This Mars was home to the sand-whalers (sentient crustaceans who rode on sand-yachts), who hunted the land-dragons (massive, fire-breathing, sauropod-like creatures). The land-dragons were armored, meaning that they must have had true predators that were probably even larger than them, that also lived on this world.
This Mars had a purplish sky.
Willis and the others discovered five monoliths on this world, which had inscriptions on them. They couldn't get close enough to them to read them, but the sand-whalers could, and so Willis got them to go and photograph them for him. They did this in exchange for the knowledge of how to make and use Stepper Boxes.