Mars: Atmospheric chemistry and astrobiology

It is nearly thirty years since the early pioneering work on the chemistry of the Martian atmosphere. Later work provided quantitative comparisons between theory and observations. The status will be reviewed. Whether the composition of the Martian atmosphere can or cannot be accounted by photochemistry and geochemistry becomes a question of great importance for Astrobiology. In 1965 Lovelock wrote a seminal paper on the detection of extraterrestrial life on the basis of departure from thermodynamic equilibrium. Unfortunately, both photochemistry and biochemistry are disequilibrium driving forces, and we must understand the system in sufficient detail to be able to distinguish the two.