Constraints on water vapor vertical distribution at the Phoenix landing site during summer from MGS TES day and night observations

We present a new method to retrieve column abundances and vertical extent of the water vapor from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) spectra. The new method enables retrievals from the nighttime TES spectra. The retrieval algorithm employs a new model of the vertical distribution of water vapor in the martian atmosphere. In this model water vapor is confined to a layer of finite height in the lower atmosphere. The atmosphere is dry above this 'wet' layer. Within the 'wet' layer the water vapor has a constant mixing ratio below the water ice cloud condensation height and is saturated above that height. The new retrieval method simultaneously fits the daytime and nighttime TES spectra for a given location using a single mixing ratio profile. We apply this new method to the TES spectra collected over the site of the Phoenix spacecraft landing during late northern spring and summer. Retrieved daytime column abundances are ∼1-5 pr-μm higher than in the previous TES retrieval. Nighttime column abundances are lower than the daytime abundances by ∼5-10 pr-μm due to assumed exchange with soil and predicted water ice cloud formation. The height of the 'wet' layer varies with season, reaching ∼18 km around Ls = 80-100° and decreasing to 7-10 km by Ls = 140°. Changes in the vertical extent of vapor are consistent with seasonal changes in the intensity of the turbulent mixing in the lower atmosphere and in the water ice cloud condensation height. Water vapor extends by several kilometers above the top of the boundary layer at ∼4 km, suggesting that vertical transport of vapor is not limited to the boundary layer.