Solar EUV and Electron-Proton-Hydrogen Atom-Produced Ionosphere on Mars: Comparative Studies of Particle Fluxes and Ion Production Rates Due to Different Processes

The total photoelectron and secondary electron fluxes are calculated at different times and altitudes along the trajectory of Mars Global Surveyor passing through the nightside and dayside martian ionosphere. These results are compared with the electron reflectometer experiment on board Mars Global Surveyor. The calculated electron spectra are in good agreement with this measurement. However, the combined fluxes of proton and hydrogen atom as calculated by E. Kallio and P. Janhunen (2001, J. Geophys. Res.106, 5617-5634) were found to be 1-2 orders of magnitude smaller than the measured spectra. We have also calculated ionization rates and ion and electron densities due to solar EUV, X-ray, and electron-proton-hydrogen atom impacting with atmospheric gases of Mars at solar zenith angles of 75°, 105°, and 127°. In the vicinity of the dayside ionization peak, it is found that the ion production rate caused by the precipitation of proton-hydrogen atom is larger than the X-ray impact ionization rate while at all altitudes, the photoionization rate is always greater than either of the two. Moreover, X-rays contribute greatly to the photoelectron impact ionization rate as compared to the photoion production rate. The calculated electron densities are compared with radio occultation measurements made by Mars Global Surveyor, Viking 1, and Mars 5 spacecraft at these solar zenith angles. The dayside ionosphere produced by proton-hydrogen atom is smaller by an order of magnitude than that produced by solar EUV radiation. X-rays play a significant role in the dayside ionosphere of Mars at the altitude range 100-120 km. Solar wind electrons and protons provide a substantial source for the nightside ionosphere. These calculations are carried out for a solar minimum period using solar wind electron flux, photon flux, neutral densities, and temperatures under nearly the same areophysical conditions as the measurements.     

Validation of martian meteorological data assimilation for MGS/TES using radio occultation measurements

We describe an assimilation of thermal profiles below about 40 km altitude and total dust opacities into a general circulation model (GCM) of the martian atmosphere. The data were provided by the Thermal Emission Spectrometer (TES) on board the Mars Global Surveyor (MGS) spacecraft. The results of the assimilation are verified against an independent source of contemporaneous data represented by radio occultation measurements with an ultra-stable radio oscillator, also aboard MGS. This paper describes a comparison between temperature profiles retrieved by the radio occultation experiments and the corresponding profiles given by both an independent, carefully tuned GCM simulation and by an assimilation of TES observations performed over the period of time from middle, northern summer in martian year 24, corresponding to May 1999, until late, northern spring in martian year 27, corresponding to August 2004. This study shows that the assimilation of TES measurements improves the overall agreement between radio occultation observations and the GCM analysis, in particular below 20 km altitude, where the radio occultation measurements are known to be most accurate. Discrepancies still remain, mostly during the global dust storm of year 2001 and at latitudes around 60° N in northern winter-early spring. These are the periods of time and locations, however, for which discrepancies between TES and radio occultation profiles are also shown to be the largest. Finally, a further direct validation is performed, comparing stationary waves at selected latitudes and time of year. Apart from biases at high latitudes in winter time, data assimilation is able to represent the correct wave behaviour, which is one major objective for martian assimilation.     

Radio occultation measurements and MGCM simulations of Kelvin waves on Mars

We have derived new results concerning thermal tides on Mars from a combination of radio occultation measurements and numerical simulations by a Mars General Circulation Model (MGCM). This investigation exploits a set of concurrent observations by Mars Express (MEX) and Mars Global Surveyor (MGS) in mid-2004, when the season on Mars was midspring in the northern hemisphere. The MEX occultations sampled the atmosphere near the evening terminator at latitudes ranging from 54° N to 15° S. The MGS occultations provided complementary coverage near the morning terminator at latitudes of 35° N and 71° S. The geopotential field derived from these measurements contains distinctive modulation caused by solar-asynchronous thermal tides. Through careful analysis of the combined observations, we characterized two prominent wave modes, obtaining direct solutions for some properties, such as the amplitude and phase, as well as constraints on others, such as the period, zonal wave number, and meridional structure. We supplemented these observations with MGCM simulations. After evaluating the performance of the MGCM against the measurements, we used the validated simulation to deduce the identity of the two tidal modes and to explore their behavior. One mode is a semidiurnal Kelvin wave with a zonal wave number of 2 (SK2), while the other is a diurnal Kelvin wave with a zonal wave number of 1 (DK1). Both modes are known to be close to resonance in the martian atmosphere. Our observations of the SK2 are more complete and less ambiguous than any previous measurement. The well-known DK1 is the dominant solar-asynchronous tide in the martian atmosphere, and our results confirm and extend previous observations by diverse instruments.     

