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(1∆g) 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.