North-south asymmetry of the polar solar wind and some characteristics of solar magnetic field

We have found correlated variations of the yearly averaged north-south asymmetry in the polar solar wind speed (_sol) and the ratio of the zonal quadrupolar to the zonal dipolar contribution in the inferred coronal magnetic field during the declining phase of sunspot cycle 21. A physically meaningful association between _sol and some polar solar magnetic field proxies is also observed during the low sunspot activity periods of the above cycle.     

Eolian controlled modification of the martian south polar layered deposits

Data from the Mars Orbiter Laser Altimeter (MOLA) and Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor (MGS) mission and the Thermal Emission Imaging System (THEMIS) aboard the Mars Odyssey mission have revealed unique surface features in a particular region of the South Polar Layered Deposits (SPLD). The dominant morphology is large-scale quasi-parallel grooves that extend for hundreds of kilometers with only tens of meters of vertical relief, that we have termed here the “Wire Brush” terrain. The grooves are also transected by disjointed, yet roughly continuous, low-relief sinuous ridges that cross roughly perpendicular to the trend-direction of the large-scale grooves and show only tens of meters of relief. We interpret these ridges to be eroded remnants of folded layers. At the northern end of the large-scale grooves there are non-symmetric mounds. They are frequently preceded by a significant depression and/or trailing grooves that are parallel to the Wire Brush trend. We find that a two-stage process involving winds that intermittently remove a low-density crust exposing the underlying ice to ablation is the interpretation that best explains the multitude of features observed here. These features appear to be currently inactive indicating higher winds in previous epochs.     

Speculation on martian north polar wind circulation and the resultant orientations of polar sand dunes

Dunes in the Martian north polar erg show two dominant orientations. When seen at frost cap minimum, dunes north of 80°N record east winds, dunes south of 80°N record west winds. Many of the transverse dunes are considered to be reversing dunes. Dunes in two fields may have reversed at least once during the lifetime of the Viking Orbiters. Poor agreement exists among published predictive models of north polar winds and the interpretations derived from the major published map of the north polar dunes. We propose that the average polar winds are: (1) strong, off-pole northwest winds in fall; (2) moderate west winds in winter; (3) latitude-dependent weak to strong off-pole northeast winds in spring; and (4) weak west winds in summer. Viking images of near-polar clouds confirm much of the hypothesis. Images discussed in other studies can be given alternative interpretations that support this hypothesis also. Over millenia, the combination of reversing west and east winds could produce the binodal distributions of dune orientations observed at the north pole.     

Subsurface radar sounding of the martian polar cap: radiative transfer approach

The problem of subsurface radar sounding of the martian polar caps [Ilyushin, 2004. Martian northern polar cap: layering and possible implications for radar sounding. Planet. Space Sci. 52, 1195–1207] is considered from the point of view of incoherent radiative transfer theory. Since it has been previously shown that the radar signal field within the polar cap has diffuse structure, there is a need for a statistical approach to the problem. Radiative transfer theory, which is now well developed, seems to be the most appropriate formalism for this approach.Several physical models of polar caps have been formulated. The asymptotic solutions for all proposed models are derived here. In the present paper only the case of orbital ground penetrating radar is considered, because it is of great interest in relationship to currently developed radar experiments. In principle, the approach is believed to be applicable to a wide class of short pulse and compressed chirp radar experiments, including both orbital and landed instruments and media more complicated than a simple plane parallel geometry. This work, however, is postponed to future papers.Techniques for retrieval of physical properties of polar caps from the radar measurements are proposed. From the observational data, the macroscopic parameters of the medium appearing in radiative transfer theory, i.e. the single scattering albedo and volume extinction coefficient can be estimated. These estimates put certain constraints on the physical parameters of the medium model introduced in the paper. With some additional information, known a priori or from other observations, these estimates can be used to retrieve physically meaningful information, for example, the average content of impurities in the ice.     

Constraints on the composition of the martian south polar cap from gravity and topography

The polar caps of Mars have long been acknowledged to be composed of unknown proportions of water ice, solid CO₂ (dry ice), and dust. Gravity and topography data are here analyzed over the southern cap to place constraints on its density, and hence composition. Using a localized spectral analysis combined with a lithospheric flexure model of ice cap loading, the best fit density of the volatile-rich south polar layered deposits is found to be 1271 kg m⁻³ with 1-σ limits of 1166 and 1391 kg m⁻³. The best fit elastic thickness of this geologically young deposit is 140 km, though any value greater than 102 km can fit the observations. The best fit density implies that about 55% dry ice by volume could be sequestered in these deposits if they were completely dust free. Alternatively, if these deposits were completely free of solid CO₂, the dust content would be constrained to lie between about 14 and 28% by volume. The bulk thermal conductivity of the polar cap is not significantly affected by these maximum allowable concentrations of dust. However, even if a moderate quantity of solid CO₂ were present as horizontal layers, the bulk thermal conductivity of the polar cap would be significantly reduced. Reasonable estimates of the present day heat flow of Mars predict that dry ice beneath the thicker portions of the south polar cap would have melted. Depending on the quantity of solid CO₂ in these deposits today, it is even possible that water ice could melt where the cap is thickest. If independent estimates for either the dust or CO₂ content of the south polar cap could be obtained, and if radar sounding data could determine whether this polar cap is presently experiencing basal melting or not, it would be possible to use these observations to place tight constraints on the present day heat flow of Mars.     

The texture of condensed CO2 on the martian polar caps

The widespread deposition of CO₂ ice on the martian polar caps in winter is readily visible from Earth and has been extensively studied from orbit. As the surface cools during polar night, CO₂ condenses directly out of the atmosphere at a rate that establishes equilibrium between radiative loss and latent heat of condensation. Since radiative loss is strongly geometry-dependent, the CO₂ frost will grow most rapidly on exposed surfaces and more slowly in depressions. Positive feedback will cause a dramatic enhancement of the relief of the underlying topography and a corresponding reduction in the average bulk density. The resulting surface will be highly textured and riddled with perforations.     

Searching for the source regions of martian meteorites using MGS TES: Integrating martian meteorites into the global distribution of igneous materials on Mars

Abstract— The objective of this study was to identify and map possible source regions for all 5 known martian meteorite lithologies (basalt, lherzolite, clinopyroxenite, orthopyroxenite, and dunite) using data from the Mars Global Surveyor Thermal Emission Spectrometer (MGS TES). We deconvolved the TES data set using laboratory spectra of 6 martian meteorites (Los Angeles, Zagami, ALH A77005, Nakhla, ALH 84001, and Chassigny) as end members, along with atmospheric and surface spectra previously derived from TES data. Global maps (16 pixels/degree) of the distribution of each meteorite end member show that meteorite‐like compositions are not present at or above TES detectability limits over most of the planet's dust‐free regions. However, we have confidently identified local‐scale (100s‐1000s km²) concentrations of olivine‐ and orthopyroxene‐bearing materials similar to ALH A77005, Chassigny, and ALH 84001 in Nili Fossae, in and near Ganges Chasma, in the Argyre and Hellas basin rims, and in Eos Chasma. Nakhla‐like materials are identified near the detection limit throughout the eastern Valles Marineris region and portions of Syrtis Major. Basaltic shergottites were not detected in any spatially coherent areas at the scale of this study. Martian meteorite‐like lithologies represent only a minor portion of the dust‐free surface and, thus, are not representative of the bulk composition of the ancient crust. Meteorite‐like spectral signatures identified above TES detectability limits in more spatially restricted areas (<tens of km) are targets of ongoing analysis.     

