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Enceladus Jet Sources
ugordan
post Oct 9 2010, 10:20 AM
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I don't know what I'd expect! That doesn't appear to be the case, though. Look at alan's enhancement. It appears to be more or less uniformly "filled".


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remcook
post Oct 9 2010, 10:25 AM
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I see a little bit of a ring, but indeed small contrast. Might be optical illusion. But very sharp boundary indeed. It's just weird smile.gif
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ugordan
post Oct 9 2010, 11:32 AM
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Perhaps the most compelling evidence that this is not an optical artifact or optical effect in the ring particles comes from comparing two frames taken at different distances.
Below's a comparison of two clear filter frames taken from 617 000 km and 540 000 km, that's a 14% difference in distance. First, both images at their original pixel scales:



And here's the match when resized to the same pixel scale:



The overall E-ring brightness drops off rapidly with lowering phase angle so it's not really surprising this can only be seen in the highest phase angle imagery Cassini has obtained so far (notwithstanding the grand Saturn eclipse sequence as that was too low resolution).


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ZLD
post Oct 9 2010, 06:29 PM
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I've just finished looking through tons of other sets of pictures that feature Enceladus and am missing any good signs of the halo around the planet in past pictures. This isn't to say that it isn't still there; it may need to be lit in a very particular angle for the diffusion of the particles to reflect it in the lens. A more interesting possibility could be that this is something that has just been generated or at least become more prominent due to the changing season.

In this image from Rev 121, only a very faint presence of this halo exists. I would be inclined to say that this is more likely an optical illusion or a processing artifact but after overlaying the other processed image on top, they clearly line up. Take that however you will.


-Edit: I Just realized I hadn't rotated the image in that overlapping example. Here is the corrected image (though only a very quick aligning was done). Interestingly, they still appear to line up.




I did however stumble onto something else interesting in looking through past photos. PIA_12693 from early in August, it looks like there is a somewhat prominent blurring/diffusing circling the moon. I've blown up the image 400% to show this more clearly. Not sure if this illustrates anything really but the diffusing effect is quite similar to the one of the much larger halo.



Strange and interesting indeed.

-Zac


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nprev
post Oct 9 2010, 06:45 PM
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Just a thought here: Maybe the halo boundary is actually at Enceladus' Roche limit distance (liquid satellite version), which is pretty small (smaller than the putative Hill sphere radius, presumably.)

EDIT: This seems to make sense. The stuff in the halo is actually in orbit around Enceladus; beyond the critical radius it goes into orbit around Saturn. The N-S elongation is caused by excess velocity imparted to some of this stuff from the south polar plumes, which is pushing some of the material into polar elliptical orbit; presumably a lot of this gets picked off by Saturn as well & joins the E-ring.


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ZLD
post Oct 9 2010, 06:57 PM
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The only problem that I see with the halo is that basically what you're describing is an atmosphere and as I understand it, moons as small as Enceladus, which is quite small compared to even our own moon, aren't supposed to be able to retain particles of this size. If the halo was made up of semi-large icy chunks, I could see it being more possible but then it should also be much more visible. The distance the halo extends away from the surface is also baffling considering the size of Enceladus. More data is necessary.


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ugordan
post Oct 9 2010, 07:09 PM
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QUOTE (ZLD @ Oct 9 2010, 08:57 PM) *
The only problem that I see with the halo is that basically what you're describing is an atmosphere

Not an atmosphere, atmospheres aren't in orbit around the parent body. These particles (not gas) *might* be in orbit around Enceladus.

FWIW, I looked at previous high phase imagery and have not seen anything of the sort. Even a wide-angle context frame taken at roughly the same time doesn't show it, though the low resolution and graininess leaves something to be desired.


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brellis
post Oct 10 2010, 05:49 AM
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QUOTE (nprev @ Oct 9 2010, 11:45 AM) *
The stuff in the halo is actually in orbit around Enceladus; beyond the critical radius it goes into orbit around Saturn. The N-S elongation is caused by excess velocity imparted to some of this stuff from the south polar plumes, which is pushing some of the material into polar elliptical orbit; presumably a lot of this gets picked off by Saturn as well & joins the E-ring.


