I suppose this is a good place to post this... I just finished up a color version of this observation and it turned out to have two bonuses to it I wasn't aware of. One is that Enceladus' shadow on the E-ring is faintly visible, a rare sight. The second one is the reason why I'm posting this. There's what looks like a dome above the plumes. If you look real closely at the (contrast-enhanced) image below you can pick it up as a discrete change in plume brightness gradient about 1 Enceladus radius above the south pole.



My initial reaction was that this had to be some kind of artifact, but now I'm not so sure. It's visible in at least 3 clear filter frames and a red and blue frame. Moreover, the position of Enceladus was different in the color frames and the feature still follows it, suggesting it's not an optics scattered light effect from Saturn or something (Cassini was in eclipse at that point IIRC, anyway). Here's a natural-ish color R+B image and a heavy unsharp enhancement.

Click to view attachment

It looks like a bow shock in the plume but my gut says the whole thing is too diffuse for that. What do you think? Artifact or real?

Hmm. Subtle, good catch!

To me, the 'dome' looks like it's produced by plume particles that are in the foreground of the general E-ring glow. In other words, I see Enceladus's circular black shadow on the E-ring in the background, and it looks like this is a perspective effect. Is there any way to make a stereo pair? That might be a good test of this idea.

The dome appears to continue through the shadow and may wrap all the way around, perhaps its an optical effect of the e-ring related to the lighting
Below is your image with the histogram equalized.
Click to view attachment

I can't see how it would be a lighting effect on the ring unless the sun happened to be precisely behind Enceladus. The phase angle was high, 174 degrees, but even that is still 17 NAC FOVs away from the sun!

It does seem to fit inside Enceladus' Hill sphere and is elongated in the N-S direction, though.

Yea, thought about that after I hit post.

Ahh...my bad, Gordan, misinterpreted what you meant by 'dome'. Just to be sure I'm on the same page now, would characterize what I think you mean as a halo or even aureole that completely encircles the moon(?)

EDIT: Wow. Does indeed look like there's an enrichment of material in what appears to be Enceladus' Hill sphere. The N/S elongation might be due to the effects of Saturn's magnetic field on what (I guess) highly diffused water vapor mixed with OH & H3O radicals. Again, wow!!!

Yet another great spot, Gordan! And a beautiful image!

Elliptical halo caused by scattering by the ring particles of light coming from Enceladus? http://www.atoptics.co.uk/fz409.htm
Whatever it is, it's pretty cool.

I wonder if it's merely the effect of particles being more stably bound to Enceladus in those directions than in the orbital plane. If it's linked to the Hill sphere at all, that is.

The problem with that is that this halo would be very very small. Remember the narrow-angle camera has a FOV of 0.35 so any ice halos we're familiar with here on Earth couldn't possibly completely fit into the frame. Not even in the wide angle camera.

I'll have to check if anything of the sort is visible in earlier observations, although they were nowhere as close to such a high phase angle.

Yeah, I don't know what angles these inner rings of these halos go to.

About the Hill sphere thing: would you expect the most material to be at the outer edge?

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".

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

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).

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

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.

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.

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.

Oxygen ends up in Titan's atmosphere. Might that be related?

Maybe. Enceladus' emissions seem to end up splattered all over the Saturnian system; the E-ring is just the densest part of it.

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?

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

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!

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

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?

Indeed. Personally, I'm not convinced by any possible explanation at the moment.