5 Discussion
5.1 Column abundance
In this work, the MCD was used to provide a priori values for the column
abundance retrievals with the uncertainty set equal to the abundance.
With a posterior-to-prior error ratio analysis, we demonstrated that the
synergy injects a significant amount of information to the retrieval,
and obtains highly robust column abundances. The climatology presented
here displays a water vapor cycle consistent with established
literature, both in terms of magnitude and seasonal and meridional
variations. Abundances are observed to peak near the edge of the
seasonal frost cap in spring, forming an annulus of vapor encircling and
following the retreating seasonal cap into the early summer. The
appearance of the water annulus is consistent with the proposed
mechanism for the seasonal cycling of the water in the Northern
hemisphere, in which vapor subliming from the seasonal water frost
annulus re-condenses on the surface of the retreating
CO2 cap. The water decrease poleward of the annulus is
observed consistently for all seasonal intervals in our composite
average, yet annual variations have been previously reported (Pankine et
al., 2010).
Although the overall behavior is well known and also agrees well with
the MCD model, significant differences do exist. The synergy column
abundances deviates most prominently from the MCD in terms of absolute
value with significantly lower abundances, particularly in the summer
NH. The observed northern sublimation maximum is 30% lower than MCD
estimates, and the sublimation season onset itself is observed to occur
later in time. In the SH, the model and observations are in better
agreement, and similar to what was reported by Clancy et al., (Clancy et
al., 2017) using CRISM occultation data, who also found that retrieved
water vapor abundances matched MCD model estimates better in the SH than
in the NH. The synergy yields slightly higher values in the southern
early summer, resulting in a somewhat asymmetrical relationship between
the synergy and MCD, where the synergy finds a lower summer peak in the
NH, but a larger peak in the SH.
When compared to previous works, the synergy northern maximum abundance
was quite consistent with PFS, SPICAM and the revised TES abundances of
60-70 pr-μm (Fouchet et al., 2007; Pankine et al., 2010; Trokhimovskiy
et al., 2015), while CRISM obtained a slightly lower sublimation peak in
MY 28 and 29 of around 50 pr-μm (Smith et al., 2009). Although the
synergy finds a smoothed average of around 50 pr-μm at 75°N and
Ls=105°-120°, some local and transient instances of abundances up to 100
pr-μm occur. Observations from the Limb and Nadir Observation channel of
the NOMAD instrument on the ExoMars TGO satellite agree well with the
synergy in terms of seasonal variations, however, the northern maximum
obtained by the synergy is significantly higher than those found by
NOMAD for the corresponding time and place (just above 30 pr-μm)
(Crismani et al., 2020).
The southern maximum coincides in time with previous results, but the
large asymmetry between the NH and SH maxima observed by SPICAM and
CRISM is not as prominent in the synergy dataset (see Figure 6, where a
few very high column abundances are observed), as the northern maximum
is normally a factor of 2 higher than the southern peak for the
corresponding season (Figure 10). On average, the synergy finds a
southern maximum of ∼33 pr-μm, significantly higher than SPICAM. It
should be noted that the location where the largest SH abundances were
observed were at latitudes not captured by previous TES and PFS studies.
It should also be pointed out that observations in the south polar
region are much sparser than elsewhere, and measurements from several
years are binned together, whereas the observations of the north polar
region are abundant and mostly from MY 27. Smith (2004) found that the
year-to-year variations can be as high as 10 pr-μm, and might thus
explain why we observe instances of high vapor abundances in the south.
Outside the summer maximums, the synergy again is most similar to SPICAM
and PFS, and agrees very well also with NOMAD. During Ls=0°-50°, the
mean low latitude (0°-30°N) CIA was 7-8 pr-μm for the synergy, SPICAM,
PFS and NOMAD, and ~5 pr-μm for CRISM. Later, during
Ls=150°-180° for the same latitudes, the mean abundances were 13-15
pr-μm for the synergy, SPICAM, NOMAD and CRISM, ~12
pr-μm for PFS.
The difference between the synergy and other datasets is most likely due
to differences in calibration and data processing techniques, even
though diurnal variations cannot be excluded. For example, NOMAD samples
local times from 08:00 to 16:00, anf PFS covers local times into the
late afternoon. TES sampled the equatorial region and mid latitudes
around 14:00 and 02:00, with only data captured during the 10:00-14:00
range being used to assemble the revised dataset presented by Pankine et
al., (2010). No
evidence supporting diurnal variations have yet been uncovered using
OMAGE or SPICAM (Maltagliati, Montmessin, et al., 2011; Trokhimovskiy et
al., 2015), and in the synergy, any diurnal variations are lost in the
averaging process as PFS and SPICAM cover a broader time interval.
Crismani et al., (2020) found no evidence for substantial diurnal
variation in the total dayside water vapor column, thus the plausibility
of diurnal variations causing such a large spread in column abundances
is still considered unlikely.
