I made a tutorial on how to remove the limb artifacts on your Mars images. You'll see in this video tools in Gimp like layers, opacity and selection tools. I hope this is useful. It's 8 minutes long, but be sure to full screen and set to 720 to see all the little things going on in it.
Others here:
http://maphilli14.webs.com/planetaryastronomytutorial.htm
