Arid, stony Mars, whose sinuous canyons and broad flood plains have long beckoned Earth-bound explorers, may never have been the warm, wet home of life that scientists have envisioned so hopefully, a new study of the planet’s past maintains. In a scenario that contradicts decades of hints from orbiting spacecraft that rivers and an ocean once covered the Martian surface during a prolonged warm period, scientists at the University of Colorado and NASA’s Ames Research Center are posing a vastly different concept.