4. Discussion.
The first evidence of water supersaturation in the Martian atmosphere was obtained during the aphelion season (from Ls 50° to 120°) by SPICAM solar occultations in MY29, when 61 water vapor vertical profiles were retrieved and reported by (Maltagliati et al., 2011). Lacking simultaneous temperature measurements, SPICAM saturation ratios relied on temperature nearly simultaneous MCS/MRO profiles, with a GCM-based correction applied to compensate for the difference in local time between the two instruments. Sixty percent of the data showed water vapor exceeding saturation and more than 10% the saturation ratios greater than 5. Later (Clancy et al., 2017) derived a proxy of water profiles by scaling LMD GCM simulated water vapor by the ratio of LMD simulated to CRISM retrieved 1.27 μm O2(1g) dayglow vertical emission rate. Using GCM simulated temperature, the saturation ratio was then averaged during the Ls60° to 140° period. They obtained saturation ratios >2.2 at northern mid-to-high latitudes between 20 and 35 km, which is roughly consistent with SPICAM supersaturation measurements in Maltagliati et al. (2011).
Figure 10 presents the comparison of SPICAM IR observations of water vapor and saturation ratio at Ls=75°-100° with ACS NIR observations in MY35 and 36. Latitudinal coverage of SPICAM and ACS NIR is shown in Fig.10A. In MY34 ACS NIR performed measurements at the same latitudes as SPICAM between Ls=80° and 94°, whereas in MY35 the observations are closer before Ls=83° and after Ls=92°. Despite the coarser vertical resolution of SPICAM IR (1-10 km for SPICAM and <1 km for NIR) and a lower SNR (~100 for SPICAM and ~1000 for NIR), the water vapor vertical profiles of SPICAM (fig.10B) are consistent with NIR profiles in both hemispheres. The increase of water to 100 ppmv below 30-40 km in the northern hemisphere repeats in all years with some temporal and spatial variations. Both SPICAM and ACS NIR have shown the increase of the saturation ratio above 30-40 km where the water v.m.r. sharply decreases to a few ppms. A strong high-latitude supersaturation detected by NIR in the southern hemisphere below 20 km (MY34-35) is supported by SPICAM in MY29 at Ls~80° where latitudes ~60°S were observed. Concluding, SPICAM and ACS NIR observations of H2O saturation state are generally in good agreement.