Portions of the Martian surface shot by Nasa's Mars Reconnaissance Orbiter show many channels from about one metre to 10 metres wide on a scarp in the Hellas impact basin.  Researchers found tell-tale fingerprints of salts that form only in the presence of water in narrow channels cut into cliff walls throughout the planet's equatorial region. Photograph:   Reuters/Nasa/JPL-Caltech/University of Arizona