of the ISS = 424.75 m3 = 424E6 cm3. Breaking into 25 cm3 particles gives 16E6 particles.

Average radius of Earth = 6371 km = 6.371E6 m
Average orbital height of ISS = 330 km = 330E3 m
Average circumference of orbital sweep = 3.141 x 2 x [6.371E6 m + 330E3 m] = 42E6 m.

Length of ISS = 58.2 m (along truss)
Width of ISS = 73.5 (along solar arrays)

Orbital volume of space swept by ISS (orbital circumference x length x width) = 42E6 m x 58.2 m x 73.5 m = 179E9 m3

16E6 particles in 179E9 m3 volume = 1E-4 particle per m3.

So we could have a ring, too.

-Mike

Actually, we *do* have some kind of ring, considering all the space debris orbiting the Earth at the moment.

Sorry, this might be stupid, but if the ring indeed exists, it would mean that a Rhea flyby by a spacraft would be really risky !
And this is especially the case if the particles are as big as suspected.
A chance that nothing happened during the Cassini close flyby !
Marc.

That is if you consider missing one particle in a 10 square km volume in the densest part of the proposed ring a chance...
I'd have more chance of winning the lottery 3 times in a row than hitting that accidentally.

My first thought was "Cassini's camera isn't that big!"

This is counterintuitive to me...

If a noticeable fraction of electrons/ions zipping through the putative ring are waylaid, it seems to me like a noticeable fraction of photons should also be waylaid.

Even if we grant that ice is a perfect absorber of electrons, but allows a fair number of photons through unmolested, there should still be diffraction going on. The rings ought to be visible in that long exposure. I can't see why a quantity of ringstuff insufficient to show up backlit should block the particles and fields stuff.

We have Saturn's rings to elucidate this point. They vary from thick to wispy. Do we see particles and fields shadows where no visible stuff is?

I recall that the Galileans create aurora blobs on Jupiter, and those blobs aren't perfectly moon-shaped.

http://hubblesite.org/newscenter/archive/r...rmat/web_print/

Is it possible that what we're seeing IS Rhea's shadow in the charged particle belts of Saturn, and that the pattern differs funkily from the obvious? For example, an electron diffraction pattern on the quantum scale:

http://cache.eb.com/eb/image?id=96828&rendTypeId=4

Granted, Rhea is a few dozen powers of 10 bigger than your typical quantum object, and we've seen quotes that other satellites don't show the same effect, but I wonder if there isn't a tricky explanation. The invisibility of said rings is odd.

Hi all....way too tired after the trip cum toddler to post something coherent....but here's a plot from Geraint Jones comparing the electron flux from the Rhea flyby with the electron flux from a Tethys flyby. You can see drop in flux w/proximity to Rhea, and the 3 symmetrical dips they're identifying as rings or ring arcs.
Click to view attachment
JRheling, I think there was somehting in the paper about the weird way electrons traverse the system that would make them more likely to be interrupted by the rings than photons would -- I'll try and remember to look for it when I've got some brain cells back, or maybe somebody else with Science access can try and find the relevant paragraph.

--Emily

Thanks for posting the graph. Clearly something interesting is happening around Rhea.

But I have some concerns.

Would concentric spheres of "whatever this is" make the same graph as rings? Or 'belts' that extend, let's say 45 degrees north and south of the Rheaic equator?

See belts and spheres aren't rings. And by using the term 'rings' we might be preconceiving too much and delay understanding whatever is going on here.

The negative imaging results are crucial information.

Wouldn't whatever is collimating or confining the particles (if there is a force) cause them to at least once in a while contact each other and create dust we can see in backlighting?

The IRAS dust bands seen in the asteroid belt I have not noticed anyone refer to as rings (but if I missed that let me know) and if this might be a ~similar structure (allowing for bigger particles) then that might be significant.


Could these 'whatever' about Rhea be gravel in horseshoe orbits (from the L4 to L5 the long way about Rhea) spalled from 100 meter diameter satellites of Rhea, so far overlooked?


(help me out here, are Lagrange effects possible this close to Rhea from satellites plausibly unobserved at this point? It's been awhile since I read up on that)

Is there any relationship with the orbit periods noted for those Rhea altitudes? I am trying to figure out how 100 meter satellites of Rhea might be stabilized over longish time spans so we could be seeing their effects now. If they are all in resonance, or constrained by nearby resonances, that might help figure out what is going on here.

I am quite concerned that long term orbits around Rhea, due to proximity to Saturn, are not stable enough.

Gyroradius, probably (I havent read the paper tho). The electrons go in little spirals around the field lines
so their interaction cross section is larger than it would otherwise be.

