Relationship Between the Ozone and Water Vapor Columns on Mars as Observed by SPICAM and Calculated by a Global Climate Model

Ozone (O₃) in the atmosphere of Mars is produced following the photolysis of CO₂ and is readily destroyed by the hydrogen radicals (HO_x) released by the photolysis and oxidation of water vapor. As a result, an anti-correlation between ozone and water vapor is expected. We describe here the O₃-H₂O relationship derived from 4 Martian years of simultaneous observations by the SPICAM spectrometer onboard the Mars Express spacecraft. A distinct anti-correlation is found at high latitudes, where the O₃ column varies roughly with the −0.6 power of the H₂O column. The O₃ and H₂O columns are uncorrelated at low latitudes. To evaluate our quantitative understanding of the Martian photochemistry, the observed O₃-H₂O relationship is then compared to that predicted by a global climate model with photochemistry. For identical model and observed abundances of H₂O, the model underpredicts observed ozone by about a factor of 2 relative to SPICAM when using the currently recommended gas-phase chemistry. Sensitivity studies employing low-temperature CO₂ absorption cross sections, or adjusted kinetics rates, do not solve this bias. Taking into account potential heterogeneous processes of HO_x loss on clouds leads to a significant improvement, but only at high northern latitudes. More broadly, the modeled ozone deficits suggest that the HO_x-catalyzed photochemistry is too efficient in our simulations. This problem is consistent with the long-standing underestimation of CO in Mars photochemical models, and may be related to similar difficulties in modeling O₃ and HO_x in the Earth's upper stratosphere and mesosphere.