Why were Opportunity and Spirit sent to Mars' surface?

First among the mission's scientific goals was to search for and characterize a wide range of rocks and soils for clues to past water activity on Mars. The rovers were targeted to sites on opposite sides of Mars that looked like they were affected by liquid water in the past. Opportunity landed at Gusev Crater, a possible former lake in a giant impact crater. Spirit landed at Meridiani Planum, a place where mineral deposits suggested that Mars had a wet history.