Where to plunk down twin Mars rovers on the Red Planet in early 2004 has sparked spirited debate among scientists and engineers. Battle lines are being drawn between these two camps, each weighing scientific payoff against technical worries in picking ultra-safe landing areas over more risky touchdown spots. Last week, here at the NASA Ames Research Center, top Mars researchers met January 24-25. They selected a handful of candidate exploration zones, including a section of Valles Marineris, an enormous canyon system on Mars.