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 H₂O₂), 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 H₂O₂ and O₃ gives a good match to the observed abundances of O₂, CO, and H₂, the three abundant photochemical trace gases. We then apply our model to Martian atmospheres with different amounts of CO₂, H₂O, and solar forcing. We find that thick, cold, dry CO₂ 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 CO₂. The possible photochemical instability of cold thick CO₂ atmospheres, and the high likelihood that CO was abundant even if CO₂ were stable, has broad implications for early Mars.