windblown sand on venus the effect of high atmospheric density the high density of the venusian atmosphere significantly affects aeolian saltation transport and may also allow rapid alterations in the radar backscatter cross section of the surface saltation flux experiments were conducted under terrestrial and simulated venusian conditions to assess the effect of atmospheric density on aeolian transport of sediment total lane flux the vertical distribution of saltating particles and the average horizontal component of particle speed were measured and used to estimate the volumetric concentration of particles above the surface results show that so many particles are set into motion when wind speeds exceed a critical value that mid air collisions are a common result a condition termed choked saltation when choking occurs there is a reduction in the rate at which the total saltation flux increases with increasing wind speed at least in part due to a reduction in the mean horizontal speed of the particles choked saltation on venus may result in a reduction of the local aeolian erosion rate the obliteration of aeolian bedforms and rapid alteration of surface radar backscatter characteristics

windblown sand on venus preliminary results of laboratory simulations abstract small particles and winds of sufficient strength to move them have been detected from venera and pioneer venus data and suggest the existence of aeolian processes on venus the venus wind tunnel vwt was fabricated in order to investigate the behavior of windblown particles in a simulated venusian environment preliminary results show that sand size material is readily entrained at the wind speeds detected on venus and that saltating grains achieve velocities closely matching those of the wind measurements of saltation threshold and particle flux for various particle sizes have been compared with theoretical models which were developed by extrapolation of findings from martian and terrestial simulations results are in general agreement with theory although certain discrepancies are apparent which may be attributed to experimental and or theoretical modeling procedures present findings enable a better understanding of venusian surface processes and suggest that aeolian processes are important in the geological evolution of venus

venus concentrations of radar reflective minerals by wind abstract areas of high radar reflectivity and low emissivity on venus may be covered with windblown material rich in fe and ti such as ilmenite a mineral common in some basaltic lava flows experiments under venus like conditions show that dense mineral grains become concentrated on the surfaces of windblown sediments when subjected to the low winds typical for venus

altimetric observations of surface characteristics of the antarctic ice sheet the aim of this paper is to investigate the geophysical characteristics of the antarctic ice sheet using radar altimetric observations to do this we use an altimetric waveform simulator in situ observations ers 1 european remote sensing satellite data and spot satellite pour l observation de la terre images the small scale study takes place at dome c terre adelie which is a relatively flat region with gentle undulations and low wind speed despite this the altimetric waveform parameters height energy leading edge and trailing edge are highly noisy the effect of undulations on the waveform parameters is found to be dominant the combination of a subsurface signal and a rough surface produces a linear effect on the altimetric backscattering or on the trailing edge of the waveform but a strongly non linear effect on the leading edge of the waveform or height estimation as a consequence the height measurement is very sensitive to the altimeter technical or orbital characteristics and is not reproducible from one mission to another observations show sastrugi fields that enhance the leading edge and affect the whole waveform observed local backscattering changes probably due to local variations in surface microroughness enhance the backscattered energy and may artificially create a topographic signal the continental scale study shows coherent patterns even if both surface and subsurface components affect the altimetric observation the large scale signal is mostly controlled by surface backscattering variations the surface or near subsurface characteristics of the snowpack may then be reached by altimetric observations

studies of snow surface characteristics by landsat tm in dronning maud land antarctica large scale melting phenomena like meltwater drainage channels and meltwater accumulation basins or frozen lakes have been surveyed on the land ice mass in dronning maud land antarctica these melting features were also detected in a landsat thematic mapper tm image recorded on 12 february 1990 image processing techniques such as principal component pc analysis band ratioing and histogram equalizing are carried out to emphasize the melting phenomena interestingly a histogram equalized single tm band 5 image appears to be a good discriminator leaving the melting phenomena as bright areas in the tm scene the largest frozen lake is close to 1km wide while some of the drainage channels stretch more than 5 km there is also a comparison of satellite derived and in situ reflectance and temperature the atmospheric corrected satellite data underestimate surface snow albedo by about 16 while tm measurements of surface temperature are 2 to 6 c lower than that measured on the ground

