good ideas, also you may want to use the better skull i think its a poser download in free-stuff, its more detailed, including teeth. try a dark overcast image with maybe little glows across the surface of raging fires. (ooh mr gloomy)

I would put some browns and purples in the cloud layer with the green to represent dirt and pollution and bump map the cloud layer. I would also use a bump map on the land mass of the planet to raise up the mountains and lower the valleys.