NASA’s one-two punch at reactivating on-site Mars exploration is near at hand. The space agency’s delivery plan is twins: two lookalike Mars Exploration Rovers — for now tagged simply MER-A and MER-B — are ready to wheel their way into scientific history. Once the dual robots begin navigating the red planet, a key job is to ferret out evidence for liquid water in the planet’s past. That data should help reveal just how suitable or unsuitable conditions have been, or might be today, for life on that baffling world.