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We report retrievals of atmospheric water vapor abundances in and around the South Polar Region (SPR) of Mars during spring and summer of Mars Years (MY) 24–26 (1999–2003). The retrieval utilizes infrared spectra collected by the Thermal Emission Spectrometer (TES) aboard the Mars Global Surveyor (MGS) spacecraft. Modifications to the existing retrieval algorithm enable the first retrievals of water vapor abundances over regions covered by the seasonal CO 2 frost during spring. We present polar maps of water vapor variability in the SPR for the time periods L s = 195°–345° in MY24–26. Water vapor behavior in the SPR in MY24 and MY26 exhibits substantial similarity in spatial distribution for the same dates and in temporal variability. Water vapor behavior in MY25 is significantly different from that in MY24 and MY26 with abundances ~30% lower around summer solstice (L s = 270°). We hypothesize that water vapor behavior in MY25 was affected by the Global Dust Storm (GDS) that started at L s ~185° of that year and significantly affected the atmosphere of the whole planet. We speculate that the reduction in the observed vapor abundances in MY25 could have resulted from disruption of the southward atmospheric vapor transport from mid-latitudes by the GDS or from reduction of the rate of desorption of vapor from thawed circumpolar terrains due to changes in surface temperature during the GDS. Through most of the spring, highest vapor abundances are observed just northward of the retreating edge of the seasonal CO 2 cap, with generally low abundances over the seasonal cap. Repeated cycles of condensation and re-sublimation of water vapor on the edge of the retreating cap could be responsible for this build-up of vapor near the cap edge. Relatively high vapor abundances that are systematically observed over some regions of the cap in late spring could be associated with venting of the mixture of water vapor, CO 2 gas and dust from underneath the seasonal cap through fractures in the frost. We explore the evolution of the water vapor in the SPR during spring and summer using two 1-dimenional models representing two limiting cases that attribute observed changes to either atmospheric transport or surface sources. The atmospheric transport model appears to be more consistent with observations, suggesting that atmospheric transport plays the dominant role in the water vapor cycle in the SPR. However, activity of surface processes such as desorption of vapor from the regolith cannot be excluded. Comparison of the water vapor cycle during spring and summer in the SPR and the Northern Polar Region (NPR) shows that, besides the difference in the highest observed abundances, the evolution of the vapor abundances shows remarkable qualitative similarity, despite the presence of a large source of vapor during summer in the north in the form of the North Polar Residual Cap (NPRC). This suggests that the processes shaping the water cycle in the polar regions are similar in nature and differ only in strength. The new data on water vapor in the SPR, together with our previous retrievals of water vapor in the NPR, constitute a new dataset that enables testing of Martian Global Circulation Models close to the planet's poles, and enables better understanding of the difference between the Martian polar regions. • Water vapor abundances in the Martian polar atmosphere retrieved for spring and summer seasons in MY24-26. • Atmospheric transport appears to play the dominant role in the water cycle at the southern polar region (SPR). • Seasonal changes and spatial distributions of vapor abundances are similar in MY24 and MY26. • Vapor abundances are 30% lower in MY25 following the global dust storm of MY25, representing substantial interannual variability. • The dust storm of MY25 could have reduced atmospheric transport of vapor to the SPR and/or reduced desorption of water from soil in the SPR. [ABSTRACT FROM AUTHOR]

Using a combination of orbital and surface observations, we constrain the seasonal vertical distribution of water vapor in the planetary boundary layer in the lowest scale-height of the Martian atmosphere at the Phoenix Mars Lander location (68°N, 234°E). Previous observations have shown significant discrepancies regarding the vapor distribution in the boundary layer, drawing conflicting conclusions as to whether water is uniformly mixed below the cloud condensation height, or whether water is mostly confined in the near surface layer. We conclude that the uniformly well-mixed assumption for water vapor up to the cloud condensation level is not always compatible with observational constraints, particularly when peak near-surface water vapor abundances are observed during L s = 110°–120°. The uniformly well-mixed assumption leads to an over-estimation of the total water vapor column abundances between L s = 80°–120°, and an under-estimation of the total water vapor column abundances between L s = 120°–150°. The overestimation of vapor in the column is particularly evident during peak surface water vapor pressures (∼L s = 110°–120°), suggesting a concentration of water vapor at the surface that actively participates in subsurface exchange. Using both TECP and column vapor abundance as constraints for the vertical vapor distribution, the maximum height of the well-mixed vapor layer ranges between ∼3.5–13.5 km, all of which falls below the cloud condensation height for the corresponding season. From L s = 120° to 150°, the maximum height of the well-mixed layer stays fairly consistent at a mean height of ∼7 km. These results are particularly important for providing insight into the seasonal transport of water and the role of regolith-atmospheric exchange. • Relative humidity along with column abundances are used to constrain the seasonally varying vertical extent of water vapor. • We conclude that water is mostly confined in the near surface layer, rather than uniformly well-mixed, as commonly assumed. • Near-surface enhancement of water suggests daytime convection is insufficient to mix water up to the cloud condensation level. [ABSTRACT FROM AUTHOR]

