This pic, recently taken by the Mars Rover Spirit, is clearly an alien skull and not a rock.
The eye sockets align perfectly with our generally accepted view of how life should be formed off of Earth. The mouth is directly below a slopping nose. Hell, even the shape of the rear of the skull is large enough to support a massive, telekinetic-capable brain. I’m telling ya, that right there, is the skull of an alien and not an f’n rock.