5.1 Partitioning index
The strongest motivation for the use of a spectral synergy retrieval
approach is to access information on the vertical distribution of water
vapor. We have shown that during the polar cap sublimation periods, the
magnitude of the near-surface vertical confinement matches model
predictions quite well, though discrepancies in the meridional
partitioning gradient are significant. For both hemispheres the vertical
partitioning remains high and fairly constant (±0.2) for all seasons and
latitudes, while displaying a wave-like behavior. Poleward of the polar
cap edge however, the hemispheres differ. In the south the partitioning
index is observed to drop for all seasonal intervals except for during
mid spring. In the north the PI seems to be decreasing at first between
70° and 80°N, and then rapidly increases beyond the polar cap edge,
especially so for mid and early spring. This polar cap behavior is well
reproduced by the global climate model used to construct the MCD, except
during spring for both hemispheres.
The largest differences in MCD and synergy vertical confinement in the
northern hemisphere are found at mid-latitudes after Ls=150° (see Figure
9). The column abundance, which never exceeds 20 pr-μm, agrees best with
the MCD in this region (though still the synergy finds a lower value),
while the obtained synergy partitioning was more than 50% higher than
model estimates. This might be indicative of less water escaping through
the hygropause than what is estimated in the MCD. For Ls=135°-150°,
Figure 11 shows that the MCD and synergy are quite consistent for high
latitudes, both finding a PI of 0.7 at 70°N. Further equatorward in the
drier low latitudes, where model and synergy agree quite well with
regard to column abundances, the partitioning differs significantly. The
model suggests the confinement decreases monotonically, reaching a
PI=0.4 at 20°N, while the synergy maintains a strong confinement,
obtaining a PI of ~0.7 at 20°N, having barely changed
despite a drastic reduction in the total water column. This could
suggest that the circulation incorporated in the current model at low
latitudes is too strong, causing the MCD partitioning to decrease more
quickly towards the equator. The difference could also possibly be due
to diurnal “breathing” of the regolith, actively exchanging water with
the atmosphere and thus maintaining a near-surface layer.
Overall, the synergy finds a more variable vertical partitioning than
what the model suggests, which corresponds well with results from solar
occultations observations with SPICAM (Maltagliati et al., 2013). This
demonstrates that the synergy is particularly useful at mid to low
latitudes where atmospheric dynamics influence the vertical
partitioning, and over the polar regions where seasonal variations in
the vertical partitioning are large and not reproduced by the model. It
would be of great interest to compare the synergistic partitioning with
high resolution vertical profiles from for example the solar occultation
instruments NOMAD and ACS on TGO. This will be included in future work,
although as mentioned earlier, the ability of these instruments to probe
the water vapor content in the very low atmosphere is not always
present. As the southern hemisphere normally has a higher dust loading
than the north, conditions are most favorable in the north high
latitudes. At low latitudes where we observe large differences between
synergy and model, continuously high dust loading will also make direct
comparisons between synergy and TGO difficult.
6 Conclusions
Presented here are the results from a spectral synergistic retrieval
method applied to water vapor nadir measurements from PFS and SPICAM
sampled over seven Martian years. The synergy produces a highly reliable
water vapor climatology with geographical and temporal patterns
consistent with established literature. When compared to the LMD MCD,
the synergy tends to retrieve lower total column abundances, in absolute
differences the deviance is biggest for the northern summer sublimation
peak, while in relative terms the most significant discrepancies are
found at mid latitudes. In the southern hemisphere the synergy and MCD
correspond very well. Other differences of note include timing and
latitudinal extent of the sublimation onset, which occurs earlier in the
MCD, and extends much further equatorward. The synergy finds very
comparable column abundances to previous works using single spectral
domain approaches with SPICAM and PFS (Fouchet et al., 2007;
Trokhimovskiy et al., 2015), somewhat higher values than CRISM (Smith et
al., 2009), and slightly lower than TES (Pankine et al., 2010; Smith,
2002).
The ability to extract information on the vertical distribution of water
vapor from nadir observations is a unique capability of the spectral
synergy approach. The synergy is unable to produce a vertical profile of
fine resolution, but it can set reliable constraints on the partitioning
of the water column, differentiating between the near-surface content
below 5 km and the rest of the column. Significant differences between
the vertical partitioning over the north and south hemispheres are
revealed, where the southern hemisphere exhibits a generally weaker
confinement coupled with a stronger seasonal dependence and latitudinal
variations than in the north. The near-surface confinement from the
synergy overall differs from the MCD especially at low and middle
latitudes where the synergy finds a stronger near-surface confinement
than MCD estimates. The synergy also finds that the meridional spread of
this strong confinement is larger than what the model suggests, with a
strong confinement far south of the polar region. There appears to be no
clear connection between a peak in total column abundance and the amount
of vertical partitioning. In general, the synergy finds that the
vertical confinement is subject to rapid and local variations, and can
change significantly even while the total column abundance remains
stable, or remain stable while the column abundance varies.
We have shown that by combining two separate spectral intervals, within
which water vapor possesses diagnostic features, increased robustness is
brought to the retrieval of column abundances as well as additional
information about the vertical content, as compared to the commonly used
single-interval retrieval approach. The combination of more accurate
column abundances and constraints on the vertical distribution is
essential for our understanding of the processes that control the
distribution and transport of volatiles in the lower atmosphere.
Considering that current knowledge of the water distribution in the
lowermost layer of the atmosphere is mainly based on GCMs, the
comparison between the synergy partitioning results and the predictions
of the MCD is of particular interest. The significant discrepancies
between the two indicate that our understanding of the physics that
shape the vertical distribution of atmospheric water on Mars is
incomplete.