3.3 Perihelion season
Figure 7 shows the same latitudinal variations of temperature, water vapor, saturation ratio and aerosol extinction as Figure 6, except for the perihelion season from Ls=180 to 360°, in 30° Ls bins for MY34 and 35.
The Martian year 34 was characterized by the GDS that began at Ls=193° and vanished around Ls=240°. This event generated a strong interannual variability between MY34 and MY35 from Ls 180 to 240°. During MY35, water is distributed from 60°S to 45°N with values of 200 ppmv found in the South up to 60 km, above which water decreases down to 30 ppmv at 80 km. In the North, these high abundances are only seen up to 40 km. The atmospheric layer where most of the decrease occurs is well correlated with aerosol extinction and temperature. This period of time corresponded to the onset of large supersaturations. Close to the poles, the 10 to 30 ppmv water layers located at 40-70 km appear correlated with warm temperatures and are likely the result of advection. Below 40 km, temperatures decrease, and water falls to <1 ppm, still exhibiting some slight supersaturation. As reported by other studies (Neary et al., 2020, Heavens et al.), the MY34 GDS warmed the atmosphere as captured by ACS in the North (Fig. 7A) where the temperature below 60 km is 30-40 K warmer than in MY35. In the southern hemisphere the region between 10°S-60°S was observed before the GDS whereas the region between 60°S-90°S could only be observed after it started (Fig. 3). This period is characterized by water observed up to 80 km with values of 200 ppmv, which sharply decrease to 30-50 ppmv above 100 km (Belyaev et al., 2021). At this season, water begins to be supersaturated only above 70-80 km where clouds actually form (Fedorova et al., 2020; Luginin et al., 2020). At Ls=210°-240° the atmosphere is much warmer than in MY35. Water mixing ratios up to 100 ppmv were observed at high altitudes, that is above 70 km in high southern latitudes and above 60 km in high northern latitudes. In both cases, water is again observed to fall rapidly down to a mixing ratio of 30 ppm. At these high altitudes, water is supersaturated, yet aerosols are present up to 100 km, indicating that cloud or aerosol presence was not sufficient to relax the atmosphere to saturation. In MY35 between 45°S-60°N, 100-200 ppmv of water prevails below 40-50 km with a decrease to 5-30 ppmv above. Again, water is strongly supersaturated up to 60 km even in the presence of aerosols. At 70-50°S, the water maximum is pushed higher, up to 80-100 km, which may actually be the consequence of the set-up of the solstitial circulation (Richardson and Wilson, 2002a, 2002b; Montmessin et al., 2007).
During the southern summer solstice Ls=240-270° and 270-300° the water distribution is very similar for two years with an asymmetry between hemispheres also observed by SPICAM/MEx (Fedorova et al., 2021). In the mid-to-high southern latitudes 200 ppmv are observed up to 60-70 km. This water top gradually decreases to 40 km in the northern hemisphere at 60°N. At these altitudes clouds are observed for both years from 60°S to 60°N. At 30-70°S above 70 km the water abundance sharply decreases to 50-70 ppmv and reaches 100 km and higher. In the northern hemisphere H2O decreases to 10-30 ppmv between 40 and 80 km and then increases again, revealing the transport of water from the southern to the northern hemisphere at high altitudes. Water is strongly supersaturated during this season for both years. But saturation is much higher for MY35 compared to MY34 and begins above 40 km near the equator and above 70 km at high latitudes . Moving to the northern vernal equinox, circulation is shaping a nearly symmetric picture that is observed both years. The regional dust storm “C” happened between Ls=315° and 330°, whose effect can be grasped in the Ls=300-330° interval in the form of a rise of water mixing ratio and aerosol extinctions up to 70-80 km in the 45°S-70°S region. In both years, supersaturation was observed at high altitudes with water mixing ratio about 30 ppmv. At Ls=330-360° in high southern polar latitudes below 20 km the supersaturation reappears despite the sub-ppmv concentration. A simple interpretation points to the local cooling of the polar atmosphere, as water is still present and condensation is unable to keep up with the fast temperature decrease.