The same as Figure 5, except showing results for the 10⁻⁶ bar surface H₂O with diffusion and photochemistry model, only contributing to the H₂O abundance above the surface layer. At these low surface H₂O concentrations, stratospheric H₂O is generated primarily by the methane oxidation photochemistry detailed in Equation (1). The posterior classification provides only an upper limit for the H₂O abundance at this low concentration. Note that this also corresponds to a surface water concentration similar to modern Mars.