measuring snow and glacier ice properties from satellite satellite remote sensing is a convenient tool for studying snow and glacier ice allowing us to conduct research over large and otherwise inaccessible areas this paper reviews various methods for measuring snow and glacier ice properties with satellite remote sensing these methods have been improving with the use of new satellite sensors like the synthetic aperture radar sar during the last decade leading to the development of new and powerful methods such as sar interferometry for glacier velocity digital elevation model generation of ice sheets or snow cover mapping some methods still try to overcome the limitations of present sensors but future satellites will have much increased capability for example the ability to measure the whole optical spectrum or sar sensors with multiple polarization or frequencies among the methods presented are the satellite derived determination of surface albedo snow extent snow volume snow grain size surface temperature glacier facies glacier velocities glacier extent and ice sheet topography in this review emphasis is put on the principles and theory of each satellite remote sensing method an extensive list of references with an emphasis on studies from the 1990s allows the reader to delve into specific topics

impact craters of venus analysis of venera 15 and 16 data from description of impact craters on venus and crater size and depth distributions models are developed that analyze the size distribution curve and radar bright haloes around fresh craters the dense venusian atmosphere has a strong influence on cratering the estimated age of the surface of the area under study is from 0 5 to 1 0 109 years the estimated rate of resurfacing during this time interval is about 0 04 to 0 8 cm 106 years the lithosphere of venus seems to be sufficiently rigid to sustain craters of 100 140 km diameter and 0 8 2 5 km depth for about 108 years

wind streaks on venus clues to atmospheric circulation magellan images reveal surface features on venus attributed to wind processes sand dunes wind sculpted hills and more than 5830 wind streaks have been identified the streaks serve as local wind vanes representing wind direction at the time of streak formation and allowing the first global mapping of near surface wind patterns on venus wind streaks are oriented both toward the equator and toward the west when streaks associated with local transient events such as impact cratering are deleted the westward component is mostly lost but the equatorward component remains this pattern is consistent with a hadley circulation of the lower atmosphere

magellan initial analysis of venus surface modification initial magellan observations reveal a planet with high dielectric constant materials exposed preferentially in elevated regions with high slopes ejecta deposits extending up to 1000 kilometers to the west of several impact craters windblown deposits and features in areas where there are both obstacles and a source of particulate material and evidence for slow steady degradation by atmosphere surface interactions and mass movements

analysis of synthetic aperture radar data collected over the southwestern greenland ice sheet analyses of the first aircraft multi frequency polarimetric synthetic aperture radar sar data acquired over the southwestern greenland ice sheet are presented data were collected on 31 august 1989 by the jet propulsion laboratory sar using the nasa dc 8 aircraft along with curvilinear patterns associated with large scale morphologic features such as crevasses lakes and streams frequency and polarization dependencies are observed in the p l and c band image products

dunes on titan observed by cassini radar thousands of longitudinal dunes have recently been discovered by the titan radar mapper on the surface of titan these are found mainly within 30 of the equator in optically near infrared and radar dark regions indicating a strong proportion of organics and cover well over 5 of titan s surface their longitudinal duneform interactions with topography and correlation with other aeolian forms indicate a single dominant wind direction aligned with the dune axis plus lesser off axis or seasonally alternating winds global compilations of dune orientations reveal the mean wind direction is dominantly eastwards with regional and local variations where winds are diverted around topographically high features such as mountain blocks or broad landforms global winds may carry sediments from high latitude regions to equatorial regions where relatively drier conditions prevail and the particles are reworked into dunes perhaps on timescales of thousands to tens of thousands of years on titan adequate sediment supply sufficient wind and the absence of sediment carriage and trapping by fluids are the dominant factors in the presence of dunes

venus global surface radar reflectivity observations of the surface of venus carried out by the pioneer venus radar mapper at a wavelength of 17 centimeters reveal a global mean reflectivity at normal incidence of 0 13 0 03 over the surface variations from a low of 0 03 0 01 to a high of 0 4 0 1 are found with theia mons previously identified as possibly volcanic showing a value of 0 28 0 07 regions of high reflectivity may consist of rocks with substantial inclusions of highly conductive sulfides

aeolian features on venus preliminary magellan results magellan synthetic aperture radar data reveal numerous surface features that are attributed to aeolian or wind processes wind streaks are the most common aeolian feature they consist of radar backscatter patterns that are high low or mixed in relation to the surface on which they occur a data base of more than 3400 wind streaks shows that low backscatter linear forms long narrow streaks are the most common and that most streaks occur between 17 s to 30 s and 5 n to 53 n on smooth plains moreover most streaks are associated with deposits from certain impact craters and some tectonically deformed terrains we infer that both of these geological settings provide fine particulate material that can be entrained by the low velocity winds on venus turbulence and wind patterns generated by the topographic features with which many streaks are associated can account for differences in particle distributions and in the patterns of the wind streaks thus some high backscatter streaks are considered to be zones that are swept free of sedimentary particles to expose rough bedrock other high backscatter streaks may be lag deposits of dense materials from which low density grains have been removed dense materials such as ilmenite or pyrite have dielectric properties that would produce high backscatter patterns wind streaks generally occur on slopes 2 and tend to be oriented toward the equator consistent with the hadley model of atmospheric circulation in addition to wind streaks other aeolian features on venus include yardangs and dune fields the aglaonice dune field centered at 25 s 340 e covers 1290 km 2 and is located in an ejecta flow channel from the aglaonice impact crater the meshkenet dune field located at 67 n 90 e covers 17 120 km 2 in a valley between ishtar terra and meshkenet tessera wind streaks associated with both dune fields suggest that the dunes are of transverse forms in which the dune crests are perpendicular to the prevailing winds dunes on venus signal the presence of sand size 60 to 2 000 m grains the possible yardangs are found at 9 n 60 5 e about 300 km southeast of the crater mead although most aeolian features are concentrated in smooth plains near the equator the occurrence of wind streaks is widespread and some have been found at all latitudes and elevations they demonstrate that aeolian processes operate widely on venus the intensity of wind erosion and deposits however varies with locality and is dependent on the wind regime and supply of particles

surface characteristics of venus derived from pioneer venus altimetry roughness and reflectivity measurements the three primary data sets for the pioneer venus orbiter radar experiment topography roughness and reflectivity contain important information about the geological and textural characteristics of the surface of venus we have subdivided the range of roughness and reflectivity values into three categories as follows roughness in degrees rms slope relatively smooth 1 2 5 transitional from smooth to rough 2 5 5 and relatively rough 5 and fresnel reflectivity surfaces dominated by soil or porous material 0 2 we have analyzed each of these data sets and their relationships to each other in order to define areas of the surface that are characterized by distinctive properties e g rough rocky surfaces smooth soil surfaces we then describe the abundance and areal distribution of such areas and locally calibrate the geological significance of some of the surface types by examining high resolution images from spacecraft and earth based observatories we find that the majority of venus is covered by regionally contiguous rock and bedrock surfaces many of the smooth surfaces we interpret to be of volcanic origin most likely lava flows while rougher surfaces are locally characterized by tectonic deformation of several types soil surfaces cover less than about 27 of the planet and are generally patchy in their distribution on the basis of the distribution of these surfaces we see no evidence for the extensive preservation of an ancient global regolith or for widespread topographically controlled erosion lateral transport and sedimentation the small percentage of the surface of venus characterized by high dielectric material appears to originate from several processes including primary lava flows probably containing enrichments of high dielectric materials such as metal or metal oxides e g theia mons in beta regio and exposure of high dielectric materials by tectonic deformation e g maxwell montes in ishtar terra these global data set correlations provide a fundamental frame work for understanding the nature of the surface of venus and will permit extrapolation of local and regional findings from future geochemical and imaging experiments to a global context

shuttle radar images of wind streaks in the altiplano bolivia shuttle imaging radar sir a coverage across bolivia shows the major physiographic provinces including the andean altiplano the altiplano contains a variety of eolian features many of which are visible as radar dark radar mottled and radar bright streaks aligned parallel to the prevailing winds the streaks form downwind from hills and are as much as 15 km long and 800 m wide dark streaks originate between closely spaced hills whereas bright streaks form in the immediate lee of hills the radar brightness of the streaks is modulated by the proportion of sand cover vegetation and exposed substrate all of which relate to the local wind regime in the radar dark streaks vegetation is sparse and the sand is organized into dune forms some of which can be recognized as barchan dunes on the radar image by outlining of the dark dune by bright substrate radar mottled zones are covered by numerous 0 5 2 m high coppice dunes and in radarbright streaks the sand forms flat sheets but also includes sand mounds and abundant vegetation measurements indicate that winds are strongest and most turbulent in the region of active sand sheets and dunes inhibiting the growth of vegetation and the formation of mounds and resulting in a radar dark zone these results are consistent with previous studies that have demonstrated the existence of turbulent eddies in the wake of topographic obstacles that produce zones of increased surface wind shear and enhanced sand mobility the relation between the radar signature and the wind regime allows inferences to be drawn for the wind regime from image interpretation

large scale patterns of eolian sediment transport on venus predictions for magellan the atmosphere of venus is likely to exert a major influence on the surface arecibo and venera 15 16 observations of the population of impact craters with bright halos give direct evidence of surface modification on the scale of centimeters to smooth the surface on time scales of 50 250 my both chemical and mechanical modifications probably occur diurnal winds in the planetary boundary layer can transport particles using boundary layer theory including the effects of topography surface stresses and resulting transport and deposition of sand size particles are calculated regional slopes are sites of largest surface stresses sand will be generally transported downhill although there is a preferential net transport from east to west in the same direction as the atmospheric superrotation it is predicted here that streaks may be seen in the magellan radar images which will indicate directions of net eolian transport on the surface of venus

transport of venusian rolling stones by wind speculation that aeolian processes operate on venus1 2 has been confirmed tentatively by analyses of results from venera spacecraft3 5 here we describe simulations of venusian wind processes which show that particles are moved by rolling at wind speeds as much as 30 lower than those required for saltation threshold this mode of wind transport is only observed for sustained periods in water on earth thus there are similarities between aqueous fluid transport on earth and atmospheric transport on venus the formation of small sand ridges and grooves oriented parallel to the wind direction is associated with the rolling of grains in venusian simulations and these structures may be unique aeolian features on venus

measurements of snow and glacier covered areas with single polarization sar climatological and hydrological investigations in mountainous regions frequently require delineation of the areas covered by snow and glacial ice active microwave sensors can discriminate snow and glacier from other targets are effective under all weather conditions and have a spatial resolution compatible with the topographic variation in alpine regions using data acquired with the nasa airsar which operates at three frequencies and four combinations of polarization we examine the usage of synthetic aperture radar sar to map snow and glacier covered areas in order to assess the available sars that operate from satellites we analyze single frequency single polarization data and we compare our results with images from the landsat thematic mapper obtained under clear conditions only a few days after the sar flight single polarization c band 5 3 ghz sar data can discriminate between areas covered by wet snow from those that are ice free but do not easily separate glacier ice from snow and rock l band sar 1 25 ghz data cannot discriminate between snow and glacier ice the accuracy 74 of sar is high enough to justify its use as the data source in areas that are too cloud covered to obtain reliable data from the thematic mapper

