venus topography a harmonic analysis a model of venusian global topography has been obtained by fitting an eighteenth degree harmonic series to pioneer venus orbiter radar altimeter data the mean radius is 6051 45 0 04 km the corresponding mean density is 5244 8 0 5 kg m 3 the center of figure is displaced from the center of mass by 0 339 0 088 km towards 6 6 10 1 n 148 8 7 7 e the figure of venus is distinctly triaxial but the orientation and magnitudes of the principal topographic axes correlate rather poorly with the gravitational principal axes however the higher degree harmonics of topography and gravity are significantly correlated the topographic variance spectrum of venus is very similar in form to those of the moon mars and especially earth we suggest that this spectral similarity simply reflects a statistical balance between constructional and degradational geomorphic processes venus and earth are particularly similar and differ from the moon and mars in that the larger bodies both exhibit a significant low degree deficit relative to the extrapolated trend of the higher harmonics

gravity field determination and characteristics retrospective and prospective gravimetry has had a long history using pendulums torsion balances and static spring gravimeters relative accuracy adequate for many geophysical problems was already attained by 1900 but it took another half century to build readily portable gravimeters calibration and datum definition remained problems until the 1970s when free fall absolute gravimeters were developed that now have a precision of 10 3 mgal the problems of geographic inaccessibility and field party costs notably in areas of greatest tectonic interest are now being overcome by airborne gravimetry that has already achieved accuracies of 1 3 mgal with resolutions of 10 to 20 km satellite techniques are the best way to determine the long wavelength variations of the gravity field the resolution of the models has steadily improved with the number of satellites and the precision of the observations the best current model includes tracking data from more than 30 satellites satellite altimetry and surface gravimetry and has a resolution of about 290 km harmonic degree 70 with the most recent improvements coming from doppler orbitography and radiopositioning integrated by satellite doris tracking of the spot 2 satellite and satellite laser ranging slr doris and global positioning system gps tracking of the topex poseidon satellite meanwhile radar altimetry has become the dominant technique to infer the marine geoid with a resolution of tens of kilometers or shorter similarly the gravity fields of the moon venus and mars have been determined to harmonic degrees 70 75 and 50 respectively although tracking limitations result in variations of spatial resolution modeling earth s gravity field from the abundance of precise data has become an increasingly complex task with which the development of computer capacity has kept pace contemporary solutions now entail about 10 000 parameters half of them for effects other than the fixed gravity field of earth temporal variations arising from tides have long been well modeled and nontidal changes are now being identified the improvement in gravitational models engendered corresponding advances in geophysical interpretation isostatic models were refined and expanded to account for regional thermal and tectonic histories interpretation of the long wavelength gravity field determined by satellite techniques has been mainly in terms of plate tectonics as a manifestation of mantle convection gravity has been significant in inferring that there must be a large increase in viscosity with depth most strongly from the apparent slow sinking of subducted slabs the prospects for increasing accuracy and resolution in the determination of earth s gravity field rest primarily with the development of new measurement systems airborne gravimetry is taking promising new steps using gps but significant global model improvement awaits a dedicated satellite gravimetry system and future satellite altimeter missions will do more for ocean dynamics studies than geoid improvement advances in interpretation will occur through the development of other data such as seismic tomography and larger scale computer modeling of tectonics and convection

yardang geometries in the qaidam basin and their controlling factors abstract the hyperarid qaidam basin features extensive fields of yardangs covering an area of 40 000km2 sculpted in tectonically folded sedimentary rocks we extracted the geometries of 16 749 yardangs such as length to width ratio l w spatial density and spacing from multi source remote sensing data provided by google earth we classified the yardangs into four types based on their l w short axis 1 2 whale back 2 6 hogsback 6 10 and long ridge 10 210 we interpreted the yardang geometries in the context of their geologic setting bedding orientation location along anticline crests or syncline troughs and lithologic heterogeneity our results show that the yardang geometries in the qaidam basin are mainly controlled by the structural geology and rheology of the sedimentary rocks e g strike and dip of bedding the presence or absence of interbedded soft and hard beds and structural position with folds the angle between geomorphically effective wind directions and the strike of bedding and the relative cumulative wind shear force where two geomorphically effective wind directions are present our analysis revealed the following 1 nearly 69 of the yardangs with long ridge and hogsback geometries are distributed in syncline areas whereas 73 of the yardangs with short axis geometries are distributed in anticline areas 2 the l w ratio of yardangs exposed along the windward limbs of anticlines is lower than that of yardangs exposed along the leeward limbs and 3 in the westernmost parts of the basin yardangs are locally sculpted into mounds by two geomorphically effective wind directions

venus gravity a harmonic analysis an improved model of venusian global gravity has been obtained by fitting an eighteenth degree and eighteenth order spherical harmonic series to 78 orbital arcs of high altitude 950 1350 km at periapsis tracking data and 351 orbital arcs of lower altitude 150 200 km at periapsis data from the pioneer venus orbiter pvo compared to a recently published tenth degree model mottinger et al 1985 which is based on the 78 high altitude arcs only the current model provides a significant improvement in resolution and fidelity as a measure of this improvement we note that for the low altitude arcs alone the variance of the residuals for the present model is reduced to 19 of the data variance compared to 51 for the tenth degree model venus differs significantly from the earth in that it exhibits a significant correlation between long wavelength topography and gravity the gravity topography spectral admittances are inconsistent with either airy or pratt isostasy but they are consistent with dynamic support by mantle convection

venus tectonics an overview of magellan observations the nearly global radar imaging and altimetry measurements of the surface of venus obtained by the magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet many areas of venus record a superposition of different episodes of deformation and volcanism this deformation is manifested both in areally distributed strain of modest magnitude such as families of graben and wrinkle ridges at a few to a few tens of kilometers spacing in many plains regions as well as in zones of concentrated lithospheric extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes ridge belts and mountain belts which have characteristic widths and spacings of hundreds of kilometers represent successive degrees of lithospheric shortening and crustal thickening the mountain belts of venus as on earth show widespread evidence for lateral extension both during and following active crustal compression venus displays two principal geometrical variations on lithospheric extension the quasi circular coronae 75 2600 km diameter and broad rises with linear rift zones having dimensions of hundreds to thousands of kilometers both are sites of significant volcanic flux but horizontal displacements may be limited to only a few tens of kilometers few large offset strike slip faults have been observed but limited local horizontal shear is accommodated across many zones of crustal stretching or shortening several large scale tectonic features have extremely steep topographic slopes in excess of 20 30 over a 10 km horizontal scale because of the tendency for such slopes to relax by ductile flow in the middle to lower crust such regions are likely to be tectonically active in general the preserved record of global tectonics of venus does not resemble oceanic plate tectonics on earth wherein large rigid plates are separated by narrow zones of deformation along plate boundaries rather tectonic strain on venus typically involves deformation distributed across broad zones tens to a few hundred kilometers wide separated by comparatively undeformed blocks having dimensions of hundreds of kilometers these characteristics are shared with actively deforming continental regions on earth the styles and scales of tectonic deformation on venus may be consequences of three differences from the earth 1 the absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well preserved 2 a high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller scale deformational features 3 a strong coupling of mantle convection to the upper mantle portion of the lithosphere probably because venus lacks a mantle low viscosity zone leads to crustal stress fields that are coherent over large distances the lack of a global system of tectonic plates on venus is likely a combined consequence of a generally lesser strength and more limited horizontal mobility of the lithosphere than on earth

