venusian channel formation as a subsurface process 1 we constructed detailed geologic maps of channels within three widely separated regions using magellan synthetic aperture radar sar imagery and altimetry radar data in order to understand the operative processes involved in venusian channel formation channels in each region cut topographic highs including ridges and shield edifices with no deflection of the channel course by surface topography we argue that these relationships are difficult to reconcile with the widely held view that venusian channels represent surface processes whether constructional or erosional we conclude that the channels evolved through subsurface fluid flow which involved local stoping and transport of overlying material that is these channels were carved from below rather than from above we postulate that at least some venusian channels form because of subsurface fluid flow along a shallow crustal interface that forms the boundary between overlying low backscatter surface materials and underlying basal materials fluid movement along the interface may facilitate piecemeal stoping and erosion of the local surface materials from below eventually resulting in the formation of channel traces exposed at the surface the low backscatter material which hosts the channels in this study also hosts abundant coalescing shields and associated deposits which may represent shield terrain

sequence of tectonic deformation in the history of venus evidence from global stratigraphic relationships analysis of local and regional stratigraphic relationships has permitted the assessment of the nature of tectonic structures and their distribution throughout the observed history of venus spanning the past several hundreds of millions of years we find that shortening characteristic of intensely deformed tessera terrain gave way to widespread distributed fracturing and extension within the tessera and early post tessera volcanic plains this phase was followed by distributed deformation of the widespread younger volcanic plains involving compression to form broad ridge belts and closely following and sometimes simultaneous extension to form fracture belts emplacement of the most areally extensive regional volcanic plains exposed today was followed by widely distributed compression forming wrinkle ridges on the plains surfaces focused extensional deformation localized linear rift systems dominated the latest stages these major temporal trends appear well established from a stratigraphic point of view and provide guidelines and constraints on models for the geologic history of venus

a method to retrieve the spectral complex refractive index and single scattering optical properties of dust deposited in mountain snow dust deposition to snow can have regionally important climatic and hydrologic impacts resulting from direct reduction of surface albedo and indirectly from the initiation of snow albedo feedbacks modeling the radiative impacts of dust deposited in snow requires knowledge of the optical properties of both components here we present an inversion technique to retrieve the effective optical properties of dust deposited in mountain snow cover from measurements of hemispherical dust reflectance and particle size distributions using radiative transfer modeling first modeled reflectance is produced from single scattering properties modeled with mie theory for a specified grain size distribution over a range of values for the imaginary part of the complex refractive index k 0 00001 0 1 then a multi step look up table process is employed to retrieve k and single scattering optical properties by matching measured to modeled reflectance across the shortwave and near infrared the real part of the complex refractive index n for dust aerosols ranges between 1 5 and 1 6 and a sensitivity analysis shows the method is relatively insensitive to the choice of n within this range 1 525 was used here using the values retrieved by this method to update dust optical properties in a snow aerosol radiative transfer model reduces errors in springtime albedo modeling by 50 70

evidence of fluid flow in microfractures in geopressured shales petrographic and microprobe analyses of geopressured shales from the oligocene frio formation brazoria county texas provide direct evidence of hydraulic fracturing and significant fluid flow in the shales the optical continuity of the fracture margins and their higher al si ratios indicate that the fracture margins have undergone more complete alteration of smectite to illite than the interiors of the fracture blocks crosscutting relationships between calcium sulfate and the organic material that fills the fractures indicate that calcium sulfate was emplaced before the organic material and thus was precipitated in the subsurface the calculated fracture permeabilities of the shales on the order of 10 13 m2 are similar in magnitude to that of he frio sandstone reservoirs which suggests that these fractured shales could support flow comparable to that in the sandstones fracture fill deposits or collapse could in turn reduce this flow in addition the more extensive alteration of the fracture margins suggests that fluid in these shales flowed preferentially through the microfractures rather than through the matrix

impact craters and venus resurfacing history venusian impact crater size frequency distributions locations and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism an atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km the size frequency distribution is close to the atmosphere free case with this result and assuming that the surface records a crater production population a catastrophic resurfacing model crm an age of cessation of rapid resurfacing of 500 ma is obtained crater locations are widely dispersed across venus and the hypothesis that they are completely spatially random csr cannot be rejected however craters that show embayment by plains materials or modification by throughgoing faults i e tectonized are preferentially found in areas with relatively few craters overall the primary region where these modified craters are found is the aphrodite volcanotectonic zone extending from ovda regio on the west to the region east of atla regio these results together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy imply that a range of surface ages are recorded by the impact crater population e g the aphrodite zone is relatively young an end member model equilibrium resurfacing model erm was developed to quantify resurfacing scenarios in the erm venus has been resurfacing at an average rate of approximately 1 km2 yr 1 however the crm and erm are idealized end member representations of possible resurfacing histories for both models the resurfacing rate can be expressed as the product of resurfacing patch area a normalized by planetary surface area and the frequency of resurfacing events numerical simulations of resurfacing showed that there are two solution branches that satisfy the csr constraint a 0 1 74 diameter circle the former range corresponds to resurfacing diameters smaller than the average intercrater distance whereas the latter is associated with large infrequent events resurfacing 10 of the planet every 50 ma to 100 every 500 ma the observed fraction of embayed and tectonized craters further constrains values of a and only values near 0 0003 are admissible the resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs in any given geological episode in only a limited number of regions on the planet

retreating alpine glaciers increased melt rates due to accumulation of dust vadret da morteratsch switzerland the automatic weather station aws on the snout of the vadret da morteratsch switzerland has delivered a unique 12 year meteorological dataset from the ablation zone of a temperate glacier this dataset can be used to study multi annual trends in the character of the surface energy budget since 2003 there has been a substantial darkening of the glacier tongue due to the accumulation of mineral and biogenic dust the typical surface albedo in summer has dropped from 0 32 to 0 15 we have analysed the implications of the lowered albedo for the energy balance and the annual ablation for the 4 year period 2003 06 the decreased albedo caused an additional removal of about 3 5 m of ice calculations with an energy balance model show that the same increase in ablation is obtained by keeping the ice albedo fixed to 0 32 and increasing the air temperature by 1 7 k our analysis confirms that for retreating glaciers the deposition of dust from exposed side moraines on the glacier surface constitutes an important feedback mechanism the mineral dust stimulates the growth of algae lowers the surface albedo enhances the melt rates and thereby facilitates the further retreat of the glacier snout

surface modification of venus as inferred from magellan observations of plains in sedna planitia clear stratigraphic relations can be discerned among volcanic flow units young flows exhibit sar specific cross section values similar to fresh terrestrial basalt flows whereas older flows exhibit backscatter signatures similar to degraded terrestrial basalt flows total degradation of 1 m depth over 0 6 b y is inferred for the sedna area from radar signatures impact crater abundances and ejecta superposition relations with respect to volcanic flow units analyses of parabolic ejecta deposits associated with the crater stuart imply that the material is typically centimeters in thickness a relatively small fraction 10 of venusian impact craters exhibit prominent parabolic ejecta deposits these craters are interpreted to be relatively young and parabolic deposits are interpreted to be dispersed by aeolian activity over at least tens of millions of years the inferred dispersal rate 10 3 m yr is too low to produce the degradation of flows at sedna planitia and it is concluded that the dominant flow modification process is in situ weathering in addition elevation dependent weathering is inferred in western ovda regio where plains above 6054 km radius have enhanced reflection coefficients as compared to adjacent plains at lower elevations the inferred rate of generation of high reflection coefficient materials is no more than 10 2 m yr based on the inability of aeolian activity to cover high reflectivity surfaces with normal reflection coefficient materials and the ubiquitous nature of high reflectivity surfaces at high elevations surface modification rates on venus are orders of magnitude lower than on earth venusian rates are also much lower than the inferred rate of aeolian dispersal of friable materials on mars but are comparable to the estimated rate of weathering and erosion of martian bedrock low surface modification rates imply that it will be possible to determine regional scale age variations on venus based on the degree of preservation of volcanic landforms and microwave signatures

