Seasonal vertical water vapor distribution at the Mars Phoenix Lander site

Using a combination of orbital and surface observations, we constrain the seasonal vertical distribution of water vapor in the planetary boundary layer in the lowest scale-height of the Martian atmosphere at the Phoenix Mars Lander location (68 degrees N, 234 degrees E). Previous observations have shown significant discrepancies regarding the vapor distribution in the boundary layer, drawing conflicting conclusions as to whether water is uniformly mixed below the cloud condensation height, or whether water is mostly confined in the near surface layer. We conclude that the uniformly well-mixed assumption for water vapor up to the cloud condensation level is not always compatible with observational constraints, particularly when peak near-surface water vapor abundances are observed during Ls = 110 degrees -120 degrees. The uniformly well-mixed assumption leads to an over-estimation of the total water vapor column abundances between Ls = 80 degrees -120 degrees, and an under-estimation of the total water vapor column abundances between Ls = 120 degrees -150 degrees. The overestimation of vapor in the column is particularly evident during peak surface water vapor pressures (-Ls = 110 degrees -120 degrees), suggesting a concentration of water vapor at the surface that actively participates in subsurface exchange. Using both TECP and column vapor abundance as constraints for the vertical vapor distribution, the maximum height of the well-mixed vapor layer ranges between -3.5-13.5 km, all of which falls below the cloud condensation height for the corresponding season. From Ls = 120 degrees to 150 degrees, the maximum height of the well-mixed layer stays fairly consistent at a mean height of -7 km. These results are particularly important for providing insight into the seasonal transport of water and the role of regolith-atmospheric exchange.