Oxygen ends up in Titan's atmosphere. Might that be related?
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nprev
post Oct 10 2010, 05:59 AM
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Maybe. Enceladus' emissions seem to end up splattered all over the Saturnian system; the E-ring is just the densest part of it.


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brellis
post Oct 10 2010, 06:09 AM
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Got it! A halo would imply a wide emission of material. Perhaps at certain points it emits more stuff towards Titan, other times it feeds the E-ring, and other times it recollects material?

re ZLD's post: Perhaps it is a trailing atmopshere, almost like a comet?
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ZLD
post Oct 10 2010, 08:17 AM
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A trailing atmosphere, similar to a tail and coma on a comet, could be a possibility but generally for those to be present even around a comet, they have to be rather close to the sun before they are present. The lack of any type of obvious tail also presents a problem with that theory. However, the data is obviously inconclusive and pretty much anything is open for possibility at the moment as this seems to be something entirely new to everyone.

-Zac


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nprev
post Oct 10 2010, 08:55 AM
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Let's step back a bit & look at the situation:

1. Enceladus is a teeny little mass very close to a very large mass.

2. Enceladus is active, and emits a steady stream of (mostly) water vapor continuously into space.

I don't think that the comet analogy really applies here. Comets are much smaller than Enceladus, and as ZLD points out their emissions are a consequence solely of solar effects on their substance. They have effectively no gravitational influence over their surroundings, so their emissions just blast off into space more or less in the same direction as the stimulating radiation.

In contrast, Enceladus is massive enough to hold onto at least some of the stuff it's venting. Also, Saturn's gravitational influence is much more influential than solar radiation pressure, esp. at the system's distance from the Sun.

So, what I think might be happening here is as follows:

1. Enceladus is surrounded by a spheroid of emission products from the south polar jets; this is Gordan's halo, and it's caused by particles in orbit around the moon that didn't achieve escape velocity.

2. The polar elongation of this spheroid is due to either magnetic effects from Saturn or the fact that some of the particles blasted out by the jets are more energetic than others & end up in elongated polar orbits, or both. (The fact that the elongation is N-S really leads me to think that the polar-orbit effect is real & dominant, and mostly an artifact of the location of th jets on the moon.)

3. Eventually, the stuff in the halo escapes (possibly aided by solar radiation pressure and/or other electromagnetic effects from Saturn), but it can't get out of Saturn's Hill sphere so it becomes the E-ring & a much more diffuse & extended torus of material that encompasses pretty much all of the inner moons out to Titan. (Titan itself probably bounds it rather effectively by sweeping up damn near all of the material before it can migrate outward any further from Saturn).


Anyhow, that's my theory. Reality of course may be completely different! wink.gif



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remcook
post Oct 11 2010, 12:51 PM
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So, it does seem to be something physical, not optical. From a gut feeling sort of thing, I would expect stuff that would hang around Enceladus to be more dense near the moon itself and not be so neatly uniformly distributed with such a sharp edge. Some (dynamical) simulations people have done for Enceladus (for those with access :S ) seem to show something similar :
http://www.scienceonline.org/cgi/reprint/311/5766/1416.pdf
http://www.sciencedirect.com/science?_ob=A...mp;searchtype=a

http://www.lpi.usra.edu/meetings/lpsc2010/pdf/2635.pdf (free)

So, something else might be going on as well? weirdness stays blink.gif
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ugordan
post Oct 11 2010, 01:27 PM
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QUOTE (remcook @ Oct 11 2010, 02:51 PM) *
So, it does seem to be something physical, not optical.

On the other hand, there's a problem with this conclusion. I would expect Cassini's sensitive fields and particles instruments to pick up this change in perceived optical density as well if it was an actual density gradient. Also, wouldn't UVIS occultations pick up this enrichment in material - although it's possible measurements typically started closer in than about 2 Enceladus radii?


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remcook
post Oct 11 2010, 02:54 PM
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Indeed. Personally, I'm not convinced by any possible explanation at the moment.
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