An epic poem differs from other forms of poetry mainly

a) it is written in iambic pentameter

b) it tells the story of a cultural hero

c) it makes extensive use of allusion

d) it always includes a comic twist

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.