Martian surface. False-colour image from NASA's Mars exploration rover Spirit of the surface of Voltaire rock outcrop, showing rounded pebbles embedded in a fine-grained matrix. The smooth pebbles must have been exposed to erosion in the past and so it is thought this layered rock was formed from deposits ejected from an impact event. Spirit and its sister rover Opportunity were launched in the summer of 2003. Spirit landed on Mars on 3 January 2004. Its mission is to analyse soil and rocks and search for water. Image taken on Spirit's 554th Martian day, 25 July 2005.