Mars has long fascinated scientists and the public. As a result, it has been the target of a number of ambitious spacecraft missions, the most recent being that of Vikings 1 and 2 in 1976. Following an initial period of intensive study, it became clear by the late 1970s that the Viking data could still yield important clues about the nature and evolution of Mars. In recognition of this potential, NASA established the Mars Data Analysis Program (MDAP) in 1979 to coordinate the funding and direction of Mars research. The first major investigation supported by MDAP was a focused 3-year study project on the origin and evolution of martian volatiles. The project, entitled “Mars: Evolution of its Climate and Atmosphere” (MECA), was initiated in 1984 under the direction of the Lunar and Planetary Institute, Houston, Tex.