on the nature and rate of resurfacing of venus crater production and obliteration are modeled for the plains of venus using 1 the observed distribution of venus crossing asteroids and comets 2 viscous relaxation of crater topography and 3 erosion and burial by atmospheric volcanic and tectonic processes crater lifetimes are assumed to be proportional to crater depths for both classes of obliterative processes although the individual criteria vary an average crater retention age between 0 4 to 2 0 ga is estimated for plains under the assumption that craters are produced and not removed the range is driven by uncertainty in identifying degraded impact as opposed to volcanic craters on the other hand crater retention ages greater than about 1 6 ga are unlikely if viscous relaxation operates without loading of crater floors by burial our preferred model has plains subject to crater production and obliteration processes that vary over both space and time in some areas radar bright crater ejecta haloes are preserved for long periods of time because volcanism tectonism and weathering occurs at rates 1 km ga viscous relaxation has probably generated numerous shallow craters in these relatively quiescent regions in other areas volcanism and tectonism have resurfaced the terrain at rates greater than several km ga the global coverage and high resolution sar and altimetry data expected from magellan will allow testing of this model based on detailed crater observations diameter depth distributions morphologic criteria surface scattering properties and their association with volcanic and tectonic features

on the relationship between isostatic elevation and the wavelengths of tectonic surface features on venus abstract regional isostatic elevation d and the widths and spacings wavelengths of tectonic features formed due to horizontal extension and compression are observed quantities on venus that are sensitive to the thermal and compositional structure of the lithosphere to investigate the relationships between these parameters and the implications for venus lithosphere structure we formulate models in which d and are functions of surface thermal gradient dt dz 0 and crustal thickness c in models that relate to dt dz 0 and c we impose a requirement that for tectonic features with multiple wavelengths of deformation to develop the growth rates of instabilities that control the dominant wavelengths must be similar in magnitude this constrains the maximum values of c and dt dz 0 that can result in multiple tectonic wavelengths on venus to be less than or equal to those determined in our previous study c 30 km and dt dz 0 25 k km m t zuber 1987 j geophys res 92 e541 e551 we also estimate the strength of venus upper crust in areas with multiple wavelengths and obtain limits that are most consistent with a compressional origin for ridge belts which are enigmatic features that have been purported to be of both extensional and compressional origin in models that relate d dt dz 0 and c we show that in areas of venus where the upper mantle is stronger than the upper crust the spacings of short wavelength 30 km features should increase with increasing d if the change in d is due to increasing c but should decrease with increasing d if the change in d is due to increasing dt dz 0 long wavelength 30 km features are not strongly correlated to variations in d due to the effect of crustal thickness on the depth distribution of lithospheric strength which influences wavelength selection in a complex manner in areas of venus where the upper mantle is weaker than the upper crust the spacings of short wavelength features should remain constant with increasing d if the change in d is due to a change in c but should decrease with increasing d if the change in d is due to increasing dt dz 0 the results demonstrate that in shallowly compensated areas of venus that contain regularly spaced tectonic features it may be possible to distinguish whether isostatic elevation differences are a consequence of variations in crustal thickness or thermal gradient we interpret the observed relationship of with d for short wavelength tectonic features in several tessera regions to reflect crustal thickness variations

fractal analysis of venus topography in tinatin planitia and ovda regio we have applied a one dimensional fractal analysis to magellan altimetry data for two 20 20 equatorial regions on venus for the ovda regio highlands the power spectra of the altimetry data correlate well with power law fractal statistics over the full range of wavelengths considered 36 km 703 km the mean fractal dimension is dovda 1 64 for the tinatin planitia lowlands the power spectra consistently show a rollover at long wavelengths 200 km the long wavelength topography in this region either never developed or has subsequently relaxed the mean of the best fit fractal dimension for tinatin is dtinatin 1 41 the measure of roughness ra correlates well with variations in relief especially at short wavelengths 50 km our quantitative analysis of venus topography can be applied on a global scale the one dimensional method can be extended to two dimensional space yielding two dimensional maps of regional d and ra allowing a more complete use of regional two dimensional altimetry data

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

viscoelastic relaxation of topographic highs on venus to produce coronae coronae on venus are believed to result from the gravitationally driven relaxation of topography that was originally raised by mantle diapirs we examine this relaxation using a viscoelastic finite element code and show that an initially plateau shaped load will evolve to the characteristic corona topography of central raised bowl annular rim and surrounding moat stresses induced by the relaxation are consistent with the development of concentric extensional fracturing common on the outer margins of corona moats however relaxation is not expected to produce the concentric faulting often observed on the annular rim the relaxation timescale is shorter than the diapir cooling timescale so loss of thermal support controls the rate at which topography is reduced the final corona shape is supported by buoyancy and flexural stresses and will persist through geologic time development of lower flatter central bowls and narrower and more pronounced annular rims and moats enhanced by thicker crusts higher thermal gradients and crustal thinning over the diapir

lithospheric flexure beneath the freyja montes foredeep venus constraints on lithospheric thermal gradient and heat flow analysis of venera 15 and 16 radar images and topographic data from the freyja montes region on venus suggest that this mountain belt formed as a result of a sequence of underthrusts of the lithosphere of the north polar plains beneath the highlands of ishtar terra the freyja montes deformation zone consists south to north of a linear orogenic belt an adjacent plateau a steep scarp separating the plateau from the north polar plains a linear depression at the base of the scarp and an outer rise the topographic profile of the depression and outer rise are remarkably similar to that of a foreland deep and rise formed by the flexure of the under thrusting plate beneath a terrestrial mountain range we test the lithospheric flexure hypothesis and we estimate the effective thickness t e of the elastic lithosphere of the under thrusting portion of the north polar plains by fitting individual topographic profiles to deflection curves for a broken elastic plate the theoretical curves fit the observed topographic profiles to within measurement error for values of flexural rigidity d in the range 0 8 3 x 1022 n m equivalent to t e in the range 11 18 km under the assumption that the base of the mechanical lithosphere is limited by the creep strength of olivine the mean lithospheric thermal gradient is 14 23 k km the portion of the north polar plains to the immediate north of freyja montes stands at a regional elevation very near the modal elevation for the planet so the inferred elastic itho sphere thickness and thermal gradient may be at least approxi mately representative of a significant fraction of the venus lithosphere that the inferred thermal gradient is similar to the value expected for the global mean gradient on the basis of scaling from earth provides support for the hypothesis that simple conduction dominates lithospheric heat transport on venus relative to lithospheric recycling and volcanism

character mass distribution and origin of tephra fall deposits from the 2009 eruption of redoubt volcano alaska highlighting the significance of particle aggregation abstract the 2009 eruption of redoubt volcano included 20 tephra producing explosions between march 15 2009 and april 4 2009 utc next generation radar nexrad data show that plumes reached heights between 4 6 km and 19 km asl and were distributed downwind along nearly all azimuths of the volcano explosions lasted between 0 8 mm thick including communities along the kenai peninsula 80 100 km and the city of anchorage 170 km trace ash on march 15 a small 4 6 km asl phreatic explosion containing minor non juvenile ash erupted through the summit ice cap the first five magmatic explosions events 1 5 occurred within a 6 hour period on march 23 plumes rose to heights between 5 5 km and 14 9 km asl during 2 to 20 minute duration explosions and were dispersed mainly along a nne trajectory trace ash fall was reported as far as fairbanks owing to a shift in wind direction and heavy snowfall during these events field discrimination among many of these layers was possible all deposits comprise a volumetrically significant amount of particle aggregates yet only event 5 deposits contain coarse clasts including glacier ice the most voluminous tephra fall was deposited on march 24 event 6 from a 15 minute explosion that sent a plume to 18 3 km asl and dispersed tephra to the wnw within 10 km of the vent this deposit contains 1 11 cm pumice clasts in a matrix of 1 2 mm aggregate lapilli a small dome was presumably emplaced between march 23 and march 26 and was subsequently destroyed during 1 14 minute magmatic explosions of events 7 8 march 26 that sent plumes between 8 2 km and 19 km asl ash fell along a broad swath to the ese covering communities along the kenai peninsula with up to 1 mm of ash proximal deposits are largely composed of aggregate lapilli of 1 2 mm with very little coarse juvenile material events 9 18 march 27 sent plumes between 5 2 km and 15 5 km asl during particle size data showing a preponderance of fine ash even in the most proximal locations along with the abundance of aggregate lapilli documented in most samples confirms that particle aggregation played a significant role in the 2009 eruption and induced premature fallout of fine ash

