Seasonal redistribution of water in the surficial Martian regolith: Results from the <Emphasis Type="Italic">Mars Odyssey</Emphasis> high-energy neutron detector (HEND)

The seasonal variation of neutron emissions from Mars in different spectral intervals measured by the HEND neutron detector for the entire Martian year are analyzed. Based on these data, the spatial variations of the neutron emissions from the planet are globally mapped as a function of season, and the dynamics of seasonal variation of neutron fluxes with different energies is analyzed in detail. No differences were found between seasonal regimes of neutron fluxes in different energy ranges in the southern hemisphere of Mars, while the regime of fast neutrons (with higher energies) during the northern winter strongly differs from that during the southern winter. In winter (L _s = 270°–330°), the fast neutron fluxes are noticeably reduced in the northern hemisphere (along with the consecutive thickening of the seasonal cap of solid carbon dioxide). This provides evidence of a temporary increase in the water content in the effective layer of neutron generation. According to the obtained estimates, the observed reduction of the flux of fast neutrons in the effective layer corresponds to an increase in the water abundance of up to 5% in the seasonal polar cap (70°–90°N), about 3% at mid-latitudes, and from 1.5 to 2% at low latitudes. The freezing out of atmospheric water at the planetary surface (at middle and high latitudes) and the hydration of salt minerals composing the Martian soil are considered as the main processes responsible for the temporary increase in the water content in the soil and upper layer of the seasonal polar cap. The meridional atmospheric transport of water vapor from the summer southern to the winter northern hemisphere within the Hadley circulation cell is a basic process that delivers water to the subsurface soil layer and ensures the observed scale of the seasonal increase in water abundance. In the summer northern hemisphere, the similar Hadley circulation cell transports mainly dry air masses to the winter southern hemisphere. The point is that the water vapor becomes saturated at lower heights during aphelion, and the bulk of the atmospheric water mass is captured in the near-equatorial cloudy belt and, thus, is only weakly transferred to the southern hemisphere. This phenomenon, known as the Clancy effect, was suggested by Clancy et al. (1996) as a basic mechanism for the explanation of the interhemispheric asymmetry of water storage in permanent polar caps. The asymmetry of seasonal meridional circulation of the Martian atmosphere seems to be another factor determining the asymmetry of the seasonal water redistribution in the “atmosphere-regolith-seasonal polar caps” system, found in the peculiarities of the seasonal regime of the neutron emission of Mars.     

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.     

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.     

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°.     

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.     

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.     

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.     

The Martian Hadley circulation: Comparison of “viscous” model predictions to observations

The predictions of an analytical steady thermally forced viscous model for the zonally averaged global circulation of the Martian atmosphere are compared to observations of atmospheric temperatures and eolian feature directions. The temperature of the winter polar atmosphere is a sensitive indicator of the value of the mean eddy diffusivity v of the atmosphere. For global dust storm season, the observed large warming of the atmosphere over the winter (north) pole during the 1977b storm as well as the distribution and azimuths of Type I(b) dust streaks can be well reproduced if v = 10⁷−10⁸cm²sec⁻¹. Observed northeasterly surface winds at high northern latitudes could be produced by plausible fluxes of CO₂ from the northern polar cap. The thermal structure at southern latitudes during this storm is satisfactorily produced, but a major discrepancy exists between observations and predictions of the temperature structure at middle northern latitudes. By comparing these results with those from previous inviscid models of the zonal mean circulation on Mars, we suggest v > 10⁷cm²sec⁻¹ at heights above 30 km and v ⪡ 10⁷cm²sec⁻¹ at lower altitudes may yield a meridional circulation which produces the dynamical warming necessary to account for the observed thermal structure at 0.6 mbar at all latitudes. For late southern summer when the dust content of the atmosphere has decreased to levels characteristic of much of the Martian year, the lack of dynamical warming of the winter pole implies v ≤ 10⁶cm²sec⁻¹. In addition, the predicted surface wind azimuths at this season agree well with the observed azimuths of Type I(d) dust streaks, which are observed to form at this season. The observed latitudinal concentration of the streaks is not explained by the model and may require additional processes which have not been included.     