Continuous monitoring of nightside upper thermospheric mass densities in the martian southern hemisphere over 4 martian years using electron reflectometry

Details are presented of an improved technique to use atmospheric absorption of magnetically reflecting solar wind electrons to constrain neutral mass densities in the nightside martian upper thermosphere. The helical motion of electrons on converging magnetic field lines, through an extended neutral atmosphere, is modeled to enable prediction of loss cone pitch angle distributions measured by the Magnetometer/Electron Reflectometer (MAG/ER) experiment on Mars Global Surveyor at 400 km altitude. Over the small fraction of Mars' southern hemisphere (∼2.5%) where the permanent crustal magnetic fields are both open to the solar wind and sufficiently strong as to dominate the variable induced martian magnetotail field, spherical harmonic expansions of the crustal fields are used to prescribe the magnetic field along the electron's path, allowing least-squares fitting of measured loss cones, in order to solve for parameters describing the vertical neutral atmospheric mass density profile from 160 to 230 km. Results are presented of mass densities in the southern hemisphere at 2 a.m. LST at the mean altitude of greatest sensitivity, 180 km, continuously over four martian years. Seasonal variability in densities is largely explained by orbital and latitudinal changes in dayside insolation that impacts the nightside through the resulting thermospheric circulation. However, the physical processes behind repeatable rapid, late autumnal cooling at mid-latitudes and near-aphelion warming at equatorial latitudes is not fully clear. Southern winter polar warming is generally weak or nonexistent over several Mars years, in basic agreement with MGS and MRO accelerometer observations. The puzzling response of mid-latitude densities from 160° to 200° E to the 2001 global dust storm suggests unanticipated localized nightside upper thermospheric lateral and vertical circulation patterns may accompany such storms. The downturn of the 11-year cycle of solar EUV flux is likely responsible for lower aphelion densities in 2004 and 2006 (Mars years 27 and 28).     

Extension of atmospheric dust loading to high altitudes during the 2001 Mars dust storm: MGS TES limb observations

Mars Global Surveyor (MGS) visible (solarband bolometer) and thermal infrared (IR) spectral limb observations from the Thermal Emission Spectrometer (TES) support quantitative profile retrievals for dust opacity and particle sizes during the 2001 global dust event on Mars. The current analysis considers the behavior of dust lifted to altitudes above 30 km during the course of this storm; in terms of dust vertical mixing, particle sizes, and global distribution. TES global maps of visible (solarband) limb brightness at 60 km altitude indicate a global-scale, seasonally evolving (over 190-240° solar longitudes, L_S) longitudinal corridor of vertically extended dust loading (which may be associated with a retrograde propagating, wavenumber 1 Rossby wave). Spherical radiative transfer analysis of selected limb profiles for TES visible and thermal IR radiances provide quantitative vertical profiles of dust opacity, indicating regional conditions of altitude-increasing dust mixing ratios. Observed infrared spectral dependences and visible-to-infrared opacity ratios of dust scattering over 30-60 km altitudes indicate particle sizes characteristic of lower altitudes (cross-section weighted effective radius, ), during conditions of significant dust transport to these altitudes. Conditions of reduced dust loading at 30-60 km altitudes present smaller dust particle sizes . These observations suggest rapid meridional transport at 30-80 km altitudes, with substantial longitudinal variation, of dust lifted to these altitudes over southern hemisphere atmospheric regions characterized by extraordinary (m/s) vertical advection velocities. By L_S=230° dust loading above 50 km altitudes decreased markedly at southern latitudes, with a high altitude (60-80 km) haze of fine (likely) water ice particles appearing over 10°S-40°N latitudes.     

