characteristics of large terrestrial impact structures as revealed by remote sensing studies abstract remote sensing data sets of a spectrum of impact structures on earth have been analyzed in an effort to determine if such structures have identifiable signatures which could be used as an aid in searching broad areas for the missing craters predicted by current estimates of the cratering flux meteor crater arizona has been used as a control site due to the extensive data available for this feature laboratory thermal infrared reflectance spectra of shocked sandstones indicate that shock processes alter the appearance of fundamental absorption features enough to permit such effects to be observed in airborne thermal infrared imagery collected with the nasa tims instrument analysis of this tims imagery suggests that shocked and displaced target rocks can be readily identified aircraft multi frequency imaging radar data collected with the nasa jpl sar polarimeter clearly reveals the textural pattern of the preserved components of the ejecta blanket spot multi spectral imagery of bosumtwi ghana illustrates the difficulties associated with remote sensing in highly vegetated terrains nevertheless the imagery reveals the subtle expression of an outer tectonic ring at the crater and may be sensitive to geobotanical variations associated with impact related suevite deposits in contrast the zhamanshin impact feature on the arid sedimentary plains of kazakhstan is ideal for multi spectral remote sensing although much of the expected morphologic expression of the complex crater is missing or subdued virtually all of the field mapped impact related deposits display a unique spectral pattern in decorrelated landsat thematic mapper imagery the extensive deposits of allogenic breccias which cap the hills defining the inner ring of the crater are very distinctive as are certain of the impact glass deposits a second or outer ring at 13 13 5 km is apparent in the decorrelated principal component tm imagery the suggestion is that spectral disruption patterns for large multi ring impact features in arid settings should be identifiable from orbit by examining a pristine 144 km diameter multi ring impact feature on the venus rolling plains an impression of the original morphology of a continental impact crater of the magnitude of the cretaceous tertiary k t boundary impact structure can be deduced the 35 ma popigai structure is the only known 100 km class impact crater on earth approximately equal in age or younger than the k t event and its morphology is poorly expressed in single band landsat imagery thus spectral anomalies associated with key impact deposits such as allogenic breccias suevite melt and glasses may have to form the basis for identifying candidate k t impact sites in the absence of preserved morphologic elements of the crater it is apparent that remote sensing is not a panacea for the discovery and validation of new impact craters but it can contribute by serving as a pathfinder both for the identification of promising structures and the possible locations of key deposits for detailed fieldwork and sampling

a wind driven isopycnic coordinate model of the north and equatorial atlantic ocean 1 model development and supporting experiments numerical approximations to the dynamic equations are given which allow basin size ocean circulation models formulated in isopycnic coordinates to accommodate variable bottom topography and irregular coastlines emphasis is placed on computational economy through the use of a split explicit time integration scheme on the proper formulation of the advection and coriolis terms in the momentum equation in case of strongly varying layer thickness and on the correct estimation of the horizontal pressure gradient force in grid boxes truncated by steep bottom slopes the algorithms are tested in a series of two and three layer double gyre experiments in cases of steady forcing leading to a steady circulation we are able to reproduce the expected motionless final state in coordinate layers that are below the direct influence of the wind forcing this includes layers intersecting topographic obstacles a long term 25 30 year vacillation tentatively associated with the outcropping of isopycnals along the edge of the cyclonic gyre in a steadily forced model is documented the paper forms the basis for a subsequent study in which circulation features obtained in a realistic north atlantic setting will be discussed

