Global seasonal atmospheric fluctuations on Mars

The Mariner 9 S-band radio occultation measurements, which were taken over half a Martian year, were examined for seasonal variations in atmospheric pressures and temperatures. Seasonally related atmospheric pressure oscillations on a global scale were discovered when the pressures were compared on equi-potential levels. There was a global increase in pressure of about 13% between northern winter and spring seasons, and a global decrease in pressure of nearly 14% between northern spring and summer seasons. The maximum global pressure occurred during the northern spring season approximately one Martian month prior to aphelion. These pressure oscillations were correlated with the seasonal growth and decay, and the total area of the polar caps.Temperatures in the mid-latitude regions near the subsolar points were highest during the northern winter season when Mars was closest to the sun. In addition, high latitudinal temperature gradients (up to 2°K per degree latitude) were found. This has important atmospheric dynamical implications, especially for the growth of baroclinic waves.Occultation observations also indicated that the average elevation of the southern hemisphere was nearly 4km higher than the northern hemisphere when referenced to an equipotential level. The occultation measurements showed that the atmospheric pressures near the surface in the southern hemisphere were 33 to 43% lower than the atmospheric pressures near the surface in the northern hemisphere. In addition to other parameters, the asymmetry in the density of the Martian atmosphere and the hemispheric altitude differences are important in understanding the seasonal dynamic processes that exist in the polar cap regions and in the Martian atmosphere generally.     

Seasonal Neutron-Flux Variations in the Polar Caps of Mars as Revealed by the Russian HEND Instrument Onboard the NASA 2001 Mars Odyssey Spacecraft

We analyze the flux of epithermal neutrons from the Martian surface recorded by the Russian High-Energy Neutron Detector (HEND) from February 19 through December 19, 2002. The HEND was installed onboard the NASA 2001 Mars Odyssey spacecraft and is designed to measure neutron fluxes with energies above 1 eV. Over the period of observations, statistically significant variations in the flux of epithermal (10–100 keV) neutrons were found in the northern and southern polar caps. The largest neutron-flux variations were found at subpolar latitudes, where the relative difference between the summer and winter values can reach severalfold. This correlation becomes weaker with increasing distance from the poles. Thus, the relative change in the neutron flux near the 60° parallel is slightly more than 10%. We assume that the detected variations result from the global circulation of atmospheric carbon dioxide in subpolar Martian regions. To additionally test this assumption, we compared the HEND neutron measurements onboard 2001 Mars Odyssey and the seasonal variations in the CO₂-layer thickness as observed by the Mars Orbital Laser Altimeter (MOLA) onboard Mars Global Surveyor (MGS).     

Tracking the edge of the south seasonal polar cap of Mars

The advance and retreat of the polar caps were one of the first observations that indicated Mars had seasons. Because a large portion of the atmosphere is cycled in and out of the seasonal caps during the year, the frost deposits play a significant role in regional and global atmospheric circulation. Understanding the nature of the seasonal polar caps is imperative if we are to understand the current Martian climate. In this study, we track the seasonal cap edges as a function of season and longitude for the fall and winter seasons (MY27), using data from the Planetary Fourier Spectrometer (PFS) onboard the Mars Express (MEX) ESA mission. Making use of the rapid rise (decrease) in surface temperature that occurs when CO₂ ice is removed (deposited), in a first approach, we defined the advancing cap edge to be where the surface temperature drops below 150 K, and the retreating cap edge where the surface temperature rises above 160 K. In this case, starting from Ls∼50°, the edge progression speed start to be longitude dependent. In the hemisphere that extends form the eastern limit of the Hellas basin to the western limit of the Argyrae basin (and containing the two) the edges progression speed is about a half than that of the other hemisphere; the cap is thus asymmetric and, unexpectedly, no CO₂ ice seems to be present inside the basins. This is because the above mentioned surface temperatures used in this approach to detect the cap edges are not adequate (too low) for the high-pressure regions inside the basins where, following the Clausius–Clapeyron's law, the CO₂ condensation temperature can be several degrees higher than that of the adjacent lower-pressure regions. In the second, final approach, special attention has been given to this aspect and the advancing and retreating cap edges are defined where, respectively, the surface temperatures drop below and rise above the CO₂ condensation temperature for the actual surface pressure values. Now, the results show an opposite situation than the previous one, with the progression speed being higher and the cap more extended (up to −30° latitude) in the hemisphere containing the two major Martian basins. During the fall season, up to Ls∼50° the South Martian polar cap consists of CO₂ frost deposits that advance towards lower latitudes at a constant speed of 10° of latitude per 15 degrees of Ls. The maximum extension (−40° latitude) of the South polar cap occurs somewhere in the 80°–90° Ls range. At the winter solstice, when the edges of the polar night start moving poleward, the cap recession has already started, in response to seasonal changes in insolation. The CO₂ ice South polar cap will recede with a constant speed of ∼5° of latitude every 25° degrees of Ls during the whole winter. The longitudinal asymmetries reduce during the cap retreat and completely disappear around Ls=145°.     

Dynamics of the Martian Atmosphere–Soil–Polar Caps System

A numerical model of the Martian atmosphere–soil–polar caps system is constructed. We consider the conditions under which the seasonal meteorological variations in the system are stable. The fact that the planetary orbit is not circular is shown to be a sufficient condition for the appearance of a residual polar cap in the southern hemisphere if all parameters of the southern and northern hemispheres are identical. A latitudinal distribution of the atmospheric pressure is given for different seasons. A high-pressure region is shown to be formed at high latitudes of the planet. An altitude–latitude temperature distribution throughout the year is constructed. The snow accumulation and evaporation in the polar caps are examined. Seasonal features of the air mass dynamics are considered in terms of a two-dimensional atmospheric model.     

Long-term observations of the evolution of the southern seasonal cap of Mars: Neutron measurements by the HEND instrument onboard the 2001 <Emphasis Type="Italic">Mars Odyssey</Emphasis> spacecraft

We present the results of five-year observations of the southern seasonal cap of Mars based on neutron spectroscopy of the surface fulfilled by the Russian HEND instrument onboard the NASA 2001 Mars Odyssey spacecraft. The numerical modeling of the observational data allowed us to reconstruct the curves of the variations of the total mass of the southern seasonal cap of Mars for different years (three Martian years) and to find the year-to-year variations of the seasonal cycle.     

Seasonal buffering of atmospheric pressure on Mars

An isothermal reservoir of carbon dioxide in gaseous contact with the Martian atmosphere would reduce the amplitude and advance the phase of global atmospheric pressure fluctuations caused by seasonal growth and decline of polar CO₂ frost caps. Adsorbed carbon dioxide in the upper ～10 m of Martian regolith is sufficient to buffer the present atmosphere on a seasonal basis. Available observations and related polar cap models do not confirm or refute the operation of such a mechanism. Implications for the amplitude and phase of seasonal pressure fluctuations are subject to direct test by the upcoming Viking mission to Mars.     

Description of the Martian polar cap breeze

The surface temperature of the Martian polar caps is about 148 K (frost point temperature of CO₂ at a surface pressure of about 6 hPa), with the “desert” (frost-free) areas adjacent to the polar caps having much greater surface temperatures. The existence of this steep meridional gradient of temperature between the polar caps and the adjacent “desert” areas may produce in the atmosphere a baroclinic instability which generates an atmospheric circulation system similar in some aspects to the terrestrial sea breeze. We have called this circulation system the Martian polar cap breeze. In this paper, the phenomenology of the Martian polar cap breeze is developed on the basis of the indirect observational evidence. Along with friction and the Coriolis force, other factors influence the polar cap breeze: the prevailing wind, topography, irregularity of the polar cap-edge, and stability of the atmosphere. These factors are studied in a qualitative form, as well as the seasonal variations. In addition, the large-scale polar cap wind is presented as a different Martian atmospheric circulation system.     

Water distribution in Martian permafrost regions from joint analysis of HEND (Mars Odyssey) and MOLA (Mars Global Surveyor) data

We jointly analyze data from the High-Energy Neutron Detector (HEND) onboard the NASA Mars Odyssey spacecraft and data from the Mars Orbiter Laser Altimeter (MOLA) onboard the Mars Global Surveyor spacecraft. The former instrument measures the content of hydrogen (in the form of H₂O or OH) in the subsurface layer of soil and the latter instrument measures the surface albedo with respect to the flux of solar energy. We have checked the presence of a correlation between these two data sets in various Martian latitude bands. A significant correlation has been found between these data at latitudes poleward of 40° in the northern hemisphere and at latitudes 40°–60° in the southern hemisphere. This correlation is interpreted as evidence for the presence of stable water ice in these regions under a dry layer of soil whose thickness is determined by the condition for equilibrium between the condensation of water from the atmosphere and its sublimation when heated by solar radiation. For these regions, we have derived an empirical relation between the flux of absorbed solar radiation and the thickness of the top dry layer. It allows the burial depth of the water ice table to be predicted with a sub-kilometer resolution based on near-infrared albedo measurements. We have found no correlation in the southern hemisphere at latitudes >60°, although neutron data also suggest that water ice is present in this region under a layer of dry soil. We conclude that the thickness of the dry layer in this region does not correspond to the equilibrium condition between the water ice table and the atmosphere.     

A numerical model of the Martian polar cap winds

An investigation of the Martian polar cap winds and their response to a variety of factors is carried out by a series of numerical experiments based on a zonally symmetric primitive equation model. These factors are the seasonal thermal forcing, mass exchange between polar caps and atmosphere, large-scale topography, and polar cap size. The thermal forcing sets up a circulation whose surface winds adjust to achieve angular momentum balance, with low-latitude easterlies and high-latitude westerlies. The maximum westerlies occur roughly where the horizontal temperature gradients are largest. This pattern changes when cap and atmosphere exchange mass. Corriolis forces acting on the net outflow or inflow produce easterlies at the surface during spring (outflow) and westerlies during winter (inflow). Topography appears to have a small effect, but cap size does play a role, the circulation intensity increasing with cap size. Peak surface winds occur when outflow or inflow is a maximum and are 20 m sec^?1 during spring and 30 m sec^?1 during winter for the northern hemisphere. The model results show that surface winds near the edge of a retreating polar cap are substantially enhanced, a result which is consistent with the Viking observations of local dust storm activity near the edge of the south polar cap during spring. The results also indicate that the surficial wind indicators near the south pole are formed during spring and those near the north pole during winter. The implication is that the high-latitude dune fields in the northern hemisphere are formed at a time when the terrain is being covered with frost. It is therefore suggested that the saltating particles are “snowflakes” which have formed by the mechanism proposed by Pollack etal. The model results for the winter simulation, which have formed by the mechanism transport by large-scale eddies, compare favorably with general circulation model (GCM) calculations. This suggests that the eddy transports may be less important than those associated with the net mass flow, and that 2-D climate modeling may be more succesful for Mars than Earth.     

The water cycle in the general circulation model of the martian atmosphere

Within the numerical general-circulation model of the Martian atmosphere MAOAM (Martian Atmosphere: Observation and Modeling), we have developed the water cycle block, which is an essential component of modern general circulation models of the Martian atmosphere. The MAOAM model has a spectral dynamic core and successfully predicts the temperature regime on Mars through the use of physical parameterizations typical of both terrestrial and Martian models. We have achieved stable computation for three Martian years, while maintaining a conservative advection scheme taking into account the water–ice phase transitions, water exchange between the atmosphere and surface, and corrections for the vertical velocities of ice particles due to sedimentation. The studies show a strong dependence of the amount of water that is actively involved in the water cycle on the initial data, model temperatures, and the mechanism of water exchange between the atmosphere and the surface. The general pattern and seasonal asymmetry of the water cycle depends on the size of ice particles, the albedo, and the thermal inertia of the planet’s surface. One of the modeling tasks, which results from a comparison of the model data with those of the TES experiment on board Mars Global Surveyor, is the increase in the total mass of water vapor in the model in the aphelion season and decrease in the mass of water ice clouds at the poles. The surface evaporation scheme, which takes into account the turbulent rise of water vapor, on the one hand, leads to the most complete evaporation of ice from the surface in the summer season in the northern hemisphere and, on the other hand, supersaturates the atmosphere with ice due to the vigorous evaporation, which leads to worse consistency between the amount of the precipitated atmospheric ice and the experimental data. The full evaporation of ice from the surface increases the model sensitivity to the size of the polar cap; therefore, the increase in the latter leads to better results. The use of a more accurate dust scenario changes the model temperatures, which also strongly affects the water cycle.     

Interannual and seasonal behavior of Martian residual ice-cap albedo

The Mars Orbiter Laser Altimeter (MOLA), functioning as a high-resolution radiometer, has observed several appearances of the Martian residual ice caps. We examine these data to quantify both seasonal behavior and interannual differences. The northern residual cap (NRC) was found to be mostly stable with the exception of one, previously identified, region of strong variability. Interannual change in the extent of the NRC appears to be small and reversible on timescales of 1 or 2 years. The NRC has an elaborate seasonal evolution of albedo. Annuli of fine-grained CO₂ and water frost, which track the inner and outer edges of the seasonal CO₂ cap, cause large temporary brightenings. The NRC albedo is stable from just after solstice to L_s 150°, after which albedo decreases steadily. This late-summer darkening can be explained by shadowing within the rough topography of the NRC, leading to a lower limit on topographic relief of 80 cm. The southern residual cap (SRC) appears stable in extent. As has been previously discovered, its seasonal frost albedo behavior appears to be correlated with insolation. However, residual CO₂ appears not to share this characteristic; we use this behavioral difference to infer net deposition of CO₂ ice on the SRC during 1 out of 3 years. Uncharacteristically, the SRC abruptly darkens at L_s 320° in 1 Martian year (year beginning April 2002). Circumstantial evidence suggests atmospheric scattering by dust is responsible. The 2001 global dust-storm appears, either, to have had no effect on the polar cap albedos, or, resulted in slightly brighter ice deposits.     

Numerical simulation of seasonal dynamics of water vapor in the Martian atmosphere

The seasonal evolution of the H₂O snow in the Martian polar caps and the dynamics of water vapor in the Martian atmosphere are studied. It is concluded that the variations of the H₂O mass in the polar caps of Mars are determined by the soil thermal regime in the polar regions of the planet. The atmosphere affects water condensation and evaporation in the polar caps mainly by transferring water between the polar caps. The stability of the system implies the presence of a source of water vapor that compensates for the removal of water from the atmosphere due to permanent vapor condensation in the polar residual caps. The evaporation of the water ice that is present in the surface soil layers in the polar regions of the planet is considered as such a source. The annual growth of the water-ice mass in the residual polar caps is estimated. The latitudinal pattern of the seasonal distribution of water vapor in the atmosphere is obtained for the stable regime.Translated from Astronomicheskii Vestnik, Vol. 38, No. 6, 2004, pp. 497–503.Original Russian Text Copyright © 2004 by Aleshin.     

