aliens do not follow our rules of science, they are a biological enigma compared to us. they use the dorsal spines located on their back as a filtration unit, sifting out the surrounding elements, be they liquids or solids, these elements are then introduced to the circulatory fluids in the alien body - showing us it can survive on any number of trace elements that are located in the environment the xenomorph is in, hence it being able to adapt to any circumstances and survive in the most hostile of places, deep space for one.
and please, to whoever pulls the strings of reality NO MORE VS MOVIES, THEY JUST DONT WORK!!
www.thorndesigns.netdarkness of acheron