This is one of the first photographs of Mars taken by the Mariner spacecraft in the late 1960s. At the time scientists were both surprised and disappointed that, like the moon, the surface of Mars was scarred by impact craters. However, closer examination reveals that many craters lack an obvious ejecta deposit. Often crater rims have also been eroded, and the interiors are shallow. We are investigating the timing and processes responsible for modifying these craters.