Personal essay differs from autobiography and memoir

by whom it's written
in length and in the amount of life it covers
because it's about imaginary events
because it's not based on the writer's experience

martians or other types of aliens represent a unique fear as opposed to zombies. vampires and other types of monsters.   in most sci-fi stories, aliens come from other worlds and have different or more advanced technology.   abductions and experimentation on human bodies that involve these technologies, or the use of these technologies against human beings and the use of unknown "powers", are a type of fear that's different from that of other types of monsters.  

different routes can lead to sammy to different places, with different roads.   if you check a map, you can see the routes.   try to make connections of the places and count how many there are.   hope this : )