the lemniscate loop comparisons with the shapes of streamlined landforms the lemniscate loop is a streamlined geometric form which has a simple algebraic equation containing a parameter whose value governs its overall length to width dimensions the lemniscate has been employed in geomorphology including comparisons with glacial drumlins and erosional streamlined islands in the channeled scabland of eastern washington and within the channels on mars these past comparisons have mainly been visual the present study involves more detailed quantitative comparisons between the lemniscate loop symmetrical airfoils drumlins and various islands formed by water flows the analyses focus primarily on the length to width ratios of these landscape features and their consistency with the analytical lemniscate shape many drumlins are found to be similar to the streamlined lemniscate but an equal number are more closely approximated by an elliptical geometric form the streamlined islands in rivers in the channeled scabland and in the martian channels all have nearly the same ave

reports venus identification of banded terrain in the mountains of ishtar terra high resolution images obtained with the arecibo radar system at a wavelength of 12 6 centimeters reveal numerous parallel 10 to 20 kilometer wide bands of high radar backscatter situated on and oriented parallel to the major mountain ranges of the ishtar terra region of venus geometric and morphological characteristics suggest that the bands represent earth like tectonic deformational features such as folds and faults

aeolian erosional lineations in the libyan desert dakhla region egypt abstract this paper presents results of a study of aeolian erosion at the landscape scale there have been few such studies in desert geomorphology compared to those focused on individual landforms and on rock surface sculpture the present study area lies in the southern part of the libyan desert in south central egypt between 25 and 27 n 29 and 30 e bedrock comprises paleogene limestone of various lithologies climate is hyperarid the following topics are treated i correspondences between rock outcrop belts and belts of terrain lineated by aeolian erosion and unlineated terrain ii the effect of lithology on aeolian lineation which works through the presence or absence of chert in the limestones lineation is produced by aeolian erosion in chert free poor rocks whereas erosion of chert rich rocks produces a desert pavement that armors the surface suppressing erosion iii the effect of large valleys eroded upwind of lineated terrain which deflect winds and trap sand ending erosion so that downwind lineations are reduced and finally erased by weathering iv sample lineated landscapes in the area show stages of evolution arranged in the time domain into a proposed cycle of aeolian erosion the cycle progresses from initial smooth plain to grooves separating long blade shaped ridges to segmentation of ridges into shorter blades along diagonal joints to streamlining of shorter blades and size reduction to final planation in any one wind parallel swath of lineated terrain stages in the cycle progress downwind in the space domain so that upwind landscapes are more advanced in the cycle v the structure of air flow responsible for lineation is still uncertain but initial grooving may respond to stable longitudinal horseshoe vortices or self organized regularity of erosion by random vortices subsequently the erosion pattern is fixed by evolving relief vi large smooth basins within the lineated terrain were carved by aeolian erosion before oxygen isotope stage ois 5 70 130 ka as were similar basins along the dakhla piedmont below the scarp little geomorphic change has occurred since so in this field of aeolian erosional lineations aels the aeolian erosion cycle may have occupied 10 times as long since continental scale aridity set in at ca 2 4 ma there have been only two cycles in this area a completed cycle earlier than the current incomplete one is indicated by rare small yardangs riding piggyback on larger ones

venus gravity anomalies and their correlations with topography this report provides a summary of the high resolution gravity data obtained from the pioneer venus orbiter radio tracking data gravity maps covering a 70 latitude band through 360 of longitude are displayed as line of sight and vertical gravity topography converted to gravity and bouguer gravity maps are also shown in both systems topography to gravity ratios are made over several regions of the planet there are markedly different ratios for the aphrodite area as compared to the beta and atla areas

evidence for divergent plate boundary characteristics and crustal spreading on venus detailed examination of the topography and morphology of western aphrodite terra reveals numerous features that are similar to terrestrial divergent plate boundary characteristics individual domains between fracturezone like discontinuities contain a variety of bilaterally symmetrical topographic elements that suggest that topographic features have been created at rise crests rifted and separated and moved laterally to their present symmetrical positions the topographic and morphologic similarities together with strikingly similar mirror image map patterns on both sides of the rise axis suggest that western aphrodite terra shares the characteristics of oceanic divergent plate boundaries and is the site of crustal spreading on venus topographic profiles are consistent with spreading rates of the order of several centimeters per year

yardangs on earth and implications to mars a review abstract yardangs as typical aeolian landforms are extensively identified in arid hyper arid areas on earth and other terrestrial bodies in this review based on analyses of morphology distribution age climate and geologic condition of yardangs worldwide we 1 assess the impacts of various controlling factors on yardang development to generalize a model based on morphology 2 illustrate the climate driven mechanism for yardangs 3 propose a geomorphological evolutionary model of yardang fields and 4 discuss potential implications of the above two models of yardang evolution on earth to mars it is demonstrated that both deflation and abrasion work on yardang development and it is the lithology to determine which one will be more effective in addition the wind action dominates the general aerodynamic form of yardangs while non aeolian factors account for the diversity in yardang morphology thus a model including four stages and initiating mechanisms is generalized for yardang processes from initiation to demise the synthetic analyses of meteorological data chronology and paleoclimatic proxies show that yardang development is mostly controlled by westerlies and would be accelerated during glacial periods and interrupted by lake formation during interglacial periods due to dry humid fluctuations and changes in atmospheric circulation driven by variations in solar radiation at orbital scales this leads to the zonal distributions of yardangs near 30 in both hemispheres depressions would be created or enlarged in yardang fields during glacial period along with activities of dust release and transport leading to aeolian deposition e g loess downwind the depressions then could be turned into lakes during wet phase of interglacial period and evolved into new yardangs in the following glacial period due to wind erosion this climate controlled geomorphological process under glacial interglacial cycles suggests that high accumulation rate in sediments from enclosed inland basins e g the qaidam basin doesn t mean high resolution climatic record due to the hiatus by wind erosion indicating the significance of robust chronology establishment before any climatic correlation under the rules of earth based yardang evolution it is potential to identify the specific level of development and explore the geomorphological processes as well as the climatic background for yardangs on mars

turbidity current sediment waves on the submarine slopes of the western canary islands two sediment wave fields have been identified on the flanks of the western canary islands of la palma and el hierro using a high quality 2 d and 3 d dataset that includes geosea and tobi imagery 3 5 khz profiles and short sediment cores the la palma sediment wave field covers some 20 000 km2 of the continental slope and rise and consists of sediment waves with wave heights of up to 70 m and wavelengths of up to 2 4 km the wave crestlines have a complex morphology with common bifurcation and a clear sinuosity waves have migrated upslope through time cores recovered from the wave field contain volcaniclastic turbidites interbedded with pelagic hemipelagic layers the wave field is interpreted as having formed beneath unconfined turbidity currents a simple previously published two layer model is applied to the waves revealing that they formed beneath turbidity currents flowing at 10 100 cm s 1 with a flow thickness of 60 400 m and a sediment concentration of 26 427 mg l the el julan sediment wave field lies within a turbidity current channel on the southwest flank of el hierro the sediment waves display wave heights of about 6 m and wavelengths of up to 1 2 km the waves are migrating upslope and migration is most rapid in the centre of the channel where the flow velocity is highest this wave field has been formed by channelised turbidity currents originating on the flanks of el hierro

mantle flow tectonics the influence of a ductile lower crust and implications for the formation of topographic uplands on venus convective flow within the interiors of terrestrial planets is a primary source of large scale topographic and tectonic features we treat the crust and mantle as a layered viscous half space and find solutions for flow driven by a buoyancy force distribution within the mantle and by relief at the surface and crust mantle boundary the crust initially responds to mantle upwelling with uplift but continued flow causes significant crustal thinning and subsidence downwelling leads to initial subsidence and subsequent crustal thickening and surface uplift changes in crustal thickness are driven by 1 vertical normal stresses due to mantle flow and 2 shear coupling of horizontal mantle flow into the crust the degree and time scale of crustal deformation are sensitive to crustal thickness and viscosity and the presence of elastic layers in the upper crust and mantle deformation can occur on time scales less than several hundred million years for reasonable values of these parameters strong or elastic upper crustal layers enhance crustal thinning or thickening while large viscosity contrasts between the lower crust and upper mantle or elastic mantle layers tend to diminish deformation for time dependent mantle flow due to a rising or sinking diapiric body crustal deformation depends upon the ratio of diapir radius to crustal thickness and the ratio of crustal to mantle viscosity a sinking diapir can lead to crustal thickening elevated topography and a specific sequence of time varying deformation while regions such as beta regio have long been suggested to result from convective upwelling in the venus mantle we suggest that a mantle downwelling origin is consistent with many of the characteristics of the compressional mountain ranges of western ishtar terra

venus global gravity and topography a new gravity field determination that has been produced combines both the pioneer venus orbiter pvo and the magellan doppler radio data comparisons between this estimate a spherical harmonic model of degree and order 21 and previous models show that significant improvements have been made results are displayed as gravity contours overlaying a topographic map we also calculate a new spherical harmonic model of topography based on magellan altimetry with pvo altimetry included where gaps exist in the magellan data this model is also of degree and order 21 so in conjunction with the gravity model bouguer and isostatic anomaly maps can be produced these results are very consistent with previous results but reveal more spatial resolution in the higher latitudes

a spreading drop model for plumes on venus many of the large scale plume related features on venus can be modeled by a buoyant viscous drop or plume head as it rises and spreads laterally below a free fluid surface the drop has arbitrary density and viscosity contrast and begins as a sphere below the surface of a fluid half space the boundary integral method is used to solve for the motion of the plume head and for the topography geoid and stress at the fluid surface as the plume approaches the surface stresses in the fluid above it cause it to spread and become thin below the surface during the spreading the surface swell above evolves through various stages whose morphologies resemble several different plume related features observed on venus when the plume head first approaches the surface a high broad topographic dome develops with a large geoid and radial extensional deformation patterns at later stages the topography subsides and becomes plateau like the geoid to topography ratio gtr decreases and the dominant stress pattern consists of a band of concentric extension surrounded by a band of concentric compression we find that a low viscosity model plume head viscosity that is 0 1 times the mantle viscosity produces maximum topography that is 20 lower and swell features which evolve faster than for an isoviscous plume we compare model results with both the large scale highland swells and smaller scale features such as coronae and novae the dome shaped highlands with large gtrs such as beta atla and western eistla regiones may be the result of early stage plume motion while the flatter highlands such as ovda and thetis regiones which have lower gtrs may be later stage features comparison of model results with gtr data indicates that the highlands result from plume heads with initial diameters of about 1000 km on a smaller scale an evolutionary sequence may begin with novae domes having radial extensional deformation followed by features with radial and concentric deformation such as arachnoids and end with coronae with mostly concentric deformation the model predicts that the highlands evolve on a timescale of order 10 ma and the smaller scale features evolve in a 100 ma timescale

quantification of soil production and downslope creep rates from cosmogenic 10be accumulations on a hillslope profile average soil transport rates over a period of 3500 yr on a convex soil mantled hillslope have been quantified using a mass balance model that incorporates the soil concentration of the cosmogenic isotope 10 be the 10 be model results support the assumption used in most geomorphic models that the soil creep rate is proportional to surface gradient the predicted diffusion coefficient is 360 55 cm 3 yr 1 cm 1 contour length and the average rate of soil production is 0 026 0 007 cm yr within the uncertainty of this technique the data do not reject g k gilbert9s hypothesis that some hillslopes may exist in a condition of dynamic equilibrium with a uniform soil production rate however the model does not require an assumption of dynamic equilibrium and may be an approach that uniquely allows the quantification of a local soil production rate law

geology of a rift zone on venus beta regio and devana chasma beta regio is a region of rifting and volcanism on venus the nature of beta a major topographic rise and rift zone is herein characterized using pioneer venus arecibo and venera 15 16 data high resolution 1 2 km arecibo and venera radar images reveal details of faulting and volcanism and pioneer venus altimetry illustrates the density and location of faults in relation to topography faults are distributed throughout beta but are concentrated in devana chasma where they are spaced 5 20 km apart the pattern of faulting and distribution and sequence of volcanic activity in beta can be used to help understand how the rift has evolved including the origin of the high topography of beta and the origin and nature of devana chasma on the basis of geologic mapping relations and map patterns beta appears to have formed as a result of doming in response to a mantle anomaly at the same time rhea mons a major shield volcano was formed formation of devana chasma followed with extensive faulting in the rift trough geometry of the trough and fault patterns suggests that some degree of lithospheric stretching has occurred later volcanism produced a second major shield volcano theia mons which is superimposed on the western bounding fault of the rift zone both uplift and extension have been involved in forming devana chasma and beta regio and may be important in the formation of other equatorial high land regions with systems of chasmata on venus such as aphrodite terra the bifurcation of devana chasma in the vicinity of theia mons and the extension of the rift system south to phoebe regio and west toward aphrodite terra suggest that their origins may be linked

structure of axisymmetric mantle plumes results of high resolution numerical calculations of axisymmetric thermal plumes in an infinite prandtl number fluid with thermally activated viscosity are used to infer the structure of axisymmetric subsolidus thermal plumes in the earth s lower mantle we calculate the velocity and temperature distribution for axisymmetric convection above a heated disk in an incompressible fluid cylinder 2400 km in height and 1200 km in diameter initially at a uniform temperature t the lower boundary representing the plume source region near the core mantle boundary is stress free and maintained at temperature tc t the upper boundary is permeable allowing the plume to escape the cylinder we determine the sensitivity of plume structure to viscosity variations c ranging from 102 to more than 104 and temperature differences t tc t in the range 400 850 k starting plumes consist of a large leading diapir and a trailing conduit connected to the thermal boundary layer above the heated surface the leading diapir expands during ascent to about 400 km diameter at 2200 km height in steady state the plume consists of a narrow high velocity conduit imbedded within a broader thermal halo the width of both these structures is proportional to height above the heated boundary and inversely proportional to t and the viscosity ratio heat transport in the plume is nearly independent of viscosity variations a simplified boundary layer model yields a heat transfer law nu ra where nu is the plume nusselt number and ra is the rayleigh number based on t and the radius of the heated circular region feeding the plume calculated steady state plume heat transports fit this law to within a few percent we present several calculations of plumes with heat transport in the range 100 400 gw similar to the advective heat transport by the hawaiian hotspot at the top of the lower mantle plumes with large viscosity variations have high velocity conduits 50 60 km in diameter and 125 km diameter thermal halos a centerline temperature anomaly of about 500 k and a centerline velocity of about 75 cm yr 1 plumes with weak viscosity variations are approximately twice as broad and have centerline temperature anomalies less than 160 k at the same height and much smaller velocity large viscosity variations and a minimum heat transport are both

hot spot evolution and the global tectonics of venus the global tectonics of venus may be dominated by plumes rising from the mantle and impinging on the lithosphere giving rise to hot spots global sea floor spreading does not take place but direct convective coupling of mantle flow fields to the lithosphere leads to regional scale deformation and may allow lithospheric transport on a limited scale a hot spot evolutionary sequence comprises i a broad domal uplift resulting from a rising mantle plume ii massive partial melting in the plume head and generation of a thickened crust or crustal plateau iii collapse of dynamic topography and iv creep spreading of the crustal plateau crust on venus is produced by gradual vertical differentiation with little recycling rather than by the rapid horizontal creation and consumption characteristic of terrestrial sea floor spreading

variations in effective compensation depth across aphrodite terra venus aphrodite terra is the largest elevated terrain on venus and it serves as a focal point in current discussions of global tectonic style using the topography and gravity data acquired by the pioneer venus orbiter pvo the authors have estimated an effective depth of airy compensation for each of 75 orbital arcs that provide fairly uniform areal coverage of the entire province the most pronounced pattern that emerges is a general increase in compensation depth to the east the most rapid change occurs near 135 degree the average west of there is 70 km while the average to the east is 230 km superimposed on this larger trend are five distinctive regional patterns four well defined peaks and one interval of widely scattered and poorly constrained depths the maxima in compensation depth are well correlated with regional topographic highs while these observations are easily reconciled with the general notion that aphrodite is a region of crustal divergence the great depth of compensation is difficult to accord with the more specific suggestion that aphrodite is a terrestrial type divergent plate margin the alternative suggestion that aphrodite and the other equatorial highlands on venus are surface manifestations of hot upwelling mantle plumes ismore consistent both with the great depths of compensation and with the pattern of regional peaks and intervening troughs the broader scale increase in effective depth of compensation from west to east is more enigmatic but it might reflect an age progression of the plumes less

geomorphology and origin of yardangs in the kumtagh desert northwest china yardangs are typical wind eroded landforms that are found in most of the world s major deserts this paper describes the morphology of yardangs in the kumtagh desert analyzes their control factors and discusses their development processes age and rates of formation yardangs in the kumtagh desert lack the ideal streamlined morphology and the length to width ratio close to 4 1 that have been previously described as being due to their unique controlling factors which include a combination of stratigraphic structures wind erosion and fluvial erosion fluvial erosion plays a particularly important role in the development of the yardangs though wind continuously sculpts their morphology the development of yardangs can be divided into four stages an embryonic stage adolescence maturation and recession most yardangs in the kumtagh desert are in the mature to receding stages we determined an approximate age for the yardangs based on the erosion rates estimated by several researchers on this basis the yardangs in the kumtagh desert appear to have formed during the holocene

statistical description of slope dependent soil transport and the diffusion like coefficient 1 for hillslopes undergoing diffusive soil transport it is often assumed that the soil flux is proportional to the local land surface gradient where the coefficient of proportionality is like a diffusion coefficient inasmuch as transport involves quasi random soil particle motions related to biomechanical mixing and similar dilational processes a slope dependent relation arises from a balance between particle fluxes that tend to loft a soil and gravitational settling of particles into available pore space a specialized form of the fokker planck equation adapted to such particle motions clarifies how the particle flux involves advective and diffusive parts this in turn contributes to a kinematic description of the diffusion like coefficient ingredients of this coefficient include an active soil thickness a characteristic particle size the porosity in excess of a consolidated porosity and the rate of particle activation as a function of depth these last two ingredients vertical porosity structure and activation rate in effect characterize the magnitude and frequency of settling particle motions related to biological activity and thereby set the rate constant of the transport process the significance of land surface slope is that it is a measure of the downslope component of slope normal lofting that is balanced by settling because the diffusion like coefficient contains the soil thickness the analysis suggests that the soil flux is proportional to the depth slope product the analysis is consistent with published profiles of soil creep displacement and with published estimates of soil flux obtained by downslope integration of soil production rates for hillslopes in california and australia

geology and structure of beta regio venus results from arecibo radar imaging arecibo radar images of a portion of the equatorial region of venus provide the first high resolution 1 5 to 2 0 km synoptic coverage of beta regio within this area tessera a complex deformed terrain is identified as a major geologic unit with the largest region corresponding to a plateau on the east flank of the highland three models are proposed to explain the origin and evolution of beta regio and are identified as mantle plume passive crust mantle plume active crust and mantle plume crustal spreading the mantle plume passive crust model appears to be the most consistent with the geology in this region and suggests that a plume disrupts a preexisting region of tessera

mantle flow tectonics and the origin of ishtar terra venus it is proposed that western ishtar terra formed due to compression and crustal thickening above a cylindrical mantle downwelling a model for crustal deformation due to downwelling successfully reproduces many observed characteristics of western ishtar although axisymmetric downwellings occur in numerical models of constant viscosity mantle convection there is no evidence for their existence in earth s mantle where downwellings are sheet like either modes of downflow in venus and earth are fundamentally different or differences in near surface conditions and material behavior selectively emphasize surface expressions of the different downwelling modes

venus comparison of gravity and topography in the vicinity of beta regio the doppler tracking data obtained from the pioneer venus orbiter when it passed near beta regio yielded a peak vertical anomaly of 150 mgal when analyzed by our two stage procedure a comparison of maps of the gravity and topography at comparable resolution shows a striking correlation a scatter plot shows that the observed gravity anomaly is approximately 0 4 of that expected from uncompensated topography of half the mean density of venus however the spectral admittance shows that the gravity anomalies can not be explained either by airy compensation at fixed depth or by a model comprising an elastic plate atop an inviscid fluid the gravity and topography variations may signify deep compensation or dynamic support for beta regio and more shallow compensation for other features

miocene reversal of bottom water flow along the pacific margin of the antarctic peninsula stratigraphic evidence from a contourite sedimentary tail a fossil mounded sedimentary body mb has been identified in the sedimentary record on the central continental rise west of adelaide island on the antarctic peninsula pacific margin the growth patterns of the mb are defined through a detailed regional stratigraphic analysis using multichannel seismic reflection profiles the mb has an elongated ne trend it overlaps and continues to the ne of an extensive cluster of seamounts and it developed between two non depositional troughs nine seismic units have been identified unit 9 the pre mb stage unit 8 mb growth stage units 7 and 6 mb maintenance stage units 5 and 4 transitional stage and units 3 2 and 1 inactive stage we interpret the mb as a patch drift plastered against the ne lee side of an obstacle as a long contourite sedimentary tail cst within a deep current that flowed northeastward this segment of the rise is however affected at present by a sw flowing branch of the lower circumpolar deep water lcdw from the weddell sea the depositional patterns of the mb growth and maintenance stages which are attributed an early miocene age on the basis of regional correlation of mcs profiles with dsdp site 325 and odp leg 178 drill sites provide the first evidence that bottom currents on the central continental rise flowed towards the ne at that time probably as part of the lower circumpolar deep water lcdw of the antarctic circumpolar current acc we suggest that significant palaeocirculation and palaeoceanographic changes occurred in this area and probably more widely during the middle miocene or at the miocene pliocene boundary although these results do not modify the regional stratigraphy of the major sediment drifts found on the continental rise of the antarctic peninsula s pacific margin they do indicate that the bottom current regime controlling the development of contourite deposits may have changed over time and also that more than one water mass has probably affected their distribution

sediment transport by wind toward a general model a general model of eolian sediment transport by both saltation and suspension is developed saltation is analyzed through calculations of single trajectories given initial conditions of liftoff speed angle and spin profiles of particle concentration and mass flux generated by assuming that all trajectories are identical display distinct maxima at the top of the trajectory by incorporating the realistic distribution of trajectories that arises from the stochastic nature of grain impacts with a granular bed the model yields monotonic decreases in particle concentration and mass flux with height in accordance with published empirical data the analysis of suspension as the balance between downward advective flux as a result of the settling of grains and their upward diffusive flux as a result of turbulence results in concentration profiles that fall off as power laws with height also in accordance with empirical data a relation for reference level concentrations is presented for both blowing snow and dust profiles the solutions for saltation and suspension are combined to yield mass flux profiles for the entire range of grain sizes in the bed the saltation layer emerges naturally as that region where the total flux of saltating grains exceeds that of suspended grains the model points clearly to the need for further research on the stochastic grain bed interactions which control both the probability distribution of liftoff velocities in saltation and the production of fine particles in suspension

venus limited extension and volcanism along zones of lithospheric weakness three global scale zones of possible tectonic origin are described as occurring along broad low rises within the equatorial highlands on venus lat 50 n to 50 s long 60 to 310 the two longest of these tectonic zones the aphrodite beta and themis atla zones extend for 21 000 and 14 000 km respectively several lines of evidence indicate that beta and atla regiones located at the only two intersections of the three major tectonic zones are dynamically supported volcanic terranes associated with currently active volcanism rift valleys south of aphrodite terra and between beta and phoebe regiones are characterized by 75 to 100 km widths raised rims and extensions of only a few tens of kilometers about the same magnitudes as in continental rifts on the earth horizontal extension on venus was probably restricted by an early choking off of plate motion by high crustal and upper mantle temperatures and the subsequent loss of water and an asthenosphere

the geology and geomorphology of the venus surface as revealed by the radar images obtained by veneras 15 and 16 a region by region condensed description of almost all of the area that was radar photographed by veneras 15 and 16 is presented using some generalizations the diversity of terrain was reduced to a discrete set from which a geological morphological map was constructed the predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types ridge and band patchy rolling plain dome and butte plain smooth plain and high smooth plain stratigraphically the ridge and band plains are the oldest and the smooth plains are the youngest the stratigraphic position of the other types is yet to be determined large sections of the plains show similarities to the mare type basaltic plains of the moon mercury and mars other types of terrain are combinations of ridges and grooves in various patterns linear parallel orthogonal diagonal or chevron like and chaotic in some places the ridge and groove terrain is stratigraphically below the plain material but in other places it appears to be plain material that has been subsequently deformed near the eastern and western boundaries of ishtar terra large several hundred kilometers in diameter ring like features can be seen that are named coronae or ovoids evidence of tectonic deformation and the presence of flow like patterns support their designation as volcano tectonic features beta regio seems to be an uplifted plain showing evidence of rifting and volcanism all types of terrain are sparesely peppered with craters of obvious impact morphology their average density gives the plain an age range of 0 5 to 1 109 years the fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking at least for the last 0 5 to 1 109 years no evidence for water erosion sedimentation processes has been found the tectonic activity of venus has no equivalent on the moon mercury or even mars and can be compared only with that of the earth intensive horizontal deformation previously known only on earth occurs on venus but in a characteristic venusian style

magellan observations of alpha regio implications for formation of complex ridged terrains on venus magellan images of alpha regio reveal previously undetected structures and details of the morphology of this region of complex ridged terrain we examine the complex ridged terrain of alpha regio using morphology and crosscutting relationships between structures to derive a sequence of tectonic events structures include broad 10 20 km wide linear and arcuate ridges fine scale 3 km wide ridges linear disruption zones ldzs up to several kilometers wide and numerous grabens 5 km wide and associated scarps and troughs based on their morphology we interpret the broad and fine scale ridges as compressional structures possibly folds ldzs appear to be due to small amounts of lateral shear which most commonly disrupts the older ridge fabric graben and associated structures are interpreted as extensional features they crosscut ridges and ldzs and thus appear to be the youngest structures in alpha regio this sequence of events and information on the orientation of these various structures are compared to the predictions of two models for the formation of complex ridged terrain and highlands on venus in general a hotspot model and a coldspot model the presence of compressional features along and parallel to the margins of alpha regio and the lack of any high elevation ring of extensional features are more consistent with a coldspot or roughly axisymmetric mantle downwelling mantle downwelling appears to be the most likely mode of formation of the upland of alpha regio and is likely to be important in other highland regions such as ovda and thetis regiones which are also dominated by complex ridged terrain

a high resolution gravity model for venus gvm 1 a spherical harmonic model of the gravitational field of venus complete to degree and order 50 has been developed using the s band doppler tracking data of the pioneer venus orbiter pvo collected between 1979 and 1982 the short wavelengths of this model could only be resolved near the pvo periapse location 14 n latitude therefore a priori constraints were applied to the model to bias poorly observed coefficients towards zero the resulting model has a half wavelength resolution of 400 km near the pvo periapse location but the resolution degrades to greater than 1000 km near the poles this gravity model correlates well with a degree 50 spherical harmonic expansion of the venus topography derived from a combination of magellan and pvo data new tracking data from magellan s gravity mission should provide some improvement to this model although a complete model of the venusian gravity field will depend on tracking of magellan after the circularization of it s orbit using aerobraking

controls on the spacing and geometry of rill networks on hillslopes rain splash detachment initial hillslope roughness and the competition between fluvial and colluvial transport 1 rill networks have been a focus of study for many decades but we still lack a complete understanding of the variables that control the spacing of rills and the geometry of rill networks e g parallel or dendritic on hillslopes in this paper we investigate the controls on the spacing and geometry of rill networks using numerical modeling and comparison of the model results to terrestrial laser scanner derived topographic data from rill networks formed in physical experiments the landscape evolution model accounts for the transport of sediment due to rain splash and fluvial entrainment as well as the deposition of sediment being advected by the overland flow in order to develop realistic rill networks in the model we find that it is necessary to incorporate the effects of raindrop impact within the fluvial sediment transport process model results are only consistent with those of experiments when raindrop aided fluvial sediment transport is accounted for dendritic networks are often predicted by the model in cases of high initial topographic roughness and high rates of advective fluvial sediment transport relative to diffusive colluvial transport parallel networks form within numerical experiments in low roughness cases under a wide range of relative advective and diffusive transport rates as well as in high roughness cases in which diffusive sediment transport is high relative to advective transport the transition from dendritic to parallel rill networks is shown to occur gradually rather than being associated with a particular threshold finally based on a balance between diffusive and advective sediment transport processes we predict that the mean spacing between parallel rills scales with the square root of the ratio of diffusivity to channel erodibility

erosion profiles due to particles entrained by wind application of an eolian sediment transport model erosion by impacts of particles entrained in the wind scales with the kinetic energy of impacting grains a recent model of eolian sediment transport permits calculation of expected erosion patterns in objects of simple geometry the modeling of vertical erosion profiles in turn provides an exacting test of the transport model realistic distributions of liftoff velocities for saltating grains give rise to kinetic energy flux profiles characterized by a strong maximum as much as 1 4 m above the bed during strong winds kinetic energy flux due to suspended grains also peaks above the bed the height and strength of the maxima depend very strongly on the grain size distribution as grain size diminishes increased particle deflection by the air flow around an obstacle reduces delivery of kinetic energy to the surface for both saltating and suspended grains kinetic energy flux scales with the fifth power of the wind shear velocity erosion profiles in man made obstacles are well modeled with only slight modification of the saltation model to account for the relatively high elasticity of actual deflationary surfaces lending considerable support for the model the modeling also elucidates several aspects of ventifaction as small ventifacts 1 2 m in diameter and the lower portions of larger ones are predominantly eroded by saltating grains upwind surface irregularities are quickly damped leading to facets that dip upwind and sharply truncate lee surfaces sandblasting by suspended grains dominates along the upper portions of large ventifacts where grain paths are partially deflected by the obstacle and may lead to radially symmetrical flutes and grooves