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

a three dimensional eulerian model for transport and deposition of volcanic ashes abstract volcanic ash fallout represents a serious threat to people living near active volcanoes because it can produce several undesirable effects such as collapse of roofs by ash loading respiratory sickness air traffic disruption or damage to agriculture the assessment of such volcanic risk is therefore an issue of vital importance for public safety and its mitigation often requires to evaluate the temporal evolution of the phenomenon through reliable computational models we develop an eulerian model named fall3d for the transport and deposition of volcanic ashes the model is based on the advection diffusion sedimentation equation with a turbulent diffusion given by the gradient transport theory a wind field obtained from a meteorological limited area model lam and the source term derived from by buoyant plume theory it can be used to forecast either ash concentration in the atmosphere or ash loading on the ground model inputs are topography meteorological data given by a lam mass eruption rate and a particle settling velocity distribution a test application to the july 2001 etna eruption is presented

volcanic degassing of argon and helium and the history of crustal production on venus we develop a new methodology linking 40ar and 4he degassing to crustal production on venus in order to examine two different scenarios for the history of magmatism on that planet both consistent with recent analyses of the cratering record of the venus surface the first scenario includes episodic global resurfacing events and modest levels of magmatism between such events while the second scenario invokes different rates of steady magmatism before and after a given transition time our degassing models include distinct mantle crustal and atmospheric reservoirs diffusive transfer of 40ar and 4he from the crust to the atmosphere is also taken into account the 40ar abundance in the present atmosphere reflects the integrated degassing and magmatism over most of planetary history in contrast helium escapes from the planetary atmosphere within a characteristic residence time of 200 myr to 1 8 gyr so the present atmospheric 4he abundance is relevant to the volume of magma produced during the last global resurfacing event and the rate of the magmatism subsequent to that event under the first scenario and to the two steady rates of magmatism and the transition time under the second unfortunately large uncertainties in mineral partition coefficients and in the ar and he mixing ratios in the lower atmosphere of venus presently prevent the use of our degassing models to distinguish among crustal formation history models we therefore explore the influence of these uncertain parameters on the degassing history in order to identify those new laboratory and in situ measurements that will most strongly constrain crustal production history as an important step in the development of the ar degassing model the k budget in the bulk silicate fraction of venus is re examined on the basis of venera and vega ray measurements of k u and th concentrations in surface materials for u and th concentrations in the bulk silicate planet of 18 29 ppb and 64 94 ppb respectively the degree of mantle melting that formed the surface materials at the venera 9 and 10 and vega 1 and 2 landing sites is calculated to lie between 0 02 and 0 16 the degree of melting calculated for materials at each landing site is combined with measurements of surface k concentration to estimate the k concentration in the bulk silicate portion of the planet at between 100 and 300 ppm these results suggest that venus and earth have similar heat production

venus southern hemisphere geologic character and age of terrains in the themis alpha lada region arecibo high resolution radar images of the southern hemisphere of venus extending to 78 s show that the surface of the themis alpha lada region is characterized by linear deformation zones with volcanoes and corona like features and by regional volcanic deposits primarily plains small shields and large edifices large scale areal deformation is limited to the tessera of alpha regio lada terra in the southern high latitudes contains several large coronae in contrast to ishtar terra in the northern high latitudes the density of craters of possible impact origin is somewhat lower than that observed in the venera 15 and 16 coverage these data extend to 43 percent of the areas of the surface of venus with ages of less than about 1 billion years

venus crater distributions at low northern latitudes and in the southern hemisphere from new arecibo observations arecibo high resolution radar data covering approximately 23 of the surface of venus was analyzed for circular features with crater like appearances overlapping venera 15 16 radar data with its more normal to the surface viewing geometry was used to verity crater diameters measured from the arecibo data a comparison of the diameter frequency distribution of the possible impact features in the arecibo data for northern latitudes south of 40 n and for the southern hemisphere with the distribution from the venera 15 16 data for the northernmost quarter of the planet indicated similar distributions for large crater like features with diameters approximately 30 km but a deficiency of smaller craters in the arecibo data the average crater density measured for all craters in the northernmost quarter of the planet from the venera 15 16 data was 1 27 per 106 km2 while the average for the 106 l06km2 covered by the arecibo data was 0 95 per 106 km2 for craters with diameters less than 22km the densities were 0 59 per million square kilometers for the venera data compared with 0 33 per 106 km2 for the average of the two arecibo data regions all circular features in the arecibo data with a crater like appearance that could not be excluded as being impact features were used in this study so the result represents an upper limit on the crater densities in the size interval considered one possible process responsible for the density differences of small craters is a difference in the rate of volcanic resurfacing an increase in the resurfacing rate of approximately 50 would explain the lower density of small craters in the area covered by the arecibo data compared with the venera data but would not account for the differences being confined to the 30 km crater diameter range

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

coronae on venus morphology classification and distribution abstract two major types of circular features with no direct morphologic analog on the terrestrial planets were identified in venera 15 16 radar images of venus coronae and corona like features coronae are irregular to circular structures characterized by a complex interior zone and maximum widths of 170 690 km corona like features resemble coronae but lack an annulus of ridges twenty one structures that fit the definition of coronae have been identified in venera 15 16 coverage along with eleven corona like features each group has been subdivided into three classes based on general morphology symmetrical coronae are characterized by circular to oval form with interiors characterized by flows domes and tectonic ridges asymmetrical coronae have similar morphology but have only a partial annulus or are asymmetrical about a central axis subdued coronae are dominated by smooth plains with flooded and embayed annuli double corona like structures are composed of two linked rings and are found along ridge belts in the region east of atalanta planitia irregular corona like features are surrounded by a rim rather than an annulus as are the double structures and are very subdued in appearance heart shaped corona like features are rounded at one end and form a wedge at the other end we interpret the classes of coronae and corona like features to have a similar mode of origin with differences in morphology due to the stage of evolution and or varying amounts and significance of geologic processes i e tectonism volcanism gravitational relaxation that occur during their evolution corona evolution is characterized by early uplift and volcanism followed by annulus formation formation of a peripheral trough may occur in the middle to late stages of corona evolution followed by extensive volcanism and lowering of topographic relief both coronae and corona like features are interpreted to have formed as a result of uplift and volcanism caused by a thermal anomaly at depth followed by gravitational relaxation

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

an overview of venus geology the magellan spacecraft is producing comprehensive image and altimetry data for the planet venus initial geologic mapping of the planet reveals a surface dominated by volcanic plains and characterized by extensive volcanism and tectonic deformation geologic and geomorphologic units include plains terrains tectonic terrains and surficial material units understanding the origin of these units and the relation between them is an ongoing task of the magellan team

extraction of a lithospheric cooling signal from oceanwide geoid data abstract the geoid an equipotential surface of earth s gravity field reflects the distribution of mass within the planet and hence a variety of geodynamic processes because these data are dominated by sublithospheric processes notably mantle convection they have not played a major role in the ongoing debate concerning models of the thermal evolution of oceanic lithosphere application of spatial filtering to the age derivative of the geoid however extracts an age dependent signal which reflects lithospheric thermal evolution the data are much better fit by a thin about 100 km thick thermal plate than by a cooling halfspace and so provide a valuable constraint complementary to and consistent with the variations in oceanic depth and heat flow with age

the magma ocean as an impediment to lunar plate tectonics potential for plate tectonics on a planet is determined by several factors besides the obvious one of temperature one of the most important of these is the ratio zc zl where zc and zl are the thicknesses of the crust and tectonic lithosphere respectively a high zc zl ratio imparts a low bulk density and thus a high buoyancy to the lithosphere cooling of the lithosphere can offset a limited degree of compositional buoyancy but for realistic assumptions regarding crustal and mantle densities lithosphere geotherms and the spatial variability of the zc zl ratio systematic lithosphere subduction i e plate tectonics is unlikely on any planet with average zc zl ratio greater than about 0 25 during a magma ocean phase zc zl must be 1 so systematic lithosphere subduction is impossible in general zl is closely linked to the average planet temperature whereas zc is only weakly related to t but is sensitive to the planet s pressure depth gradient dp dz i e its size which governs the fate of al during deep melting crystallization processes the moon has probably always had zc zl too high or in later times lithosphere thickness too high to sustain plate tectonics conceivably plate tectonics operated over the first few million years of lunar evolution particularly if the early interior was cooler than generally assumed and a true magma ocean never developed or only developed after gradual warming of the deep interior however weighing against this scenario in addition to the wide array of evidence in favor of the magma ocean hypothesis are the consistently moderate to young by lunar standards ages of geochemically evolved lunar rocks

three dimensional spherical models of convection in the earth s mantle three dimensional spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation thus subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs which are cooler than the surrounding mantle cylindrical mantle plumes that cause hotspots such as hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large scale mantle circulation active sheetlike upwellings that could be associated with mid ocean ridges did not develop in the model simulations a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs

global distribution and characteristics of coronae and related features on venus implications for origin and relation to mantle processes coronae on venus range from 60 to over 2000 km across and are characterized by a complex range of morphologies the annuli around coronae range from about 10 to 150 km across and have tectonic features ranging from extensional to compressional to a combination of both topographically coronae are domes plateaus plateaus with interior lows and rimmed depressions a subset of features classified here as coronae corresponds to depressions and is interpreted to consist of large scale calderas a number of features have been identified with many of the basic characteristics of coronae similar interior deformation associations with volcanism high topography but lacking a distinct tectonic annulus these features tend to be somewhat smaller than coronae and may represent failed coronae or coronae in an early stage of evolution the size distribution of coronae and coronalike features with maximum widths greater than about 250 km is well represented by a power law of the form n d kd where n is the number of coronae with maximum widths greater than d km and 3 05 the spatial distribution of coronae is not random the features are concentrated in a few groups and along several chains coronae are similar in many morphologic characteristics to major volcanic shield structures and volcanic rises such as western eistla regio the largest corona artemis is actually larger than several volcanic rises on venus coronae and volcanic rises appear to be surface manifestations of mantle plumes there is no evidence of any systematic variation in age along chains of coronae as occurs in hotspot chains on earth instead a number of multiple and overlapping coronae may indicate limited movement of the surface above a hotspot or mantle plume the morphology and size distribution of coronae highlands and major shields suggest that mantle upwelling on venus operates either on several spatial scales with coronae representing smaller scale upflows and major volcanic rises representing larger convective upwellings or on several temporal scales with coronae representing shorter duration upflows and major volcanic rises representing long term upwellings

oceanic eddy formation and propagation southwest of taiwan 1 oceanic eddies are active and energetic southwest of taiwan the formation and propagation of eddies in this area were investigated using 17 year satellite altimeter data cyclonic eddies ces and anticyclonic eddies aces often coexisted but there were more ces than aces generated during the period from october 1992 to october 2009 aces were stronger and in general lived longer than ces aces occurred more often in winter than in other seasons while ces were more frequent in summer compared with the direct local wind forcing the kuroshio path variability appears to be a dominant factor for eddy formation in this area a conceptual model of an eddy kuroshio interaction is proposed in summer there exists an outflow northwest of luzon island and the kuroshio likely leaps across the luzon strait to the north of the outflow and left of the kuroshio axis ces are often formed which in turn induce aces to the west of ces in winter under the influence of northeasterly monsoon the kuroshio current loop kcl appears southwest of taiwan more frequently than in other seasons and aces are frequently shed from the kcl most of the aces propagate westward and as a result ces are often spun up to the east of the aces the surface south china sea outflow in summer and the kcl in winter are however likely related to the monsoons therefore the indirect effects of monsoon winds are also evident in the seasonal variations of eddy occurrence

fundamental issues in the geology and geophysics of venus a number of important and currently unresolved issues in the global geology and geophysics of venus will be addressable with the radar imaging altimetry and gravity measurements now forthcoming from the magellan mission among these are the global volcanic flux and the rate of formation of new crust the global heat flux and its regional variations the relative importance of localized hot spots and linear centers of crustal spreading to crustal formation and tectonics and the planform of mantle convection on venus and the nature of the interactions among interior convective flow near surface deformation and magmatism

venus volcanism and rift formation in beta regio a new high resolution radar image of beta regio a venus highland area confirms the presence of a major tectonic rift system and associated volcanic activity the lack of identifiable impact craters together with the apparent superposition of the theia mons volcanic structure on the rift system suggest that at least some of the volcanic activity occurred in relatively recent geologic time the presence of topographically similar highland areas elsewhere on venus aphrodite terra dali chasma and diana chasma suggests that rifting and volcanism are significant processes on venus

blocking and westward passage of eddies in the luzon strait satellite observations have shown the abundance of generally westward propagating eddies in the subtropical regions in the north pacific ocean especially north of 10 n eddies transport mass and can significantly impact the circulation as well as the heat salt and nutrient balances of the western pacific marginal seas this paper uses a numerical model to examine the conditions when eddies can or cannot freely propagate westward through the luzon strait into the south china sea scs composite analyses on the 10 year model data show that the fates of eddies depend on the strength and path of the kuroshio in one path that exists mostly during fall and winter the kuroshio loops westward into the scs the potential vorticity pv across the current is weak and eddies are likely to propagate freely through the luzon strait in another path which exists mostly during spring and summer the kuroshio tends to leap directly northward bypassing the scs the pv across it strengthens and eddies are then blocked and are constrained to also follow the northward path nonlinear eddy current interaction and the existence of a cyclone north of the luzon island during the looping phase explain why eddies of both signs can pass through the strait it is shown also that the upstream state of the kuroshio in the western tropical pacific plays an important role in dictating the different paths of the kuroshio the looping leaping path is caused by a weakened stronger kuroshio transport related to the northward southward shift of the north equatorial current in wintertime summertime

formation of mountain belts on venus evidence for large scale convergence underthrusting and crustal imbrication in freyja montes ishtar terra the freyja mottles venus deformation zone consists of a distinctive linear orogenic belt freyja montes an adjacent plateau a steep scarp separating the plateau from the north polar plains lying about 3 km below the plateau and a volcanically flooded foredeep and rise at the base of the scarp on the basis of mapping of radar images and interpretation of the geologic units and topography the freyja montes deformation zone is interpreted to be the site of large scale crustal convergence and imbrication involving the underthrusting of the north polar plains beneath ishtar terra this deformation is interpreted to have begun with north south regional compressional deformation causing flexure buckling and underthrusting of the crust and lithosphere of the north polar plains beneath the relatively higher proto ishtar lakshmi further convergence and underthrusting caused crustal thickening and uplift and created compressive stresses in the upper part of the underthrusting layer failure of this layer led to a new site of underthrusting outboard of the rise and a new detachment surface the detached layer and the wedge of volcanic flows in the intervening foredeep were further deformed and incorporated sutured onto the plateau and mountain range this crustal underthrusting and imbrication caused crustal thickening and corresponding crustal uplift the continuing process of convergence underthrusting detachment and imbrication produced large scale crustal thickening uplift and the formation of the plateau behind the asymmetrical front of freyja montes the present configuration of topography and units and the distinctive elevation of the scarp and freyja montes suggest that the process continues to operate at the present time

a mechanism for tectonic deformation on venus in the absence of identifiable physiographic features directly associated with plate tectonics alternate mechanisms are sought for the intense tectonic deformation observed in radar images of venus one possible mechanism is direct coupling into an elastic lithosphere of the stresses associated with convective flow in the interior spectral green s function solutions have been obtained for stresses in an elastic lithosphere overlying a newtonian interior with an exponential depth dependence of viscosity and a specified surface density distribution driving the flow at long wavelengths and for a rigid elastic fluid boundary condition horizontal normal stresses in the elastic lid are controlled by the vertical shear stress gradient and are directly proportional to the depth of the density disturbance in the underlying fluid the depth and strength of density anomalies in the venusian interior inferred by analyses of long wavelength gravity data suggest that stresses in excess of 100 mpa would be generated in a 10 km thick elastic lid unless a low viscosity channel occurring beneath the lid or a positive viscosity gradient uncouples the flow stresses the great apparent depth of compensation of topographic features argues against this however thus supporting the importance of the coupling mechanism if there is no elastic lid stresses will also be very high near the surface providing also that the viscosity gradient is negative

coronae of mnemosyne regio morphology and origin abstract coronae are circular to elliptical structures 150 600 km across characterized in venera 15 16 and arecibo radar images by an annulus of concentric ridges the interiors of the structures are geologically complex displaying volcanic flows domes and radial concentric and or obliquely trending ridges and are generally distinct from the exterior the mnemosyne regio area to the west of ishtar terra contains seven coronae the largest cluster of coronae identified on venus on the basis of the morphologic and topographic characteristics of the coronae in the mnemosyne region we assess several possible models of corona origin using cross cutting and stratigraphic relationships we have been able to identify a sequence of features and events for this group of coronae and have determined that a variety of processes contribute to the origin and evolution of coronae we find that several processes associated with diapirism or hotspots and gravitational relaxation of high topography are most consistent with observed corona topography and morphology and are thus likely to have been significant in the formation and evolution of coronae on the basis of these observations and stratigraphic relationships the following sequence of events is outlined for the formation and evolution of coronae in this region 1 regional volcanic and tectonic activity 2 localized uplift and volcanism caused by diapirism or hotspots 3 annulus and trough formation related to gravitational relaxation accompanied by further volcanic activity 4 continued local volcanic flooding of the corona annulus and trough differences in morphology between coronae reflect variations in amount and significance of these geologic processes coronae in other regions exhibit the same overall characteristics as the mnemosyne coronae and support a similar origin for the majority of coronae

the thermal regime of venus models for the thermal evolution of venus are constructed numerically the mantle is proposed to consist of two independently convecting layers upper and lower mantles the floating crust keeps the temperature of the upper surface of the convecting mantle near 1200 c and the core may solidify at temperatures below the liquidus previously these models were calculated with the usual parameterization of convection based mostly on investigations of constant viscosity convection now this parameterization is modified to take into account new numerical investigations of convection in media with complicated rheology the thermal evolution of venus is assumed to have begun 4 6 billion years b y ago it can be divided into three periods 1 adjustment of the upper mantle to the thermal regime of the lower mantle 0 5 b y 2 transition of the entire mantle to the asymptotic regime 3 4 b y 3 asymptotic regime parameters of the convecting planet in the asymptotic regime do not depend on initial conditions the planet forgets its initial state and are found analytically the present day heat flux is equal to 50 erg cm 2s 1 the results are close to those obtained previously comparison of the present thermal models of venus and earth with the recent studies of melting of the fe fes system suggest that the absence of a dipole magnetic field is closely connected to the absence of solidification of the venusian core at present the position of the iron triple point e liquid makes this conclusion very probable the thermal regime of the venusian crust is also discussed convection in the lower part of the crust is shown to play a role in certain regions with specific crustal composition the prevailing mechanism of heat transfer to the surface is advection by magmas produced by melting the lower layers of the crust

