Goals: NASA's Mars Exploration Rovers - Spirit and its twin Opportunity - were designed to study the history of climate and water at sites on Mars where conditions may once have been favorable to life. Each rover is equipped with a suite of science instruments to read the geologic record at each site, to investigate what role water played there and to determine how suitable the conditions would have been for life.
Accomplishments: Both rovers far exceeded their design specifications and returned science results that transformed our understanding of Mars.
Spirit: Described as a "wonderful workhorse," Spirit explored for years beyond its original 92 day mission. The rover revealed an ancient Mars that was very different from the Mars we see today. Spirit uncovered strong evidence that Mars was much wetter than it is now in a silica patch apparently produced by hot springs or steam vents. The rover captured movies of dust devils in motion, leading to a better understanding of Martian wind. Spirit continued to make discoveries even as it was stuck in deep sand at a spot dubbed Troy at Gusev Crater on Mars. On 25 May 2011, NASA ended efforts to contact the marooned rover and declared its mission complete. The rover had been silent since March 2010.
Opportunity: The second rover to land on Mars has returned dramatic evidence that its area of Mars stayed wet for an extended period of time long ago, with conditions that could have been suitable for sustaining microbial life. Scientists believe that Opportunity's Meridiani Planum landing site "was once the shoreline of a salty sea on Mars." Opportunity also has analyzed exposed rock layers recording how environmental conditions changed over time. Opportunity holds a Martian driving record: more than 20 km as of March 2010.