Panoramic 'true-color' view of Rub al Khali, by Opportunity rover, May 2005. The name is taken from the 'Empty Quarter' in Saudi Arabia. Shallow sand waves stretch, vast and desolate, beyond the horizon. Wind-blown dust and sand is the predominant factor in sculpting the Martian surface in the present era, and has been, for the last three and a half billion years. (Mars Exploration Rover Mission, JPL, NASA, apod050822)