A moon for Maxwell? Options

[jasedm]

The ciclops 'looking ahead' article for rev 60 (February 25th to March 7th) has just been posted, and alas no Titan or icy moons this revolution (bar a very distant wink at Tethys).
However, on the slate of observations is a 250 frame azimuthal scan of the Maxwell gap in the outer C ring, either side of periapsis.
This gap is around 220km across - more than enough to harbour a sizeable moon, and hence I suspect the reason for this observation.
AFAIK no scalloping either side of the gap has been detected thus far, but an irregular ringlet resides in the gap (as does one within the Encke division )
Anybody willing to predict yes or no to a new discovery? - any dynamicists here with sound reasons why one's unlikely to be found? does the gap coincide with any major resonances with other moons?

My guess is that no moon will be found (let's arbitrarily define 'moon' here as having a diameter 1km or greater) but I'd love to be proved wrong, and that Cassini is to find Saturn's 61st moon on the 61st orbit of the planet

[nprev]

My guess is that no moon will be found (let's arbitrarily define 'moon' here as having a diameter 1km or greater) but I'd love to be proved wrong, and that Cassini is to find Saturn's 61st moon on the 61st orbit of the planet

Glad you added that qualifier; I've been waiting for a debate to open up about what is & ain't a moon based on Saturn's population of ring particles...although, it seems that people are a lot more comfortable with a term like 'moonlet' then they ever seem to be with 'planetoid' or its various equivalents (we are strange beasts indeed!)

Mmm...I'm willing to bet that there's something in there, but it's probably very small. Has anybody calculated the minimum size needed for a body to clear out its own gap in the rings? (Lots of variables there, esp. the average size of ring particles in a given region; doesn't seem like an easy computation).

[siravan]

I guess that in a very simplistic way, a moon(let) clears a path equal to its Hill sphere. In the distant of the Maxwell gap (87500 km) from Saturn and assuming a moon density of 0.7 g/cm^3 (similar to Janus or Epimetheus), the Hill sphere is approximately the same size as the moon. But this is assuming an eccentricity of 0. If the orbit is eccentric, the size of the gap depends more on the eccentricity rather than the moon size itself. That close to Saturn, eccentricity in turn depends on the orbital resonance with other moons. Now, I've no idea which orbital resonance sustains the Maxwell gap or what eccentricity to expect.

[jasedm]

Hmmm - interesting Siravan... IIRC the Voyagers detected an irregular ringlet within the Maxwell gap although no images of it were made. (photopolarimeter results??) I hadn't connected the eccentricity of a moon's orbit with it's subsequent tendency to clear out a wider gap as a result.
The Maxwell ringlet's width varies by several tens of kilometres within the ~250km gap, and perhaps it lies along the elliptical orbit of a small yet-to-be-discovered moon(let).
Comparisons to the dynamics elsewhere in the Saturn system are irresistible - Encke gap - irregular ringlet within the gap along the orbit of Pan, and the very diffuse rings following the orbits of Janus/Epimetheus, and Methone.

[zvezdan]

Hello folks!

This seems to be a nice place to give an example of Encke
gap ringlets (plural, there is 4 of those).

Long time ago I took >100 NAC images from 013RI_AZSCNHIPH
(2005/08/20) sequence, calibrated them with cisscal from PDS-rings,
used NAIF Spice + hand corrected geometry
and arranged into a panorama (orbital motion between images was
subtracted).




1st picture is linear ==> I/F mapped linearly onto grayscale (min = 0.0080, max = 0.020)


2nd picture is logarithmic ==> I/F mapped logarithmically onto grayscale (min = 3e-4, max= 0.020)


Typical image from the sequence is:




At the gap edges in panoramas you can see regular sine-like pattern due to Pan.
From those wiggles Jeff Cuzzi and Mark Showalter inferred
the existence of Pan in Voyager images.
The wavelength of the edge wavy pattern is =~ 3 Pi * distance =~ 1500km =~ 0.6deg in longitude.

And I would love to know what the hell is making the wiggles in ringlets.
My guess: not Pan, but other smaller chunks (~1km size).


And here are details:

X coordinate = azimuth (+X == orbital motion)
Y coordinate = radius betwen [133384,133784]km from Saturn (-Y == towards Saturn, +Y == away from Saturn)

radial pixel size = 1km (total = 400 pixels in [133384,133784]km radius)
azimuthal pixel size = 12km (total = about 44 deg of circumference, orbital motion subtracted)


(Oh hell, I had to convert the images to JPEG to reduce the file size...)

[jasedm]

Welcome zvezdan - great first post!

[jasedm]

The images for the last periapsis pass are now up, and although I haven't checked all the NAC shots of the Maxwell gap, the edges appear unscalloped. In fact there seems to be a remarkable smoothness to the gradations in the rings at this range, and very sharp edge boundaries.
Needless to say no moon(let)s are visible (I don't know what the limiting resolving power of the NAC is at a range of ~260,000km, but I Imagine anything larger than around a kilometre would have been picked up).
Perhaps a number of very faint/very dark rocks in the range of a few hundreds of metres in various elliptical orbits are responsible for clearing out the gap.

[jasedm]

Although this image did get me going for a while..........

[ugordan]

"That's no moon...."

Sorry, couldn't resist.

[jasedm]

Unfortunately Gordan, it seems you're right - the search for a Maxwell gap moon (thus far) has proved fruitless. I wonder what the constraining resolution was on that last series of obs? From the limited amount I know of the Cassini NAC, I'm tempted to conclude that there's nothing there >1km. But this gap has pin-sharp edges at the resolutions achieved so far - could they possibly be maintained by gravitational resonances with moons further out?
My feeling is no. There's something(s) in that gap (or do I mean division ) clearing out a space.

[zvezdan]

First, let me correct few statements in this topic.

Two examples of an embedded moonlets (Pan and Daphnis) have their Hill spheres equal to their radius. An explanation for that was offered by Porco et al.
Such a moon maintains a gap that is several times their Hill sphere (factor of 3-10).

The outer moon resonances (either Linblad or vertical) can and do maintain some sharp ring-gap edges (examples: B ring outer edge by Mimas, A ring outer edge, bunch of such edges in C ring...).
There are also isolated ringlets created by strong resonances, for instance Titan 1:0 makes Titan ringlet in the C ring.

But the Maxwell gap is strange as there are no strong resonances anywhere close to it.
The ringlet inside the gap is eccentric and has variable width. Such a shape was neatly explained by Esposito et al.: the self-gravity of the ringlet keeps it eccentric and dictates the variable width (an eccentric ringlet without such effect would soon spread around, as particles with different distance to Saturn will have different precession rates due to the oblateness of Saturn and will make a circular spreaded ring).
However, that still leaves the Maxwell gap edges in question: what maintains them?
Well, there are many other edges which don't have an associated moon resonance. Are we going to propose un-observed embedded moonlet for each one of those? Plus, those moonlets would be deep inside the Roche zone - how would they survive the tides?
My 0.02$: it could be some interplay of ring (or isolated ringlets) self-gravity that helps confinement, similar to self-gravity keeping the eccentricity and shape of the Maxwell ringlet.