There should be no "natural laws" which are certain. Space could be passable because it is filled with breathable air. On the other hand the stars could be tiny lights only a few hundred miles away. Some areas of land could be gates into other worlds, dimensions, times, or whatever. Mars is given in these rules, but some other fantastic world or setting could be equally as possible. This function is up to the referee, and what he wishes to do with it is necessarily limited by his other campaign work. However, this factor can be gradually added, so that no sudden burden will be placed upon the referee.One of the things I lament about AD&D is the way it codified what the Dimensions were. I mean I love the concept for a specific campaign world, but rather than every campaign world being unique, the infinite Prime Material planes in AD&D suggested that every world must fit within the Gygaxian concordance. You can see in the original edition there was a much more free wheeling spirit to the whole concept of worlds, cosmos, and other dimensions. This single paragraph inspires me perhaps more than any other in the original booklets.
I enjoy collecting images that inspire ideas for other cosmos ideal for a fantasy campaign: