If experts hope to prove that Mars harbors some form of life, or once did, they will probably have to retrieve pieces of soil and rock and return them to Earth for analysis. That task will likely fall to a robotic spacecraft sometime in the next decade. Though part of a long-term effort by NASA to solve that mystery, neither the Spirit nor Opportunity missions is equipped to make that determination. Instead, the robotic rovers will look for evidence in the rocks and soil of Mars that the arid planet was once wet and warm.
