You are an enemy of the Daleks...
You will be exterminated!!!
Sorry, it's just that swivelling gun turret reminds me of a dalek. ;)
Everything about that little game was great.
Graphics: Simple, but didn't need to be complex. The mars-like red terrain was a nice touch; really gave the game that "alien planet" feel. Did you make those backgrounds yourself?
Sound: Nice choice of music, I especially like how you added a volume slider. Too often in these games/movies the music drowns out all other sound.
Interactivity: What's a game without interactivity? You seem to have thought of everything. The HUD-type menu really gave it an authentic game feel.
One small suggestion: For any future edition perhaps you could change to limited ammo. Add a little more challenge and teach those trigger happy players some ammo conservation. ;)