Well, that certainly took care of them. One thing that bothers me though... have these marines been around the whole time, or did they just arrive? If they just arrived, that means another dropship must've landed. Might be a way off the planet for me, but more likely Mastaba will try to escape in it.
In any case, I don't have time to lose. I can see light coming from the doorway, so that must be the way back outside.
Looks like the exit is right next to the alien tower structures. I should be able to use the alien cube from before to work my way across and back to the catwalk.