Cool concept and composition; I especially like the mountain in the back, how you shaded it, and the moon and the valley floor (based off of Death Valley, perhaps? It has that cool, endless look).
A critique: the robot's proportions? I don't mind that he's sketchy, but it looks like somewhere around his arms and hands he gets disproportionally skinny. It just seems to contrast too much with the size of his torso? If the size of his arms and legs were changed around a bit, it could even him up and make this even cooler! Especially with the shading you use; it's like he's sorta translucent, reflecting the night and his inner potential...