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Rev 153 - Sep 3-22, 2011 - Titan T78, Also Pallene, Tethys, Enceladus, and Hyperion
Bjorn Jonsson
post Sep 14 2011, 12:43 AM
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Seems partially due to Titan's own shadow but the strange thing is that the haze gets brighter just outside the shadow (which might be consistent with ngunn's suggestion above).

Beautiful images/color composites by the way - some of the best ones I've seen.
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remcook
post Sep 14 2011, 07:12 AM
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Of course there's some shadows/darkness behind the bright bit, but the fact that the bright 'line' seems to go up in altitude significantly looks quite strange to me. There seems to be either a big pile-up of haze at the pole (polar cap?) + shadow, or a local increase in haze scattering, like the detached haze layer.
Very nice images by the way (yet again!) smile.gif
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ugordan
post Sep 14 2011, 07:39 AM
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I was thinking the increase in brightness could come from some funky forward-scattering properties of the orange haze ahead. To my eye it only appears to be pronounced in longer wavelengths. I can't think of a mechanism that would do that, though.

Also, did anyone actually try measuring pixel values to verify it really is brighter and not some optical illusion? I was too lazy to do that myself yesterday.


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remcook
post Sep 14 2011, 08:10 AM
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As far as I can tell from stretching etc. it's actually brighter, especially at these very high altitudes.
Since the upper haze looks blue (although the line looks perhaps less blue than the rest), it seems to me the light there is mostly scattered only once by the haze, which would mean in this case that the scattering angle is about 100 degrees or so.
There is a very interesting paper by Lavvas et al. ( http://adsabs.harvard.edu/abs/2009Icar..201..626L ) which argues that the detached haze is indeed some sort of 'optical illusion' (not really) and is caused by the formation of these fractal paricles by clumping together of small particles. Because many small particles scatter more light than less bigger particles, the detached haze layer looks brighter, although there is not in fact more material there. I think that's a pretty cool idea. But there still needs to be some dymanics to explain some of the changes with time (see http://www.sp.ph.ic.ac.uk/~ingomw/Titan_Me..._files/West.pdf , last slide)
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Juramike
post Sep 14 2011, 02:13 PM
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There actually IS more material at the inner boundary of the detached haze layer. Monomers (small particles) are bopping around in the uppermost atmosphere >500 km or so, increasing in density as you get lower. But at around 500 km (inner boundary of the detached haze layer) they start to clump together, get much bigger, and drop out of the layer, leaving a gap until you get to the lower haze deck. The haze layer at the critical monomer density. Any more dense, and the stuff sticks and drops out.

There was a very recent paper also by Lavvas et al. (2011) [I think it is this one (pay for article)] that describes the agglomeration and does a good job of modeling the profile of Titan's layers and the location of the detached haze layers.



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remcook
post Sep 14 2011, 07:05 PM
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Very cool paper. smile.gif It's amazing how much Cassini has increased the knowledge about Titan.

But about the region below 500 km it says: "In this altitude region, the high atmospheric density and pressure decreases even further the particle settling velocity. ... This leads to a pile-up of the particles, demonstrated by the local increase of the particle density right below 500 km. The particle density decreases again at lower altitudes because of the enhanced coagulation rate, which enhances the particle growth" And: "We should note though that because of the small sedimentation velocity of the particles, the effects of atmospheric mixing and advection are expected to have a larger impact in the aerosol evolution for this region and this is not included in our model." The particles are still tiny there of course. So, I'm not sure that 'dropping out' actually takes place. In any case, the optical effect would be there.
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Bjorn Jonsson
post Sep 14 2011, 08:15 PM
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QUOTE (ugordan @ Sep 14 2011, 07:39 AM) *
Also, did anyone actually try measuring pixel values to verify it really is brighter and not some optical illusion? I was too lazy to do that myself yesterday.

Yes, I did - this is not an illusion.
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ngunn
post Sep 15 2011, 07:36 PM
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QUOTE (ugordan @ Sep 14 2011, 08:39 AM) *
I was thinking the increase in brightness could come from some funky forward-scattering properties of the orange haze ahead. To my eye it only appears to be pronounced in longer wavelengths.


That sounds very plausible to me, and is a more economical hypothesis than my earlier suggestion as it does not invoke diurnal changes. We know that the orange haze scatters photons predominantly into a narrow forward cone. Near the limb the uppermost part of that cone would miss the surface and escape tangentially into the upper haze layers. An observer inside the bright streak looking back toward the rising or setting sun would be doubly dazzled - by the sun itself and also by an exceptionally bright patch in the orange haze just below it, looking almost like a specular reflection.

It works for me (in a purely handwaving way!) until the experts come up with a better explanation.

That said, the possibility of diurnal changes in the haze layers cannot be altogether discounted as far as I'm aware.

Gordan - a direct question/request: You say the streak looks brighter at longer wavelengths. Can you construct an image, maybe in stretched colours, that shows this? The spectrum of the streak would be a clincher, I think.
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ugordan
post Sep 15 2011, 09:07 PM
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QUOTE (ngunn @ Sep 15 2011, 09:36 PM) *
Gordan - a direct question/request: You say the streak looks brighter at longer wavelengths. Can you construct an image, maybe in stretched colours, that shows this? The spectrum of the streak would be a clincher, I think.

Yeah, I don't think that impression holds up to closer scrutiny. Here's a CB3 (940 nm narrowband), 570 nm green and 440 nm violet filter combination and magnified 2x.
Attached Image


I don't see any obvious color shift in the haze color, as much as can be inferred from a raw image with scattered light effects. This composite does give a hint of the "normal" detached layer continuing normally behing "sunset" as seen by a faint reddish band and the brightening sort of splits off upwards.

Note also one lower density "crack" at higher altitude seems to pass through uninterrupted. I would expect it to be bent up as well if this was a major updraft.


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Ian R
post Sep 16 2011, 11:39 PM
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This shot of Tethys has it all: a giant crater, a gaping chasm, and a tantalizing blue streak:

Attached Image


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PDP8E
post Sep 17 2011, 02:02 AM
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Fabulous image Ian!
It is so tempting to say that the giant crater (Odysseus) is related to the chasmata on the other side of this frozen ice ball (boom ..stretch!)
But those pesky crater counters insist that chasm system is older! Who is right?
Great image!


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Juramike
post Sep 17 2011, 03:29 AM
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Another version of Tethys RGB[IR1,GRN,UV3]:

Attached Image


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Juramike
post Sep 17 2011, 03:34 PM
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5-frame mosaic of images taken along Enceladus' terminator through clear filters:

Attached Image



(note crater that got cut in half)


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Floyd
post Sep 17 2011, 04:11 PM
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Thanks Mike. I was hoping someone would do something with the Enceladus images too.


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Gladstoner
post Sep 17 2011, 08:42 PM
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