EXACTLY! I have long had a feeling that the asteroid belt was a debris field from a cataclysmic collision between two bodies in our solar system. I strongly suspect that that Mars was somewhat 'shotgunned' with debris, with one massive projectile generating the Hellas/Mariner/Tharsis, and other massive chunks of debris becoming Mars' remarkably asymetrical moons.

I believe there WAS indeed another planet between Mars and Jupiter that was destroyed; I don't believe the orthodox idea that the asteroid belt was simply primordial material that 'just didn't ' coalesce into a planetary body...that seems a bit more fantastic considering all the nice, symmetrical planetary bodies we have in our system.

Now, just what were the two impactors? There's another question.