Mars is a real life planet. It's completly red and named after the Roman god of war.
Mars was once a habitable planet with wildlife and flora, somewhat like Earth. During sometime in the Cretaceous Period of planet Earth however, Mars was was dying, it's ozone layer was oozing away. Both Crayak and Ellimist knew is was a doomed world.
- It's not mentioned if Mars was once home to an intelligent race, as the Ellimist doesn't mention anything about the natives. It was likely only a world with wild animals and plants.