an equilibrium relationship for shear velocity and apparent roughness lenght in aeolian saltation abstract an equilibrium relationship between shear velocity and apparent surface roughness is established for aeolian saltation velocity profiles of wind over blowing sand were digitized from eleven published data sets the 106 profiles were analyzed to obtain estimates of shear velocity and apparent surface roughness lenghts these values were used to test charnock s relationship for suitability in aeolian environments the results indicate that z 0 0 0091 u 2 g modifying charnock s model to describe wind over an equilibrium saltation field yields z 0 2d 50 30 0 0252 u u t 2 g the latter result is used to demonstrate that the focal point concept proposed by bagnold stems from the increase in apparent roughness with increased shear velocity the results demonstrate an excellent example of threshold controlled equilibrium in dynamic systems

erosion by planar turbulent wall jets new scaling laws are presented for the spatial variation of the mean velocity and lateral extent of a two dimensional turbulent wall jet flowing over a fixed rough boundary these scalings are analogous to those derived by wygnanski et al 1992 for the flow of a wall jet over a smooth boundary they reveal that the characteristics of the jet depend weakly upon the roughness length associated with the boundary as confirmed by experimental studies rajaratnam 1967 these laws are used in the development of an analytical framework to model the progressive erosion of an initially flat bed of grains by a turbulent jet the grains are eroded if the shear stress exerted on the grains at the surface of the bed exceeds a critical value which is a function of the physical characteristics of the grains after the wall jet has been flowing for a sufficiently long period the boundary attains a steady state in which the mobilizing forces associated with the jet are insufficient to further erode the boundary the steady state profile is calculated separately by applying critical conditions along the bed surface for the incipient motion of particles these conditions invoke a relationship between the mobilizing force exerted by the jet the weight of the particles and the local gradient of the bed use of the new scaling laws for the downstream variation of the boundary shear stress then permits the calculation of the shape of the steady state scour pit the predicted profiles are in good agreement with the experimental studies on the erosive action of submerged water and air jets on beds of sand and polystyrene particles rajaratnam 1981 the shape of the eroded boundary at intermediate times before the steady state is attained is elucidated by the application of a sediment volume conservation equation this relationship balances the rate of change of the bed elevation with the divergence of the flux of particles in motion the flux of particles in motion is given by a semi empirical function of the amount by which the boundary shear stress exceeds that required for incipient motion hence the conservation equation may be integrated to reveal the transient profiles of the eroded bed there is good agreement between these calculated profiles and experimental observations rajaratnam 1981

aerodynamic roughness lengths over movable beds comparison of wind tunnel and field data 1 we evaluate the increase in apparent roughness length during aeolian saltation using a data set comprising 291 wind profiles measured in wind tunnels or in the field we tested the charnock 1955 modified charnock sherman 1992 and raupach 1991 relationships for the entire data set and for segregated wind tunnel and field data sets we assessed the abilities of the models to predict enhanced roughness and investigated scaling differences between wind tunnels and field environments for the entire data set the charnock relationship worked best with an r2 of 0 25 for the segregated data sets the charnock relationship also worked best with an average r2 of 0 675 as opposed to 0 66 for the modified charnock and 0 61 for the raupach model for conditions with shear velocity 1 0 ms 1 the comparable average values of r2 are 0 54 0 625 and 0 645 respectively there was an order of magnitude difference in the roughness length responses for wind tunnel and field data for each model evaluated convincing evidence of a fundamental scaling constraint in the wind tunnel data the scaling differences are attributed to conceptual and bias errors that include the use of the law of the wall versus the wake defect law or differences in turbulence characteristics especially regarding the role of large coherent structures raupach s 1991 a was found to be an extremely volatile constant making the modified charnock relationship most suitable for predicting aerodynamic roughness lengths over movable sand beds

venus gravity a high resolution map the doppler data from the radio tracking of the pioneer venus orbiter pv0 have been used in a two stage analysis to develop a high resolution map of the gravitational potential of venus represented by a central mass and a surface mass density the two stage procedure invokes a kalman filter smoother to determine the orbit of the spacecraft and a stabilized linear inverter to estimate the surface mass density the resultant gravity map is highly correlated with the topographic map derived from the pv0 radar altimeter data however the magnitudes of the gravity variations are smaller than would be expected if the topography were uncompensated indicating that at least partial compensation has taken place

effect of the brittle ductile transition on the topography of compressive mountain belts on earth and venus the coulomb critical taper model has been very successful in explaining the large scale topography of a number of terrestrial accretionary wedges however this model is limited to cases of purely brittle frictional deformation in this paper we extend the range of applicability of the critical taper model by explicitly including the effects of temperature dependent ductile deformation the new model includes temperature dependent power law flow an assumed velocity field and linear thermal gradients in the atmosphere and within the crust flexural isostasy is also incorporated so that the decollement geometry is computed as a response to the applied load of the wedge material we assume that ductile deformation within the decollement zone is controlled primarily by diffusion flow whereas ductile deformation within the wedge itself is controlled by dislocation creep the topographic profiles predicted by the model are very similar to those of a number of fold and thrust belts on both earth and venus a typical wedge profile includes three distinctive topographic regions a narrow taper toe where both the wedge and the decollement zone deform in a brittle frictional manner a region of relatively steep slope where the wedge base deforms ductilely and the decollement zone is still frictional and a flat plateau region where both the wedge base and the decollement zone are deforming by ductile flow we have applied the model to two fold and thrust belts on venus maxwell montes and uorsar rupes and to the andes on earth and we find good agreement between observed and predicted topography using reasonable parameter values the model accounts for the observed positive correlation between relief and elevation of venusian fold and thrust belts on the basis of different thermal environments at different elevations it is also able to explain the first order differences between terrestrial and venusian fold and thrust belts fundamentally this difference is due to a combination of the lower temperatures and the presence of water on earth

biotic origin for mima mounds supported by numerical modeling article i nfo mima mounds are 1 m high hillocks found on every continent except antarctica despite often numbering in the millions within a single field their origin has been a mystery with proposed explanations ranging from gla cialprocesses to seismic shaking one hypothesisproposes that mounds innorth america are built by burrowing mammals to provide refuge from seasonally saturated soils we test this hypothesis with a numerical model parameterized with measurements of soil transport by gophers from a california mound field that couples animal behavior with geomorphic processes the model successfully simulates the development of the mounds as well as key details such as the creation of vernal pools small intermound basins that provide habitat for en demic species furthermore we demonstrate that the spatial structure of the modeled mound fields is similar to actual mound fields and provides an example of self organized topographic features we conclude that scaled by body mass mima mounds are the largest structures built by nonhuman mammals and may provide a rare example of an evolutionary coupling between landforms and the organisms that create them

bottom currents near a small hill on the maderia abyssal plain abstract near bottom currents at depths in exceeds of 5000 m have been measured in the great meteor east study area near 31 30 n 25 w over a 3 year period the sites selected were on top of a small abyssal hill on its flank and on the abyssal plain 30 km distant from the hill the magnitude of the mean current 10 m above the seabed was 1 2 cm s 1 but its direction was quite different at the three sites and reflected the presence of a clockwise vortex trapped over the hill on the plain the mean flow direction was to the west and directly opposed to that furnished by a spiral analysis of the density field it is suggested that time dependent variations in the large scale density field are more important than hitherto supposed for periods greater than 120 days the variance of the current on the plain is concentrated in the east component and for periods 50 120 days the variance is concentrated in the north component fluctuations propagate westward at speed 1 10 cm s 1 but are more complex than bar