venus geology and tectonics hotspot and crustal spreading models and questions for the magellan mission spacecraft and ground based observations of venus have revealed a geologically young and active surface with volcanoes rift zones orogenic belts and evidence for hotspots and crustal spreading yet the processes responsible for these features cannot be identified from the available data the magellan spacecraft will acquire an unprecedented global data set which will provide a comprehensive and well resolved view of the planet this will permit global geological mapping an assessment of the style and relative importance of geological processes and will help in the understanding of links between the surface geology and mantle dynamics of this earth like planet

geologic evidence of hotspot activity on venus predictions for magellan a number of distinctive types of geologic features have been identified on venus that are interpreted to be related to thermal plumes including domal rises coronae and major composite shield volcanoes we document the basic characteristics of these features as well as their distribution the three types of features have related morphologies and we interpret them to represent a continuum of features formed by mantle plumes at scales from 100 s to over 1000 km the artemis structure located in aphrodite terra is proposed to be a large corona if crustal spreading processes are operating on venus hotspot features should form chains on the surface as seen in terrestrial ocean basins on the basis of current data on hotspot related feature distribution on venus no clear evidence exists for hotspot chains the complete distribution of hotspot features in magellan data will be used to understand better the relationship between interior processes and surface features as well as to provide a test for the crustal spreading hypothesis

limits on modes of lithospheric heat transport on venus from impact crater density we present a formalism by which the size frequency distribution of impact craters on venus may be used to estimate upper bounds on the mean global rates of volcanic resurfacing and lithospheric recycling on that planet over the past several hundred million years the principal assumptions are that an upper bound on the rate of crater production on venus may be estimated and that craters are volcanically obliterated only when flows completely cover their rims the impact crater density reported from venera 15 16 observations if valid for the entire venus surface then indicates a mean volcanic flux no greater than 2 km y corresponding to a maximum average rate of resurfacing of about 4 km b y the fraction of global heat loss due to such a rate of volcanic resurfacing is negligible for the lowest estimated crater retention age of the surface imaged by venera 15 16 if globally representative the rate of lithospheric recycling on venus does not exceed 1 5 km y corresponding to 25 of the global heat loss more likely estimates of the mean surface age limit the contribution to global heat loss of any lithospheric recycling to be less than 10 these results support the hypothesis that simple conduction dominates heat transport at lithospheric levels in the venus interior

blob tectonics a prediction for western aphrodite terra venus it is proposed that the western aphrodite terra region of venus was formed by four diapirs of heated mantle material or blobs rising to the surface in a region of convective upwelling the four blobs are in different stages of evolution producing different topography gravity anomalies and morphology we show that this model is consistent with available data and make specific predictions that can be tested with magellan radar data

localization of gravity and topography constraints on the tectonics and mantle dynamics of venus summary we develop a method for spatio spectral localization of harmonic data on a sphere and use it to interpret recent high resolution global estimates of the gravity and topography of venus in the context of geodynamical models our approach applies equally to the simple spatial windowing of harmonic data and to variable length scale analyses which are analogous to a wavelet transform in the cartesian domain using the variable length scale approach we calculate the localized rms amplitudes of gravity and topography as well as the spectral admittance between the two fields as functions of position and wavelength the observed admittances over 10 per cent of the surface of venus highland plateaus and tessera regions are consistent with isostatic compensation of topography by variations in crustal thickness while admittances over the remaining 90 per cent of the surface rises plains and lowlands indicate that longwavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere the global average crustal thickness is less than 30 km but can reach values as large as 40 km beneath tesserae and highland plateaus we also note that an earth like radial viscosity structure cannot be rejected by the gravity and topography data and that without a mechanical model of the lithosphere admittance values cannot constrain the thickness of the thermal boundary layer of venus modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae the effective elastic plate thickness k was less than 20 km estimates of t at highland rises are consistently less than 30 km our inability to find regions with t 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models

albedo of dirty snow during conditions of melt the evolution of spectrally averaged albedo wavelengths between 0 28 m and 2 8 m of snow surfaces treated with known initial concentrations of particles of submicron sized soot and air fall volcanic ash was investigated during conditions of natural melt depending on the particle type and concentration the initial applications reduced the surface albedo to values ranging from 0 18 to 0 41 which were substantially lower than the albedo of the untreated natural snow about 0 61 many of the soot particles flushed through the snowpack with the meltwater and surface concentrations of soot greater than about 5 10 7 kg kg did not persist for more than a few days the migration of particles to depth caused the snow to brighten after the initial application thus limiting the amount of albedo reduction and the consequent effects on melting nevertheless the soot remaining near the surface had a substantial long term effect the residual concentration of 5 10 7 kg kg persisted for several weeks and compared to the untreated surface reduced the albedo by about 30 and increased melting by 50 particles of volcanic ash with diameters larger than about 5 m remained at or near the snow surface although many of the smaller particles flushed through the snow with the meltwater the surface albedo was not changed significantly by their removal the different behaviors of the ash and soot are probably related to the difference in their particle size distributions in relation to the thickness of water films that form the transport paths under conditions of partial saturation that are characteristic of melting snow

assessment of depositional ash loadings from the 2009 eruption of mt redoubt abstract volcanic ash deposited onto ice and snow in the arctic has the potential to perturb the regional radiation balance by altering the surface reflectivity in order to determine changes in surface albedo loading fields of deposits must be quantified loading fields are defined as the total mass of ash deposited over an area we assessed the areal extent and loading of ash deposits from the 2009 eruption of mt redoubt using an eulerian volcanic ash transport and dispersion model fall3d combined with satellite and deposit observations because direct observations are often limited in remote arctic regions we devised a novel method for modeling ash deposit loading fields for the entire eruption based on best fit parameters of a well studied eruptive event the major land depositing eruptive events events 2 6 on march 23 24 were chosen for analyses realistic ranges of model parameters were selected based on the well described event 5 march 23 and were estimated for other events the model results were validated against nasa a train satellite data and field measurements reported by the alaska volcano observatory overall good to moderate agreement was found a total cumulative deposit area of 3 7 106 km2 was produced and loadings ranged from 7000 3000 g m 2 near the vent to

a mechanism for episodic subduction on venus we propose a mechanism previously developed as a hypothetical cause of the initiation of subduction in the earth s mantle to describe a situation where such subduction may occur transiently at irregular intervals of time it has been suggested that tectonics on venus may be described by such a scenario in our model a subduction event is followed by resumption of high rayleigh number mantle convection below a stagnant lithosphere which thickens due to conductive cooling as it thickens differential buoyancy causes large lithospheric stresses which eventually lead to plastic failure in the upper portions of the lithosphere this plastic zone thickens faster than the lithosphere so that at some critical time it reaches the base of the lithosphere at this point the effective lithosphere viscosity decreases to that of the underlying mantle and subduction can occur we suggest that this is mechanistically consistent with the postulated venusian tectonic style

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

magma reservoirs and neutral buoyancy zones on venus implications for the formation and evolution of volcanic landforms we examine the production of magma reservoirs and neutral buoyancy zones nbzs on venus and the implications of their development for the formation and evolution of volcanic landforms the high atmospheric pressure on venus reduces volatile exsolution and generally serves to inhibit the formation of nbzs and shallow magma reservoirs for a range of common terrestrial magma volatile contents magma ascending and erupting near or below mean planetary radius mpr should not stall at shallow magma reservoirs such eruptions would be characterized by relatively high total volumes and effusion rates for the same range of volatile contents at 2 km above mpr about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones in general neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on venus than on earth for a fixed volatile content nbzs become deeper with increasing elevation over the range of elevations treated here 1 km to 4 4 km depths differ by a factor of 2 4 these analyses reveal several factors that may help to account for the low height of volcanoes on venus larger primary reservoirs cause the wide dispersal of conduits building edifices models of the position of the shallow nbz reservoir during edifice growth show that for earth the magma chamber center remains at a constant depth below the growing edifice summit thus keeping pace with the increasing elevation while on venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude therefore neutral buoyancy zones and magma reservoirs on venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all the lower rate of vertical migration implies that magma reservoirs would tend to stabilize undergo greater lateral growth and become larger on venus than earth the proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on venus than earth large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions in large reservoirs positively buoyant materials are likely to be produced from differentiation substrate remelting and volatile exsolution nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on earth and when this gas enriched melt emerges at the surface it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations leading to anomalous more viscous rheological properties inhibition of disruption of volatile rich magma for both basaltic and more evolved compositions can lead to the production of 1 more bubble rich vesicular flows characterized by higher viscosity and greater thicknesses the venus equivalent of effusions that would have lost much of their volatiles in the fire fountaining process on earth 2 high volume high discharge rate extremely gas rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent and 3 more evolved compositions that are gas rich but do not undergo disruption upon effusion and produce domes and flows of high viscosity the venus equivalent of terrestrial ignimbrites on the basis of these analyses we predict that intrusions and dike emplacement should be common particularly around both shallow and deep magma reservoirs and the tops of dikes should be manifested as radial fractures there should be a range of diameters of circular features associated with the presence of both shallow nbz related and deep plumerelated magma reservoirs large volume lava outpourings should be favored at lower elevations and shallow reservoirs and edifices should be more common at intermediate elevations at the highest elevations magma reservoirs are predicted to be large and relatively deep with low associated edifices direct ascent of magma where neutral buoyancy zones are unlikely low elevations and the propensity for relatively larger magma reservoirs on venus when they do form are both factors which will tend to favor high volume eruptions independent of variations in magma temperature or chemistry individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on venus