wrinkle ridges stress domains and kinematics of venusian plains wrinkle ridges are nearly ubiquitous landforms on the plains of venus by analogy with similar structures on other planets venusian wrinkle ridges are inferred to trend normal to the direction of maximum principal compression in the crust an inference that is verified by geometrical relationships with positive and negative relief features on venus because plains are the dominant terrain on venus wrinkle ridges provide an excellent opportunity to determine the orientations of shallow crustal principal stress trajectories over most of the planet in most places there are two or more sets of wrinkle ridges and commonly one of these persists over a very large area defining a regional stress domain intersection relationships indicate that these domains differ in age

kinematic analysis of radial structures around irnini mons venus abstract radial structures to the north east of irnini mons are discernable as ridges at the full magellan image resolution of 75 m pixel using fmap tiles while a magma source modeled as a pressurized hole in an elastic plate usually allows for only extensional radial structures the perturbation of regional stresses around the hole can explain the presence of radial ridges sixteen numerical models were run to see what effect changes in magma pressure and regional stresses had on the magnitude of the maximum principal stresses and the orientation of potential features around a hole in a plate these models were arranged into four groups based on model input and setup 1 uniaxial compressive stress 2 uniaxial tensile stress 3 biaxial stress and 4 multiple holes nine of the sixteen models result in radial ridges in the orientation observed around irnini mons all of these models incorporate a regional n s compression flows from irnini mons are superimposed on an older regional plains material deformed by a regional set of east west trending wrinkle ridges implying a regional north south compression affected the area the existence of radial ridges on the irnini flows implies that the regional n s compression that caused the e w trending wrinkle ridges was still active during the formation of irnini mons however the magnitude of the regional compressive stress required for radial ridge formation could be 1 5 mpa less than the compressive strength of the material indicating that radial ridges could form after wrinkle ridge formation has ceased

determining heights and slopes of fault scarps and other surfaces on venus using magellan stereo radar this report presents relationships and techniques for determining the heights and slopes of discretely dipping surfaces such as normal fault scarps on the surface of venus from measurements of their widths in magellan stereo synthetic aperture radar sar images these surfaces are clearly recognizable as distinct bands of increased or decreased radar backscatter relative to flat lying areas in the 120 to 280 m resolution magellan sar images but are not generally imaged in the 10 km resolution magellan altimetry our methods take into account radar distortion effects and allow one to determine whether a slope is foreshortened laid over elongated or in radar shadow the techniques make use of graphs constructed for the magellan incidence angle profiles so that investigators can determine the local height and slope of an individual surface in a straightforward manner but the techniques are applicable to any stereo radar data set if the radar incidence angles are known additionally the techniques can be used to improve digital elevation models constructed using radar stereoscopy in areas that have steep local terrain where stereoscopic fusion may be impossible due to the effects of layover or radar shadow using these techniques we show that there are normal fault scarps on venus with heights in the 700 m range that have remarkable topographic slopes of close to 60 something unheard of on earth and which suggests a high effective cohesive strength of the venusian crust

radiating graben fissure systems in the ulfrun regio area venus abstract radiating graben fissure systems are common on venus most are thought to be underlain by mafic dykes fed by centrally located magmatic centres from previous work it has been shown that these magmatic plumbing systems can extend out up to 2000 km or more and that interaction between neighbouring systems can provide insight into the relative chronology of their magmatic centres systematic mapping of graben fissure systems has potential as a tool for regional magmatic chronology and correlation on venus this methodology is applied to the ulfrun regio area 200 240 e 0 25 n where we mapped 47 000 graben and fissures from these 66 radiating systems comprised of 13 000 individual graben and fissures and having radii of up to 2000 km have been identified and are interpreted to be underlain by dyke swarms focussed on magmatic centres cross cutting relationships among these systems and with the hecate chasma rift zone have been examined to provide a relative chronology for the magmatic centres two trends emerged a an apparent younging from the southwest to northeast of the study area and b a cluster of older ages in the southwest linked to the atla regio mantle plume

hierarchical clustering of pit crater chains on venus composed of a series of circular to elliptical bowl shaped depressions pit crater chains are common on the surface of many of our solar system s terrestrial planets and moons using magellan synth

plains tectonism on venus the deformation belts of lavinia planitia high resolution radar images from the magellan spacecraft have revealed the first details of the morphology of the lavinia planitia region of venus a number of geologic units can be distinguished including volcanic plains units with a range of ages transecting these plains over much of the lavinia region are two types of generally orthogonal features that we interpret to be compressional wrinkle ridges and extensional grooves the dominant tectonic features of lavinia are broad elevated belts of intense deformation that transect the plains with complex geometry they are many tens to a few hundred kilometers wide as much as 1000 km long and elevated hundreds of meters above the surrounding plains two classes of deformation belts are seen in the lavinia region ridge belts are composed of parallel ridges each a few hundred meters in elevation that we interpret to be folds typical fold spacings are 5 10 km fracture belts are dominated instead by intense faulting with faults in some instances paired to form narrow grabens there is also some evidence for modest amounts of horizontal shear distributed across both ridge and fracture belts crosscutting relationships among the belts show there to be a range in belt ages in western lavinia in particular many ridge and fracture belts appear to bear a relationship to the much smaller wrinkle ridges and grooves on the surrounding plains ridge morphology tends to dominate belts that lie more nearly parallel to local plains wrinkle ridges and fracture morphology tends to dominate belts that lie more nearly parallel to local plains grooves we use simple models to explore the formation of ridge and fracture belts we show that convective motions in the mantle can couple to the crust to cause horizontal stresses of a magnitude sufficient to induce the formation of deformation belts like those observed in lavinia we also use the small scale wavelengths of deformation observed within individual ridge belts to place an approximate lower limit on the venusian thermal gradient in the lavinia region at the time of deformation

regional topographic rises on venus geology of western eistla regio and comparison to beta regio and atla regio regional topographic rises in the equatorial region of venus western eistla regio beta regio and atla regio form a unique class of structures which on the basis of geologic mapping and geophysical data are interpreted to be sites of mantle upwelling or plumes the geologically least complex of these regions is western eistla regio an elongated domical rise that contains the volcanoes sif and gula montes sif mons is a large volcano diameter 100 km whose local stratigraphy indicates that it has undergone a history marked by initial radial fracturing widespread outpouring of lava additional fracturing along its northern flank and extrusion of lava that is either extremely fluid or was erupted at a high rate of effusion sif contains a 40 km diameter summit caldera within which are nested smaller calderas and pit structures theoretical considerations suggest multiple levels of magma emplacement within the volcano with the primary reservoir residing in the crust below the edifice gula mons is made up of multiple volcanic source regions and is associated with guor linea a zone of lithospheric stretching faulting and rifting the geologically complex highland of beta regio is a tectonic junction with three arms of devana chasma intersecting at theia mons magellan data reveal 1 the details of rifting and volcanism concentrated at theia mons 2 that rhea mons is highly deformed and possesses characteristics of tesserae and whose origin as a volcano is placed in question 3 that tesserae are a major unit arrayed in a pattern around theia from east to west starting on the eastern flank of beta and continuing westward to the northern part of asteria regio 4 that the northern periphery of the highland contains coronae connected by fractures forming chains and 5 the presence of a deformed crater within devana chasma split and extended in an east west direction indicating that part of the rift has undergone as much as 20 30 extension like beta atla regio is a complex area of converging rifts five separate arms that are centered on the volcano ozza mons the magellan images show that volcanism and rifting have occurred contemporaneously at ozza with lava flows both covering and being crosscut by faults a second volcano maat mons contributes to the infilling of the rift dali chasma and does not appear to be deformed by faulting two of the rifts at atla contain chains of coronae suggesting that upwelling along linear zones of extension has occurred beta atla and western eistla are similar in that they are broad regional rises are associated with major rift zones have large apparent depths of isostatic compensation and contain large volcanoes their associations with features mapped as coronae and tesserae are quite variable suggesting that the detailed characteristics of the individual highlands are most likely linked to each areas local geologic environment