isostatic compensation of equatorial highlands on venus abstract spherical harmonic models for venus global topography and gravity incorporating magellan data are used to test isostatic compensation models in five 30 30 regions representative of the main classes of equatorial highlands the power spectral density for the harmonic models obeys a power law scaling with spectral slope 2 brown noise for the topography and 3 kaula s law for the geoid similar to what is observed for earth the venus topography spectrum has lower amplitudes than earth s which reflects the dominant lowland topography on venus observed degree geoid to topography ratios gtrs on venus are significantly smaller than degree gtrs for uncompensated topography indicative of substantial compensation assuming a global airy compensation most of the topography is compensated at depths greater than 100 km suggesting a thick lithosphere on venus for each region considered we obtain a regional degree of compensation c from a linear regression of bouguer anomaly versus bouguer gravity data geoid anomaly n versus topography variation h data for each sample were compared in the least squares sense to theoretical correlations for pratt airy and thermal thinning isostasy models yielding regional gtr zero elevation crustal thickness h and zero elevation thermal lithosphere thickness y l 0 respectively we find the regional compensation to be substantial c 52 90 and the h n data correlations in the chosen areas can be explained by isostasy models applicable on the earth and involving variations in crustal thickness airy and or lithospheric thermal thinning thickness however a thick crust and lithosphere y l 0 300 km must be assumed for venus compensation mechanisms are distributed as follows for the high plateaus of ovda gtr 9 m km and thetis gtr 11 m km mainly airy isostasy with h 50 60 km for the chasmata area sample gtr 15 m km airy with perhaps a thermal component finally a very strong thermal component for the broad swells of beta gtr 32 m km and atla gtr 25 m km with the lithosphere thinned to 100 km aphrodite terra which shows a marked eastward increase in pratt and airy model parameter values could be a candidate for thermal isostasy decay from east to west

the morphology and evolution of coronae on venus coronae on venus are large circular to ovoidal surface features that have distinctive tectonic volcanic and topographic expressions they range in diameter from less than 200 km to at least 1000 km new data from the magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail typical coronae are dominated by concentric tectonic features and have a raised rim a central region higher than the surounding plains but in many instances lower than the rim and commonly a peripheral depression or moat some coronae also show significant amounts of radial tectonic structure and in most cases this predates the concentric features in addition there are other features on venus recognized for the first time in magellan data that consist of domical rises with intense radial tectonic patterns and little or no concentric structure all of these features commonly are associated with moderate to large quantities of volcanism in fact some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography we explore a model of corona formation that links these forms into a genetic sequence the model begins with the ascent of a mantle diapir upward mantle flow driven by its ascent forces the lithosphere above the diapir upward producing a gentle dome with a radiating pattern of extensional fractures as the diapir impinges on the underside of the lithosphere it flattens and spreads transforming the uplift to a more flat topped shape in this flattened near surface configuration the diapir can cool rapidly with the resultant loss of buoyancy the raised plateau can relax to form a central sag a raised rim and a depressed moat concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs our analyses to date suggest that this scenario is broadly consistent with many of the coronae on venus however there is enormous diversity in corona morphology and features are present that require substantial deviations from this simple model in particular some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the venusian lithosphere

40ar 39ar ages of six apollo 15 impact melt rocks by laser step heating we have obtained 15 high resolution 21 51 step 40ar 39 age spectra on six apollo 15 impact melt rocks of different compositions using a continuous laser system on submilligram subsamples and on single crystal plagioclase clasts four of the six samples gave reproducible age spectra with well defined intermediate temperature plateaus over 48 or more of the 39ar released the plateaus are interpreted as crystallization ages samples 15304 7 69 15294 6 21 and 15314 26 156 gave virtually identical plateau ages whose weighted mean is 3 870 6 ma these three melt rocks differ in composition and likely formed in three separate impact events sample 15356 9 gave replicate plateau ages that average 3 836 12 ma and date a fourth and younger impact event the age spectra for samples 15308 9 and 15414 3 36 increase with increasing increment temperature and may have been formed in or affected by impacts at about 2 700 ma and 3 870 ma respectively so far there continues to be no convincing evidence in the lunar record for impact melts older than about 3 9 ga

venus topography and kilometer scale slopes during the first 8 months of the magellan mission the radar altimeter has made some three million measurements of the surface of venus covering the latitude range from 85 n to 80 s methods involving range correlation doppler filtering multi burst summation and range migration are used to focus the observations and to achieve high surface resolution results are presented as maps of the global distribution of topography meter scale roughness and power reflection coefficient the results are similar to those reported in previous experiments surface heights exhibit a unimodal distribution with more than 80 of the surface lying within 1 km of the 6051 84 km mean radius but the higher resolution of the magellan altimeter has disclosed several surprisingly steep features e g the southwest face of the maxwell montes the southern face of the danu montes and the chasmata to the east of thetis regio where average kilometer scale slopes of greater than 30 are not uncommon this conclusion is corroborated by close inspection of synthetic aperture radar imagery

ventifacts on earth and mars analytical field and laboratory studies supporting sand abrasion and windward feature development abstract terrestrial ventifacts rocks that have been abraded by windblown particles are found in desert periglacial and coastal environments on mars their abundance suggests that aeolian abrasion is one of the most significant erosional processes on the planet there are several conflicting viewpoints concerning the efficacy of potential abrasive agents principally sand and dust and the relationships between wind direction and ventifact form our research supported by a review of the literature shows that sand rather than dust or other materials is the principle abrasive agent on earth and mars relative to dust sand delivers about 1000 the energy onto rock surfaces on a per particle basis even multiple dust collisions will do little or no damage because the stress field from the impact is much smaller than the spacing of microflaws in the rock the abrasion profiles of terrestrial ventifacts are consistent with a kinetic energy flux due to saltating sand not airborne dust furthermore scanning electron microscope images reveal surfaces that are fractured and cleaved by sand grain impact with respect to their distribution ventifacts are found in regions that contain sand or did so in the past but are not found where only dust activity occurs contrary to some published reports our evidence from field studies analytical models and wind tunnel and other experiments indicates that windward not leeward abrasion is responsible for facet development and feature formation pits flutes and grooves leeward abrasion is confined to fluvial conditions in which the high viscosity and density of water are able to entrain sand size material in vortices therefore ventifacts and abraded terrain provide an unambiguous proxy for the direction of the highest velocity winds and can be used to reconstruct palaeowind flow

evolution of the yardangs at rogers lake california yardangs are streamlined wind eroded hills common to most deserts yardangs at rogers lake mojave desert california have streamlined forms characteristic of objects eroded by moving fluids a teardrop shape that approaches an ideal 1 4 width to length ratio in wind tunnel simulations miniature forms of various shapes changed sequentially by 1 erosion of the windward corners 2 erosion of the windward slope 3 erosion of the leeward corners and flanks and 4 erosion of the leeward slope prominent mechanisms in yardang evolution apparently are abrasion at the windward end and deflation and reverse air flow near the middle and at the downstream end width to length ratios of yardangs are grossly similar to those of some fluvial and glacial streamlined landforms the low kinetic energy of wind relative to ice and water the erosional resistance to wind of most rocks the rarity of long term unidirectional winds and the presence of running water topographic roughness and vegetation all limit the abundance of yardangs