Yearly comparisons of the martian polar caps: 1999-2003 Mars Orbiter Camera observations

The Mars Global Surveyor Mars Orbiter Camera wide-angle cameras were used to obtain images of the north and south seasonal and residual polar caps between 1999 and 2003. Wide-angle red camera images were used in assembling mosaics of the north and south polar recessions and regression rates were measured and compared. There are small variations in the north polar recession between 2000 and 2002, especially between LS=7° and LS=50°, however there is no evidence for the plateau in the recession curves that has been observed in some prior years. The south polar recession changes very little from year to year, and the 2001 dust storm had little if any effect on the average cap recession that year. Albedo values of the geographic north pole were measured using wide-angle red and blue camera images, and the residual south polar cap configuration was compared between the three years observed by MOC. The albedo of the geographic north pole generally varies between 0.5 and 0.6 as measured from MOC wide-angle red camera images. There were only minor variations near the edges of the residual south polar cap between the three years examined.     

Modeling of major martian magnetic anomalies: Further evidence for polar reorientations during the Noachian

Maps of the vector components of the Mars crustal magnetic field are constructed at the mapping altitude (360 to 410 km) using a selected set of data obtained with the Mars Global Surveyor magnetometer during 2780 orbits of the planet in 1999. Forward modeling calculations are then applied to six relatively strong and isolated, dominantly dipolar, magnetic anomalies for the primary purpose of estimating bulk directions of magnetization. Assuming that the magnetizing field was a (dipolar) core dynamo field centered in the planet, paleomagnetic pole positions are calculated for the six primary source bodies together with that for a seventh anomaly analyzed earlier. In agreement with several previous studies, it is found that six of the seven pole positions are clustered in what is now the northern lowlands in a region centered northwest of Olympus Mons (mean pole position: 34°±10° N, 202°±58° E). Assuming that the dynamo dipole moment vector was approximately parallel to the rotation axis, the modeling results therefore suggest a major reorientation of Mars relative to its rotation axis after magnetization was acquired. Such a reorientation may have been stimulated by internal mass redistributions associated with the formation of the northern lowlands and Tharsis, for example. A comparison of the mean paleo (magnetic) equator to the global distribution of crustal fields shows that magnetic anomalies tend to occur at low paleolatitudes. The same appears to be true for the Noachian-aged valley networks, which exhibit a broad spatial correlation with the magnetic anomalies. A possible interpretation is that the formation of magnetic anomalies and the valley networks was favored in the tropics where melting of water ice and snow was a stronger source of both surface valley erosion and groundwater recharge during the earliest history of the planet. This would be consistent with models in which hydrothermal alteration of crustal rocks played a role in producing the unusually strong martian magnetic anomalies.     

Limits on the trapping of atmospheric CH4 in martian polar ice analogs

Recent detection of methane (CH₄) on Mars has generated interest in possible biological or geological sources, but the factors responsible for the reported variability are not understood. Here we explore one potential sink that might affect the seasonal cycling of CH₄ on Mars - trapping in ices deposited on the surface. Our apparatus consisted of a high-vacuum chamber in which three different Mars ice analogs (water, carbon dioxide, and carbon dioxide clathrate hydrates) were deposited in the presence of CH₄ gas. The ices were monitored for spectroscopic evidence of CH₄ trapping using transmission Fourier-Transform Infrared (FT-IR) spectroscopy, and during subsequent sublimation of the ice films the vapor composition was measured using mass spectrometry (MS). Trapping of CH₄ in water ice was confirmed at deposition temperatures <100 K which is consistent with previous work, thus validating the experimental methods. However, no trapping of CH₄ was observed in the ice analogs studied at warmer temperatures (140 K for H₂O and CO₂ clathrate, 90 K for CO₂ snow) with approximately 10 mTorr CH₄ in the chamber. From experimental detection limits these results provide an upper limit of 0.02 for the atmosphere/ice trapping ratio of CH₄. If it is assumed that the trapping mechanism is linear with CH₄ partial pressure and can be extrapolated to Mars, this upper limit would indicate that less than 1% is expected to be trapped from the largest reported CH₄ plume, and therefore does not represent a significant sink for CH₄.     

