[X] My Assistant

[Floyd]

From Looking Ahead: Rev153: Sep 3 - Sep 22 '11

A lot going on this revolution:

Sixty-one ISS observations are planned for Rev153, the majority designed to monitor cloud systems in Saturn's atmosphere. The spacecraft also will encounter a number of Saturn's moons, including Titan, Pallene, Tethys, Enceladus, and Hyperion, for which ISS will acquiring imaging.

ISS will image the L5 Lagrange point (60 degrees behind satellite) regions of the moons Iapetus, Rhea and Dione to see if they have Trojan’s like Dione/Polydeuces and Tethys/Calypso.

Titan encounter Sept 12. CIRS, VIMS, UVIS, INMS, CAPS, ISS
Imaging Enceladus polar plume from night side at 42,224 km and two mosaics of sub-Saturn hemisphere. Sept 13
Imaging Tethys from 300,00 km Sept 13
Image Pallene from 25,960 km (38x26 pixels at that distance as only 3.6 x 2.5 miles) Sept 13
Image Hyperion from 58,015 km—a little further than last month. Sept 16.
Color image of Tethys passing in front of Titan's south polar hazes. Sep 16.
Mutual events Enceladus in front of Titan with Rings and Pandora in background Sept 17.

[remcook]

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!)

[ugordan]

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.

[ngunn]

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.

[ugordan]

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.


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.