NASA's rover Spirit has found a mineral that is the surest hint yet of water on Mars, scientists said. Spirit discovered a so-called water-signature mineral called goethite in a section dubbed "Columbia Hills," NASA scientists said."Goethite, like the jarosite that Opportunity found on the other side of Mars, is strong evidence for water activity," said Goestar Klingelhoefer of the University of Mainz, Germany. He is the lead scientist for the iron-mineral analyzer on each rover, the Moessbauer spectrometer.