Rotation of the magnetic elements in polar regions on the Sun

Using the Michelson Doppler Imager (MDI) data from Solar and Heliospheric Observatory (SOHO), the rotation rate of the unipolar magnetic regions in North high-latitude regions of the Sun is estimated by tracking individual magnetic elements. The analysis reveals a strong spin down near the pole, which is greater than the Doppler and magnetic rotation rates estimated by Snodgrass & Ulrich (1990), and rotation rate inferred from helioseismology (Birch & Kosovichev 1998), and is probably related to variation of velocity gradient in the subsurface shear layer. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

THEMIS-VIS observations of clouds in the martian mesosphere: Altitudes, wind speeds, and decameter-scale morphology

We present measurements of the altitude and eastward velocity component of mesospheric clouds in 35 imaging sequences acquired by the Mars Odyssey (ODY) spacecraft’s Thermal Emission Imaging System visible imaging subsystem (THEMIS-VIS). We measure altitude by using the parallax drift of high-altitude features, and the velocity by exploiting the time delay in the THEMIS-VIS imaging sequence.We observe two distinct classes of mesospheric clouds: equatorial mesospheric clouds observed between 0° and 180° L_s; and northern mid-latitude clouds observed only in twilight in the 200–300° L_s period. The equatorial mesospheric clouds are quite rare in the THEMIS-VIS data set. We have detected them in only five imaging sequences, out of a total of 2048 multi-band equatorial imaging sequences. All five fall between 20° south and 0° latitude, and between 260° and 295° east longitude. The mid-latitude mesospheric clouds are apparently much more common; for these we find 30 examples out of 210 northern winter mid-latitude twilight imaging sequences. The observed mid-latitude clouds are found, with only one exception, in the Acidalia region, but this is quite likely an artifact of the pattern of THEMIS-VIS image targeting. Comparing our THEMIS-VIS images with daily global maps generated from Mars Orbiter Camera Wide Angle (MOC-WA) images, we find some evidence that some mid-latitude mesospheric cloud features correspond to cloud features commonly observed by MOC-WA. Comparing the velocity of our mesospheric clouds with a GCM, we find good agreement for the northern mid-latitude class, but also find that the GCM fails to match the strong easterly winds measured for the equatorial clouds.Applying a simple radiative transfer model to some of the equatorial mesospheric clouds, we find good model fits in two different imaging sequences. By using the observed radiance contrast between cloud and cloud-free regions at multiple visible-band wavelengths, these fits simultaneously constrain the optical depths and particles sizes of the clouds. The particle sizes are constrained primarily by the relative contrasts at the available wavelengths, and are found to be quite different in the two imaging sequences: r_eff = 0.1 μm and r_eff = 1.5 μm. The optical depths (constrained by the absolute contrasts) are substantial: 0.22 and 0.5, respectively. These optical depths imply a mass density that greatly exceeds the saturated mass density of water vapor at mesospheric temperatures, and so the aerosol particles are probably composed mainly of CO₂ ice. Our simple radiative transfer model is not applicable to twilight, when the mid-latitude mesospheric clouds were observed, and so we leave the properties of these clouds as a question for further work.     

Possible role of brines in the darkening and flow-like features on the Martian polar dunes based on HiRISE images