interannual variations of kuroshio transport in the east china sea and its relation to the pacific decadal oscillation and mesoscale eddies results of a data assimilative ocean model jcope2 from 1993 to 2012 were used to examine the correlation between the pacific decadal oscillation pdo index and interannual variations of the kuroshio transport in the east china sea ecs and the influences of mesoscale eddies on this correlation in a period from 1993 to 2002 the kuroshio transport estimated from the jcope2 reanalysis has a positive correlation with the pdo index this well known correlation became weak or even disappeared when the analysis period was extended from 1993 2002 to 1993 2012 this occurs because the variation range of the pdo index became small during enhanced mesoscale eddy activity southeast of taiwan in years after 2002 the eddies caused a larger variation in the kuroshio transport in the years after 2002 than before 2002 and therefore changed the correlation between the pdo index and kuroshio transport in the ecs the influence of mesoscale eddies on the kuroshio transport has strong regional dependence the kuroshio transport from the area east of taiwan to the midway along the shelf break in the east china sea depends mainly on eddies arriving from southeast of taiwan while transport from the midway along the shelf break to the tokara strait depends mainly on the eddies arriving from northeast of okinawa island the combination of pdo related signals and eddy related signals determines the interannual variations of the kuroshio transport in the ecs and sufficient attention must be paid to the spatial dependence of the kuroshio transport in the ecs on eddies

impact craters on venus initial analysis from magellan magellan radar images of 15 percent of the planet show 135 craters of probable impact origin craters more than 15 km across tend to contain central peaks multiple central peaks and peak rings many craters smaller than 15 km exhibit multiple floors or appear in clusters these phenomena are attributed to atmospheric breakup of incoming meteoroids additionally the atmosphere appears to have prevented the formation of primary impact craters smaller than about 3 km and produced a deficiency in the number of craters smaller than about 25 km across ejecta is found at greater distances than that predicted by simple ballistic emplacement and the distal ends of some ejecta deposits are lobate these characteristics may represent surface flows of material initially entrained in the atmosphere many craters are surrounded by zones of low radar albedo whose origin may have been deformation of the surface by the shock or pressure wave associated with the incoming meteoroid craters are absent from several large areas such as a 5 million square kilometer region around sappho patera where the most likely explanation for the dearth of craters is volcanic resurfacing there is apparently a spectrum of surface ages on venus ranging approximately from 0 to 800 million years and therefore venus must be a geologically active planet

tectonics of lakshmi planum venus tests for magellan the origin of lakshmi planum and its surrounding mountain belts remains an important unresolved element in the global tectonic framework of venus from the perspective of gravity signature and potential driving forces the mantle upwelling model is the simplest and its principal failure that it cannot produce radial shortening on the uplift periphery may be resolved if the lithosphere is laterally heterogeneous our preferred model consists of a hot mantle plume rising beneath a pre existing block of tessera the lithosphere is weakened at this hotter and presumably thicker crust and the outward near surface flow is attenuated at the peripheral discontinuity in lithospheric strength crustal thickening and mountain belt formation occur there we propose several criteria to test this tessera plume model together with its competitors at the higher resolution in both imaging and gravity afforded by the magellan mission

lithospheric flexure on venus summary topographic flexural signatures on venus are generally associated with the outer edges of coronae with some chasmata and with rift zones using magellan altimetry profiles and grids of venusian topography we identified 17 potential flexure sites both 2 d cartesian and 2 d axisymmetric thin elastic plate models were used to establish the flexural parameter and applied load bending moment these parameters can be used to infer the thickness strength and possibly the dynamics of the venusian lithosphere numerical simulations show that the 2 d model provides an accurate representation of the flexural parameter as long as the radius of the feature is several times the flexural parameter however an axisymmetric model must be used to obtain a reliable estimate of load bending moment 12 of the 17 areas were modelled with a 2 d thin elastic plate model yielding best fit effective elastic thicknesses in the range 12 to 34 km we find no convincing evidence for flexure around smaller coronae though five possible candidates have been identified these five features show circumferential topographic signatures which if interpreted as flexure yield mean elastic thicknesses ranging from 6 to 22 km we adopt a yield strength envelope for the venusian lithosphere based on a dry olivine rheology and on the additional assumption that strain rates on venus are similar to or lower than strain rates on earth many of the flexural signatures correspond to relatively high plate bending curvatures so the upper and lower parts of the lithosphere should theoretically exhibit brittle fracture and flow respectively for areas where the curvatures are not too extreme the estimated elastic thickness is used to estimate the larger mechanical thickness of the lithosphere the large amplitude flexures in aphrodite terra predict complete failure of the plate rendering mechanical thickness estimates from these features unreliable one smaller corona also yielded an unreliable mechanical thickness estimate based on the marginal quality of the profile data reliable mechanical thicknesses found by forward modelling in this study are 21 km 37 km significantly greater than the 13 km 20 km predictions based on heat flow scaling arguments and chondritic thermal models if the modelled topography is the result of lithospheric flexure then our results for mechanical thickness combined with the lack of evidence for flexure around smaller features are consistent with a venusian lithosphere somewhat thicker than predicted dynamical models for bending of a viscous lithosphere at low strain rates predict a thick lithosphere also consistent with low temperature gradients recent laboratory measurements indicate that dry crustal materials are much stronger than previously believed corresponding time scales for gravitational relaxation are 108 109 yr making gravitational relaxation an unlikely mechanism for the generation of the few inferred flexural features if dry olivine is also found to be stronger than previously believed the mechanical thickness estimates for venus will be reduced and will be more consistent with the predictions of global heat scaling models

ridge belts evidence for regional and local scale deformation on the surface of venus we evaluate models for the formation of the ridge belt fan assemblage on venus through consideration of the orientation spatial distribution topographic expression and wavelengths of observed tectonic surface features we favor a compressional mechanism for long wavelength deformation corresponding to the spacing of ridge belts 300 400 km however short wavelength ridges and grooves 10 20 km that are contained within belts and trend parallel to them are likely to be compressional and extensional and to reflect both regional and local stress fields we hypothesize that large scale ridge spacing early stage mantle downwelling is the source of regional compression responsible for the establishment of the long wavelength of deformation

water mass and throughflow transport variability in the taiwan strait 1 variability of water mass and throughflow transport in the taiwan strait are analyzed using strait wide conductivity temperature depth 1985 2003 and sectional acoustic doppler current profiler 1999 2001 data results from a cluster analysis temperature salinity diagrams and direct transport calculations indicate that the strong northeast monsoon drives the brackish mixed china coastal water into the northern strait and hinders the northward intruded saline kuroshio branch water kbw in the southeastern strait from december to january the mean throughflow transport across the central strait is about 0 1 sv southward during this period thus supporting previous observations that there is no persistent northward flowing current throughout the strait in winter the weakening of the northeast monsoon in february march however emancipates the kbw to intrude northward into the east china sea ecs in june the increase in this northward transport accompanied by the decrease in the westward intrusion of kuroshio through the luzon strait leads to the replacement of the kbw by the less saline south china sea water scsw the northward transports ranging from 1 16 to 2 34 sv between march and august yield 0 131 0 238 1015 w and 53 33 81 74 106 kg s of temperature and salt transports respectively toward the ecs the inception of the northeast monsoon in october marks both the change of water mass from fall to winter patterns and the decrease of the northward throughflow transport our results also reveal that the subsurface kbw and scsw remain consistent throughout the observation periods between 100 and 200 m and below 200 m depths respectively in the southeastern strait

styles of volcanism on venus new arecibo high resolution radar data arecibo high resolution 1 5 to 2 km radar data of venus for the area extending from beta regio to western eisila regio provide strong evidence that the mountains in beta and eisila regiones and plains in and adjacent to guinevere planitia are of volcanic origin recognized styles of volcanism include large volcanic edifices on the beta and eisila rises related to regional structural trends plains with multiple source vents and a mottled appearance due to the ponding of volcanic flows and plains with bright features surrounded by extensive quasi circular radar dark halos the high density of volcanic vents in the plains suggests that heat loss by abundant and widely distributed plains volcanism may be more significant than previously recognized the low density of impact craters greater than 15 km in diameter in this region compared to the average density for the higher northern latitudes suggests that the plains have a younger age