a new view of the stratigraphic history of venus abstract studies of venus using magellan data have so far generated two views about the way venus has evolved on the one hand venus has been suggested to have had a history in which there was a series of epochs each represented by a different volcanic or tectonic process on a global scale basilevsky and head 1995 j w head et al 1996 lunar planet sci 27th 525 526 p t basilevsky et al 1997 in venus ii univ of arizona press tucson this we define as a directional history on the other hand there is evidence to suggest that coronae rifts wrinkle ridges small and large edifices and large flow fields have each formed throughout the portion of venus history revealed by presently exposed rock units we propose that the plains have been built up by lavas erupted in a number of different styles each occurring throughout the history represented by the exposed stratigraphy of the planet dates derived from crater counts accumulated from the combined area of specific types of feature such as coronae e g m h price et al 1996 j geophys res 101 4657 4672 must be interpreted with care as the method is based upon the assumption that features of like morphology have the same age detailed studies from full resolution magellan data indicate that venus has had a complex history in which most geologic processes have operated in a nondirectional fashion to a greater or lesser extent throughout the planet s history

evidence for rapid regional plains emplacement on venus from the population of volcanically embayed impact craters we have reexamined the population of craters on venus believed to be embayed by lava flows and discuss the reliability of criteria for determining the volcanic embayment state of a crater presenting nonvolcanic processes which may produce similar crater morphologies we conclude that 29 craters have been embayed by lava flows from the exterior and an additional 27 craters are ambiguous cases 3 6 of the total crater population all of the embayed craters are embayed by the stratigraphically youngest lava flows except for five to 11 craters embayed by stratigraphically older regional plains deposits which are typically deformed by wrinkle ridges through geologic mapping of an area covering over 8 of the surface we examined the spatial relationships between wrinkle ridged plains and stratigraphically older plains units and estimate that a large portion of the wrinkle ridged plains are thinner than 500 m which is thin enough for the rims of the largest half of preexisting craters to be preserved there are not enough craters embayed by the wrinkle ridged plains to support a model in which the plains were erupted directly onto an ancient surface given the number of large craters embayed by thin deposits of wrinkle ridged plains we develop a simple statistical model to overestimate the number of craters on the preplains surface this model suggests that the average age of the surface beneath the plains plus the average length of time over which the plains were emplaced was of the order of tens of millions of years the short exposure age for the preplains surface indicates that some event or events must have erased the ancient population of craters from the surface before the emplacement of the wrinkle ridged plains

the global distribution of wrinkle ridges on venus abstract new digital mapping of over 65 000 wrinkle ridges of the plains of venus shows a strong correlation between the location and orientation of these long low amplitude compressive anticlines and major features of the geoid and long wavelength topography regions with wrinkle ridges occupy 43 of the plains and are strongly skewed toward low elevations and negative geoid anomalies about 93 of the wrinkle ridge plains lie below mean planetary radius and 72 have negative geoid anomalies in contrast the extensional rift zones of venus are strongly skewed toward high elevations and positive geoid anomalies thus compressive deformation dominates the topographic and geoid lows whereas extensional rifting is generally restricted to the highs these observations are consistent with geoid based stress models that predict compression in geoid lows and extension in geoid highs the orientations of wrinkle ridges are generally consistent over regions extending for 1000 10 000 km many regions have multiple sets of wrinkle ridges of different orientations reflecting multiple episodes and directions of compression however about 80 of the regions display a single dominant wrinkle ridge orientation with secondary orientations subordinate the dominant wrinkle ridge orientations in many regions follow the contours of long wavelength topography and geoid or lie along the axes of troughs in the geoid thus the maximum horizontal compression recorded by the folds is commonly parallel to the present day gradient in geoid and topography although some regions are strong exceptions perhaps reflecting changes in topography and geoid with time the dominant wrinkle ridge trends ring several major geoid and topographic swells especially western aphrodite terra and lada terra with ring diameters of 75 120 8000 13 000 km in addition there are smaller rings of wrinkle ridges around the swells in themis eistla and bell regiones especially gula mons with ring diameters of 25 45 2500 5000 km in contrast there are no rings of compressive deformation surrounding some other geoid topographic highs including beta and atla regiones which have been proposed to be the youngest swells and display the largest peak geoid topography ratios and largest geoid gradients these observations are consistent with the idea that not all the major topographic geoid swells existed on venus at the time of wrinkle ridge formation

artemis venus the largest tectonomagmatic feature in the solar system new geologic mapping reveals that artemis a unique 2400 km diameter feature on venus is much larger than previously recognized including a wide outer trough 5000 km diameter a radial dike swarm 12 000 km diameter and a concentric wrinkle ridge suite 13 000 km diameter artemis s evolution included formation of its interior and chasma accompanied by lateral propagation of radial dikes the escape of dike magma to the surface formed local cover deposits that buried parts of the remaining radial fracture suite cover deposits are cut in turn by wrinkle ridges that likely formed by coupling of convective mantle flow with the lithosphere the outer trough formed late relative to radial fractures cover deposits and wrinkle ridges we suggest that artemis represents the magmatic signature of a deep mantle plume acting on relatively thin lithosphere as such it appears to represent the largest tectonomagmatic feature in the solar system the newly recognized vast extent of artemis holds implications for the formation of giant radial dike swarms wrinkle ridge formation terrestrial planet mantle lithosphere coupling and venus s surface and geodynamic evolution

graben fissure systems in guinevere planitia and beta regio 264 312 e 24 60 n venus and implications for regional stratigraphy and mantle plumes detailed mapping in a 14 000 000 km 2 area of northwestern guinevere planitia and northern beta regio bounded by 264 312 e 24 60 n has revealed thousands of long extensional lineaments graben fissures and related fractures these can be grouped into radiating circumferential and linear systems thirty four radiating systems have been identified of which 16 have radii greater than 300 km and eight have radii greater than 1000 km twenty six linear straight systems with a length greater than 300 km have been distinguished of which six have a length greater than 1000 km linear systems are generally associated with rifts although some may represent distal portions of radiating systems in addition 19 circumferential systems some associated with coronae have been identified the distribution of each system is compared with the host geology in order to place the graben fissure systems in a regional stratigraphic framework the majority of systems are 1 younger than tesserae ridge belts and densely fractured plains 2 coeval with and in many cases define fracture belts 3 partially flooded by wrinkle ridged plains units and 4 older than smooth and lobate plains units and young rifts the inventory of radiating graben fissure systems that we catalogue represents a database of tectono magmatic centers that complements the centers defined using other criteria e g large volcanoes coronae and shield fields we have attempted to identify those systems that are underlain by dike swarms in order to evaluate their relationship to mantle plumes at least 11 of the radiating systems extend well beyond any central topographic uplift and are therefore interpreted to be underlain by dike swarms

geological evolution of venus rises plains plumes and plateaus crustal plateaus and volcanic rises major physiographic features on venus both formed over mantle plumes crustal plateaus were produced by large degrees of plume melting beneath thin lithosphere the oldest tectonic features in crustal plateaus are ribbon like troughs indicating early uplift and tensile stretching their shallow depths suggest that surface temperature there was higher in the past widespread volcanic plains derived from the broad upwellings of internally heated mantle convection were continually erupted during the time of crustal plateau formation discrete volcanic rises younger than crustal plateaus formed over a thicker lithosphere with less plume partial melting the presence of only one transitional form indicates that the lithosphere thickened rapidly thermal and magmatic models show that the formation of these major features can be tied to an expected change in mantle convective style about 1 billion years ago

contact spectroscopy for determination of stratigraphy of snow optical grain size we present a technique for in situ measurement of the vertical and spatial stratigraphic distribution of snow optical grain size with a coupled contact illumination probe and field spectroradiometer accurate measurements of optical equivalent grain size are critical for modeling radiative properties of snow such as spectral albedo and microwave emission measurements of the spectral reflectance of the snow pit surface are made at 2 cm intervals in the vertical plane under constant illumination and view geometries we invert the integral of the continuum normalization of the ice absorption feature with maximum at 1 03 mm wavelength for optical equivalent grain size using the validated model of nolin and dozier 2000 that has accuracy of 10 50 mm across the grain size range 50 900 mm results are presented for an alpine site in southwest colorado usa across the ablation season and for a greenland ice sheet site at the onset of snowmelt these results suggest that traditional measurements of grain size using a hand lens are nearly accurate only for rounded grains r 2 0 41 rmse 160 mm for polycrystals and faceted grains the hand lens approach is very inaccurate r 2 0 03 and 0 24 rmse 1206 and 1010 mm respectively we demonstrate an order of magnitude improvement in modeling of shortwave spectral albedo and net shortwave flux with contact spectroscopy measurements of grain size stratigraphy over those from a hand lens