Polar warming in the middle atmosphere of Mars

We report ground-based laser heterodyne spectroscopy of non-thermal emission in the cores of the 10.33-μmR(8) and 10.72-μmP(32) lines of ¹²C¹⁶O₂, obtained at 23 locations on the disk of Mars during the 1984 opposition, at L_s = 130°. The data were obtained at a sub-Doppler spectral resolution, and the temperature of the middle Martian atmosphere (50–85 km) is derived from the frequency width and intensity of the R(8) emission, and from the total intensity of the P(32) emission. We find that the temperature of the middle Martian atmosphere varies with latitude. Near the subsolar latitude, the average 50- to 85-km temperature is close to the radiative equilibrium value for a CO₂ atmosphere. However, at high latitudes in both the northern (summer) and southern (winter) hemispheres the 50- to 85-km temperature exceeds the CO₂ radiative equilibrium value; a meridional gradient in the range of 0.4 – 0.9°K per degree of latitude is indicated by our data. The highest temperatures are seen at high latitudes in the winter hemisphere, reminiscent of the seasonal effects seen at the Earth's mesopause. As in the terrestrial case, this winter polar warming in the Martian middle atmosphere necessitates departures from radiative equilibrium; dynamical heating of order 4 × 10² ergs g⁻¹ sec⁻¹ is required at the edge of the winter polar night. A comparison with 2-D circulation models shows that the presence of atmospheric dust may enhance this dynamical heating at high winter latitudes, and may also account for heating at high latitudes in the summer hemisphere.     

H2O- and OH-bearing minerals in the martian regolith:: analysis of 1997 observations from HST/NICMOS

We have analyzed observations of the Acidalia hemisphere of Mars taken by the Hubble Space Telescope's Near-Infrared Camera Multi-Object Spectrograph (HST/NICMOS) during July of 1997 (L_s=152°, northern martian summer). The data consist of images at ∼60 km/pixel resolution, using both narrow- and medium-band filters specifically selected to allow us to study the hydration state of the martian surface. Calibration was performed by comparison to Phobos-2 ISM observations of overlapping regions, and atmospheric gas correction was performed by modeling the atmosphere for each pixel using a line-by-line radiative transfer code coupled with the MOLA altimetry data. Our results indicate the presence of at least three spectrally different large-scale (>1000 km diameter) terrains corresponding to the dark regions of northern Acidalia, the southern hemisphere classical dark terrain, and the classical intermediate terrain adjacent to southern Acidalia. We also identified two other spectrally unique terrains, corresponding to the northern polar ice cap, and to the southern winter polar hood. Comparisons with mineral spectra indicate the possibility of different H₂O- or OH-bearing (i.e., hydroxides and/or hydrates) minerals existing both in northern Acidalia and in the nearby intermediate albedo terrain. Hydrated minerals do not appear to be spectrally important components of the southern hemisphere dark terrains imaged by HST in 1997.     

Measurements of water vapor in Mars' Antarctic

A complete Mars year of measurements of atmospheric water vapor in the south arctic have been obtained from the Viking Orbiters. Analysis of the observations indicates that, except for the south remnant cap, the southern hemisphere of Mars is devoid of any substantial reservoirs of water in contact with the atmosphere, and that, in the summer time, the top layer of soil is desiccated. Small amounts of water ice are incorporated into the annual CO₂ cap; this water is released at the cap margin when it retreats in the spring. The first global dust storm resulted in heating of the south arctic atmosphere and a transport of water in from the equatorial region. The second global dust storm had a negligible effect on atmospheric water vapor; the dust contained little water.     

