NASA wants to bring back samples of Martian rock and soil to check again for signs of lifeforms; this is the first time since 1976 that a mission is specifically designed to search for the chemical/biological clues of life, as well as any possible fossilized microorganisms.
NASA wants mission to bring Martian rocks to Earth | Science Headlines | Comcast.net (http://www.comcast.net/articles/news-science/20100518/US.SCI.Mars.Search.for.Life/ - broken link)
Mars Exploration Program .