Round Thing Near South Pole Options

[antoniseb]

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.

[Guest_Myran_*]

Interesting, I took the opportunity to lighten that dark part and found concentric lines running around the rim.
Yes inspired by the great work of others here Myran have done the first ever image enhancement, nothing added but not entirely true to he original either.

In lack of a better explanation I would guess its a crater, if its impact or a volcanic caldera im unable to judge, but the enhanced image makes me go for a wild guess at caldera.