consequences of a hydrous mantle for venus tectonics predictions for magellan we will consider the consequences of a venus mantle enriched in water relative to the earth if the moon forming impact hypothesis is correct earth may have lost substantial volatiles venus however experienced no such impact and is therefore likely to be relatively more water rich we demonstrate that a higher abundance of water in the venus mantle has the effect of inhibiting the rise of crustforming melts and volatiles to the surface which results in a thin stable crust a dry stiff upper mantle and restricted magmatism conduction through a shallow thermal lithosphere dominates the removal of heat from the interior and flow in the mantle is strongly coupled to the surface deformation current observations of venus are consistent with this model a review of the consequences of this model allows us to make predictions for the magellan radar images these include 1 no evidence for plate tectonic processes 2 younger apparent surface ages and enhanced crater relaxation for highland areas 3 limited quantity of hot spot volcanism and 4 occasional evidence of localized magmatism diatremes associated with a volatile rich mande source the model has implications to the earth as well in the absence of the moon forming impact crustal formation in the water rich earth mantle may have been arrested and plate recycling would have ceased early in the planet s history

terrestrial spreading centers under venus conditions evaluation of a crustal spreading model for western aphrodite terra abstract assessment of terrestrial spreading centers under venus environmental conditions indicates that the major difference between earth and venus would be the influence of the enhanced surface temperature on upper mantle temperatures on venus and the resulting increase in crustal thickness and elevation of isostatically compensated topography crust on venus produced at average spreading centers would be about 15 km thick in contrast to the average crustal thickness on earth of about 5 km we have used this model and the pioneer venus altimetry and gravity data to test the hypothesis of head and crumpler 1987 6 that western aphrodite terra represents the location of crustal spreading on venus the topographic data are processed such that all points within equally spaced 100 km wide bins parallel to the proposed axis are stacked together and the average value of topography in each bin is calculated providing average profiles out to a distance of 2000 km on either side of the axis the topography is shown to be very symmetric about the axis with data points less than 1000 km from the axis providing a fit to thermal boundary layer topography comparable to that seen in spreading centers on earth at 1000 km on either side of the axis steep topographic slopes exist this change in topography could be explained by variations of about 15 km in crustal thickness with the crust being thicker in the central domain by a ridge jump or by mechanisms unrelated to crustal spreading for processing line of sight los acceleration data we first show that static and dynamic calculations give the same results for track numbers 427 449 this allows us to determine a synthetic orbit track parallel to the cross strike dis continuities csd s and to compare it with observed los accelerations gravity data for this domain were then processed in a manner similar to that for topography models give a reasonable fit to the data for half spreading rates between 0 3 and 0 5 cm yr we conclude that a spreading center model for ovda regio could account for the observed topography and los gravity anomalies if it had the following characteristics a central domain with a 30 km thick crust that has been spreading at about 0 5 cm yr for the last 200 myr over a mantle whose temperature is about 1600 c at about 200 myr ago a change in upper mantle temperature of about 100 c is required prior to this time the decreased upper mantle temperature would have produced a thinner crust averaging about 15 km at the spreading center this thinner crust would now lie on the flanks of ovda regio at distances greater than 1000 km from the spreading center and would be in excess of 200 myr old on the basis of our analysis we conclude that the hypothesis that ovda regio in western aphrodite terra is the site of crustal spreading on venus is consistent with the topography and gravity data

injection transport and deposition of tephra during event 5 at redoubt volcano 23 march 2009 abstract among the events of the 2009 eruption at redoubt volcano alaska event 5 was the best documented by radar satellite imagery and deposit mapping we use the new eulerian tephra transport model ash3d to simulate transport and deposition of event 5 tephra at distances up to 350 km the eruption which started at about 1230 utc on 23 march 2009 sent a plume from the vent elevation estimated at 2 3 0 1 km above sea level or a s l to about 16 2 km above sea level in 5 min the plume was a few kilometers higher than would be expected for the estimated average mass eruption rate and atmospheric conditions possibly due to release of most of the eruptive mass in the first half of the 20 minute event the eruption injected tephra into a wind field of high shear with weak easterly winds below 3 km elevation strong southerly winds at 6 10 km and weak westerlies above 16 km model simulations in this wind field predicted development of a northward migrating inverted v shaped cloud with a southwest trending arm at a few kilometers elevation which was not visible in ir satellite images due to cloud cover and a southeast trending arm at 10 km elevation that was clearly visible simulations also predicted a deposit distribution that strongly depended on plume height a plume height below 15 km predicted ash deposits that were located west of those mapped whereas good agreement was reached with a modeled plume height of 15 18 km field sampling of the deposit found it to contain abundant tephra aggregates which accelerated the removal of tephra from the atmosphere we were able to reasonably approximate the effect of aggregation on the deposit mass distribution by two methods 1 adjusting the grain size distribution taking the erupted mass 2 isomass lines that was within a few tens of percent of mapped area however they under predicted deposit mass at very proximal 250 km locations modeled grain size distributions at sample locations are also generally coarser than observed the mismatch may result from a combination of limitations in field sampling approximations inherent in the model errors in the numerical wind field and aggregation of particles larger than 0 063 mm

gravity driven deformation of the crust on venus intense horizontal deformation features are observed on the surface of venus despite the apparent absence of terrestrial style plate tectonics high surface temperature low erosion rate and regions of elevated terrain suggest that gravity spreading may be responsible for some of these features gravity spreading on the earth is known to cause extensive deformation in certain thin skinned tectonic regions where the uppermost layers become detached along a decollement while the high pore pressures and soft sedimentary rocks frequently observed along decollements on the earth are not plausible for venus temperature induced regions of ductile deformation may exist simple viscous models of gravity spreading above a ductile region of the crust are developed to examine the effects of rheology thermal gradient surface slope and crustal thickness on deformation for a reasonable range of parameters the models show that surface slope driven ductile deformation at relatively shallow depths in the crust is possible and may be capable of detaching the uppermost rigid portion of the crust resulting in horizontal translation

aerosol radiative forcing from the 2010 eyjafjallajokull volcanic eruptions although the 2010 volcanic eruptions of eyjafjallajokull did not exert a large climate forcing several features of their emissions favored weaker aerosol cooling or stronger warming than commonly attributed to volcanic events these features include a high ratio of fine ash to so2 occurrence near reflective surfaces exposed to strong insolation and the production of very little stratospheric sulfate we derive plausible ranges of optical properties and top of atmosphere direct radiative forcing for aerosol emissions from these events and find that shortwave cooling from sulfate was largely offset by warming from ash deposition to cryospheric surfaces and longwave warming from atmospheric ash and sulfate shortwave forcing from atmospheric ash was slightly negative in the global mean under central estimates of optical properties though this forcing term was uniquely sensitive to the simulated distribution of clouds the forcing components sum to near climate neutral global mean 2010 instantaneous 1 9 mwm 2 and effective 0 5 mwm 2 radiative forcing where the latter is elevated by high efficacy of snow deposited ash ranges in net instantaneous 7 3 to 2 8 mwm 2 and effective 7 2 to 4 9 mwm 2 forcing derived from sensitivity studies are dominated by uncertainty in ash shortwave absorptivity forcing from airborne ash decayed quickly while sulfate forcing persisted for several weeks and ash deposits continued to darken snow and sea ice surfaces for months following the eruption despite small global forcing monthly averaged net forcing exceeded 1 wm 2 in some regions these findings indicate that ash can be an important component of climate forcing from high latitude volcanic eruptions and in some circumstances may exceed sulfate forcing

the role of rifting in the generation of melt implications for the origin and evolution of the lada terra lavinia planitia region of venus a rift zone over 6000 km in total length runs along the border of lada terra a highland in the southern hemisphere of venus and lavinia planitia a basin that has been interpreted as a site of early stage mantle downwelling along the length of the rift are a number of volcanic centers of widely varying morphology and volcanic output these include coronae radially fractured domes and large flow fields similar in scale to terrestrial flood basalts we develop a model for the origin of extension related to passive rifting in response to stresses created by the adjacent downwelling volcanism and extension at other rifts on venus such as devana chasma have been attributed to deep seated mantle plume activity in contrast we interpret the origin of extension and volcanism along the lada rift to be linked to upwelling and decompression melting of mantle material due to rifting and possibly to counterflow associated with downwelling extension occurred generally prior to the formation of volcanic centers and the eruption of large scale flow fields although most of the volcanic centers have been fractured by continued extension along the rift current debate over the formation of terrestrial flood basalts centers on the necessity of preexisting extension and stretched and thinned lithosphere to produce enhanced decompression melting within a large plume head or mantle thermal anomaly our studies of large scale flow fields associated with the lada rift and coronae on venus indicate that extension is a prerequisite for the formation of the majority of large scale flow units on venus

