No, brain, it’s Martin Guerre, not Martian Guerre
OMG it would be a perfect match: Astronaut goes to Mars, comes back…different. Is it really him? Is an alien presence hitchhiking in his mind? Or is it a shape-changer like The Thing? Or did he simply have a life-changing experience on the mission?
It could be played for comedy, horror (body or psychological), or anything in between.