In Alien's universe (depicted in Alien, Predator & Prometheus movies, games and books) Mars is the first planet colonized by humans in mid-21st century. There are many colonies and military bases on the planet established by both national and commercial entities and housing millions of people. On Phobos there is a commercial research facility experimenting with xenomorphs or aliens - a highly aggressive endoparasitoid extraterrestrial species.
Spaceship leaving Phobos' spaceport. Note the partially terraformed Mars below: