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