Titan’s cloud seasonal activity from winter to spring with Cassini/VIMS

Since Saturn orbital insertion in July 2004, the Cassini orbiter has been observing Titan throughout most of the northern winter season (October 2002–August 2009) and the beginning of spring, allowing a detailed monitoring of Titan’s cloud coverage at high spatial resolution with close flybys on a monthly basis. This study reports on the analysis of all the near-infrared images of Titan’s clouds acquired by the Visual and Infrared Mapping Spectrometer (VIMS) during 67 targeted flybys of Titan between July 2004 and April 2010.The VIMS observations show numerous sporadic clouds at southern high and mid-latitudes, rare clouds in the equatorial region, and reveal a long-lived cloud cap above the north pole, ubiquitous poleward of 60°N. These observations allow us to follow the evolution of the cloud coverage during almost a 6-year period including the equinox, and greatly help to further constrain global circulation models (GCMs). After 4 years of regular outbursts observed by Cassini between 2004 and 2008, southern polar cloud activity started declining, and completely ceased 1 year before spring equinox. The extensive cloud system over the north pole, stable between 2004 and 2008, progressively fractionated and vanished as Titan entered into northern spring. At southern mid-latitudes, clouds were continuously observed throughout the VIMS observing period, even after equinox, in a latitude band between 30°S and 60°S. During the whole period of observation, only a dozen clouds were observed closer to the equator, though they were slightly more frequent as equinox approached.We also investigated the distribution of clouds with longitude. We found that southern polar clouds, before disappearing in mid-2008, were systematically concentrated in the leading hemisphere of Titan, in particular above and to the east of Ontario Lacus, the largest reservoir of hydrocarbons in the area. Clouds are also non-homogeneously distributed with longitude at southern mid-latitudes. The n = 2-mode wave pattern of the distribution, observed since 2003 by Earth-based telescopes and confirmed by our Cassini observations, may be attributed to Saturn’s tides.Although the latitudinal distribution of clouds is now relatively well reproduced and understood by the GCMs, the non-homogeneous longitudinal distributions and the evolution of the cloud coverage with seasons still need investigation. If the observation of a few single clouds at the tropics and at northern mid-latitudes late in winter and at the start of spring cannot be further interpreted for the moment, the obvious shutdown of the cloud activity at Titan’s poles provides clear signs of the onset of the general circulation turnover that is expected to accompany the beginning of Titan’s northern spring. According to our GCM, the persistence of clouds at certain latitudes rather suggests a ‘sudden’ shift in near future of the meteorology into the more illuminated hemisphere. Finally, the observed seasonal change in cloud activity occurred with a significant time lag that is not predicted by our model. This may be due to an overall methane humidity at Titan’s surface higher than previously expected.     

Argon in the Martian atmosphere: Do the results of Mars 6 agree with the optical and radio occultation measurements?

Mars 6 discovered an inert gas (probably argon) in the Martian atmosphere. An analysis is carried out for the available spectroscopic observations, radio occultation results, and other data with the aim of determining the maximum argon content with which they are consistent. Possible seasonal variations of pressure are taken into account. It is shown that 25–30% of argon is consistent with all these data. The presence of argon slightly increases the mean pressure estimates; 25% argon increases pressures derived from radio occultation data by 0.5 mbar.     

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).     

Evidence for temporal change at Uranus' south pole

Analysis of Hubble Space Telescope images of Uranus taken between 1994 and 2002 shows evidence for temporal changes in zonal brightness patterns in the south polar region. Between 1994 and 2002, a relatively bright ring developed near 70° S. The pole itself, which was the brightest area of the southern hemisphere in 1994, has become relatively dark. The polar collar at 45° S has also become brighter relative to the rest of the southern polar region. Comparison of images through different filters suggests that the change is occurring at pressures of 2-4 bars in the atmosphere. A change at this depth is consistent with radio measurements which indicate seasonal variability in Uranus' deep atmosphere. Disk-integrated photometry at visible wavelengths also exhibits variability on seasonal (∼?decades) timescales. The observed changes are not predicted by existing dynamical models of Uranus' atmosphere.     

