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zvezdan
Posted on: Mar 22 2008, 10:03 PM


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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.



  Forum: Cassini general discussion and science results · Post Preview: #111176 · Replies: 10 · Views: 6270

zvezdan
Posted on: Mar 11 2008, 06:44 PM


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QUOTE (ynyralmaen @ Mar 11 2008, 03:07 AM) *
Thanks for the plots. Closest approach was 5737km... wasn't that just inside the Hill sphere?

If that number is altitude then not.
  Forum: Cassini general discussion and science results · Post Preview: #110711 · Replies: 87 · Views: 38035

zvezdan
Posted on: Mar 11 2008, 01:19 AM


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And Dione flyby:
Attached Image

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

Edit: forgot to write that Hill sphere radius is about RH=~ 3250km
  Forum: Cassini general discussion and science results · Post Preview: #110680 · Replies: 87 · Views: 38035

zvezdan
Posted on: Mar 11 2008, 12:59 AM


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

And Mimi lemms electron data from PDS:
Attached Image
Attached Image

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.
  Forum: Cassini general discussion and science results · Post Preview: #110678 · Replies: 87 · Views: 38035

zvezdan
Posted on: Mar 11 2008, 12:49 AM


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

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):
Attached Image


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:
Attached Image

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.
  Forum: Cassini general discussion and science results · Post Preview: #110677 · Replies: 87 · Views: 38035

zvezdan
Posted on: Mar 10 2008, 01:23 AM


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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).
Attached Image
Attached Image
Attached Image


For additional details see the post by Emily:
QUOTE (elakdawalla @ Mar 7 2008, 10:31 PM) *

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?
  Forum: Cassini general discussion and science results · Post Preview: #110654 · Replies: 87 · Views: 38035

zvezdan
Posted on: Feb 28 2008, 11:22 PM


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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)
Attached Image


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


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...)
  Forum: Cassini general discussion and science results · Post Preview: #110142 · Replies: 10 · Views: 6270


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