impact crater air fall deposits on the surface of venus areal distribution estimated thickness recognition in surface panoramas and implications for provenance of sampled surface materials 1 during their formation impact craters on venus larger than 11 km in diameter acquired parabolic features composed of air fall deposits that are radar dark in magellan synthetic aperture radar images a model of the parabola planimetric shape and size the latter being a function of crater diameter was constructed from the data of campbell et al 1992 reconstructing model parabolas around all impact craters with d 11 km in the regions of the venera vega sites revealed that parabolas cover 80 90 of the surface these regions are representative of the majority of the venus surface in terms of general geology and impact crater abundance this implies that a very significant part of the surface of venus was covered at least once with parabola deposits and thus it is logical to look for them in the tv panoramas of the venus surface taken by the venera landers these panoramas show the presence of soil and layered rocks with the rocks shown by the data of several instruments consisting of porous mechanically weak material implying an altered or sedimentary in a broad sense including volcanic tuffs and impact deposits nature we propose the hypothesis that 1 the layered rocks may be the partly lithified and then partially eroded parabola deposits while 2 the soil may partly be unlithified crater deposits and partly a product of the degradation of layered rocks if this hypothesis is correct then the interpretations related to the provenance of the materials studied geochemically by the landers may need revision in particular it is very possible that deep seated 1 3 km rocks the lower units of the regional plains complex rocks of the plains basement and plutonic rocks are a part perhaps significant of the material analyzed by the landers the possible presence of this deeply derived material should also be taken into account in selecting landing sites for future missions to venus

surface formation rates and impact crater densities on venus impact crater density has been used to estimate relative ages of major venus geologic units typically based on the assumption of short globally synchronous formation periods the total number density of craters is the only parameter which may be quantified for most venus subregions in contrast to the size frequency analysis often used for other planetary studies this paper examines the statistical limits on such crater counts and the predicted crater density for various possible time varying surface formation rate functions these results show that 1 the true mean crater density is poorly constrained due to the limited sample size provided by aggregate geologic units and 2 any given crater density can be accommodated by a very short pulse of unit formation at some age t gaussian distributed formation functions of mean age t exponential functions with initiation ages between t and 2t or a host of other more complicated rate functions these results when coupled with the unknown rate of crater removal by tectonic or volcanic activity indicate that the crater counts for specific landforms on venus offer little constraint on the relative timing or duration of their development the hypothesis of globally synchronous formation of landforms such as volcanoes or coronae cannot be validated with the available crater data

displacement of sand grains in sandy gypsum crystals sand grains enclosed in gypsum crystals which grew in otherwise uncemented sand commonly either float in the gypsum or are arrayed in sand bearing and sand free zones in a manner which indicates that they were displaced during growth of the gypsum corroborative evidence of displacement of included sand grains is found in some sandy gypsum crystals from coastal sabkhas in abu dhabi u a e in these crystals dust particles which formerly coated the sand grains are strung out as festoons behind the grains and thus record the directions and extent of the displacement the direction of displacement was outwards normal to the traces of what had been former crystal faces the displacement also involved some pivoting of elongate grains which now lie with their long axes in the direction of movement

chemical dieferentiation of a convecting planetary interior consequences for a one plate planet such as venus partial melting to generate the crust of a planet creates compostionally buoyant residual mantle in the absence of mantle flow associated with plate tectonics this buoyant refractory layer may collect at the top of the mantle with important implications for the evolution of the interior and surface in this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer as the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases as less depleted mantle with less positive compositional buoyancy is added negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy the depleted layer then sinks into and mixes with the convecting interior the top of the convecting mantle then moves to a shallower depth larger degrees of melting resume and a new depleted layer accumulates this accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planet s evolution with a period of 300 500 myr on venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 myr we speculate that the mechanism described above may explain this episodic global resurfacing of venus

tessera terrain and crustal plateaus venus many workers assume that tessera terrain marked by multiple tectonic lineaments and exposed in crustal plateaus comprises a global onion skin on venus a growing body of structural mechanical magmatic gravitational topographic and geologic evidence indicates that tesserae record the local interaction of individual deep mantle plumes with an ancient globally thin venusian lithosphere resulting in local regions of thickened crust

comparative hypsometric analysis of earth and venus the comparison between earth and venus global hypsometric distributions is reinvestigated on the basis of the high resolution data provided by the recent magellan global topographic coverage of venus surface and the detailed etopo 5 topographic database for the earth s surface the study of the cumulative hypsometric curve for both planets reveals that there exists a domain of elevation for which the relationship between the elevation and the square root of cumulative area percentage is linear this domain covers about 80 of the mapped venusian surface and nearly all the terrestrial oceanic surface 55 of the earth s surface and corresponds to an elevation range of about 2000 m for both planets proposing the upper threshold elevation of this domain as a common geophysical reference level it is shown that the terrestrial oceanic peak and the venusian peak coincide very closely in addition the terrestrial and venusian hypsometric distributions for elevations lower than this reference level are quite similar in their amplitude modal distribution and pronounced symmetry the correspondence of the venusian peak with the terrestrial oceanic peak is a striking feature but it does not necessarily imply the occurrence of plate tectonics on venus the present observation is consistent with the existence of a similar thermal isostasy acting at the planetary scale it places an upper bound at 90 10 km for the modal thermal lithosphcric thickness of venus if the venus mantle thermal conditions are earth like this estimate is reduced if hotter conditions occur

lips on venus venus a planet similar to earth in heat budget lacks plate tectonics yet displays ample evidence of extensive volcanic and tectonomagmatic processes including regions large enough to be considered lips thus venus provides an excellent opportunity to examine large scale magmatic processes outside a plate tectonic framework i discuss four groups of venus largest tectonomagmatic provinces volcanic rises large coronae artemis heng o quetzalpetlatl atahensik crustal plateaus and plains with wrinkle ridges unit pwr unit pwr widely interpreted as representing catastrophic resurfacing has been suggested to be the solar system s largest lip i argue herein that these features each covering 1 million km 2 or more and recording extensive volcanic activity record different evolutionary processes including both endogenic and exogenic processes volcanic rises represent surface manifestations of deep mantle plumes on thick lithosphere large coronae may record different evolutionary paths artemis represents a plume signature on thin lithosphere more mapping is required to determine if heng o formed by endogenic or exogenic processes preliminary mapping suggests quetzalpetlatl represents a young mantle plume atahensik marks a diapiric structure though buoyancy mode is unconstrained crustal plateaus record solidification of huge lava ponds formed by massive partial mantle melting caused by large bolide impact on thin lithosphere the status of unit pwr as the largest lip within the solar system is challenged based on geological considerations and results of a growing body of geologic mapping 2007 elsevier b v all rights reserved

