A principal scientific objective for sending spacecraft to Mars has been to explore its geology as interpreted from data returned to Earth (see Chapter 2 for a review of the history of Mars exploration). This exploration was spurred on when early spacecraft revealed a surface reflecting a long-lived, dynamic interplay of geologic forces, in many ways similar to those evident in our own Earth–Moon system. Thus, scientific investigation has focused on topics such as: What are Martian rocks made of, and how old are they? What do volcanic and tectonic features reveal about the interior of the planet? What is the history of impact bombardment? How much water has there been at or near the surface, and were there times of abundant surface water on the planet? Have there been gradual or sudden changes in the planet’s climate? Is Mars geologically active today? Could life survive in environmental niches on the surface or below the surface, now or in times past?