Seasonal and interannual variability of the water vapor vertical distribution in the Martian lower atmosphere

We apply a novel method to retrieve water vapor vertical distributions in the Martian atmosphere to spectra collected by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES). The new method exploits differences in sensitivity of the daytime and nighttime TES nadir spectra to water vapor content in different parts of the lower atmosphere (0-40 km). Based on the test retrievals from simulated environments, vertical profiles of water vapor can be retrieved with accuracy similar to 20-40 % depending on the season and altitude above surface. Retrievals from observations collected by the MGS TES between L-s = 135 degrees in MY24 and L-s = 75 degrees in MY27 enable exploration of the seasonal and interannual variabilities of the vertical distribution of water vapor. Retrieved vapor distributions and their seasonal variability are generally consistent with those predicted by a numerical circulation model. Vapor is concentrated near the surface in the northern polar region and the mid-latitudes during the northern summer (L-s = 90 degrees). During the southern summer (L-s = 270 degrees) vapor extends higher in the atmosphere in the southern polar region, reflecting both sublimation of the seasonal surface ice and atmospheric transport by the upper branch of the southern Hadley cell. During both equinoxes (L-s = 0 degrees and L-s = 180 degrees) vapor is found in the lower part of the tropical atmosphere. At the same time the retrieved vertical distributions exhibit notable interannual variability. Following the global dust storm of MY25, vapor is confined to lower altitudes in the southern polar region during the southern summer. During the southern spring in MY26 (L-s = 180 degrees-270 degrees) vapor extends higher in the atmosphere and mixing ratios increase faster with height at the equator and in the northern tropics. These examples suggest that Martian atmosphere may experience interannual changes in the atmospheric transport, some of which could be in response to global dust storm events.