Propagation of tropospheric gravity waves into the upper atmosphere of Mars

We study the propagation of gravity waves in the martian atmosphere using a linearized one-dimensional full-wave model. Calculations are carried out for atmospheric parameters characteristic of Mars Orbiter Laser Altimeter (on Mars Global Surveyor MGS) observations of apparent gravity waves in high latitude clouds and MGS radio occultation measurements of temperature variations with height suggestive of gravity wave activity. Waves that reach the thermosphere produce fluctuations in density comparable in amplitude with the density variations detected in Mars Odyssey aerobraking data. Gravity waves of modest amplitude are found to deposit momentum and generate significant heating and cooling in the martian atmosphere. The largest heating and cooling effects occur in the thermosphere, at altitudes between about 130 and 150 km, with heating occurring at the lower altitudes and cooling taking place above.     

A statistical study of flux ropes in the Martian magnetosphere

Using minimum variance analysis of the circular mapping data from the Mars Global Surveyor (MGS) spacecraft during four selected weeks of observation, we identify 360 magnetic field structures in the Martian topside ionosphere with characteristic signatures of flux ropes. Physical parameters including size, peak field strength, helicity, orientation, and external conditions at the time of each observation are compiled for the events in each population. We observe that Martian flux ropes typically have a peak field amplitude of ∼15 nT and a diameter of ∼80–100 km assuming they are stationary. Flux ropes tend to be aligned approximately parallel to the planetary surface, and perpendicular to the direction from which the solar wind flows. They are more frequently observed during times of low solar wind pressure, but do not show a clear preference for a particular Interplanetary Magnetic Field (IMF) draping direction. Flux rope characteristics of peak field amplitude, diameter, and helicity vary with solar zenith angle. Amplitudes tend to be higher during periods of high solar wind pressure. The events are sorted into three populations based on the location at which they were observed, possibly corresponding to distinct formation mechanisms. Flux ropes observed in eclipse tend to have smaller peak amplitudes and are larger than those observed in sunlight, and are less likely to be oriented parallel to the planetary surface. Proximity to crustal fields does not appear to influence the characteristics of flux ropes observed at the 400 km spacecraft altitude. The frequent observation of flux rope structures near Mars in a variety of locations suggests that the low-altitude plasma environment is quite dynamic, with magnetic shear playing a prominent role in determining magnetic field structure near the planet.     

Correction of the ionospheric distortion on the MARSIS surface sounding echoes

Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft has made numerous measurements of the Martian surface and subsurface. However, all of these measurements are distorted by the ionosphere and must be compensated before any analysis. We have developed a technique to compensate for the ionospheric distortions. This technique provides a powerful tool to derive the total electron content (TEC) and other higher-order terms of the limited expansion of the plasma dispersion function that are related to overall shape of the electron column profile. The derived parameters are fitted by using a Chapman model to derive ionospheric parameters like n₀, electron density primary peak (maximum for solar zenith angle (SZA) equal 0), and the neutral height scale H.

Our estimated ionospheric parameters are in good agreement with Mars Global Surveyor (MGS) radio-occultation data. However, since MARSIS does not have the observation geometry limitations of the radio occultation measurements, our derived parameters extend over a large range of SZA for each MEX orbit.

The first results from our technique have been discussed by Safaeinili et al. [2007, Estimation of the total electron content of the Martian ionosphere using radar sounder surface echoes. Geophys. Res. Lett. 34, L23204, doi:10.1029/2007GL032154].     

Response of peak electron densities in the martian ionosphere to day-to-day changes in solar flux due to solar rotation

Photochemical Chapman theory predicts that the square of peak electron density, N_m, in the dayside ionosphere of Mars is proportional to the cosine of solar zenith angle. We use Mars Global Surveyor Radio Science profiles of electron density to demonstrate that this relationship is generally satisfied and that positive or negative residuals between observed and predicted values of are caused by periods of relatively high or low solar flux, respectively.Understanding the response of the martian ionosphere to changes in solar flux requires simultaneous observations of the martian ionosphere and of solar flux at Mars, but solar flux measurements are only available at Earth. Since the Sun's output varies both in time and with solar latitude and longitude, solar flux at Mars is not simply related to solar flux at Earth by an inverse-square law. We hypothesize that, when corrected for differing distances from the Sun, solar fluxes at Mars and Earth are identical when shifted in time by the interval necessary for the Sun to rotate through the Earth–Sun–Mars angle.We perform four case studies that quantitatively compare time series of N_m at Mars to time series of solar flux at Earth and find that our hypothesis is satisfied in the three of them that used ionospheric data from the northern hemisphere. We define a solar flux proxy at Mars based upon the E_10.7 proxy for solar flux at Earth and use our best case study to derive an equation that relates N_m to this proxy. We discuss how the ionosphere of Mars can be used to infer the presence of solar active regions not facing the Earth.Our fourth case study uses ionospheric observations from the southern hemisphere at latitudes where there are strong crustal magnetic anomalies. These profiles do not have Chapman-like shapes, unlike those of the other three case studies. We split this set of measurements into two subsets, corresponding to whether or not they were made at longitudes with strong crustal magnetic anomalies. Neither subset shows N_m responding to changes in solar flux in the manner that we observe in the three other case studies.We find many similarities in ionospheric responses to short-term and long-term changes in solar flux for Venus, Earth, and Mars. We consider the implications of our results for different parametric equations that have been published describing this response.     

Day-side ionospheric conductivities at Mars

We present estimates of the day-side ionospheric conductivities at Mars based on magnetic field measurements by Mars Global Surveyor (MGS) at altitudes down to ∼100 km during aerobraking orbits early in the mission. At Mars, the so-called ionospheric dynamo region, where plasma/neutral collisions permit electric currents perpendicular to the magnetic field, lies between 100 and 250 km altitude. We find that the ionosphere is highly conductive in this region, as expected, with peak Pedersen and Hall conductivities of 0.1-1.5 S/m depending on the solar illumination and induced magnetospheric conditions. Furthermore, we find a consistent double peak pattern in the altitude profile of the day-side Pedersen conductivity, similar to that on Titan found by Rosenqvist et al. (2009). A high altitude peak, located between 180 and 200 km, is equivalent to the terrestrial peak in the lower F-layer. A second and typically much stronger layer of Pedersen conductivity is observed between 120 and 130 km, which is below the Hall conductivity peak at about 130-140 km. In this altitude region, MGS finds a sharp decrease in induced magnetic field strength at the inner magnetospheric boundary, while the day-side electron density is known to remain high as far down as 100 km. We find that such Titan-like behaviour of the Pedersen conductivity is only observed under regions of strongly draped magnetospheric field-lines, and negligible crustal magnetic anomalies below the spacecraft. Above regions of strong crustal magnetic anomalies, the Pedersen conductivity profile becomes more Earth-like with one strong Pedersen peak above the Hall conductivity peak. Here, both conductivities are 1-2 orders of magnitude smaller than the above only weakly magnetised crustal regions, depending on the strength of the crustal anomaly field at ionospheric altitudes. This nature of the Pedersen conductivity together with the structured distribution of crustal anomalies all over the planet should give rise to strong conductivity gradients around such anomalies. Day-side ionospheric conductivities on Mars (in regions away from the crustal magnetic anomalies) and Titan seem to behave in a very similar manner when horizontally draped magnetic field-lines partially magnetise a sunlit ionosphere. Therefore, it appears that a similar double peak structure of strong Pedersen conductivity could be a more general feature of non-magnetised bodies with ionised upper atmospheres, and thus should be expected to occur also at other non-magnetised terrestrial planets like Venus or other planetary bodies within the host planet magnetospheres.     

A plasma flow velocity boundary at Mars from the disappearance of electron plasma oscillations

