What made this movie be rated R?

I don't get why it was rated R? Was it all a marketing ploy to get people to think "Prometheus" is scarier then it really is? I've seen tons of movies that had more blood/gore that were not rated R. IMO they tried to con Alien fans into thinking it was going to be more suspensful and scary then it was. Something like the original Alien or Aliens but in reality it resembled more a PG rated Star Trek film.