geology of venus mapping of a global geotraverse at 30 n latitude we mapped a swath of terrain on venus that extends circumferentially completely around the globe at 30 n latitude representing about 11 of the surface of the planet in order to address the following questions do presently defined stratigraphic units prove useful for broad application to regional and global mapping do units described in one area occur in other regions of the planet and how laterally continuous are they if similar sequences of units are found in different parts of the planet do they correlate temporally e g time correlative or do they represent similar sequences formed at different times e g time transgressive or independently locally repetitive we found that similar sequences of units are seen in different parts of the global geotraverse and that key stratigraphic units could be traced across almost the entire circumference of venus at this latitude providing evidence that these units are not only global in extent but also laterally contiguous this gives confidence that the stratigraphic sequence can be extended to other parts of venus for further testing and refinement on the basis of this mapping we find that several areas including tessera massifs and beta regio show a concentration of older units and appear to have been topographically high over much of the visible history of venus tessera terrain is not uniformly distributed in some areas where these older units are concentrated ridge belts and fracture belts are not randomly distributed throughout the area suggesting that they represent regional patterns of deformation the altitude distribution of the stratigraphic units and structures shows that the regional plains tend to be the lowest with older units tessera and earlier plains at intermediate to high elevations consistent with regional plains emplacement following the formation of the most significant regional topography sixty corona and corona like features were found in the geotraverse and the vast majority show evidence of being initiated in the earliest posttessera time early guineverian in an extensional tectonic environment few show evidence of postregional plains volcanism and no positive evidence was found for the initiation of coronae in the postregional plains period of the history of venus regional mapping shows that shield plains represent a unit occurring primarily prior to the emplacement of regional plains with wrinkle ridges early part of the middle guineverian perhaps overlapping somewhat with its lower member evidence is found for changes in styles of volcanism with time and tectonic activity changed in time in terms of pervasiveness style and areal distribution on the basis of mapping of this globally continuous swath representing about 11 of the planet we find that our analysis supports the presence of changes in the style and relative importance of different types of geological activity as a function of time rather than the nondirectional history proposed by guest and stofan 1999

contemporary geochemical composition and flux of aeolian dust to the san juan mountains colorado united states 1 dust deposition in the rocky mountains may be an important biogeochemical flux from upwind ecosystems seasonal winter spring dust mass fluxes to the san juan mountains during the period from 2004 to 2008 ranged from 5 to 10 g m 2 with individual deposition events reaching as high as 2 g m 2 dust deposited in the san juan mountains was primarily composed of silt and clay sized particles indicating a regional source area the concentrations of most major and minor elements in this dust were similar to or less than average upper continental crustal concentrations whereas trace element concentrations were often enriched in particular dust collected from the san juan mountain snowpack was characterized by enrichments of heavy metals including as cu cd mo pb and zn the mineral composition of dust partially explained dust geochemistry however based on results of a sequential leaching procedure it appeared that trace element enrichments were associated with the organic and not the mineral fraction of dust our observations show that the dust derived fluxes of several nutrients and trace metals are substantial and because many elements are deposited in a mobile form could be important controls of vegetation soil or surface water chemistry the flux measurements reported here are useful benchmarks for the characterization of ecosystem biogeochemical cycling in the rocky mountains

a high resolution record of atmospheric dust composition and variability since a d 1650 from a mount everest ice core abstract a mount everest ice core analyzed at high resolution for major and trace elements sr cs ba la ce pr nd sm eu tb dy ho er tm yb lu bi u tl al s ca ti v cr mn fe co and spanning the period a d 1650 2002 is used to investigate the sources of and variations in atmospheric dust through time the chemical composition of dust varies seasonally and peak dust concentrations occur during the winter spring months significant correlations between the everest dust record and dust observations at stations suggest that the everest record is representative of regional variations in atmospheric dust loading back trajectory analysis in addition to a significant correlation of everest dust concentrations and the total ozone mapping spectrometer toms aerosol index indicates that the dominant winter sources of dust are the arabian peninsula thar desert and northern sahara factors that contribute to dust generation at the surface include soil moisture and temperature and the long

venus s shield terrain plains planitiae or lowlands expanses of gentle long wavelength 1000 km basins cover 80 of venus s surface these regions are widely accepted as covered by volcanic fl ows although the mechanism s responsible for resurfacing remains elusive in addition a volcanic origin for the lowland surface may be open to question lowland resurfacing is typically attributed to catastrophic emplacement 10 100 m y of globally extensive thick 1 3 km fl ood type lava this model of resurfacing has been postulated on the basis of impact crater distribution taken together with a lack of obvious volcanic fl ows or edifi ces and a lack of viable alternative models ongoing geologic mapping of 15 000 000 km2 0 25n 90 150e using 225 m pixel and 75 m pixel nasa magellan sar synthetic aperture radar data indicates that small edifi ces called shields 1 15 km diameter edifi ces 1 km high play a major role in lowland resurfacing individual shields are radar smooth or rough quasi circular to circular features with or without a central pit shield shapes which have been previously documented range from shield dome or cone to fl at topped or fl at shield deposits typically coalesce forming a thin regionally extensive but discontinuous mechanically strong layer herein called shield paint shield paint conforms to delicate local topography providing evidence of its thin character and indicating generally low viscosity during emplacement shield terrain shields and shield paint covers more than 10 000 000 km2 within the study area detailed mapping of fi ve 2 2 regions using coregistered normal and inverted right and left illumination sar imagery indicates shield densities of 3500 33 500 shields 106 km 2 thus the map area hosts more than 35 000 335 000 shields shield terrain generally postdates but is also locally deformed by fractures and wrinkle ridges indicating time transgressive formation relative to local deformation and or reactivation the regional scale crust was strong throughout shield terrain formation shield terrain may extend across much of venus s surface

small volcanic edifices and volcanism in the plains of venus the most widespread terrain type on venus consists of volcanic lowland plains several styles of volcanism are represented in the plains the most extensive volcanic units consist of flood lavas the largest of which have volumes of the order of thousands of cubic kilometers as with terrestrial flood lavas they are inferred to have erupted at high effusion rates they show a range of radar backscatter characteristics indicating different surface textures and ages small edifices on the plains occur mainly in clusters associated with fracture belts the majority are shield volcanoes that may be up to a few tens of kilometers across but are generally 10 km or less in diameter volcanic cones have the same size range volcanic domes have diameters up to several tens of kilometers and volumes of the order of 100 km3 these are interpreted as being constructed of lava erupted with a relatively high effective viscosity and thus possibly composed of more silicic lava for many domes the flanks were unstable during and afte eruption and suffered gravity sliding that produced steep scalloped outer margins because of the high atmospheric pressures on venus explosive activity is less likely to occur than on earth however n a few plains areas there is evidence of pyroclastic deposits surrounding craters indicating that volatile contents in some of the magmas may be high in comparison to earth the clusters of small volcanic edifices are considered to be analogous to plains volcanism similar to that of the snake river plain of idaho there may also be analogues with terrestrial volcanic clusters associated with mid oceanic ridges

climate change as a regulator of tectonics on venus tectonics volcanism and climate on venus may be strongly coupled large excursions in surface temperature predicted to follow a global or near global volcanic event diffuse into the interior and introduce thermal stresses of a magnitude sufficient to influence widespread tectonic deformation this sequence of events accounts for the timing and many of the characteristics of deformation in the ridged plains of venus the most widely preserved volcanic terrain on the planet

geologic constraints on crustal plateau surface histories venus the lava pond and bolide impact hypotheses 1 venusian crustal plateau formation is hotly debated crustal plateaus are large 1500 2500 km quasi circular 0 5 to 4 km high plateaus that host distinctive tectonic fabrics called ribbon tessera debate centers on plateau support and fabric formation detailed geologic mapping of an 360 000 km2 region eastern ovda regio provides critical clues for plateau evolution ribbon tessera fabrics record broadly synchronous layer contraction extension and flooding as an initially strong thin layer 100 m developed across ovda as the layer underwent ductile shortening and brittle extension subsurface lava flooded structural lows with progressive deformation the layer thickened leading to formation of progressively longer wavelength structures early formed shorter wavelength structures and lava filled valleys were carried piggyback on longer wavelength folds this history indicates an extremely large strength contrast that is inconsistent with a conductive geotherm reflecting instead the presence of a near surface molten layer i propose the lava pond hypothesis ribbon tessera fabrics represent progressively solidified scum of a huge lava pond leading to crustal plateau formation i further propose that individual lava ponds formed as a result of large bolide 20 30 km diameter impact with ancient thin lithosphere causing massive partial melting in the upper mantle melt rose buoyantly to form huge 1500 2000 km diameter lava ponds completely resurfacing the local area in the upper mantle depleted residuum would be compositionally more buoyant and stronger than adjacent undepleted mantle isostatic adjustment resulted in pond uplift and plateau formation subsequent secular cooling could freeze a plateau in place or mantle convection could strip away the residuum root leading to plateau collapse