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft is capable of measuring ionospheric electron density by the use of two main methods: remote radar sounding and from the excitation of local plasma oscillations. The frequency of the locally excited electron plasma oscillations is used to measure the local electron density. However, plasma oscillations are not observed when the plasma flow velocity is higher than about 160 km/s, which occurs mainly in the solar wind and magnetosheath. As a consequence, in many passes, there is a sudden disappearance of the plasma oscillations as the spacecraft enters into the magnetosheath. This fact allows us to identify a flow velocity boundary on the dayside, between the ionosphere of Mars and the shocked solar wind. This paper summarizes the results of the measurement of 552 orbits mostly over a period from August 4, 2005 to August 17, 2007. The boundary points found using MARSIS have been verified by measurements from the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) Electron Spectrometer (ELS) instrument on Mars Express. The average position of the flow velocity boundary is compared to flow velocity simulations computed using hybrid model and other boundaries. The boundary altitude is slightly lower than the magnetic pile-up boundary determined using Phobos 2 and Mars Global Surveyor (MGS) crossings, but it is in good agreement with the induced magnetospheric boundary determined by ASPERA-3. Investigation of the effect of the crustal magnetic field revealed that the flow velocity boundary is raised at the locations with strong crustal magnetic fields.     

Upper thermal boundary layer of planetary atmosphere: An attempt of developing a general model

In any planetary atmosphere there is an uppermost layer in which the molecular thermal conduction is a significant mechanism of forming the thermal structure of the atmosphere. In this paper, the similarity approach is first used to develop the 1-D general model of aforementioned layer. The main concepts of the model are (i) the radiative equilibrium condition at the lower boundary of the layer and (ii) taking into account a single rovibrational band for radiative cooling of the layer. Five dimensionless parameters of the model characterize both “strengths” and altitudinal distributions of heat sources and sinks in the layer, including an effect of the atmosphere under the layer. By fitting the modeled temperature profile to the mean empirical profile, both the magnitudes of the parameters and the relations between them have been determined for the Earth and Mars. Distinctions between these planets in both the parameter magnitudes and relationships can be accounted for by distinction in composition of their atmospheres. For both planets the model shows weak sensitivity of the modeled temperature profile to significant changes in the state of the underlying atmosphere. The model demonstrates some prognostic capabilities. Namely, the fitting reveals presence of O in the martian thermosphere. (However, the fractional O abundance is overestimated.) From drag deceleration of the MGS orbiter the mean temperature profile of the martian thermosphere between 115 and 170 km has been derived for the solar zenith angle of 45°-70°, the solar longitude of 30°-80°, and the latitude range from −10° to 60°at a moderate level of solar activity.     

The effects of topographically-controlled thermal tides in the martian upper atmosphere as seen by the MGS accelerometer

Mars Global Surveyor accelerometer observations of the martian upper atmosphere revealed large variations in density with longitude during northern hemisphere spring at altitudes of 130-160 km, all latitudes, and mid-afternoon local solar times (LSTs). This zonal structure is due to tides from the surface. The zonal structure is stable on timescales of weeks, decays with increasing altitude above 130 km, and is dominated by wave-3 (average amplitude 22% of mean density) and wave-2 (18%) harmonics. The phases of these harmonics are constant with both altitude and latitude, though their amplitudes change significantly with latitude. Near the South Pole, the phase of the wave-2 harmonic changes by 90° with a change of half a martian solar day while the wave-3 phase stays constant, suggesting diurnal and semidiurnal behaviour, respectively. We use a simple application of classical tidal theory to identify the dominant tidal modes and obtain results consistent with those of General Circulation Models. Our method is less rigorous, but simpler, than the General Circulation Models and hence complements them. Topography has a strong influence on the zonal structure.     

Dust storms originating in the northern hemisphere during the third mapping year of Mars Global Surveyor

Data from the third Mars Global Surveyor (MGS) mapping year (MY 26, 2003-2005) are used to investigate dust storms originating in the northern hemisphere. Flushing dust storms, which originate as frontal dust storms at the northern polar vortex edge and propagate southward through topographic channels, are observed immediately before and after a quiescent period that occurs around the northern winter solstice (240°<Ls<300°). Both the pre- and post-solstice active periods can be further divided into two sub-periods. The most vigorous of these flushing storms occurred during L_s 210-220° and L_s 310-320°. The lifted dust crossed the equator and accumulated in the southern hemisphere. These major dust storms enhanced the Hadley circulation and suppressed the lower-level baroclinic eddies in the northern mid and high latitudes. The 2-3 sol wave number m=3 traveling waves show the best correlation with flushing dust storms and can combine with other wave modes to produce storm tracks and fronts within individual sub-periods.     