atmospheric effects on ejecta emplacement laboratory experiments allow the investigation of complex interactions between impacts and an atmosphere although small in scale they can provide essential first order constraints on the processes affecting late stage ballistic ejecta and styles of ejecta emplacement around much larger craters on planetary surfaces the laboratory experiments involved impacting different fine grained particulate targets under varying atmospheric pressure and density different gas compositions during crater formation ballistic ejecta form the classic cone shaped profile observed under vacuum conditions as atmospheric density increases for a given pressure however the ejecta curtain bulges at the base and pinches above this systematic change in the ejecta curtain reflects the combined effects of deceleration of ejecta smaller than a critical size and entrainment of these ejecta within atmospheric vortices created as the outward moving wall of ejecta displaces the atmosphere additionally a systematic change in emplacement style occurs as a function of atmospheric pressure largely independent of density contiguous ejecta rampart superposing ballistically emplaced deposits 0 06 to 0 3 bar ejecta flow lobes 0 3 to 0 7 bar and radial patterns 0 8 bar underlying processes controlling such systematic changes in emplacement style were revealed by observing the evolution of the ejecta curtain by changing target materials including layered targets and low density particulates by varying atmospheric density by changing impact angle and by comparing the ejecta run out distances with first order models of turbidity flows three distinct ejecta emplacement processes can be characterized ejecta ramparts result from coarser clasts sorted and driven outward by vortical winds behind the outward moving ejecta curtain this style of wind modified emplacement represents minimal ejecta entrainment and is enhanced by a bimodal size distribution in the ejecta such eddy supported flows are observed to increase in run out distance scaled to crater size with increasing atmospheric pressure by analogy with turbidity flows this scaled distance should increase as r1 2 for a given atmospheric pressure and degree of entrainment ejecta flows with much greater run out distances develop as the turbulent power in atmospheric response winds increase such flows overrun and scour the inner ejecta facies thereby producing distinct inner and outer facies the degree of ejecta entrainment depends on the dimensionless ratio of drag to gravity forces acting on individual ejecta and the intensity of the winds created by the outward moving curtain entrainment increases with increasing atmospheric density and ejection velocity crater size but decreases with ejecta density and size the intensity of curtain generated winds increases with ejection velocity crater size the dimensionless drag ratio characterizing the laboratory experiments can be applied to mars since the reduced atmospheric density is offset by the increased ejection velocities for kilometer scale events for a given crater size ejection velocity and atmospheric conditions a wide range of nonballistic ejecta emplacement styles could occur simply by varying ejecta sizes even without the presence of water alternatively the onset crater diameter for nonballistic emplacement styles can reflect the range of ejecta sizes possible from the diverse martian geologic history massive basalts to fine grained aeolian deposits scaling considerations further predict that ejecta run out distances scaled to crater size on mars should increase as r1 2 hence long run out flows dependent on crater diameter need not reflect depth to a buried reservoir of water on venus however the dense atmosphere maximizes entrainment and results in ejecta flow densities approaching a constant fraction of the atmospheric density under such conditions ejecta run out distances should decrease as r 1 2

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

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

postimpact modification by volcanic or tectonic processes as the rule not the exception for venusian craters 1 we present three investigations that use the venusian impact crater population to constrain the planet s resurfacing history we evaluate stereo derived topography for 91 venusian craters that have a diameter d greater than 15 km craters with radar bright floors have greater rim floor depths and rim heights than craters with radar dark floors for the bright floored craters rim floor depths are dbf 0 483 d0 165 and rim heights are rhbf 0 056 d0 483 trends for dark floored craters are ddf 0 424 d0 108 and rhdf 0 181 d 0 025 for a 60 km crater this represents differences of 290 m in rim floor depth and 240 m in rim height we interpret these results to indicate that dark floored craters have experienced postimpact volcanic embayment and filling we examine the population of craters with d 20 km that have radar dark halos surrounding their continuous ejecta 114 craters we find that a portion of the halo has been removed for almost all dark floored craters consistent with our interpretation that dark floored craters have been affected by postimpact volcanism finally we assessed geologic histories of 12 large impact structures with stereo coverage all but one of these structures has experienced postimpact volcanism or tectonic deformation often in multiple episodes in summary widespread volcanic and tectonic activity occurred throughout the time period of emplacement of the crater population postulated resurfacing histories that consider the majority of craters to be at the top of the stratigraphic column are invalid and the mean surface age of venus is young 150 my

impact craters of venus a continuation of the analysis of data from the venera 15 and 16 spacecraft additional studies of venusian impact craters have been made based on an analysis of venera 15 16 radar imagery and altimetry the crater population on venus has been subdivided into groups representing different morphological classes and types the craters display the size dependent variations in morphology which are well known from other planets assuming a crater production rate based on estimates by hartmann et al 1981 their areal density indicates an age for the total population of approximately 1 b y alternative estimates based on the assumption that the recent cratering rates on the earth and venus were similar would give a significantly lower age the areal and size frequency distribution of a number of circular features of unclear origin have also been analyzed and it is proposed that some of these features may be highly degraded and modified impact craters corresponding to a population approximately 3 b y in age a suggested model of the effect of the atmosphere on the crater diameter frequency curve indicates that cometary impacts played only a minor role in forming the contemporary venus environment a comparative study of craters in the area where the arecibo earth based radar and venera 15 16 images overlap indicates that the radar dark inner zones of the craters in the high incidence angle arecibo images correspond to the flat bottoms of the craters seen in the venera images

karst channelled intertidal submarine groundwater discharge sgd conditions the form of the rock pool sessile assemblage abstract freshwater submarine groundwater discharge sgd frequently occurs in areas of karst coastline where karst aquifers are hydraulically connected to the sea though up to 25 of the world s coastline is karst little work has been done to determine the ecological impacts of karst channelled freshwater sgd reported herein are the effects of karst channelled freshwater sgd on the rock pool sessile assemblage in the locale of olhos de agua on the south portuguese coast two quadrat surveys were conducted in eight intertidal rock pools which ranged in freshwater sgd content from 0 to 57 3 salinity and nitrogen no3 and no2 concentrations were measured in the eight surveyed rock pools as well as a further three rock pools on the same plateau three inland groundwater boreholes and the adjacent coastal marine environment there was a strong negative correlation between salinity used as a proxy for freshwater sgd and nitrogen concentration r2 0 98 p

