This is really interesting. I noticed something similar in some of the raw images back in 2006, and asked a few friends, but nobody seemed to have a good guess. The images are here: http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=60698 http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=60699 http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=60702 http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=60703 I'd love to hear if anyone has any ideas.

Alan, the consensus back then was that that is an optical artifact in the camera, an internal reflection of sunlight in the optics. Those images were taken by the wide-angle camera and at a high phase angle and that camera is much more susceptible to stray light at those angles. If you look at that ring closely, you can see it doesn't move consistently with the rest of Titan and it doesn't change apparent size as Titan does when its distance changes between subsequent frames. The current effect on the other hand is very much real and first hints of something "funny" happening at the south pole started appearing about a year ago. Several months ago it was becoming obvious there was a separate layer of orange haze piling up where normally just the blue, detached haze would persist.

Thanks Gordan, that makes sense. Still very curious about this new phenomenon.

We may merely be witnessing the way a polar "hood" appears over the winter pole. Note the structure over the north pole is now fading and there were prominent layers back when Cassini arrived, when it was the winter pole - whereas the south pole looked unremarkable. Very cool to watch this transition unfold and it makes these crescent images even more interesting.

Cassini has photographed similar features in the atmosphere of Titan in the past. This is not an optical artifact of some sort, because the picture has been turned by 27° and it would't make sense for some kind of artifact to appear in that location. I don't think that the cap represents the haze that Gordon mentions either. If you look very closely at the photo, you can see a thin layer in the atmosphere over the south pole, which is probably the haze that Gordon refers to. I think there are two more likely scenarios. As Gordon says, Cassini has taken some pretty stunning images of massive methane cloud formations over the north pole, and other pictures imply that similar things exist over the south pole during a different season, so it might very well be that what we see is a reflection off them. Nevertheless, we also shouldn't exclude the possibility (I think this is quite likely), that what we're seeing is a reflection off a lake on the surface of Titan. There is a very interesting picture of a reflection off the surface of a lake near the north pole, so this might be the same thing, just off the south pole.

Hi Emily we also have noticed the peculiarity of these images and finally we find someone who talk about it. In the sequence N00190936 - 50 the lenticular cap was even more visible. If I can report, this's one of our processing http://farm8.staticflickr.com/7072/7169979253_f6f4c2624d_o.jpg and recent images of June 26 are equally curious http://farm8.staticflickr.com/7276/7462638124_7bca776a22_o.jpg. Thanks for reporting the news. We hope to get more information.