morphometry of small shield volcanoes on venus implications for the thickness of regional plains the morphometry of small volcanic shields on venus is used to assess stratigraphic relationships and the thickness of regional volcanic plains comparison of shield width and height in three different regions supports the interpretation that plains with shields form a distinctive stratigraphic unit psh that is subsequently embayed and flooded by younger regional plains with wrinkle ridges pwr leaving lava shield kipukas among the plains near the contact differences in shield morphometry in the two units show that pwr averages less than 150 m in these three embayed areas and where the underlying shields disappear the minimum thickness of pwr is likely to be in excess of 400 m this technique can be applied elsewhere on venus in order to provide improved estimates of the total volume of regional plains

evidence for rapid and widespread emplacement of volcanic plains on venus stratigraphic studies in the baltis vallis region geological mapping of the area around baltis vallis the longest 6 800 km canali type channel on venus has led to the determination of stratigraphic relations which appear to be consistent over this entire region comprising 20 x 106 km 2 or 5 of the total surface of venus the most abundant unit in the study area plains with wrinkle ridges pwr consists of two subunits separated by the formation of baitis vailis a feature which is interpreted by most workers to have formed essentially geologically instantaneously the wrinkle ridges deform both subunits of pwr while ridge belts are embayed by the pwr plains consistent age relationships between the pwr subunits wrinkle ridges baltis vallis and ridge belts along the entire 6 800 km channel length is evidence that unit pwr is a real stratigraphic marker unit and not a mosaic of plains emplaced at significantly different times at least in the 5 of the surface of venus comprising the study area the baltis stratigraphic sequence was compared with stratigraphic relations found in earlier studies of thirty six 1 000 x 1 000 km sites randomly distributed over the surface of venus basilevsky and head 1995 and was found to be comparable this supports the interpretation that these stratigraphic units represent generally globally quasi synchronous geologic events rather than the products of geologic activity in different areas of the planet at different times stratigraphic complexes and a number of structures typically formed at several different periods during the observed geologic history this model may correspond to either of two different options for the geologic history of venus 1 the proposed stratigraphic units and equivalent geologic events were generally synchronous over the whole planet or 2 the proposed stratigraphic sequence represents typical sequences of local events which occurred in different places at different times the first option is consistent with the global resurfacing model and the second is consistent with the equilibrium resurfacing model in our previous observations we never saw a case of superposition of one local stratigraphic sequence on another one with repetition of all or part of the sequence of units which would seem to be inevitable at the boundaries of neighboring local activity zones if the second scenario is correct on the basis of this we concluded that the first option consistent with the global resurfacing model agrees better with the observations than the second one however the validity of this conclusion remains tentative for other areas and requires additional testing stratigraphic studies in the area of baltis vallis is a further step in this direction because of the potential for the use of canali type channels as stratigraphic time markers

venus crater distribution and plains resurfacing models detailed analysis of the distribution of craters on venus using mth nearest neighbor analysis coupled with models based upon surface morphology constraints indicates that the hypothesis of complete spatial randomness csr cannot be rejected but is not a unique model of the observed crater distribution based on morphologic mapping the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0 5t the mean surface age of the planet this four unit plains model along with its derivatives produce test statistics that indicate such models also cannot be rejected further the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis model corresponds to venus is true is lowest for the csr model there is no particular reason to pick a csr model along with its implications for catastrophic resurfacing as a constraint on the evolution of venus and there are geological reasons to choose the multiage models we find that we cannot distinguish statistically among models that have two three or four distinct production ages within the plains however the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons first the binomial probability that such a process could exist within each of the plains units is 0 05 except the smallest and youngest unit pl1 second using a chi squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 4 indicating confident rejection of the hypothesis thus csr cannot be used as a constraint on models of resurfacing or planetary evolution of venus because of the non uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age geological and geophysical constraints provide our best clues for understanding venus

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

the geologic history of venus a stratigraphic view on the basis of regional and global stratigraphic analyses we outline the major events in the geologic history of venus determined by photogeological study of surface features because the morphological signatures of terrain emplaced prior to the time of tessera formation are not preserved the stratigraphic record presented comprises only the last 10 20 of the total history of venus the estimated range of the mean crater retention age of the surface from 200 to 1600 million years leads us to describe the timing and duration of different events in terms of fractions of the mean surface age t the beginning of the observed history of venus was characterized by intensive tectonic deformation of global or semi global scale which formed the tessera terrain termination of the compressional stage is estimated to have occurred at about 1 4t while the tensional stage lasted for another 0 1 0 2t after tessera formation several stages of extensive volcanism occurred burying vast areas of tessera and forming what are now observed as regional plains the combined duration of the emplacement of these plains is estimated to be about 0 2 0 3t with an implied average global rate of volcanism of a few cubic kilometers per year regional plains forming materials can be subdivided and are separated from each other and from underlying and overlying units by unconformities these unconformities are formed from oldest to youngest by tessera forming deformation dense fracturing broad ridging and finally wrinkle ridging these tectonic episodes are interpreted to be generally globally synchronous and to represent successive episodes characterized by the dominance of compression then tension then again compression and finally tension the last global scale tectonic episode extensive wrinkle ridging happened at about time t which was very close in time to the emplacement of the most areally abundant plains unit this marked the transition to the present stage of the history of venus which is characterized by a predominance of regional rifting and related volcanism this stage appears to have lasted from about time t to the present making it the longest time duration among the stratigraphic units considered although the resulting tectonic and volcanic features and deposits cover only 10 20 of the surface of venus these observations mean that the general intensity of tectonics and the flux of volcanism a few tenths of a cubic kilometer per year in this latest period were much lower than those in earlier times in summary the morphologically observable part of the history of venus was characterized by two key characteristics that stand in contrast to the comparable period of earth history approximately the phanerozoic when global geodynamic processes were dominated by plate tectonics 1 venus shows no signature of plate tectonics instead its global tectonic environment passed from an initial dominance of compression through tension then again compression and finally tension with the density of deformational structures and the strain rate declining with time 2 in the beginning of this period of time on venus plains forming volcanism occurred at a rate comparable to volcanism at mid ocean ridges but was emplaced in an entirely different style the predominant component of volcanism on earth during this time was the extrusive volcanism at mid oceanic ridges for the last few hundred million years venus has been dominated primarily by rift associated volcanism emplaced at a production rate comparable to or even lower than present intraplate volcanism production rates on earth

capillary geochemistry in non saturated zone of soils water content and geochemical signatures abstract the unsaturated zone uz retains aqueous solutions against gravity by capillary forces this suction state corresponds to a decreasing internal pressure of the water which modifies its thermodynamic properties accordingly the speciation of solutes and the solubility of solids and gases in such capillary solutions change the volumetric capillary water content of the soil at high suction can be calculated extrapolating the water retention curves wrc with the rossi nimmo model interestingly several tens of liters per cubic meter of soil can be thus suctioned a sufficiently large volume to support that 1 capillary water is not restricted to nanosized pores which means it disobeys the young laplace law and is metastable with respect to vapor superheating and 2 the geochemistry of capillary solutions might significantly influence the subsurface mass transfer two field situations are here interpreted using the capillary thermodynamic properties 1 the trapping of sand grains during the growth of desert roses gypsum and 2 the development of abnormal paragenetic sequences in some saprolites the capillary approach is extended to the soil solids so that the micro mineralogy can be explicitly though sketchily integrated in the calculations the key conclusion is that capillarity changes the saturation indexes and so the reaction rates at given solution composition in a way consistent with the field observations this perspective amounts to geochemically distinguishing the capillary and percolating solutions which is interestingly analogous to the immobile and mobile water distinction already often integrated in uz flow models

