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:
- 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.
- 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.
- 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.
- 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.
- 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.