The social constructionist position is pretty simple (but mind, Butler is +criticizing_ social con here). Mars exists. It's materiality is not in doubt. But what Mars means to us is something we, as a society, culture create and this has varied historically. The red planet, Barsoom, the stuff of mythic stories or science fiction, home of ETs, an object consulted in astrological forecasts, the object of scientific investigation, a future home for those fleaing a dying planet earth, etc.
All those ways of thinking of mars, more or less shaped by it's physical properties and how we perceived them, will matter as to our attitude toward, thinking about, concern with, etc. this planet. Some would counter that we can break through the mythological, ill-founded beliefs about Mars that have dominated the past. We do that, they say, via science. But someone like Butler is arguing that even science, shot through as it is with relations of power shaped by capitlist social relations, is going to present to us a mars that is not simply the mars of some neutral descriptive observation language, but through a language which, by its very operations, encourages us to see it as, for instance, a planet devoid of human life to, therefore, be exploited for resource, to be colonized for human use, etc. and whatever.
Where the social constructinist break from others is with the idea that, if we just keep trying harder and honing our tools, we can eventually tear off the veil of ideology and get at the +real+ mars lying the to be grasped by us, unmediated by power. That what we call mars will, someday, be described and perceived by us as what it really is, without us bringing any *interests* to the understanding. We would be, like Spock, observing phenom and cooly announcing, "interesting."