backscatter model for the unusual radar properties of the greenland ice sheet a number of planetary objects exhibit large radar reflectivity and polarization ratios and more recently a similar behavior has been observed over a vast portion of the earth s surface the percolation facies of the greenland ice sheet surface based ranging radar data and snow stratigraphy studies demonstrated that the radar properties of that portion of greenland are caused by enhanced scattering from massive large solid ice bodies buried in the top few meters of the dry cold clean snowy surface of the ice sheet and created by seasonal melting and refreezing events from author

radially fractured domes a comparison of venus and the earth radially fractured domes are large tectonic and topographic features discovered on the surface of venus by the magellan spacecraft they are thought to be due to uplift over mantle diapirism and to date are known to occur only on venus since venus and the earth are grossly similar in size composition and structure we seek to understand why these features have not been seen on the earth we model the uplift and fracturing over a mantle diapir as functions of lithospheric thickness and diapir size and depth we find that lithospheres of the same thickness on the earth and venus should respond similarly to the same sized diapir and that radially fractured domes should form most readily in thin oceanic lithospheres on earth if diapiric activity is similar on the two planets however our current knowledge of the earth s oceanic floors is insufficient to confirm or deny the presence of radially fractured domes we compute the expected dimensions for these features on the earth and suggest a search for them to determine whether mantle diapirism operates similarly on the earth and venus

interpretations of the jebar term abstract in diagnostic calculations of the ocean s circulation the so called jebar joint effect of baroclinicity and relief term may induce a significant depth average current field as has been noted in the oceanographic literature here we present two consistent interpretations of this term in the equation governing the vorticity of the depth averaged current a topographic vortex stretching term proportional to the dot product of depth averaged velocity and the depth gradient arises we show that jebar corrects this term by removing the contribution of the geostrophic flow referenced to the bottom which cannot generate topographic vortex stretching in the evolution equation for the vorticity of the depth integrated flow a bottom torque term is present we show that the jfbar term discussed above enters here as a contribution to the bottom torque emphasizing the role of jebar as a forcing term we also show that when a diagnostic calculation is formulated completely in terms of the transport str

regional linear cross strike discontinuities in western aphrodite terra venus western aphrodite terra venus is characterized by a series of parallel linear structural discontinuities 2000 4000 km in length and 100 200 km wide which strike at high angles to the general topographic trend of the highlands the detailed characteristics of these features are similar to terrestrial oceanic fracture zones and strike slip fault zones either interpretation implies the presence of large scale horizontal tectonic movement on venus

a dynamic model of venus s gravity field unlike earth long wavelength gravity anomalies and topography correlate well on venus venus s admittance curve from spherical harmonic degree 2 to 18 is inconsistent with either airy or pratt isostasy but is consistent with dynamic support from mantle convection a model using whole mantle flow and a high viscosity near surface layer overlying a constant viscosity mantle reproduces this admittance curve on earth the effective viscosity deduced from geoid modeling increases by a factor of 300 from the asthenosphere to the lower mantle these viscosity estimates may be biased by the neglect of lateral variations in mantle viscosity associated with hot plumes and cold subducted slabs the different effective viscosity profiles for earth and venus may reflect their convective styles with tectonism and mantle heat transport dominated by hot plumes on venus and by subducted slabs on earth convection at degree 2 appears much stronger on earth than on venus a degree 2 convective structure may be unstable on venus but may have been stabilized on earth by the insulating effects of the pangean supercontinental assemblage

orogenic belts on venus recent radar observations of venus show evidence for three types of deformational features in the linear mountain belts of ishtar terra linear ridges and troughs oriented parallel to the strike of the mountains and interpreted to be anticlines and synclines broad low arches similar to mare ridge type features seen on the moon and interpreted to represent low angle thrusting and buckling and linear discontinuities that cut across the strike of the ridges and troughs and are interpreted to represent strike slip movement on the basis of the localized concentration of these features their orientations and patterns of distribution and their association with linear mountainous topography we conclude that they mark the location of concentrated horizontal compressional deformation and that akna and freyja montes represent erogenic belts on venus

the effect of a shallow low viscosity zone on the apparent compensation of mid plate swells abstract a simple model for mid plate swells is that of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region finite element calculations have been used to determine the effects of the viscosity contrast the layer thickness and the rayleigh number on the flow and on the perceived compensation mechanism for the resulting topographic swell as the viscosity decreases in the low viscosity zone the effective local rayleigh number for the top boundary layer of the convecting cell increases also because the lower viscosity facilitates greater velocities in the low viscosity zone the low viscosity layer produces proportionally greater horizontal flow near the conducting lid causing the base of the conducting lid to appear like a free boundary the change in the local rayleigh number and in the effective boundary condition both cause the top boundary layer to thin through a green s function analysis we have found that the low viscosity zone damps the response of the surface topography to the temperature anomalies at depth whereas it causes the gravity and geoid response functions to change sign at depth counteracting the positive contributions from the shallower temperature variations by increasing the viscosity contrast the conbined effects of the thinning of the boundary layer and the behaviour of the response functions allow the apparent depth of compensation to become arbitrarily small therefore shallow depths of compensation cannot be used to argue against dynamic support of mid plate swells furthermore we compared the distribution of the effective compensating densities which is used to obtain the geoid to that of pratt compensation which is often used to calculate the depth of compensation from geoid and topography data for mid plate swells for all of our calculations including those with no low viscosity layer the effective gravitational mass distribution is more complex than assumed in simple pratt models so that the pratt models are not an appropriate gauge of the compensation mechanism

relating modis derived surface albedo to soils and rock types over northern africa and the arabian peninsula 1 we use the moderate resolution imaging spectroradiometer modis aboard the terra spacecraft to derive surface albedo for the arid areas of northern africa and the arabian peninsula albedo in seven modis spectral bands for land and three broad bands for shortwave near infrared and visible portions of the spectrum is produced surface albedo is derived from modis observations during a sixteen day period and is analyzed at 1 km spatial resolution modis data show considerable spatial variability of surface albedo in the study region that is related to soil and geological characteristics of the surface for example solar shortwave white sky albedo varies by a factor of about 2 5 from the darkest volcanic terrains to the brightest sand sheets vegetation contribution to surface reflectance is essentially negligible since we only considered pixels with under 10 percent fractional canopy cover few if any coupled land atmosphere global or regional models capture this observed spatial variability in surface reflectance or albedo here we suggest a scheme that relates soil groups based on the united nations food and agriculture organization fao soil classification and rock types based on the united states geological survey usgs geological maps to modis derived surface albedo statistics this approach is a first step towards the incorporation of the observed spatial variability in surface reflective properties into climate models

effects of bubbles cracks and volcanic tephra on the spectral albedo of bare ice near the transantarctic mountains implications for sea glaciers on snowball earth 1 spectral albedo was measured along a 6 km transect near the allan hills in east antarctica the transect traversed the sequence from new snow through old snow firn and white ice to blue ice showing a systematic progression of decreasing albedo at all wavelengths as well as decreasing specific surface area ssa and increasing density broadband albedos under clear sky range from 0 80 for snow to 0 57 for blue ice and from 0 87 to 0 65 under cloud both air bubbles and cracks scatter sunlight their contributions to ssa were determined by microcomputed tomography on core samples of the ice although albedo is governed primarily by the ssa and secondarily by the shape of bubbles or snow grains albedo also correlates highly with porosity which as a proxy variable would be easier for ice sheet models to predict than bubble sizes albedo parameterizations are therefore developed as a function of density for three broad wavelength bands commonly used in general circulation models visible near infrared and total solar relevance to snowball earth events derives from the likelihood that sublimation of equatorward flowing sea glaciers during those events progressively exposed the same sequence of surface materials that we measured at allan hills with our short 6 km transect representing a transect across many degrees of latitude on the snowball ocean at the equator of snowball earth climate models predict thick ice or thin ice or open water depending largely on their albedo parameterizations our measured albedos appear to be within the range that favors ice hundreds of meters thick

magellan a new view of venus geology and geophysics within the past fifteen years the surface of venus has gone from being the least well understood of all the terrestrial planets to the most thoroughly mapped surface of any terrestrial planet including the earth this is primarily due to the magellan mission which has collected a variety of data on the surface morphology physical properties and interior density structure of venus amounting to more than 1 terabit 1012 bits of data synthetic aperture radar images have been obtained for over 95 of the surface their high resolution reveals most surface features larger than 100 200 meters across using its radar altimeter magellan has collected data on surface elevations sub meter scale roughness and radar reflectivity at a resolution of approximately 10 km further information on the physical properties of the surface was gathered by measuring the passive microwave emissivity of the surface pettengill et al 1992 two way doppler tracking of the spacecraft has yielded line of sight los gravity data and a spherical harmonic model of gravity and geoid out to degree 75 collection of high resolution gravity data has been aided by an innovative aerobraking maneuver which used venus atmosphere to brake the spacecraft and lower it from a highly elliptical orbit to a near circular orbit

