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All three terrestrial planets with atmospheres support O3 layers of somethickness. While currently only that of Earth is substantial enough to beclimatically significant, we hypothesize that ancient Mars may also havesupported a thick O3 layer during volcanically quiescent periods whentheatmosphere was oxidizing. To characterize such an O3 layer and determine thesignificance of its fedback on the Martian climate, we apply a 1D line-by-lineradiative-convective model under clear sky conditions coupled to a simplephotochemical model. The parameter space of atmospheric pressure, insolation,and O2 mixing fraction are explored to find conditions favorable to O3formation. We find that a substantial O3 layer is most likely for surfacepressures of 0.3-1.0 bar, and could produce an O3 column comparable to that ofmodern Earth for O2 mixing fractions approaching 1%. However, even for thinnerO3 layers, significant UV shielding of the surface occurs along with feedbackon both the energy budget and photochemistry of the atmosphere. In particular,CO2 condensation in the middle atmosphere is inhibited and the characteristicsof H2O dissociation are modified, shifting from a direct photolysis dominatedstate similar to modern Mars to a more Earth-like state controlled by O(1D)attack.

Characterization of a Thick Ozone Layer in Mars' Past