Photochemical instability of the ancient Martian atmosphere

We develop a 1‐D steady state photochemical model of the modern Martian atmosphere and apply it to possible Martian atmospheres present and past. A unique feature of our model is that the major current sink of oxygen is dry deposition (surface reactions) of highly reactive, oxidized molecules (chiefly H 2 O 2 ), rather than oxygen escape to space. Another difference is that we allow hydrogen to escape to space at the diffusion limit, which gives H escape fluxes ∼70% higher than in other models. What results is a model with one free parameter: a dry deposition velocity to describe the surface sink of reactive molecules. An effective global average deposition velocity of 0.02 cm s −1 for H 2 O 2 and O 3 gives a good match to the observed abundances of O 2 , CO, and H 2 , the three abundant photochemical trace gases. We then apply our model to Martian atmospheres with different amounts of CO 2 , H 2 O, and solar forcing. We find that thick, cold, dry CO 2 atmospheres are photochemically unstable with respect to conversion to CO. This may be pertinent to ancient Mars when the Sun was faint and O escape rates were likely high, for which the tipping point is computed to be ∼10 mbar of CO 2 . The possible photochemical instability of cold thick CO 2 atmospheres, and the high likelihood that CO was abundant even if CO 2 were stable, has broad implications for early Mars.