Dayside induced magnetic field in the ionosphere of Mars

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) onboard the Mars Express spacecraft has occasionally displayed surprising features. One such feature is the occurrence of a series of broadband, low-frequency echoes at equally spaced delay times after the sounder transmitter pulse. The interval between the echoes has been shown to be at the cyclotron period of electrons orbiting in the local magnetic field. The electrons are believed to be accelerated by the large voltages applied to the antenna by the sounder transmitter. Measurements of the period of these “electron cyclotron echoes” provide a simple technique for determining the magnitude of the magnetic field near the spacecraft. These measurements are particularly useful because Mars Express carries no magnetometer, so this is the only method available for measuring the magnetic field magnitude. Using this technique, results are presented showing the large scale structure of the draped field inside the magnetic pile-up boundary. The magnitude of the draped field is shown to vary from about 40 nT at a solar zenith angle of about 25°, to about 25 nT at a solar zenith angle of 90°. The results compare favorably with similar results from the Mars Global Surveyor spacecraft. A fitting technique is developed to derive the vector direction and magnitude of the draped magnetic field in cases where the spacecraft passes through regions with significant variation in the crustal field. The magnetic field directions are consistent with current knowledge of the draping geometry of the magnetic field around Mars.     

A sensitive search for SO2 in the martian atmosphere: Implications for seepage and origin of methane

Mars was observed near the peak of the strongest SO₂ band at 1364-1373 cm⁻¹ with resolving power of 77,000 using the Texas Echelon Cross Echelle Spectrograph on the NASA Infrared Telescope Facility. The observation covered the Tharsis volcano region which may be preferable to search for SO₂. The spectrum shows absorption lines of three CO₂ isotopomers and three H₂O isotopomers. The water vapor abundance derived from the HDO lines assuming D/H = 5.5 times the terrestrial value is 12±1.0 pr. μm, in agreement with the simultaneous MGS/TES observations of 14 pr. μm at the latitudes (50° S to 10° N) of our observation. Summing of spectral intervals at the expected positions of sixteen SO₂ lines puts a 2σ upper limit on SO₂ of 1 ppb. SO₂ may be emitted into the martian atmosphere by seepage and is removed by three-body reactions with OH and O. The SO₂ lifetime, 2 years, is longer than the global mixing time 0.5 year, so SO₂ should be rather uniformly distributed across Mars. Seepage of SO₂ is less than 15,000 tons per year on Mars which is smaller than the volcanic production of SO₂ on the Earth by a factor of 700. Because CH₄/SO₂ is typically 10⁻⁴-10⁻³ in volcanic gases on the Earth, our results show seepage is unlikely to be the source of the recently discovered methane on Mars and therefore strengthen its biogenic origin.     

Mapping of Mars O2 1.27 μm dayglow at four seasonal points

The O₂ dayglow at 1.27 μm is formed by high-altitude ozone on Mars and is a sensitive tracer of Mars photochemistry. Mapping of this dayglow using the IRTF/CSHELL long-slit spectrograph requires the extraction of weak emission lines against a strong continuum of the reflected solar light. Some new tools are suggested to improve the data processing. The observed O₂ dayglow intensities at L_S=67°, 112°, 148°, and 173° show a decrease from late spring (aphelion) to fall equinox by a factor of ≈5 at low latitudes (±30°). This decrease agrees with that predicted by a model of Clancy and Nair (1996, J. Geophys. Res. 101 (12) 12785-12790), although the dayglow intensities are weaker than those based on that model. The measured dayglow variations with latitude are rather low at L_S=67°, 112°, and 148° and unexpectedly high at 173°. The dayglow intensity peaks near noon and is smaller at 9:00 and 16:30 LT by a factor of 2. Some data on the ozone profile near aphelion are obtained from a combination of the dayglow and ozone observations. It is hardly possible to detect the O₂ night airglow at 1.27 μm on Mars using the existing ground-based and on-orbit instruments. The O₂ dayglow intensity as a function of latitude and season from aphelion to fall equinox has been obtained. Our goal is to extend this distribution to the full martian year and get a database for Mars photochemistry to complement the MGS/TES observations of water vapor, atmospheric temperature, and dust and ice aerosol.     

