What unique fear do martians or other aliens represent as opposed to zombies vampires and types of monsters

Martians or Aliens, oppose to Zombies and other monsters, cause a unique fear because of the fact that outer space scares us. I mean, why shouldn't it? Space is a cold desolate place bigger than we could possibly imagine, whereas the earth, where we live, is mostly explored. That's not even including all the rumors about Alien cover-ups by the government.