geologic evolution of southern rusalka planitia venus geologic mapping of southern rusalka planitia venus reveals interactions of volcanism tectonism and topography we recognize three regional plains units prr1 prr2 and prr3 based on crosscutting structural relations embayment patterns radar brightness and surface roughness data delineation of secondary tectonic structures allows us to constrain the relative temporal relations between the three material units unit prrl a radar dark smooth unit exposed in local topographic highs hosts ne trending extension fractures low viscosity lava flows of prr2 the most areally extensive unit fill local topographic lows and the ne trending fractures a shield sourced lava unit prr3 overlies prr2 on the basis of embayment relations and radar brightness nw trending wrinkle ridges deform all three plains units and record regional contraction locally flood lava flows that fill ne trending fractures are structurally inverted to form short stepped ne trending wrinkle ridges map patterns indicate that prr2 comprises a thin layer 50 m thick much thinner than previous estimates of 1 3 km therefore previously proposed estimates of plains flood lava flow volumes and effusion rates are much too high the local geologic history of southern rusalka planitia is inconsistent with global stratigraphy models our study supports the view of plains evolution occurring through discrete volcanic processes working at local and regional but not global scales

ice free glacial northern asia due to dust deposition on snow during the last glacial maximum lgm 21 kyr bp no large ice sheets were present in northern asia while northern europe and north america except alaska were heavily glaciated we use a general circulation model with high regional resolution and a new parameterization of snow albedo to show that the ice free conditions in northern asia during the lgm are favoured by strong glacial dust deposition on the seasonal snow cover our climate model simulations indicate that mineral dust deposition on the snow surface leads to low snow albedo during the melt season this in turn caused enhanced snow melt and therefore favoured snow free peak summer conditions over almost the entire asian continent during the lgm whereas perennial snow cover is simulated over a large part of eastern siberia when glacial dust deposition is not taken into account

tessera terrain on venus a survey of the global distribution characteristics and relation to surrounding units from magellan data the tessera terrain on venus comprised of areas of high radar backscatter complex deformation patterns relative to other units and topography standing higher than surrounding plains covers 35 33 106 km2 about 8 of the surface of venus and is nonrandomly distributed being preferentially located at equatorial and higher northern latitudes with a distinct paucity below about 30 s individual tessera occurrences range in area from the lower limits of our measurements about 200 km2 up to the largest tessera ovda with an area of about 8 6 106 km2 or about 2 of the surface area of venus the size frequency distribution of tessera patches is strongly unimodal and skewed toward smaller sizes reflecting the great abundance of small tessera fragments modes of occurrence include 1 large clusters e g aphrodite terra and ishtar terra 2 arc like segments which may extend for thousands of kilometers and are either concave inward toward the major tessera cluster development or away from it 3 areas where tesserae are rare or absent which occur both as low lying plains e g guinevere planitia and as elevated regions e g atla regio tessera terrain has a bimodal elevation frequency distribution with the main peak at about 0 1 km and an additional peak at about 3 km above mean planetary radius in terms of number of occurrences however tesserae do not display a correlation with elevation at the global scale since small tessera patches commonly occupy low lying regions although tessera exhibit a range of gravity signatures many occurrences are interpreted to represent relatively shallow crustal levels of compensation tessera boundaries include type i sinuous embayed dominated by adjacent lava plains embaying tessera massifs 73 of total tesserae boundaries and type ii linear tectonic only a small percentage of the length of all boundary types show no lava embayment and could be interpreted as tectonically active for long periods subsequent to initial tessera formation occurrence of broad slopes of post tessera embayed plains away from tessera boundaries suggests that regional tilting occurred subsequent to final tessera deformation in some places several lines of evidence suggest the possibility that a widespread tessera like basement comprising at least 55 of the surface of venus is buried under a cover of lava plains a few hundred meters to as much as 2 4 km thick a wide variety of deformational structures and patterns is observed within the tessera including those representing extension compression shear and transpression in some cases the apparently complex patterns can be resolved into single event kinematic interpretations involving noncoaxial deformation e g itzpapalotl while in other cases polyphase deformation is more likely e g central ovda and thetis where relations can be determined stratigraphically earliest deformation within the tessera is primarily related to crustal shortening and compression phase i followed by pervasive extensional deformation commonly oriented normal to the strike of phase i features generally along the same principal stress direction phase ii evidence also exists for the contemporaneous formation of these distinctive deformation patterns lava plains within and adjacent to the tessera embay both of these fabrics but sometimes overlap in time with phase ii extensional deformation and with regional tilting tessera terrain as a geologic unit occupies the lowest portion of the stratigraphic column in all areas that we have observed an observation consistent with many other mapping studies we see no evidence for transitional stages between tessera and volcanic rises and or lowlands that might represent a long term sequence of upwelling or downwelling followed by crustal deformation and tessera formation no impact craters deformed by phase i deformation have yet been observed on tessera suggesting that phase i tessera deformation sufficiently intense to eradicate earlier impact craters ceased relatively abruptly somewhat before 300 500 m y ago however the starting time and thus duration of tessera formation is unknown on the basis of the very small number of on tessera craters deformed by phase ii extensional deformation this period probably did not last more than several tens of millions of years after the cessation of phase i little observable deformation of the tessera terrain appears to have taken place in the last several hundred million years during which time the vast volcanic plains were emplaced although tilting of early plains along some tessera margins is observed building on the global synthesis presented here future analyses of individual tessera occurrences will provide the detailed descriptions kinematic interpretations and strain histories necessary to assess and distinguish between the several catastrophic and uniformitarian models for tessera formation

implications of a global survey of venusian impact craters abstract we present a global survey of the areal distribution size frequency distribution and morphometric properties of the venusian impact cratering record we explore the resurfacing history of venus crater degradation ejecta emplacement and cratering mechanics the number of volcanically embayed and tectonically deformed craters from 0 5 to 1 0 km above mean planetary radius is disproportionately high for an otherwise crater deficient elevation range more resurfacing occurred in this range an elevation range dominated by volcanic rises rifts and coronae than elsewhere on venus although the majority of craters appear to be relatively undisturbed and have intact ejecta blankets some craters appear particularly fresh because they have radar bright floors a radar dark halo surrounding the ejecta blanket and a west facing parabola of low radar return 20 35 and 8 respectively of craters with diameters 22 6 km have these features statistical correlations support the idea that such craters are younger than the remaining crater population characteristics of ejecta deposits for venusian craters change substantially with size particularly at 20 km crater diameter which marks the transition at which the boundaries of ejecta blankets go from ragged to lobate and the slope of the ejecta distance vs diameter curve steepens we suggest that this transition may be similar to the transition from inertial granular flow to fluidized granular flow for terrestrial debris avalanches a diameter independent variation in extent of the ejecta deposits is not related to impact angle or target properties but may be related to the shape distribution and dispersion of impacting fragments secondary craters are a ubiquitous part of the ejecta blanket for craters over 50 km but occur infrequently as isolated rays about smaller craters the large distances secondary craters extend from the crater rim imply that the secondary forming projectiles traveled in an atmosphere significantly altered by the impact process comparison of complex craters found on venus with those of other planets gave results that were consistent with the idea that inter planetary differences in complex crater shape are controlled by interplanetary differences in gravity and crustal strength the interplanetary comparison indicates that venus the moon and mercury appear to have stronger crusts than do mars and ganymede callisto

daily evolution in dust and black carbon content snow grain size and snow albedo during snowmelt rocky mountains colorado light absorbing impurities lai initiate powerful snow albedo feedbacks yet due to a scarcity of observations and measurements lai radiative forcing is often neglected or poorly constrained in climate and hydrological models to support physically based modeling of lai processes daily measurements of dust and black carbon bc stratigraphy optical grain size snow density and spectral albedo were collected over the 2013 ablation season in the rocky mountains co surface impurity concentrations exhibited a wide range of values 0 02 6 0 mg g 1 pptw with 98 of mass being deposited by three episodic dust events in april even minor dust loading initiated albedo decline and the negative relationship between dust concentrations and albedo was log linear as melt progressed individual dust layers coalesced and emerged at the snow surface with minimal mass loss to meltwater scavenging the observations show that the convergence of dust layers at the surface reduced albedo to 0 3 and snow depth declined 50 faster than other years with similar depth but less dust the rapid melt led to an unexpected reduction in both grain size and density in uppermost surface layers bc concentrations co varied with dust concentrations but were several orders of magnitude lower