Dust aerosols above the south polar cap of Mars as seen by OMEGA

The time evolution of atmospheric dust at high southern latitudes on Mars has been determined using observations of the south seasonal cap acquired in the near infrared (1-2.65 μm) by OMEGA/Mars Express in 2005. Observations at different solar zenith angles and one EPF sequence demonstrate that the reflectance in the 2.64 μm saturated absorption band of the surface CO₂ ice is mainly due to the light scattered by aerosols above most places of the seasonal cap. We have mapped the total optical depth of dust aerosols in the near-IR above the south seasonal cap of Mars from mid-spring to early summer with a time resolution ranging from one day to one week and a spatial resolution of a few kilometers. The optical depth above the south perennial cap is determined on a longer time range covering southern spring and summer. A constant set of optical properties of dust aerosols is consistent with OMEGA observations during the analyzed period. Strong variations of the optical depth are observed over small horizontal and temporal scales, corresponding in part to moving dust clouds. The late summer peak in dust opacity observed by Opportunity in 2005 propagated to the south pole contrarily to that observed in mid spring. This may be linked to evidence for dust scavenging by water ice-rich clouds circulating at high southern latitudes at this season.     

The role of the wind-transported dust in slope streaks activity: Evidence from the HRSC data

Slope streaks are gravity-driven albedo features observed on martian slopes since the Viking missions. The debated mechanism of formation could involve alternatively dry granular flow or wet mass wasting. A systematic mapping of slope streaks from the High Resolution Stereo Camera is presented in this paper. Two regions known for their slope streaks activity have been studied, the first one is located close to Cerberus lava flow, and the second one is inside the Olympus Mons Aureole. The statistics of slope streaks shapes measured from orthorectified images confirm previous results from Mars Orbiter Camera surveys. Preferential orientations of slope streaks are reported. Slope streaks occur preferentially on west facing slopes at latitudes lower than 30° N for Olympus and on south-west facing slopes for Cerberus. Wind directions derived from a General Circulation Model during the dusty season correlate with these orientations. Furthermore, west facing slopes at Olympus have a thicker dust cover. These observations indicate that slope streaks are dust avalanches controlled by the preferential accumulation of dust in the downstream side of the wind flow. The paucity of slope streaks at high latitudes and their preferential orientation on south-facing slopes have been presented as an evidence for a potential role of H₂O phase transition in triggering or flow. The potential role of H₂O cannot be ruled out from our observations but the dust avalanche model together with the atmospheric circulation could potentially explain all observations. The role of H₂O might be limited to a stabilizing effect of dust deposits on northward facing slopes at intermediate latitudes (30° N-33° N) and on all slopes further north.     

MOC observations of the 2001 Mars planet-encircling dust storm

From 15 September 1997 through 21 January 2006, only a single planet-encircling martian dust storm was observed by MGS-MOC. The onset of the storm occurred on 26 June 2001 (Ls=184.7°), earliest recorded to date. It was initiated in the southern mid-to-low latitudes by a series of local dust storm pulses that developed along the seasonal cap edge in Malea and in Hellas basin (Ls=176.2°-184.4°). The initial expansion of the storm, though asymmetric, was very rapid in all directions (3-32 m s⁻¹). The main direction of propagation, however, was to the east, with the storm becoming planet encircling in the southern hemisphere on Ls=192.3°. Several distinct centers of active dust lifting were associated with the storm, with the longest persisting for 86 sols (Syria-Claritas). These regional storms helped generate and sustain a dust cloud (“haze”), which reached an altitude of about 60 km and a peak opacity of τdust∼5.0. By Ls=197.0°, the cloud had encircled the entire planet between 59.0° S and 60.0° N, obscuring all but the largest volcanoes. The decay phase began around Ls∼200.4° with atmospheric dust concentrations returning to nominal seasonal low-levels at Ls∼304.0°. Exponential decay time constants ranged from 30-117 sols. The storm caused substantial regional albedo changes (darkening and brightening) as a result of the redistribution (removal and deposition) of a thin veneer of surface dust at least 0.1-11.1 μm thick. It also caused changes in meteorological phenomena (i.e., dust storms, dust devils, clouds, recession of the polar caps, and possibly surface temperatures) that persisted for just a few weeks to more than a single Mars year. The redistribution of dust by large annual regional storms might help explain the long period (∼30 years) between the largest planet-encircling dust storms events.