Saturday night, if the heat shield holds, if the retro-rockets fire at just the right time, and if the airbags survive an estimated 40-foot drop, NASA’s rover Opportunity will emerge from its protective shell to look upon a Mars unlike any humans have ever seen. Opportunity is now descending toward a Martian Mordor barely imaginable, where much of the characteristic salmon soil has been swept away and dark fields of puzzling ridges ripple as far as the eye can see.