The astronomical theory of climatic change on Mars

We examine the response of Martian climate to changes in solar energy deposition caused by variations of the Martian orbit and obliquity. We systematically investigate the seasonal cycles of carbon dioxide, water, and dust to provide a complete picture of the climate for various orbital configurations. We find that at low obliquity (15°) the atmospheric pressure will fall below 1 mbar; dust storms will cease; thick permanent CO₂ caps will form; the regolith will release CO₂; and H₂O polar ice sheets will develop as the permafrost boundaries move poleward. At high obliquity (35°) the annual average polar temperature will increase by about 10°K, slightly desorbing the polar regolith and causing the atmospheric pressure to increase by not more than 10 to 20 mbar. Summer polar ground temperatures as high as 273°K will occur. Water ice caps will be unstable and may disappear as the equilibrium permafrost boundary moves equatorward. However, at high eccentricity, polar ice sheets will be favored at one pole over the other. At high obliquity dust storms may occur during summers in both hemispheres, independent of the eccentricity cycle. Eccentricity and longitude of perihelion are most significant at modest obliquity (25°). At high eccentricity and when the longitude of perihelion is close to the location of solstice hemispherical asymmetry in dust-storm generation and in polar ice extent and albedo will occur.The systematic examination of the relation of climate and planetary orbit provides a new theory for the formation of the polar laminae. The terraced structure of the polar laminae originates when eccentricity and/or obliquity variations begin to drive water ice off the dusty permanent H₂O polar caps. Then a thin (meters) layer of consolidated dust forms on top of a dirty, slightly thicker (tens of meters) ice sheet and the composite is preserved as a layer of laminae composed predominately of water ice. Because of insolation variation on slopes, a series of poleward- and equatorward-facing scarps are formed where the edges of the laminae are exposed. Independently of orbital variations, these scarps propagate poleward both by erosion of the equatorward slopes and by deposition on the poleward slopes. Scarp propagation resurfaces and recycles the laminae forming the distinctive spiral bands of terraces observed and provides a supply of water to form new permanent ice caps. The polar laminae boundary marks the furthest eqautorward extension of the permanent H₂O caps as the orbit varies. The polar debris boundary marks the furthest equatorward extension of the annual CO₂ caps as the orbit varies.The Martian regolith is now a significant geochemical sink for carbon dioxide. CO₂ has been irreversibly removed from the atmosphere by carbonate formation. CO₂ has also benn removed by regolith adsorption. Polar temperature increases caused by orbital variations are not great enough     

Response of the intermediate complexity Mars Climate Simulator to different obliquity angles

A climate model of intermediate complexity, named the Mars Climate Simulator, has been developed based on the Portable University Model of the Atmosphere (PUMA). The main goal of this new development is to simulate the climate variations on Mars resulting from the changes in orbital parameters and their impact on the layered polar terrains (also known as permanent polar ice caps). As a first step towards transient simulations over several obliquity cycles, the model is applied to simulate the dynamical and thermodynamical response of the Martian climate system to different but fixed obliquity angles. The model is forced by the annual and daily cycle of solar insolation. Experiments have been performed for obliquities of φ=15^° (minimum), φ=25.2^° (present), and φ=35^° (maximum). The resulting changes in solar insolation mainly in the polar regions impact strongly on the cross-equatorial circulation which is driven by the meridional temperature gradient and steered by the Martian topography. At high obliquity, the cross-equatorial near surface flow from the winter to the summer hemisphere is strongly enhanced compared to low obliquity periods. The summer ground temperature ranges from 200 K (φ=15^°) to 250 K (φ=35^°) at 80^°N in northern summer, and from 220 K (φ=15^°) to 270 K (φ=35^°) at 80^°S in southern summer. In the atmosphere at 1 km above ground, the respective range is 195-225 K in northern summer, and 210-250 K in southern summer.     

Spatial inhomogeneity of the martian subsurface water distribution: implication from a global water cycle model