Martian atmospheric water vapor observations: 1972–1974 apparition

The patrol of Martian water vapor carried out with the echelle-coudé scanner at McDonald Observatory during the 1972–1974 apparition has produced 469 individual photoelectric scans of Doppler-shifted Martian H₂O lines. Almost an entire Martian year was covered during the 1972–1974 period (L_s = 118?269° and 301?80°). Three types of coverage have been obtained: (1) regular—the slit placed pole to pole on the central meridian; (2) latitudinal—the slit placed parallel to the Martian equator at various latitudes; (3) diurnal—the slit placed parallel to the terminator at several times during a Martian day measured from local noon.Both the seasonal and diurnal effects seem to be controlled by the insolation and not the local topography with respect to the 6.1 mb surface. A slight negative correlation with elevation was noted which improved during the seasons of greater H₂O content. The previous seasonal behavior has been confirmed and amplified. The following are the primary conclusions: (1) The planetwide abundance is low (5?15 μm of ppt H₂O) during both equinoctical periods. (2) The maximum abundance of about 40 μm occurs in each hemisphere after solstice at about 40° latitude in that hemisphere. (3) The latitude of the maximum amount in the N-S distribution precedes the latitude of maximum insolation by 10–20° of latitude. (4) During the “drier” seasons (5–20 μm) near the equinoxes on Mars, the atmospheric water vapor changes by a factor of 2–3x over a diurnal cycle with the maximum near local noon. (5) The effects of the 1973 dust storm during the southern summer reduced the amount of water vapor over the southern hemisphere regions to 3–8 μm.     

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.     

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.     

The influence of the Martian polar caps on the diurnal tide

The large horizontal heating gradients that exist near the edge of the Martian polar caps during spring are shown to be capable of exciting large oscillations in the diurnal tide. To a lesser extent, the daily mass cycling between cap and atmosphere can also contribute. The calculations which demonstrate this are based on classical tidal theory as applied to the cylindrical coordinate system. This is done to facilitate the representation of the heating function. Results are presented for the horizontal surface winds only. They indicate a circulation at the cap edge somewhat analogous to the smaller scale terrestrial sea breeze. The amplitude of the zonal component is largest and is increased from 1 to 10 m sec^?1 by the modeled influence of the polar cap. When coupled with the basic flow these cap-edge tides can produce strong surface winds during spring. Such a mechanism may contribute to the ability of the south polar cap winds to generate the local dust storms observed near the cap edge at this season.     

Loss of fine-scale surface texture in Viking orbiter inages and implications for the inferred distribution of Debris Mantles

The absence of fine-scale surface features in Viking Orbiter images must be interpreted with caution. A reduction in contrast due to the presence of atmospheric haze will preferentially obscure small features. Two sets of images of the same region, taken with similar viewing geometry but under different atmospheric conditions, allow us to demonstrate that a single scattering model quantitatively accounts for the effects of the atmosphere. Craters five to seven times the size of the camera picture element should be resolved in Viking Orbiter images if the atmosphere is clear. When atmospheric haze effects dominate, larger craters are obscured and crater size-frequency distributions appear to be depleted in small-sized craters in a predictable way. Twelve crater size-frequency counts in the northern hemisphere behave in the manner predicted for hazy conditions. Their characteristics also follow the pattern of increasing atmospheric opacity with latitude in spring and summer deduced from other measurements of cloudiness. Loss of surface resolution due to the nearly ubiquitous atmospheric obscuration in the northern mid to high latitudes makes it difficult to assesst, with existing images, the validity of suggestions that fine-scale surface features have been preferentially degraded by surface processes. However, the atmosphere in the southern mid and high latitudes is relatively clear during autumn and winter, and a preliminary review of existing image data suggests that features with sizes down to five to seven picture elements can be detected in this region. No evidence for a recent circumpolar debris mantle can be found in the southern hemisphere data. Because the smallest craters that can be resolved in Viking images are several tens of meters in diameter, we cannot rule out the occurrence of debris deposits less than a few tens of meters thick, regardless of atmospheric clarity.     