This work identifies and describes features of the changing seasonal frost-covered surface of Mars based on HiRISE images, and analyses the possibility that ephemeral liquid brine formation produces them. Because some of these dark features show flow-like appearance, and salts on Mars are present, liquid brines might be also present, possibly accounting for the changing droplet-like features on the Phoenix lander.We observed in-situ darkening and movement of dark features (or movement of the darkening front) on seasonal frost-covered polar dunes. Darkening and brightening may happen within several meters from each other during local spring. Darkening always starts from the bottom and moves up, while brightening progresses from top and moves toward the bottom between the small dune ripples. Brightening occurs during the springtime warming on time scales of several days close to the sites of darkening; therefore, dark material falling from the air, and refreezing of bright ice on it, does not adequately explain the observations. Interpreting the observations as brine-related melting or refreezing also poses problems, but because brine may engulf salt grains or ice blocks, phase changes here could be influenced by factors other than temperature values, and could produce the observations.Analysis of absolute albedo changes indicates that the flow-like features are the darkest at their lower frontal end, sometimes darker than the dark spot from which they originate. A bright halo (white collar) also forms around these spots, possibly due to refreezing. Inside the observed larger spots an outer gray area surrounds the central darkest cores, which is about 10 cm lower than the surrounding bright CO₂ ice. At those places, most or all of the CO₂ ice deposited earlier has disappeared, and H₂O ice is present. Observations of dark flow features moving on the top of this H₂O rich layer suggest even if the flow features start as dry dune avalanches of rolling grains, their dark material heated by solar insolation is in contact with H₂O ice and may produce brines.     

Methane adsorption on a martian soil analog: An abiogenic explanation for methane variability in the martian atmosphere

Recent observations suggest methane in the martian atmosphere is variable on short spatial and temporal scales. However, to explain the variability by loss reactions requires production rates much larger than expected. Here, we report results of laboratory studies of methane adsorption onto JSC-Mars-1, a martian soil simulant, and suggest that this process could explain the observations. Uptake coefficient (γ) values were measured as a function of temperature using a high-vacuum Knudsen cell able to simulate martian temperature and pressure conditions. Values of γ were measured from 115 to 135 K, and the data were extrapolated to higher temperatures with more relevance to Mars. Adsorptive uptake was found to increase at lower temperatures and larger methane partial pressures. Although only sub-monolayer methane surface coverage is likely to exist under martian conditions, a very large mineral surface area is available for adsorption as atmospheric methane can diffuse meters into the regolith. As a result, significant methane may be temporarily lost to the regolith on a seasonal time scale. As this weak adsorption is fully reversible, methane will be re-released into the atmosphere when surface and subsurface temperatures rise and so no net loss of methane occurs. Heterogeneous interaction of methane with martian soil grains is the only process proposed thus far which contains both rapid methane loss and rapid methane production mechanisms and is thus fully consistent with the reported variability of methane on Mars.     

Line asymmetry of spatially resolved spectra in quiet and plage regions

Photographic spectra of three photospheric lines with an approximate resolution of 0 _. 8 were used to evaluate average line profiles of granules and the intergranular space in quiet and plage regions. The bisectors of the profiles of the quiet regions were compared with numerical calculations from the literature. The calculated bisectors resemble the observed ones. The corresponding bisectors of the plage regions were found to be very different from those of the quiet regions. An increase of the FWHM may indicate a concentration of magnetic flux in the plage regions.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

On empirical models of the upper atmosphere in the polar regions

The expression for the exospheric temperature in Jacchia's static diffusion models of the upper atmosphere has a discontinuous gradient at the poles. Therefore it cannot describe the true state of the upper atmosphere in the polar regions. Furthermore, it cannot be used to calculate quantities that depend on the derivative of the exospheric temperature, or the density, like pressure gradients, horizontal forces or horizontal heat fluxes. A modified expression for the exospheric temperature is suggested. This modification yields variables of state of the upper atmosphere that deviate little from Jacchia's values, but it has continuous gradient at the poles and is therefore more suitable for treating dynamical problems like the global wind pattern.     

