Figure 10. Water vapor vertical profiles and saturation ratio obtained
by SPICAM IR occultations in MY29 (B) and ACS NIR observations in MY35
(C) and MY36 (D) at Ls=75°-100° in northern and southern hemispheres.
Panel A presents the latitudinal coverage of occultations.
On the contrary, Poncin et al. (2022) did not find evidence for large or
widespread supersaturation in the Martian atmosphere. From CRISM
analysis they conclude that during the aphelion season the atmospheric
water is close to saturation when clouds are present and can reach the
supersaturation ratio of 2 to 3 at most at 10–50 km. During the dusty
season subsaturation was prevalent, in agreement with previous CRISM
study by Clancy et al. (2017). Also, comparison of NOMAD water profiles
and MCS atmospheric temperatures during the GDS decay and the “C”
storm of MY34 has shown one case of saturation ratio reaching 5 at the
top of the cloud layer and below 2 in other cases. These results were
more in line with traditional understanding that water vapor is close to
saturation in presence of water ice clouds. However, the analyzed
water-temperature combinations from CRISM-MCS or NOMAD-MCS datasets were
not completely simultaneous. Also, results of Poncin et al. (2022) and
ACS TGO do not exclude each other. As shown in Figure 10, the water
vapor mixing ratio and its saturation state are very dynamic and change
dramatically from occultation to occultation with latitude and Ls not
only in the dusty season presented in Fedorova et al. (2020) but in the
aphelion season as well. Solar occultation observations at terminator
may reflect both the morning and evening conditions where a strong
temperature contrast can stimulate fast processes. Connour et al. (2020)
have reported twilight cloud bands routinely forming past the evening
terminator (18:00–19:00 local time) during the MY 34 GDS as a result of
rapidly changing temperatures. The cloud bands in the IUVS/MAVEN images
were often latitudinally continuous, spanning over 6,000 km, and
reaching 40–50 km altitudes. Such clouds were also detected in the ACS
observations during GDS (Fedorova et al., 2020; Luginin et al., 2020),
together with strong lifting of supersaturated water. The
morning-to-evening difference of the saturation state observed by ACS
(Fig. 9) supports that supersaturation cases can be related to
semidiurnal thermal tides in the atmosphere.
The recent study of the MY34 GDS with Mars Assimilated Remote Soundings
(OpenMars) dataset of Mars GCM group in the Open University (Holmes et
al., 2021a) has shown that discrete layers of supersaturation above 60
km exist across all latitudes during the MY34 dusty season with diurnal
variation in the saturation state of the atmosphere. This is consistent
with ACS NIR observations not only for MY34 but also for MY35. General
consistency of the assimilation and observations gives promising
perspective of the water vapor saturation modeling.