thermal evolution of venus abstract the theory of three dimensional and finite amplitude convection in a viscous spherical shell with temperature and pressure dependent physical parameters is developed on the basis of a modified boussinesq fluid assumption the lateral dependences of the variables are resolved through their spherical harmonic representations whereas their radial and time dependences are determined by numerical procedures the theory is then applied to produce thermal evolution models for venus the emphasis is on illustrating the effects of certain physical parameters on the thermal evolution rather than proposing a specific thermal history for the planet the main conclusions achieved in this paper are 1 a significant portion of the present temperature in the mantle and heat flux at the surface of venus is probably owing to the decay of a high temperature established in the planet at the completion of its core formation 2 the effective rayleigh number of the mantle is so high that even the lower order modes of convection cool the planet sufficiently and maintain an almost adiabatic temperature gradient in the convecting region and high temperature gradients in the thermal boundary layers 3 the convection is oscillatory with avalanche type properties which induces oscillatory features to the surface heat flux and the thickness of the crustal layer and 4 a planetary model with a recycling crust cools much faster than those with a permanently buoyant crust the models presented in this paper suggest that venus has been highly convective during its history until 0 5 ga ago the vigorous convection was bringing hot and fresh material from the deep interior to the surface and dragging down the crustal slags floating on the surface in to the mantle the rate of cooling of the planet was so high that its core has solidified in the last 0 5 ga the vigour of convection diminished considerably and the crustal slags developed into a global and permanently buoyant crustal layer the tectonic style on venus has consequently changed from the recycling of crustal plates to hot spot volcanics at the present time the planet is completely solid except in the upper part of its mantle where partial melting may occur

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

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

the geological evolution of atai mons venus a volcano corona hybrid atai mons venus is classified as a large volcano yet displays characteristics that are typical of coronae coronae are widespread volcano tectonic features that are apparently unique to venus detailed analysis of atai mons using photogeological and geophysical techniques illustrates that it has experienced a complex history and may still be active we show that processes found at both coronae and large volcanoes have occurred throughout the history of atai mons with no definite temporal preference it does not have an evolution that can be described simply as the evolution of one feature into another these results suggest that large volcanoes and coronae are more intimately linked than previously thought and that detailed mapping and gravity surveys are reliable ways of establishing the evolution of hybrid features

fortuna tessera venus evidence of horizontal convergence and crustal thickening structural and tectonic patterns mapped in fortuna tessera are interpreted to reflect a change in the style and intensity of deformation from east to west beginning with simple tessera terrain at relatively low topographic elevations in the east and progressing through increasingly complex deformation patterns and higher topography to maxwell montes in the west these morphologic and topographic patterns are consistent with east to west convergence and compression and the increasing elevations are interpreted to be due to crustal thickening processes associated with the convergent deformational environment using an airy isostatic model crustal thicknesses of approximately 35 km for the initial tessera terrain and crustal thicknesses of over 100 km for the maxwell montes region are predicted detailed mapping with magellan data will permit the deconvolution of individual components and structures in this terrain

growth of planetary crusts the planets and satellites of the solar system show much diversity but most have formed crusts which differ substantially from their bulk compositions three principal types of crust may be broadly distinguished primary formed after accretional heating e g lunar highlands secondary formed following partial melting in planetary mantles e g lunar maria terrestrial oceanic crust and tertiary formed by processing of secondary crusts of which the continental crust of the earth is the only identifiable example crustal growth before 3800 m y was complicated by the heavy planetesimal bombardment active erosion on the earth is likely to have destroyed the brecciated rubble resulting from over 200 mare orientale scale events in this period the mercurian crust is probably primary perhaps analogous to the lunar highlands in contrast the observable martian and venusian crusts are secondary dominated by basaltic volcanism the icy crusts of the satellites of the outer planets are mixtures of both primary and secondary crusts with complex histories ganymede is of special interest in displaying a secondary grooved terrain water ice crust which has split an older more heavily cratered possibly primary crust slight expansion has resulted probably from polymorphic transitions from high density ice viii to low density ice i the primary lunar crust 12 of lunar volume grew in 108 years secondary crusts derived by partial melting of mantles grow at a much slower rate total production of terrestrial oceanic crust over 4000 m y is only 2 of planetary volume whereas terrestrial continental crustal growth is even slower producing over the same period a volume only about 0 33 that of the earth crustal growth on the other planets appears to be an irreversible process without evidence of recycling no evidence for massive planetary expansion as required by expanding earth hypotheses is apparent on the other planets or satellites of the solar system

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

venus tectonics initial analysis from magellan radar imaging and altimetry data from the magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the venus surface the plains record a superposition of different episodes of deformation and volcanism strain is both areally distributed and concentrated into zones of extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression venus displays two geometrical patterns of concentrated lithospheric extension quasi circular coronae and broad rises with linear rift zones both are sites of significant volcanism no long large offset strike slip faults have been observed although limited local horizontal shear is accommodated across many zones of crustal shortening in general tectonic features on venus are unlike those in earth s oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide and separated by stronger and less deformed blocks hundreds of kilometers in width as in actively deforming continental regions on earth

surface radiative impacts of ash deposits from the 2009 eruption of redoubt volcano the solar broadband albedo change surface radiative forcing and snowmelt rate associated with ash deposits based on those from the 2009 eruption of redoubt volcano were calculated using the field corroborated loadings from the fall3d and the snow ice and aerosol radiation models the optical properties of ash were calculated from mie theory using size information from the fall3d model two sizes of snow grains were used in order to simulate a young and old snowpack the results show concentrations of aerosol sized ash in snow range from 6 9 104 to 1 108 ppb for the distal edge of the deposits located 100 570 km from the vent to the vent and integrated solar albedo reductions of 0 59 for new snow and 0 85 for old snow these albedo reductions are much larger than those typical for black carbon and are of the same order of magnitude as those reported for volcanic deposits in antarctica the daily mean surface shortwave forcings associated with ash deposits on snow were 0 96 w m 2 from the most distal deposits to the near vent deposits we show that forcings caused by ash deposits can be greater than those caused by dust deposits there were no accelerated snowmelts calculated for the edges of the deposits however for areas of higher ash concentrations daily melting rates were conservatively estimated to be 140 160 higher than those of pure snow we find that ash deposits from midsized volcanic eruptions can be a major agent of deposit induced snowmelt

parameterized thermal convection in a layered region and the thermal history of the earth parameterized convective schemes for constant viscosity fluids are used to investigate the influence of layering on the relationship between the rate of heat loss from and heat supply to a number of convecting layers of different viscosity and thicknesses simple analytic expressions are obtained which accurately predict the behavior to a step decrease in the heat supply and show that layering considerably increases the response time the same arguments are applied to a planet consisting of spherical shells they show that the response of the earth s heat loss to the decay of radioactive elements depends strongly on whether or not mantle convection extends from the core to the surface if it does so the surface heat flux is only about 50 greater than the heat generation rate however if upper and lower mantle convection occurs in two separate shells as seismological and geochemical arguments suggest the response time is increased the present surface heat flux is then principally controlled by the thermal structure of the earth at the time it was formed the best estimate of the present ratio of the heat loss to heat generation is about two and this value is consistent with the behavior of an earth model consisting of a two layered mantle overlying a core

effect of mesoscale eddies on the taiwan strait current abstractthis study shows that mesoscale eddies can alter the taiwan strait current the 20 yr data assimilated japan coastal ocean predictability experiment 2 jcope2 reanalysis data are analyzed and the results are confirmed with idealized experiments the leading wind forced seasonal cycle is excluded to focus on the effect of the eddy the warm eddy southwest of taiwan is shown to generate a northward flow whereas the cold eddy produces a southward current the effect of the eddy penetrates onto the shelf through the joint effect of baroclinicity and relief jebar the cross isobath fluxes lead to shelfward convergence and divergence setting up the modulation of the sea level slope the resulting along strait current anomaly eventually affects a wide area of the taiwan strait the stronger eddy leads to larger modification of the cross shelf flows and sea level slope producing a greater transport anomaly the composite sea viewing wide field of view sensor chlorophyll a chl a serves as an indic

venus volcanism initial analysis from magellan data magellan images confirm that volcanism is widespread and has been fimdamentally important in the formation and evolution of the crust of venus high resolution imaging data reveal evidence for intrusion dike formation and cryptodomes and extrusion a wide range of lava flows also observed are thousands of small shield volcanoes larger edifices up to several hundred kilometers in diameter massive outpourings of lavas and local pyroclastic deposits although most features are consistent with basaltic compositions a number of large pancake like domes are morphologically similar to rhyolite dacite domes on earth flows and sinuous channels with lengths of many hundreds of kilometers suggest that extremely high effusion rates or very fluid magmas perhaps komatiites may be present volcanism is evident in various tectonic settings coronae linear extensional and compressional zones mountain belts upland rises highland plateaus and tesserae volcanic resurfacing rates appear to be low less than 2 km 3 yr but the significance of dike formation and intrusions and the mode of crustal formation and loss remain to be established