planetary boundary layer and slope winds on venus few constraints are available to characterize the deep atmosphere of venus though this region is crucial to understand the interactions between surface and atmosphere on venus based on simulations performed with the ipsl venus global climate model the possible structure and characteristics of venus planetary boundary layer pbl are investigated the vertical profile of the potential temperature in the deepest 10 km above the surface and its diurnal variations are controlled by radiative and dynamical processes the model predicts a diurnal cycle for the pbl activity with a stable nocturnal pbl while convective activity develops during daytime the diurnal convective pbl is strongly correlated with surface solar flux and is maximum around noon and in low latitude regions it typically reaches less than 2 km above the surface but its vertical extension is much higher over high elevations and more precisely over the western flanks of elevated terrains this correlation is explained by the impact of surface winds which undergo a diurnal cycle with downward katabatic winds at night and upward anabatic winds during the day along the slopes of high elevation terrains the convergence of these daytime anabatic winds induces upward vertical winds that are responsible for the correlation between height of the convective boundary layer and topography

venera 13 and venera 14 sedimentary rocks on venus venera 13 and venera 14 transmitted almost complete panoramic views of their landing sites analyses of the photographs show the presence of rock formations undergoing geomorphic degradation the formations display ripple marks thin layering differential erosion and curvilinear fracturings some of them are interpreted as lithified clastic sediments the lithification could have taken place at depth or at the surface resulting in a type of duricrust the origin of the sediments is unknown but could be aeolian volcanic or related to impacts or to turbidity currents

estimation of ice sheet motion using satellite radar interferometry method and error analysis with application to humboldt glacier greenland satellite radar interferometry provides glaciologists with an important new tool for determining the motion and topography of large ice sheets we examine the sources of error in interferometrically derived ice motion measurements including those errors due to inaccurate estimates of the interferometric baseline several simulations are used to assess baseline accuracy in terms of tie point error and the number and distribution of tie points these results give insight into how best to select tie points and also demonstrate the level of accuracy that can be achieved examination of two representative cases likely to occur in mapping ice sheet motion leads to the conclusion that with adequate tie point information ice velocity can be measured accurately to within a few meters per year a method to correct horizontal velocity estimates for the effect of vertical displacement using surface slopes is also developed finally we estimate the single component velocity field for an area on humboldt glacier northern greenland using interferograms formed from ers i sar images we estimate that these velocity measurements are accurate to within 2 3 m year 1

wind streaks geological and botanical effects on surface albedo contrast abstract two wind streaks in the eastern mojave desert of california were examined to gain insight into the origin of the surface brightness contrast that makes them visible both on the ground and in remote sensing data the two localities are a 4 km long dark streak oriented s43e from the amboy cinder cone 34 32 n 115 46 w located on a quaternary basalt flow covered with aeolian sand and a 2 km long dark streak oriented s22e from a low hill near the southwestern base of sleeping beauty mountain 34 48 n 116 20 w located on a sand covered alluvial surface in both cases the dark streaks have enhanced rock abundances on the streak surface relative to the surroundings at the amboy streak slope wash likely contributed to the rock concentration on the streak surface shielded from burial under aeolian sand by the cinder cone at the sleeping beauty streak the relative albedo contrast is strongly emphasized by the presence of big galleta grass only outside of the streak the albedo contrast of the sleeping beauty streak can be effectively eliminated by the seasonal presence of annual grass preferentially within the streak some plants may have reflectances that are strongly dependent upon viewing and illumination geometry raising the possibility that certain terrestrial aeolian features may appear variable on a diurnal basis alluvial processes appear to have been important at both localities for redistributing surface materials even given the infrequent rain conditions present in the mojave desert

atmospheric tides on venus iii the planetary boundary layer abstract diurnal solar heating of venus surface produces variable temperatures winds and pressure gradients within a shallow layer at the botton of the atmosphere the corresponding asymmetric mass distribution experiences a tidal torque tending to maintain venus slow retrograde rotation it is shown that including viscosity in the boundary layer does not materially affect the balance of torques on the other hand friction between the air and ground can reduce the predicted wind speeds from 5 to 1 m sec in the lower atmosphere more consistent with the observations from venus landers and descent probes implications for aeolian activity on venus surface and for future missions are discussed