terrestrial and planetary gravity fields the determination of the gravity field together with knowledge of the surface topography provides one of the primary means of inferring the density structure and dynamics of planetary interiors phillips and lambeck 1980 hager 1985 after removing the topographic gravity signal the distribution of internal density anomalies caused by thermal or compositional differences can be studied gravity field models can also be used to study the compensation of surface topography which can provide information on the mechanical properties and state of stress of the lithosphere the determination of these models has been accomplished using a wide variety of different measurement types and solution techniques the near earth measurement types can be divided into satellite tracking measurements surface gravity measurements and satellite altimeter measurements in both near earth and interplanetary contexts tracking data are used to measure gravitational perturbations affecting satellites with the accuracy and spatial temporal distribution of the data being the most important factors in the resulting field accuracy and resolution surface gravity data provide a more direct measurement of the gravity field but acquiring data uniformly over the earth has always been difficult satellite altimetry provides precise measurements of the marine gravity field provided that satellite orbit errors and non geoidal sea surface height variations can be adequately modeled comprehensive gravity field solutions must incorporate these disparate data types in order to estimate mathematical parameters describing the gravity field such as spherical harmonic coefficients the proper combination of these data along with the desire to acquire an accurate representation of the model errors require that complex procedures be developed for computing these solutions

the spectra of the topography of the earth venus and mars the global topography of the terrestrial bodies has been more or less determined from past planetary missions and from ground measurements and the behaviour of their spectra studied over the low frequencies only except the earth s case we here perform new spherical harmonic analysis of the topography of the earth venus and mars by using existing sometimes recently made available data sets to the highest possible degree and order so as to establish the decay of these spectra over a larger range of frequencies than before study the limit of validity of the used data sets and provide some new inputs to further geophysical analysis

stress analysis of tellus regio venus based on gravity and topography comparison with venera 15 16 radar images the tectonics of the tellus regio highland of venus have been examined using pioneer venus altimetry and gravity data and venera 15 16 radar images a thin elastic shell model has incorporated the pioneer venus data to calculate both global long wavelength and regional shorter wavelength stresses for various assumed values of crust lithosphere and mantle thickness and modes of compensation the effects of these parameters on the calculated stress magnitudes have been examined the model also allows calculation of crustal thickness and mantle density anomalies the resulting calculated stress fields have been compared to the surface morphology observed in the venera 15 16 radar images and interpreted in terms of the stress history the best fitting parameters are consistent with minor amounts of lithospheric flexure being necessary to produce the observed surface features the highland region is underlain by a thick 15 20 km crust which is responsible for most of the topographic support a positive density anomaly exists below this crust the denser mantle may represent the down welling portion of a flow field in which the horizontal flow feeding the downwelling has entrained mobile crustal material dynamically maintaining a thickened crustal root near the stagnation point of the flow below tellus regio this hot crustal material may have intruded the overlying cooler crust causing lithospheric inflation and surface volcanism this scenario is in agreement with the available gravity and radar data and can explain the general surface morphology of tellus regio

the spin and inertia of venus one of the most remarkable rotational features of venus is the presence of a relatively large offset between spin and maximum inertia axis according to the most recent gravity field models this offset may amount to 0 5 considerably larger than the one which characterizes the earth s chandler wobble 0 3 at present it is not clear whether the offset of venus is the result of the limited resolution of the gravity models or alternatively it may be regarded as an evidence in support to the existence of a wobble purpose of this note is to demonstrate on quantitative grounds that an offset between spin axis and maximum inertia axis is to be expected on the time scales of mantle convection due to the peculiar rotational characteristics of venus we also show that the inertia perturbation associated to the rising of hot thermal plumes from the core mantle boundary to the surface is large enough to maintain offsets ranging between 0 1 and 1 of course this does not preclude that other processes such as venusquakes or periodic atmospheric variabilities may contribute significantly to the observed offset on shorter time scales

comparison of observed and predicted gravity profiles over aphrodite terra venus the authors compare observed pioneer venus orbiter pvo gravity profiles over aphrodite terra to profiles predicted from models of thermal isostasy mantle convection and airy compensation similar approaches are used in order to investigate how well the models can be distinguished with the pvo data topography profiles across aphrodite are compared to model spreading ridge profiles in order to further assess this model airy compensation depths and convection layer thicknesses are greater under eastern aphrodite than western aphrodite compensation depths in the east are greater than most estimates of lithospheric thickness suggesting that this part of the ridge is dynamically supported in parts of western aphrodite the spreading ridge model gravity provides a better fit to the data than either airy compensation or mantle convection best fit spreading rates are between 0 3 and 1 6 cm yr airy compensation and mantle convection cannot be distinguished in most places using only pvo data

shapes of streamlined islands on earth and mars experiments and analyses of the minimum drag form streamlined islands in rivers and comparable landforms in the channeled scabland of washington and in the outflow channels on mars have shapes that are closely similar to symmetrical airfoils with length width ratios averaging 3 to 4 a review of measurements of drag on airfoils demonstrates that a minimum total drag is achieved at such length width ratios supporting the hypothesis that these islands acquired shapes that minimized the drag or resistance to the flowing fluid when they were formed this minimum drag is achieved by the reduction of the wake while maintaining a small skin friction flume experiments were undertaken with model islands to examine how they achieve their streamlined form when the island is not fully submerged a combined bow wave and hydraulic drop play a major role in their erosion but most of the streamlining is brought about by deposition filling the wake region when the island is submerged the flow over the top can become supercritical producing a hydraulic jump in the island lee and focusing the erosion in that region rapidly eroding the island to a streamlined shape these experiments are consistent with the prototype streamlined islands duplicating many of their features

yardang evolution from maturity to demise yardangs are enigmatic wind parallel ridges sculpted by aeolian processes that are found extensively in arid environments on earth and mars no general theory exists to explain the long term evolution of yardangs curtailing modeling of landscape evolution and dynamics of suspended sediment release we present a hypothesis of yardang evolution using relative rates of sediment flux interyardang corridor downcutting yardang denudation substrate erodibility and substrate clast content to develop and sustain yardangs corridor downcutting must exceed yardang vertical denudation and deflation however erosion of substrate yields considerable quantities of sediment that shelters corridors slowing downcutting we model the evolution of yardangs through various combinations of rates and substrate compositions demonstrating the life span suspended sediment release and resulting landscape evolution we find that yardangs have a distinct and predictable evolution with inevitable demise and unexpectedly dynamic and autogenic erosion rates driven by subtle differences in substrate clast composition

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core

how do vegetation bands form in dry lands insights from numerical modeling and field studies in southern nevada usa 1 vegetation bands are periodic bands of vegetation separated by interband spaces devoid of vegetation oriented parallel to the topographic contour in some gently sloping arid to semiarid environments models of vegetation band formation attribute their formation to positive feedbacks among vegetation density soil porosity permeability and infiltration rates here we present an alternative model based on field measurements at our study sites in southern nevada in this model interband spaces between vegetation bands form because topographic mounds beneath vegetation bands detain water upslope from vegetation bands leading to hydrologic and sedimentologic conditions that inhibit the survival of plants in interband spaces we used terrestrial laser scanning tls to create high resolution 10 cm2 pixel raster data sets of bare earth topography and canopy height for four study sites analyses of the tls data in addition to measurements of soil shear strength and particle size document the potential for detention in interband spaces and a near inverse proportionality between band spacing and regional slope we describe a cellular automaton model herein called model 1 for vegetation band formation that includes just two user defined parameters and that generates vegetation bands similar to those at our field sites including the inverse proportionality between spacing and regional slope a second model model 2 accurately predicts the width of vegetation bands in terms of the number and spacing of plants and the geometry of individual plant mounds we also present a gis based analysis that predicts where bands occur within a region based on topographic and hydroclimatic controls