Water flowed on Mars for a million years, a senior National Aeronautics and Space Administration official asserted on Wednesday, lending credence to the theory there was once life on the Red Planet. Wesley Huntress, Nasa's associate administrator for space science, said that recent photos from the Mars Global Surveyor orbiting Mars show an ancient riverbed at the bottom of a 2.5-km wide canyon called the Nanedi Vallis, near the planet's equator.