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

response of the northern north atlantic and arctic oceans to a sudden change of the north atlantic oscillation 1 the influence of a switch from a long time high north atlantic oscillation nao state to an enduring nao situation is investigated with a coupled sea ice ocean model we compare the response to a sustained shift to nao conditions with that to a short lived transition to nao and back to nao forcing observed changes in sea ice and oceanic variables between high and low nao states are well captured in the model response the ocean circulation adjustment includes a fast barotropic anomaly accompanied by an enhancement of the meridional overturning and the northward heat transport at 48 n the slow response contains a substantial decrease of the northward heat transport which is caused by a reduction of the transport of subpolar and subtropical gyres after a delay of around 2 years the oceanic heat transport reacts to counteract the meridional temperature gradient imposed by the surface forcing the propagation time of baroclinic rossby waves is irrelevant for the delay between the forcing and the oceanic response rather the strength of the subpolar gyres is determined by the rapid spin up of a topographic sverdrup circulation and the subsequent slower changes through the jebar joint effect of baroclinicity and relief in an experiment where only the wind stress is changed to nao conditions the adjustment is dampened strongly by the nao thermohaline forcing the gyres decrease slower than in the nao case comparison with an experiment where only the wind stress is changed to nao conditions indicates that the thermohaline forcing is most important the heat fluxes counteract the wind stress forcing such that the meridional overturning and the northward heat transport decrease only slightly compared to the control run

atmospheric effects on ejecta emplacement and crater formation on venus from magellan the venus cratering record provides a unique environment for assessing the effects of both gravity and an atmosphere on impact crater formation this contribution uses surface signatures of energy partitioning as a framework for testing extrapolations from laboratory experiments and other planetary settings seven general conclusions can be drawn first the dense lower atmosphere of venus takes on the role of a low density target for bodies smaller than about 4 km in diameter air blasts created by cratering in the atmosphere create distinctive surface signatures that allow the derivation of an independent assessment of impactor energy at the limit of break up second dynamic pressures during entry of larger bodies will exceed their strength limit but may not prevent penetration of the atmosphere due to aerodynamic reshaping that minimizes the drag coefficient such a process may account for the formation of unusually small craters 1 3 km third the dense atmosphere of venus preserves signatures of early time cratering processes on the surface that are typically lost on atmosphere free surfaces such signatures not only provide another estimate of impactor energy but also include a distinctive record of the impactor i e comet versus asteroid in distinctive run out flows created before the crater has finished formation strong winds and turbulence associated with the atmospheric disturbance at later times create wind streaks behind topographic barriers fourth ballistic ejection of excavated debris occurs from craters on venus just as it does on planets without an atmosphere thereby underscoring the fundamental mechanical transfer of energy from impactor to target fifth ejecta emplacement is nonballistic due to the large dynamic forces acting on the advancing curtain and its constituent ejecta the outward moving ejecta curtain induces strong response winds that entrain ejecta and drive a ground hugging debris flow outward without returning to the cavity flow separation creates an overriding run out ejecta flow further sustained by atmospheric turbulence and identified as radar dark lobate lobes sixth radar dark parabolas are proposed to be late time fallout deposits created as the downrange fireball evolves aloft perhaps analogous to terrestrial tektite strewn fields and seventh the response of crater formation to the atmosphere is conversely expressed by a reduction in cratering efficiency as revealed by the unusually large central peak complexes and the unexpected diameter depth relations of craters hence surface ages may be significantly underestimated

