Figure
7: Seasonal evolution of column-integrated abundances of water vapor
during summer of MY 27. Comparison between abundances predicted by the
MCD used as prior, the synergistic retrievals and the single spectral
domain approaches for SPICAM and PFS. Abundances are not normalized to
an equivalent surface pressure. Data is averaged on the latitude
interval 15°-45°, and in bins of 5° Ls. Triangles represent the average
values for each bin, the solid curves are the smoothed abundance values,
and the shaded areas represent the uncertainty interval of the bin
averages.
4.3 Vertical partitioning of water vapor
When water vapor is retrieved simultaneously from PFS/TIR and
SPICAM/NIR, the degree of vertical confinement can be estimated by
taking the ratio of the partial column from the surface up to 5 km, to
the total column. The result is a dimensionless partitioning index (PI)
representing the amount of water vapor confined within the first 5 km of
the atmosphere compared to the rest. Average trends in the PI during the
northern summer are shown in Figure 8, with focus on the latitudes
between 45°S and the North Pole where the observation density is
highest.
As the seasonal polar ice is subliming in early northern summer, the CIA
increases drastically north of 60°N. There is no clear immediate
reaction in the PI, which is fairly high (PI typically greater than 0.7)
and stable from 30°N and northward during Ls=90°-160°. At polar
latitudes between Ls=100°-130°, when the CIA is at its highest, a local
PI maximum is observed slightly southward of the CIA maximum. The
confinement in the polar region remains strong at least until Ls=170°, a
period during latitudes above 50°N undergo extreme variations in CIA,
transitioning from the north polar summer maximum to a very dry late
summer, as can be seen from the top left panel in Figure 8. Extremely
strong partitioning (PI=0.9) is seen at Ls=165°, when almost no water
remains in the far north. This indicates that after most of the water
has sublimed and been transported south, what water vapor remains at
high latitudes is kept close to the surface for the duration of the
summer.
South of the equator the water vapor is more homogeneously distributed
with altitude with a PI of around 0.5, with some regions at low
latitudes showing signs of a drier boundary layer (PI
~0.2). The PI is highly variable and related to
topography in an anticorrelated fashion when compared to the CIA. Even
after pressure normalization, there are local variations in CIA related
to varying elevation, previously found to likely be linked to
atmospheric dynamics (Fouchet et al., 2007). Geographical variations
stand out in the bottom panels, where the PI is enhanced over drier,
elevated regions such as the Tharsis and Terra Sabaea regions (centered
around -120° and 30° longitude respectively), while the confinement is
small over low-elevation regions such as Hellas Planitia at longitudes
between 60°-90°. The PI index is a ratio of water columns, and should
inherently be independent of topography, yet the correlation with
elevation remains.
PIs smaller than 0.5 are rarely seen in the NH, suggesting that sublimed
water vapor might be transported southward at low altitudes. At low
latitudes however, the water is transported across the equator over
regions of low elevations