We present a new method to retrieve column abundances and vertical extent of the water vapor from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) spectra. The new method enables retrievals from the nighttime TES spectra. The retrieval algorithm employs a new model of the vertical distribution of water vapor in the martian atmosphere. In this model water vapor is confined to a layer of finite height in the lower atmosphere. The atmosphere is dry above this ‘wet’ layer. Within the ‘wet’ layer the water vapor has a constant mixing ratio below the water ice cloud condensation height and is saturated above that height. The new retrieval method simultaneously fits the daytime and nighttime TES spectra for a given location using a single mixing ratio profile. We apply this new method to the TES spectra collected over the site of the Phoenix spacecraft landing during late northern spring and summer. Retrieved daytime column abundances are ∼1–5 pr-μm higher than in the previous TES retrieval. Nighttime column abundances are lower than the daytime abundances by ∼5–10 pr-μm due to assumed exchange with soil and predicted water ice cloud formation. The height of the ‘wet’ layer varies with season, reaching ∼18 km around L s = 80–100° and decreasing to 7–10 km by L s = 140°. Changes in the vertical extent of vapor are consistent with seasonal changes in the intensity of the turbulent mixing in the lower atmosphere and in the water ice cloud condensation height. Water vapor extends by several kilometers above the top of the boundary layer at ∼4 km, suggesting that vertical transport of vapor is not limited to the boundary layer. [ABSTRACT FROM AUTHOR]

Highlights: [•] First retrieval of dust and ice cloud opacities from nighttime nadir TES spectra. [•] No diurnal variability in dust opacities within retrieval uncertainty. [•] Ice clouds opacity and spatial extent increase at night. [•] Nighttime ice clouds confined to few distinct geographic regions. [•] Nighttime ice clouds attain maximum extent during northern summer season. [Copyright &y& Elsevier]

Abstract: We report on new retrievals of water vapor column abundances from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) data. The new retrievals are from the TES nadir data taken above the ‘cold’ surface areas in the North polar region (T surf <220K, including seasonal frost and permanent ice cap) during spring and summer seasons, where retrievals were not performed initially. Retrievals are possible (with some modifications to the original algorithm) over cold surfaces overlaid by sufficiently warm atmosphere. The retrieved water vapor column abundances are compared to the column abundances observed by other spacecrafts in the Northern polar region during spring and summer and good agreement is found. We detect an annulus of water vapor growing above the edge of the retreating seasonal cap during spring. The formation of the vapor annulus is consistent with the previously proposed mechanism for water cycling in the polar region, according to which vapor released by frost sublimation during spring re-condenses on the retreating seasonal CO2 cap. The source of the vapor in the vapor annulus, according to this model, is the water frost on the surface of the CO2 at the retreating edge of the cap and the frost on the ground that is exposed by the retreating cap. Small contribution from regolith sources is possible too, but cannot be quantified based on the TES vapor data alone. Water vapor annulus exhibits interannual variability, which we attribute to variations in the atmospheric temperature. We propose that during spring and summer the water ice sublimation is retarded by high relative humidity of the local atmosphere, and that higher atmospheric temperatures lead to higher vapor column abundances by increasing the water holding capacity of the atmosphere. Since the atmospheric temperatures are strongly influenced by the atmospheric dust content, local dust storms may be controlling the release of vapor into the polar atmosphere. Water vapor abundances above the residual polar cap also exhibit noticeable interannual variability. In some years abundances above the cap are lower than the abundances outside of the cap, consistent with previous observations, while in the other years the abundances above the cap are higher or similar to abundances outside of the cap. We speculate that the differences may be due to weaker off-cap transport in the latter case, keeping more vapor closer to the source at the surface of the residual cap. Despite the large observed variability in water vapor column abundances in the Northern polar region during spring and summer, the latitudinal distribution of the vapor mass in the atmosphere is very similar during the summer season. If the variability in vapor abundances is caused by the variability of vapor sources across the residual cap then this would mean that they annually contribute relatively little vapor mass to significantly affect the vapor mass budget. Alternatively this may suggest that the vapor variability is caused by the variability of the polar atmospheric circulation. The new water vapor retrievals should be useful in tuning the Global Circulation Models of the martian water cycle. [Copyright &y& Elsevier]

Abstract: Atmospheric water vapor abundances in Mars’ north polar region (NPR, from 60° to 90°N) are mapped as function of latitude and longitude for spring and summer seasons, and their spatial, seasonal, and interannual variability is discussed. Water vapor data are from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) and the Viking Orbiter (VO) Mars Atmospheric Water Detector (MAWD). The data cover three complete northern spring–summer seasons in 1977–1978, 2000–2001 and 2002–2003, and shorter periods of spring–summer seasons during 1975, 1999 and 2004. Long term interannual variability in the averaged NPR abundances may exist, with Viking MAWD observations showing twice as much water vapor during summer as the MGS TES observations more than 10 martian years (MY) later. While the averaged abundances are very similar in TES observations for the same season in different years, the spatial distributions in the early summer season do vary significantly year over year. Spatial and temporal variabilities increase between L s ∼80–140°, which may be related to vapor sublimation from the North Polar Residual Cap (NPRC), or to changes in circulation. Spatial variability is observed on scales of ∼100km and temporal variability is observed on scales of <10 sols during summer. During late spring the TES water vapor spatial distribution is seen to correlate with the low topography/low albedo region of northern Acidalia Planitia (270–360°E), and with the dust spatial distribution across the NPR during late spring–early summer. Non-uniform vertical distribution of water vapor, a regolith source or atmospheric circulation ‘pooling’ of water vapor from the NPRC into the topographic depression may be behind the correlation with low topography/low albedo. Sublimation winds carrying water vapor off the NPRC and lifting surface dust in the areas surrounding the NPRC may explain the correlation between the water vapor and dust spatial distributions. Correlation between water vapor and dust in MAWD data are only observed over low topography/low albedo area. Maximum water vapor abundances are observed at L s =105–115° and outside of the NPRC at 75–80°N; the TES data, however, do not extend over the NPRC and thus, this conclusion may be biased. Some water vapor appears to be released in plumes or ‘outbursts’ in the MAWD and TES datasets during late spring and early summer. We propose that the sublimation rate of ice varies across the NPRC with varying surface winds, giving rise to the observed ‘outbursts’ at some seasons. [Copyright &y& Elsevier]