In an undated handout photo, portions of the Martian surface shot by NASA's Mars Reconnaissance Orbiter show many channels on a scarp in the Hellas impact basin. Scientists reported on Monday definitive signs of liquid water on the surface of present-day Mars, a finding that will fuel speculation that life, if it ever arose there, could persist to now. (Jet Propulsion Laboratory/University of Arizona/NASA) -- NO SALES; FOR EDITORIAL USE ONLY WITH STORY SLUGGED MARS WATER BY KENNETH CHANG. ALL OTHER USE PROHIBITED. --