Further observations of regional dust storms and baroclinic eddies in the northern hemisphere of Mars

We have investigated the near-surface meteorology in the northern hemisphere of Mars through detailed analysis of data obtained with Mars Global Surveyor in January-August 2005. The season in the northern hemisphere ranged from midsummer through winter solstice of Mars Year (MY) 27. We examined composite, wide-angle images from the Mars Orbiter Camera and compiled a catalog of the dust storms that occurred in this interval. As in previous martian years, activity in the northern hemisphere was dominated by regional “flushing” dust storms that sweep southward through the major topographic basins, most frequently in Acidalia Planitia. We also used atmospheric profiles retrieved from radio occultation experiments to characterize eddy activity near the surface at high northern latitudes. There are strong correlations between the two sets of observations, which allowed us to identify three factors that influence the timing and location of the regional dust storms: (1) transitions among baroclinic wave modes, which strongly modulate the intensity of meridional winds near the surface, (2) storms zones, which impose strong zonal variations on the amplitude of some baroclinic eddies, and (3) stationary waves, which further modulate the wind field near the surface. The flushing dust storms ceased abruptly in midautumn, possibly in response to source depletion, CO₂ condensation, a shift in the period of the baroclinic eddies, and changes in the tidal wind field near the surface. Our results extend the meteorological record of the northern hemisphere, substantiate the findings of previous investigations, and further illuminate the climatic impact of baroclinic eddies.     

Water ice clouds in the Martian atmosphere: Two Martian years of SPICAM nadir UV measurements

The SPICAM instrument onboard Mars Express has successfully performed two Martian years (MY 27 and MY28) of observations. Water ice cloud optical depths spatial and temporal distribution was retrieved from nadir measurements in the wavelength range 300–320 nm. During the northern spring the cloud hazes complex distribution was monitored. The clouds in the southern hemisphere formed a zonal belt in the latitude range 30–60°S. The edge of the retreating north polar hood merged with the northern tropical clouds in the range 250–350°E. The development of the aphelion cloud belt (ACB) started with the weak hazes formation (cloud optical thickness 0.1–0.3) in the equatorial region. At the end of the northern spring, the ACB cloud optical thickness reached already values of 0.3–1. The ACB decay in the end of the northern summer was accompanied with a presence of clouds in the north mid-latitudes. The expanded north polar hood merged with the north mid-latitude clouds in the eastern hemisphere. The interannual comparison indicates a decrease in cloud activity immediately after a strong dust storm in southern summer of MY28. The strong dust storms of the MY28 may also be a reason of the observed north polar hood edge shifting northward by 5°.     

Forced waves in the martian atmosphere from MGS TES nadir data

We have analyzed the temperature retrievals from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) nadir spectra to yield latitude-height resolved maps of various atmospheric forced wave modes as a function of season for a full Mars year. Among the isolated wave modes is the zonal mean, time mean temperature, which we used to derive zonal mean zonal winds and stationary wave quasi-geostrophic indices of refraction, diagnostic of their propagation. The diurnal Kelvin wave was isolated in the data, with results roughly consistent with models (Wilson and Hamilton, 1996, J. Atmos. Sci. 33, 1290-1326). The s = 1 and s = 2 stationary waves were found to have significant amplitude in ducts extending up the winter polar jets, while the s = 3 stationary wave was found to be confined to near the surface. The s = 1 stationary wave was found to have little phase tilt with height during northern winter, but significant westward phase tilt with height in the southern winter. This indicates that the wave carries heat poleward, slightly more than that found in Barnes et al. (1996; J. Geophys. Res. 101, 12,753-12,776). The s = 1 stationary wave is likely the dominant mechanism for eddy meridional heat transport for the southern winter. We noted that the phase of the s = 2 stationary wave is nearly constant with time, but that the s = 1 stationary wave moved 90° of longitude from fall to winter and back in spring in the North. While interannual variability is not yet addressed, overall, these results provide the first comprehensive benchmark for forced waves in Mars’s atmosphere against which future atmospheric models of Mars can be compared.