The "Face of Mars" was not created by aliens. The "face" is a mesa, an elevated piece of land similar to a plateau. The mesa is about 2 miles long, and the face is created by the shadows from protruding rock. The features of this landform could have been created by aliens if they had the ability to carve a two mile long mesa to look like a face, not knowing what it looked like while they were carving it. Since they were on the ground they we we carving, they would have had to either cut into the rock blindly, or have a method to see what the face looked like. That method, be it mechanical or optical should still be visible. The aliens might not be humanoid, and might not know what a human face looked like. They wouldn't be able to cut out an image of something if they don't know what it looks like. Even if aliens could cut a face into the rock, there would be fragments of that rock lying all around the base of the landform. Most of the debris might be swept away over time, but there would still be some. If the carving was done by aliens, they would have to have some way to get on top of the mesa, and that might be visible. It could be stairs, or a ladder, or a rope, but it should be visible to the satellite pictures. If the aliens did cut the face into the mesa, then it they would have gone about it in a different way. Assuming that the aliens created the face is assuming that they are intelligent beings. Intelligent beings would most likely have created finer details or carved a symbol or word to show who created the face. If they are unintelligent beings, how did they get on top of the mesa and how did they cut out the face.