Btw - while rings around Rhea sounded at first to me as wierd as they doubtless do to everyone, and I am
not used to 'seeing with electrons', there is a bit of a pedigree of this sort of thing that some folk
might want to follow up on. there was a similar (but larger) bite-out of the electron distribution observed
by Voyager at I think a couple of L4/L5 points of satellites. the speculation was that clumps of dust or
whatever were blocking out the electrons, much like the rings or ring arcs here.

The paper (in JGR, IIRC) was called 'The Mimas Ghost......'

Ah. So, it doesn't take very much material at all to produce detectable effects, then? Interesting.

Maybe "dust belts", or maybe even "gas belts" would be a better term then "rings". Beginning to doubt that there's anything as solid as what we think of as ring debris there that's detectable in conventional imaging, unless perhaps the material fluoreseces (sp.) at least weakly? Looks like it's absorbing a significant amount of energy, you'd think that this might cause a release of some kind.

Assuming that the bulk of the original material is/was water, and drawing an analogy to terrestrial auroras, then it might be worth looking for a green peak from excited atomic oxygen around 558 nm, with maybe another red one @ 630 nm.

Do we have any magnetospheric images of the Rhean environs ?

Might be interesting to image Rhea this way as it moves (relatively) to the Saturnian magnetic field (or vice versa) through a substantial angle and compare to Tethys with it's more 'normal' interactions seen in the electron data.

I don't have the remaining flyby data handy, do we have any chance of transmitting Cassini radar instrument power either at the Rhean material and studying the reflection with Cassini, or illuminating Rhea with it from behind and observing the signals interaction with this material from earth ??

Tasp, read my article and you'll see that a debris sphere is elminated by the RPWS and CAPS data. They determined the stuff had to lie somewhere in the Rhea system but not at the southern latitude through which Cassini flew. I'm not sure how non-equatorial belts would be gravitationally stable. Jones et al did do their homework to see if long term orbits were stable and they concluded that they were, for particles that are NOT strongly influenced by non-gravitational effects -- which would explain the lack of dust-sized particles.

Not that I'm utterly convinced; I'd like to see at least one other instrument provide a positive detection.

--Emily

Yes,

I covet having another instrument detect this stuff. A few dips in a graph, while wonderfully and deliciously provocative, just doesn't flesh out enough about what is different around Rhea compared to other moons.

IIRC, the IRAS dust bands detected in the asteroid belt span +/- 10 degrees from the ecliptic. I am not sure how much inclination objects in Trojan relationships can have, it seems some of the leading Jupiter Trojans have inclinations of ~20 degrees. I don't know if this is pertinent to objects/debris in horseshoe orbits, though.

Anything curious turn up around Iapetus in the electron data ??

[wink]

By the way, central ring period exactly equals the 1/10th of Rhea's orbital period. I wonder if there are any dips in the MIMI plot corresponding to 1/5, 1/3 or 1/2 periods?

Hi everyone,

Just to make some more issues on the charged particle motion in dipole fields and the concept of the study understandable:



Here are the three elements of motion in dipole fields:

1) Particles gyrate perpendicular to the field lines (the higher the energy and the mass, the higher the gyration scales)
2) Particles that have a velocity component parallel to the field lines, will also perform a bounce motion along the field line, with mirror points at some latitude
3) Particles will have drift motion, opposite for electrons and ions and faster with increasing energy. In the sketch the drift direction of ions and electrons is opposite to the one that we have at Saturn, as at Saturn the dipole field is oriented southward. This drift motion is also “superimposed” to an additional drift (which is called the corotational drift), which, in (very) simple words, is induced by the dipole field rotation with the planet. This drift is in the same direction for ions and electrons. Effectively, energetic ions cross faster the Rhean environment than electrons and the latter have lots of time to interact with the surrounding environment. Energetic ions, actually, don’t even “feel” Rhea.

Specifically for the case of Rhea, the depletions were seen in energetic electrons - those that have relatively large gyration scales (in the order of 10 km - equal to the inferred mean distance between the large grains). Low energy electrons did not show these depletion effects, as the gyration scales are so small (tens of meters) that one can imagine those zipping through the grains with a very small impact probability (just like Cassini does).

The bounce motion of energetic electrons is also very rapid. During the time they need to drift across Rhea's Hill sphere, they cross the equatorial plane more than 10 times, and "sample" the Hill sphere northwards and southwards. In this way, they perform some kind of a "tomography" of the Rhean environment. So if the distribution of absorbing material also has some finite thickness, this will also contribute to the electron depletion profile. Note also that gyration is so rapid (milliseconds or less) that energetic electrons gyrate several times while crossing the equatorial plane. On the other hand, an energetic ion needs about 1000 sec to complete a bounce motion. In that time most of the ions have “jumped” above or below Rhea and its Hill sphere, with little or no interaction.

Note that due to the bounce motion, the effective column mass crossed by an electron can increase with the disk’s thickness, while in an edge-on picture of Rhea, the thickness does not contribute to the opacity of the candidate absorbing medium. Only the line-of-sight column mass will determine the opacity.

