Gameplayer has an exclusive interview up with Red Faction: Guerrilla's Senior Producer Rick White in which he talks about many aspects of the game. This includes a lack of aliens in the Mars-set sci-fier.
"We wanted the destruction to be meaningful and have impact on the player and game play. So open world and our Blastech engine has allowed us to do just that. Since it is all above ground we are able to make large structures that can be flattened or used as a weapon to defeat the EDF."