Evaluating the meaning of “layer” in the martian north polar layered deposits and the impact on the climate connection

Using data from the High Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter, we reassess the methods by which layers within the north polar layered deposits (NPLD) can be delineated and their thicknesses measured. Apparent brightness and morphology alone are insufficient for this task; high resolution topographic data are necessary. From these analyses, we find that the visible appearance of layers depends to a large degree on the distribution of younger, mantling deposits (which in turn is partially influenced by inherent layer properties) and on the shape and location of the particular outcrop. This younger mantle partially obscures layer morphology and brightness and is likely a cause of the gradational contacts between individual layers at this scale. High resolution images reveal that there are several layers similar in appearance to the well-known marker bed discovered by Malin, M., Edgett, K., 2001. J. Geophys. Res. 106, 23429-23570. The morphology, thicknesses , and separation distances of these marker beds, as gleaned from a high resolution stereo digital elevation model, lend insight into the connection between stratigraphy and climate.     

The origin and specific features of the martian satellites in the context of the euruption concept

The numerous problems related to the origin and evolution of Phobos and Deimos, as well as to specific features of their topography, are readily accounted for in the context of a concept presupposing the possibility of detonation (or burning) of electrolyzed ices.The explosion of an icy asteroid of mass m _A \> 10²³ g within the gravitational sphere of Mars resulted in the capture of secondary fragments into satellite orbits and the formation of a ring of icy and rocky particles. The motion of satellites in the ring reduced the eccentricities of their orbits and rendered their mutual collisions impossible. The thick regolith of Deimos is the material captured from the ring.The impact responsible for the Stickney crater on Phobos initiated a detonation in the material in the crater, and the detonation, in its turn, ignited the products of electrolysis in the bulk of the satellite. Phobos lost its regolith because of detonation-induced acceleration. As a result of the burning out of Phobos's ices, its density somewhat exeeds that of Deimos.Different grooves on Phobos have differing origins. The grooves of groups B, E, D (as specified by P. Thomas et al., J. Geophys. Res., 84, 8457–8477, 1979) are due to fracturing caused by the Sickney event; group C (which is normal to the minor axis of the Phobos ellipsoid) reflects the layered nature of the parent asteroid; while the closely arranged group A grooves, which are normal to Phobos's major axis, were formed during the contraction of the satellite as a result of the loss of burnt-out ices. A number of conclusions are drawn which can be tested by future missions to the satellites.     

Quantitative compositional analysis of martian mafic regions using the MEx/OMEGA reflectance data: 2. Petrological implications

The primary objectives of this paper are to determine the modal mineralogy of selected low albedo terrains of different ages ranging from Noachian to Amazonian exposed on the surface of Mars. This analysis is conducted using the spectral modeling of the Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) reflectance data. Results from this work are consistent with the major results of previous spectroscopic studies: plagioclase (40-60% in volume) and high calcium pyroxene (20-40%, HCP) are the dominant minerals of the most regions. Low calcium pyroxene (10-15%, LCP) and minor amounts of olivine are also present. The oldest terrains are characterized by the largest amount of LCP and the lowest concentration of plagioclase. These overall compositions are consistent with two-pyroxene basalts. The particle sizes are in the range of a few hundreds of micrometers, which is in good agreement with the thermal inertia of the martian low albedo regions. In the region around the Nili Fossae, localized concentrations of olivine up to 40% with millimeter particle size similar to picritic basalts observed in situ by the Spirit rover in the Gusev crater are inferred. Chemical compositions are calculated for the first time from OMEGA spectra. They are quite consistent with Gusev rocks and shergottite compositions but they appear to be significantly SiO₂-poorer than Thermal Emission Spectrometer data. A decreasing low calcium pyroxene abundance with the decreasing age of the low albedo regions is reported. This may be indicative of decreasing degree of partial melting as thermal flux decreases with time. We propose that the ancient Noachian-aged, LCP-rich terrains could have been formed from H₂O-bearing melts. Then, dry, basaltic volcanism occurred leading to decreasing LCP abundance with time due to decreasing degree of partial melting. The olivine-bearing material modeled in Nili Fossae resembles the composition of ALH77005 and Chassigny meteorites consistent with prior studies. Implications on the formation of the basaltic Shergottites are discussed.