The picture at Rhea is actually no small deviation from the typical, expected plasma interaction. Its quite large, given the lack of atmosphere of that moon, as at least 4 instruments have shown with data from this flyby and elsewhere. Its not just the tiny depletion signatures on either side of the moon. Having only these, one could maybe talk about some tiny deviations. But the depletion of electrons extends on either side of Rhea by about 8 Rhea radii. It’s a huge region. Furthermore, this is very close to the Hill sphere scales, a quite fundamental boundary. That is rather peculiar for an obstacle not much different (in electromagnetic terms) than the Earth’s moon or Tethys. The coincidence of several interaction features with the Hill sphere scales is what makes everyone suspicious, about gravitationally bound material being present there. The behavior of the absorption regions as depletion regions is also peculiar.

The provided numbers have large error bars. These are rough order-of-magnitude estimations, that simply highlight the fact that it is the mass of the absorbing material that matters for the electron depletion, not the number of dust grains. You can either distribute this mass to many small particles (with small impact area and small mass), or to few large grains (with large impact area and column mass). This has been shown also in other studies, the quite recent ones relating to the possible presence of material along the orbit of Methone and Anthe and the G-ring arc, where similar depletion regions in electrons have been identified by MIMI. Note also that similar depletions have relvealed the presence of the G-ring, Epimetheus, the F-ring and the rings of Jupiter (by Pioneer 10, 11). What is more definite is that small particles cannot have stable orbits. So if what we see is actually absorption by grains, these have to be in the mm to m size range. Of course, there is a limit on how low the opacity of the absorbing medium can be. If the extracted densities where such that the mean distance between the large grains was in the order of 100 km, the proposed solution would have not worked.

Of course all these observations could be simply misleading coincidences. No one is absolutely convinced with the presented scenario, but it's the only one found by the authors that agrees with the observations, and is shown to be generally feasible. Definitely, plasma dropout does not directly mean plasma absorption. Plasma physics is not as direct as photometry in that sense. That’s why data from all plasma instruments was considered for this study and thanks to that several other candidate known plasma dropout scenarios have been excluded. The proposed scenario is the best one that the authors were able to come up with. Even if this turns out not to be the solution, it doesn’t modify the fact that these interaction features at the vicinity of an inert moon, are definitely unique, interesting and unusual.


I hope this helps the discussion

Wow! Thanks a lot!

So, to eliminate doubt, what would the best followup experiment be? Is there anything Cassini itself can do, or will it have to wait for the next Saturn probe?

--Greg

I have no problems with seeing with electrons. But to be confirmed it really needs another instrument.

Curiosity and cat... The Mimi Lemms data are on PDS/PPI

I downloaded the stuff, found the needed files (2005-330), converted the counts into the physical units (got exactly the same values as in
Jones paper: Y axis in my plots is in 1/(cm^2 sr s keV)) and I plotted all electron channels in various
resolutions (closer to Rhea resolution is better, plus channels C1 and C5 come in yet higher sampling rates).
Click to view attachment Click to view attachment Click to view attachment

For additional details see the post by Emily:
In short: I took timings from Jones et al, and the leftmost white dotted line is entering Hill sphere, the rightmost is exiting. Blue dotted vertical lines denote location of isolated 6 "ring" features.

Surprisingly the profiles look very wiggly in general, at least to my untrained eye.
For instance, what are those two dips at around 22:45 and 22:46?
I am not expert in this, but following the logic from Jones et al.: the two dips are present in most channels and they are broader and deeper with increasing energy (until they are lost in noise in higher energy channels). Thus they would also appear real?

Does anybody know the date of the Tethys flyby they used for comparison? Or any other moon flyby where Cassini entered the Hill sphere?

Cool, it didn't occur to me to check the PDS for this data, but it should have!

Geraint Jones told me that the Tethys flyby was the 2005 day 267 (Sept. 24) flyby.

I don't know of any other specific flybys but there aren't that many flybys of non-Titan icy sats that are within the Hill spheres of the moons. They'd mostly be targeted ones. The only other targeted flyby I find for biggish non-Titan icy moons is Dione Tue, Oct 11, 2005, (2005-284T17:52).

I suppose, for completeness, it'd be interesting to check on a Titan one.

--Emily

This analysis cannot be done at Titan, without avoiding lots of complexity. Titan is at the location in the magnetosphere where the dipole field is strongly distorted by the interaction with the solar wind (sometimes Titan can be in the SW). The magnetosphere is much more variable there and a lots of non-dipolar effects (stohastic or not) could affect the charged particle motion. All these add lots of complexity.

Iapetus is mostly in the SW, there are not many energetic particles there to be able to observe something relevant (unless an interplanetary energetic SW event takes place).

Nothing similar (like at Rhea) was seen at Dione or Tethys.

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):
Click to view attachment

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:
Click to view attachment
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.