NASA's twin robot geologists, the Mars Exploration Rovers, launched toward Mars on June 10 and July 7, 2003, in search of answers about the history of water on Mars. They landed on Mars January 3 and January 24 PST, 2004 (January 4 and January 25 UTC, 2004).
The Mars Exploration Rover mission is part of NASA's Mars Exploration Program, a long-term effort of robotic exploration of the red planet.
Primary among the mission's scientific goals is to search for and characterize a wide range of rocks and soils that hold clues to past water activity on Mars. The spacecraft are targeted to sites on opposite sides of Mars that appear to have been affected by liquid water in the past. The landing sites are at Gusev Crater, a possible former lake in a giant impact crater, and Meridiani Planum, where mineral deposits (hematite) suggest Mars had a wet past.
After the airbag-protected landing craft settled onto the surface and opened, the rovers rolled out to take panoramic images. These images give scientists the information they need to select promising geological targets that tell part of the story of water in Mars' past. Then, the rovers drive to those locations to perform on-site scientific investigations.