Here is an image which I think is from Mars Odyssey, but I'm not certain. It is from one of the orbiters. Thanks to a friend in another forum for pointing it out.
It shows a dark, very close to circular, ring near the extremities of the polar cap.
http://www.msss.com/moc_gallery/e13_e18/me...14/E1401276.jpg
http://www.msss.com/moc_gallery/e13_e18/im...4/E1401276.html
It may be that this feature has already been discussed here, if so can someone please point me to the discussion. Otherwise, I am curious to know what you think may have caused this.