In an effort to test and to understand the global hydrogen distribution in the shallow subsurface of Mars retrieved by the Mars Odyssey gamma-ray spectrometer, the present state and movement of water are investigated by a coupled global subsurface-atmosphere water cycle model. It was found that the observed global subsurface hydrogen distribution is largely consistent with the modeled global water cycle, so a large fraction of hydrogen is likely to exist as water, at low and mid latitudes in the form of adsorbed water. Under the present climate the water content in the shallow subsurface becomes higher in the northern hemisphere than in the southern hemisphere as a result of global water cycle, regardless of the initial water distribution in the soil or adsorptive capacity. The higher annual maximum soil temperature in the south, stronger net northward transport of atmospheric water vapor, and the emission of vapor from the northern residual polar cap in northern summer contribute to this hemispheric asymmetry. The generally higher adsorptive capacity of clay minerals in the northern plains may further increase this bias. The longitudinal inhomogeneity is caused by several factors, such as thermal inertia, adsorptive capacity, and atmospheric surface pressure. The water abundance is locally high in low thermal inertia regions (e.g., Arabia Terra) and at deep places where the surface pressure is high (e.g., Hellas); it is low in soil with a low adsorptive capacity (e.g., Tharsis) and high thermal inertia regions (e.g., Solis Planum). Most of the soil humidity near the surface at low and mid latitudes may originate from the atmosphere. The model implies that the upper soil layer should be largely ice-free because otherwise an excessive sublimation and vapor emission into the atmosphere in warm seasons would violate the observational constraints. Moreover, the more uniform latitudinal variation of the observed hydrogen abundance near the surface compared to that of deeper layers is indicative of the presence of adsorbed water instead of ground ice because the adsorbed water content does not as steeply depend on latitude as the ground ice stability. Concerning the regolith mineralogy, montmorillonite can much better account for the observed water cycle than palagonite. While the presence of permanent ground ice appears likely in the polar region below a thin layer, large seasonal cycle of phase change between pore ice and adsorbed water may be possible. Regolith adsorption/desorption is neither negligible nor crucial for the seasonal atmospheric water cycle, but the surface-atmosphere coupling is a major prerequisite for the long-term evolution of subsurface water distribution.     

Seasonal Carbon Dioxide Depositions on the Martian Surface as Revealed from Neutron Measurements by the HEND Instrument Onboard the 2001 Mars Odyssey Spacecraft

We present the results of eighteen months of observations of the seasonal caps of Mars based on data from the neutron spectroscopy of the surface by the Russian HEND Instrument mounted aboard the NASA 2001 Mars Odyssey spacecraft. A four-dimensional model of the Martian seasonal caps was developed on the basis of these observation data. The model shows how the thickness of the frozen carbon dioxide changes in different surface regions. Using the results of the model, we estimated the total mass of the seasonal caps for the period of maximal accumulation of seasonal depositions and the rates of condensation and sublimation of the seasonal cover.     

Modelling of the combined late-winter ice cap edge and slope winds in Mars Hellas and Argyre regions

Towards the end of southern hemisphere winter (L_s ≈ 180°) the Martian southern polar cap extends equatorward to 40°S and covers at least, the southern slopes of the Hellas and Argyre impact basins. Subsequently, during retreat of the seasonal ice cap, varying configurations of ice coverage on these slopes occur. Since both sloping topography and ice-edge effects can independently drive mesoscale circulations, the superposition of these two processes may then generate interesting wind patterns. A set of numerical experiments has been performed with the University of Helsinki 2-D Mars Mesoscale Circulation Model (MMCM) in order to study the characteristics of circulations driven by these combined forcings. A model-centre latitude of 57°S and a slope angle of 0.6°, both representative of Hellas southern slope, are used. When compared with the winds arising in the ice-free slope case, ice coverage in the upper extent of the slope results in diminished upslope (daytime) winds, while the down-slope (nighttime) flow is enhanced. Ice coverage in the lower section of the slope in turn causes enhanced upslope (daytime) and attenuated downslope (nocturnal) flows. This arises due to the daytime off-ice near-surface flow induced by the thermal contrast at the ice cap edge. The surface winds are persistently downslope over a fully ice-covered slope. Inclusion of atmospheric dust (τ = 0.3) reduces the ice-edge forcing. In comparison with the dust-free situation, the resulting circulation is almost unchanged in the case of ice-covered upper part of the slope, in the opposite case the daytime flow is attenuated and the nocturnal downslope flow enhanced. When the entire slope is ice-covered, the flow is amplified due to the increased direct atmospheric heating. Inclusion of a large scale circulation component (7 m⧸s southerly wind) in conjunction with an ice-covered slope top results in the generation of a downslope windstorm (föhn, or bora-type of event) with near surface winds exceeding 30 m⧸s. Winds of this magnitude, not realised in any of the other experiments, approach speeds deemed capable of lifting dust from the surface.