6. Conclusions
We analyzed two Martian years dataset of H2O profiles consisting of solar occultation observations by ACS NIR. The data have been collected from April 2018 (Ls=163° of MY 34) to January 2022 (Ls=170° of MY 36) and contain about 8500 water vertical profiles with simultaneous retrieval of CO2 density and temperature. The latter was obtained from 1.43 μm and 1.65 μm CO2bands (orders 49 and 54 of the spectrometer) in the range of altitudes from 0 to 110 km and the H2O mixing ratio in the 1.38 μm water band (order 56 of the spectrometer) in the range of altitudes from 0 to 100 km. To provide the comparison with aerosol loading, we used the aerosol extinction at 1.4 µm from the same ACS NIR observations and H2O ice extinction from simultaneous ACS MIR observation at 3.3 µm H2O ice band.
With this dataset, we have presented the first detailed analysis of the vertical distribution of water vapor saturation state. Based on a two Martian years dataset, we studied the seasonal, latitudinal and local time variations and found the following:
  1. We confirm previous observations (Fedorova et al., 2021) that water vertical distribution is variable with season and reaches 100 km in perihelion and 60 km in the aphelion season. The simultaneous measurement of atmospheric temperature allows us to calculate the saturation state of water vapor and the supersaturation was found nearly ubiquitous above aerosol layers. This finding implies that water escape processes rely on complex interactions between thermodynamics, dynamics, chemistry and microphysics that only fully coupled climate models can address.
  2. In the aphelion season, water mixing ratio above 40 km in the mid-to-high latitudes is observed to be lower than 3 ppmv and is found to be supersaturated in low-to-mid latitudes from Ls=0° to 180°. Also supersaturation was found in southern and northern polar regions below 20-30 km close to both equinoxes, correlating the region where water vapor transported from low latitudes meet cold polar vortex.
  3. Around perihelion, water is also supersaturated with a mixing ratio of 30-50 ppmv that is typical of altitudes above 60 km. Stronger saturation is observed during the dusty season in MY35 compared to what was observed in MY34 during the Global Dust Storm and around perihelion. In the dusty season the polar maximum of supersaturation at 20 km is presented in both hemispheres and the most prominent close to equinoxes.
  4. Water vapor abundance and its saturation state was found to vary between evening and morning terminators in response to temperature modulation imparted by thermal tides. Although water vapor is found to be more abundant in the evening, colder temperatures observed in the morning induce a daily peak of saturation.
  5. The observations at Ls=50°-120° during the aphelion season of MY34 and MY35 have shown a good consistency with SPICAM IR observations on Mars-Express in MY29 that demonstrates that supersaturation is repeatable state of water vapor in the Martian atmosphere from year to year.