Two robots called Spirit, and Opportunity are exploring Mars. These robots are "rovers," and these robots move across the surface of the planet. Scientists wanted the rovers to study different areas, but Spirit landed on one side of Mars, and Opportunity landed on the other. Their mission was to explore the surface, and the rocks on the surface. The rovers took pictures and studied layers of rocks and minerals. The rovers' research shows that there used to be water on Mars' surface, so there isn't anymore.