Double albums are just cool, plain and simple. In this case, you get a great picture of Jupiter and Mars (wonderful for astronomy buffs), a neat little foldout case that looks great on a bookshelf, and lots of lyrics written over images of fire and jumping band members.
If you want to explore beyond that, and develop some sort of theme, I did find the following on some obscure astrology site...
Mars is the first superior planet, it orbits at a greater distance than the earth from the Sun. For this reason it is considered as representing the urge to separate from home. Much like that of an adolescent, Mars represents our personal journey into the outer world to achieve our goals.
Jupiter is far further out from the Sun than the inner planets and lies on the other side of a major asteroid belt. It is of a different magnitude to the inner planets and represents a different principle. Once the inner planets have been understood and personalised the individual can now deal with collective issues and not lose themselves in the process. For this reason Jupiter is considered less of a personal planet than a planet to do with how we relate to the society we find ourselves in.