Lovely work Freddy

I have been looking at some pic's of Olympus Mons, and wonder if it is the caldera or the dark colouring surrounding the calderawe are seeing

http://www.olympusmons.com/mars/photo03.html
http://www.olympusmons.com/mars/photo02.html
http://www.olympusmons.com/mars/photo06.html
The dark surround doesn't seem to appear in all hi res images