Pretty picture: Halo on a halo?

Check out this curious picture taken recently by Cassini:

This image is cool to begin with because it's taken at such a high phase that we can see sunlight scattering through Titan's atmosphere completely around its globe. You're seeing every sunrise and sunset happening all over Titan, all at once. What makes it curious is the little lenticular cap at the bottom of the image, at Titan's southern winter pole. The cap is also visible in images of Titan taken as Cassini was departing the flyby, like this one. If something like this has been visible in Cassini's images of Titan before, I don't recall seeing it. (Please point it out in the comments if it's been seen before.) I don't have any idea what it is. Hopefully someone will tell me in a talk at the Division of Planetary Sciences meeting in the fall.

Cassini has been at Saturn for very nearly eight years, and has flown past Titan a hundred times, and there are still new things to see!