LOS ANGELES - Two new studies are challenging the notion that the desolate Martian plains once brimmed with salty pools of water that could have supported some form of life. Instead, the studies argue, the layered rock outcrops probed by NASA's robot rover Opportunity and interpreted as signs of ancient water could have been left by explosive volcanic ash or a meteorite impact eons ago. That would suggest a far more violent and dry history than proposed by the scientists operating Opportunity and its twin rover, Spirit, on the other side of the planet.The new scenarios, published in Thursday's journal...