... APOD: 2007 February 20- White Ridges on Mars . ... Credit: HiRISE , MRO , LPL (U. Arizona) , NASA Explanation: What created these white ridges on Mars? The images showing the white ridges, including some of the highest resolution images ever taken from Martian orbit, were recorded last year by the Mars Reconnaissance Orbiter (MRO). A current leading hypothesis is that the white ridges formed as water flowed through underground cracks and bleached and hardened the edges of surrounding rocks. ...