Ok, so I'm working on an idea I had for a cool steampunk mars map for a forum strategy game. Trouble is, I'm not sure how to proceed with making the map.

My source is this elevation map of Mars (warning, big image)


I'm a bit puzzled as how to proceed. My experience is with ArcGIS. With that, I would import the image as a raster, then select all values under a certain height to make ocean polygons, along with the appropriate altitudes and elevations to turn into icecaps. I'd then use line tools to draw in the desired canals, points for the cities, and change the color of the land to be more lifelike.

ArcGIS doesn't work for any planet other than Earth AFAIK. I'm not sure how to go about that process with GIMP, if it's even possible.

What I'd like to do is have everything lower than the very lightest green be ocean, with ice caps on the poles and on the high volcanoes. These can then feed into canals, ringing the highlands to catch seasonal melt and direct water to what would be the mother of all rain shadows.