  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 whenthe
atmosphere was oxidizing. To characterize such an O3 layer and determine the
significance of its fedback on the Martian climate, we apply a 1D line-by-line
radiative-convective 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(1D)
attack.