structural and kinematic analysis of eastern ovda regio venus implications for crustal plateau formation crustal plateaus on venus constitute one of that planet s most intriguing tectonic features they host so called tessera terrain or deformed crust exhibiting two or more sets of intersecting tectonic lineaments which comprises 8 of venus surface major questions surrounding crustal plateaus and tessera terrain include 1 what is the nature of crustal plateau deformation what structures are present and what are their spatial and temporal relations 2 what is the global distribution of tessera terrain does it form a global layer under the regional plains cropping out in crustal plateaus or is it confined to crustal plateaus and 3 what are the implications of 1 and 2 for understanding the processes responsible for formation and deformation of crustal plateaus structural and kinematic analyses reveal that deformation at ovda regio venus largest crustal plateau is characterized by linear shallow steep sided troughs ribbons long arcuate long wavelength folds short mid wavelength folds and wide complex graben temporal relations between structures lead to a three phase regional strain history consisting of 1 layer normal flattening of a thin competent membrane resulting in radially oriented distributed extension ribbon formation 2 modest contraction of a thicker layer concentric folding at plateau margins interference folding in the plateau interior and 3 limited extension localized at fold crests graben formation wavelength analysis of ribbons and folds places limits on the thickness of the competent layer during each stage of deformation and reflects an early shallow brittle ductile transition bdt that deepened with time ovda s deformation topography and gravity are all spatially correlated strongly suggesting a genetic relationship between surface deformation and processes causing crustal thickening and plateau uplift thus tessera terrain at ovda regio is likely confined to the topographically high plateau and does not extend beneath the surrounding regional plains similar patterns persist at other crustal plateaus indicating that tessera terrain is in general confined to crustal plateaus and does not form a globally continuous layer ovda s structures their spatial and temporal relations the resulting strain history and the behavior of the bdt through time support a model of mantle plume upwelling in a thin lithospheric regime for crustal plateau formation and contradict previous downwelling models similarities between structural and kinematic trends at ovda regio and other crustal plateaus lend support to this idea c 1999 academic press

poludnista dorsa venus history and context of a deformation belt 1 rolling lowlands comprise 80 of venus s surface and thus developing a geological understating of resurfacing and tectonism is critical for our understanding of venus evolution in this paper global regional and local approaches to interpreting magellan synthetic aperture radar and altimetry radar data are used to constrain modes of lowland evolution convection processes and lithospheric structure detailed geologic mapping is combined with altimetry data test models of deformation belt evolution in venus s lowlands poludnista dorsa a complex 2000 km long deformation belt is highly segmented and broadly time transgressive long wavelength deformation is spatially independent of local short wavelength deformation within the deformation belt and temporally predates regional deformation marked by wrinkle ridges a mechanically layered lithosphere of regional extent is not required by the local geologic history local lithospheric thinning or progressive regional thickening of the mechanical layer better explains the observed sequence of events

hyperspectral analysis of snow reflectance to understand the effects of contamination and grain size reflectance data for contaminated and different grain size snow were collected using a spectroradiometer ranging from 350 to 2500 nm contamination was predominantly due to soil the radiometer data were binned at 10 nm intervals by averaging and then principal component analysis shape size and strength of the absorption peak first and second derivatives were computed providing information about the effect of grain size and contamination on snow reflectance relative strength for contamination and grain size showed a distinct reverse pattern at 1025 nm after continuum removal band absorption depth at 1025 nm showed an increase with increasing snow grain size whereas the band depth was found to decrease with increased soil contamination the curve shape was right asymmetric and showed a change to left asymmetry with increase in contamination the first derivative of reflectance in the visible region showed a shift of peak due to contamination soil contamination significantly reduced the albedo of snow at a low level of contamination but showed little influence at higher level relative strength shape of curve and reflectance characteristics have shown the potential to identify the influence of contamination and grain size based metamorphism using satellite based hyperspectral remote sensing

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

beta regio venus evidence for uplift rifting and volcanism due to a mantle plume abstract geophysical data have led to the interpretation that beta regio a 2000 25000 km wide topographic rise with associated rifting and volcanism formed due to the rise of a hot mantle diapir interpreted to be caused by a mantle plume we have tested this hypothesis through detailed geologic mapping of the v 17 quadrangle which includes a significant part of the beta regio rise and reconnaissance mapping of the remaining parts of this region our analysis documents signatures of an early stage of uplift in the formation of the agrona linea fracture belts before the emplacement of regional plains and their deformation by wrinkle ridging we see evidence that the theia rift associated volcanism occurred during the first part of post regional plains time and cannot exclude that it continued into later time we also see evidence that devana chasma rifting was active during the first and the second parts of post regional plains time these data are consistent with uplift rifting and volcanism associated with a mantle diapir geophysical modeling shows that diapiric upwelling may continue at the present time together these data suggest that the duration of mantle diapir activity was as long as several hundred million years the regional plains north of beta rise and the area east and west of it were little affected by the beta forming plume but the broader area at least 4000 km across whose center northern part includes beta regio could have experienced earlier uplift as morphologically recorded in formation of tessera transitional terrain

stratigraphy of small shield volcanoes on venus criteria for determining stratigraphic relationships and assessment of relative age and temporal abundance 1 small volcanic edifices shields with a diameter less than about 20 km are common and sometimes very abundant features on the plains of venus typically they form tight or loose clusters of features known as shield fields small shields are interpreted to be formed due to small scale eruptions through numerous and distinct sources a mode of formation apparently significantly different from the mechanism thought to be responsible for the emplacement of the vast regional plains of venus did the eruption style of small shields occur repeatedly throughout the visible part of the geologic record of venus or was this style more concentrated in a specific epoch or epochs of geologic history do the clusters of shields represent localized development of sources over a thermal anomaly such as a plume or do they represent exposures or kipukas of a more regional unit or units a major step toward answering these questions is an understanding of small shield stratigraphy multiple criteria have been developed to assess the stratigraphic relationships of individual small shields and that of shield fields with the adjacent units in our analysis we expanded and developed the previous criteria and added detailed criteria to describe specific patterns of deformation within shield fields cross cutting and embayment relationships between shield fields and surrounding units we also used secondary characteristics of shield fields such as radar albedo difference changes in shield density and size etc in our study we applied these criteria and analyzed in detail stratigraphic relationships of shield fields in a random sample of features 64 fields and in the global geotraverse along 30 n 77 fields the total number of analyzed shield fields 141 represents about 22 of the general population of these features catalogued by crumpler and aubele 2000 the majority of the fields 98 or 69 predate emplacement of material of vast regional plains with wrinkle ridges fifteen fields 11 appear to be synchronous with regional plains and eleven fields 8 postdate the plains nine fields 6 display ambiguous relationships with regional plains and their relative age is uncertain and eight fields 6 represent unclear cases when fields are covered by crater related materials or by young lava flows or are not in contact with regional plains the results of our study provide evidence for a distinct change of volcanic style from the mode of formation of globally abundant small shields to the mode of emplacement of vast regional plains in many areas on venus this systematic change of volcanic style appears to be inconsistent with the nondirectional or quasi steady state character of the geologic record of venus although individual small shields were formed throughout the majority of the visible geologic history of venus in the syn and postregional plains time the small shields style of volcanism was significantly reduced in abundance the shield fields that predate regional plains do not display a strong tendency to form a single group or a few groups and can be found virtually in all places on venus we interpret this observation to mean that these shield fields were globally distributed before the emplacement of regional plains this interpretation means that the shield fields embayed by regional plains represent exposures of a specific globally widespread unit shield plains psh in contrast shield fields that postdate regional plains occur preferentially in the beta atla themis region on venus well known for its concentration of relatively young volcanic and tectonic activity the spatial association of relatively young fields with the large centers of young volcanism suggests a genetic link of these fields with the formation and development of the large scale volcanic centers the abrupt decrease of the number of shields that postdate the formation of shield plains psh strongly suggests a major change of the style of volcanism following their emplacement as a globally distributed unit