Characterization of a Thick Ozone Layer in Mars' Past

All three terrestrial planets with atmospheres support O3 layers of some thickness. While currently only that of Earth is substantial enough to be climatically significant, we hypothesize that ancient Mars may also have supported a thick O3 layer during volcanically quiescent periods when the atmosphere was oxidizing. To characterize such an O3 layer and determine the significance of its feedback on the Martian climate, we apply a 1D line-by-line radiativeconvective model under clear-sky conditions coupled to a simple photochemical model. The parameter space of atmospheric pressure, insolation, and O2 mixing fraction are explored to find conditions favorable to O3 formation. We find that a substantial O3 layer is most likely for surface pressures of 0.3–1.0 bar, and could produce an O3 column comparable to that of modern Earth for O2 mixing fractions approaching 1%. However, even for thinner O3 layers, significant UV shielding of the surface occurs along with feedback on both the energy budget and photochemistry of the atmosphere. In particular, CO2 condensation in the middle atmosphere is inhibited and the characteristics of H2O dissociation are modified, shifting from a direct photolysis dominated state similar to modern Mars to a more Earth-like state controlled by O(D) attack.