direct determination of total and fresh groundwater discharge and nutrient loads from a sandy beachface at low tide cape henlopen delaware total groundwater discharge from a sandy seepage site to the delaware bay at cape henlopen delaware was determined together with the associated nutrient loads by trapping the discharge from a length of the shoreline in a tidal pond at low tide and measuring the discharge of the trapped water through a weir at steady state salinity was used to parse the total groundwater discharge from the beachface at low tide into a recycled estuarine component and a new fresh groundwater component based on 16 measurements over 18 months average total discharge and average freshwater discharge at low tide were found to be 2 76 6 1 08 and 0 87 6 0 43 l min21 m21 of shoreline respectively the estuarine component of discharge varied with the maximum height of the immediately preceding high tide owing to the control of freshwater discharge by the average upland hydraulic gradient away from the beach individual measurements of freshwater discharge at low tide represent a good estimate of this discharge component over time scales of hours to days based on the nutrient concentrations in the discharge waters nutrient loads from the beach to the adjacent estuary at low tide were 148 6 79 7 5 6 5 1 and 197 6 101 mmol min21 m21 of shoreline for total dissolved n p and si respectively groundwater discharge and these nutrient fluxes contribute significantly to the unusually productive benthic communities that inhabit the seepage and nearby intertidal zones estuaries receive their primary inputs from fluvial discharges and exchange with the ocean but it is increasingly evident that atmospheric deposition and direct groundwater discharges from uplands also play a significant role in estuarine biogeochemical processes paerl 1997 burnett et al 2003 although groundwater discharge is thought to represent only a small fraction of the total freshwater discharge to estuaries and the ocean there is increasing evidence that nutrients and contaminants associated with this discharge may play a more significant ecological role than implied by the relatively small discharge valiela et al 1992 burnett et al 2003 shaw 2003 this is particularly true in coastal plain settings where nutrient and contaminant inputs to the groundwater from domestic industrial municipal and agricultural practices may be high the hydraulic conductivity of soils and aquifer sediments is large and the topographically driven groundwater flow to shallow upland streams is low submarine groundwater discharge in many settings appears to be associated with productive benthic communities johannes and hearn 1985 bussmann et al 1999 zipperle and reise 2005 and may influence sediment water exchange through the imposition of hydraulic gradients the modification of subbottom flow paths and the alteration of diagenetic reactions and rates moore

floor fractured impact craters on venus implications for igneous crater modification and local magmatism regional tectonism and volcanism affect crater modification and crater loss on venus but a comparison of venusian craters to lunar floor fractured craters suggests that a third style of more localized crater controlled magmatism also may occur on venus based on lunar models for such magmatism venusian crustal conditions should generally favor crater filling volcanism over crater centered floor fracturing nevertheless three craters on venus strongly resemble extensively modified craters on the moon where deformation can be attributed to failure over large crater centered intrusions models for crater modification over igneous intrusions indicate typical magmatic pressures beneath these three craters of 200 300 bars and intrusion depths of the order of 1 6 km all three craters also share common settings and low elevations whereas craters embayed by regional volcanism preferentially occur at much higher elevations on venus we suggest that the style of igneous crater modification on venus thus may be elevation dependent with crater centered intrusions primarily occurring at low elevations on venus this interpretation is consistent with theoretically predicted variations in magmatic neutral buoyancy depth as a function of atmospheric pressure suggested by other authors

orogeny and large scale strike slip faulting on venus tectonic evolution of maxwell montes the ishtar terra highland region of venus contains mountain belts morphologically similar to terrestrial orogenic belts akna montes and freyja montes are long linear mountain belts with about 4 km of relief above the 3 to 4 km high lakshmi planum the maxwell montes mountain belt however is more rectangular in plan view and has relief of approximately 8 km we have investigated the nature of this latter range using complementary radar images obtained by the arecibo observatory and the venera 15 16 spacecraft as well as allimetry data obtained by the venera 15 16 and pioneer venus spacecraft geological and structural maps have been compiled on the basis of these data the individual features and their stratigraphic relationships interpreted and a model for the origin and evolution of maxwell montes is proposed the dominant features of this mountain range are long parallel ridges and valleys that are intepreted to be anticlines and synclines and to have resulted from ene wsw oriented compression two major shear zones bound maxwell montes on the north and south converging toward the west nine narrow linear parallel features several hundreds of kilometers long identified as cross strike discontinuities csds cut obliquely across the strike of the ridges and valleys in a nw se direction and disrupt the structural fabric of the mountain range dividing maxwell montes into 10 crustal domains among several possible origins we find that the data are most consistent with their interpretation as right lateral strike slip faults with offsets of up to 125 km retrodeformation and reconstruction of maxwell montes using the offsets determined for individual domains produces a long linear mountain range similar to akna montes using geologic unit maps and topographic maps of the present configuration of maxwell similar reconstructions were made these reconstructions restored several linear tectonic elements topographic trends and sinuous unit boundaries to more contiguous positions on the basis of these data and observations we suggest that akna montes may represent the initial form of compressional orogenic belts on venus while maxwell montes attained its present morphology through a second stage of deformation involving large scale strike slip faulting we examine several models for this second stage and favor the interpretation that it occurred as the akna montes like proto maxwell montes was transported to the west between two converging shear zones accompanied by rotation of the entire mountain range thirty percent shortening of the mountain range is implied by the geometries of the present and reconstructed maxwell montes on the basis of the deforamation in maxwell montes and the evidence for large scale transport we conclude that large scale horizontal a movement of crustal materials is a significant aspect of the tectonics of venus

a hydrogeological and hydrochemical explanation of the groundwater composition under irrigated land in a mediterranean environment algarve portugal in the campina de faro in the south of portugal agricultural practices have a large impact on groundwater composition these practices involve pumping of water for irrigation from combinations of large diameter shallow wells noras and small diameter deep boreholes furos excess irrigation water returns to the aquifer and mixes with water from the regional groundwater flow system this irrigation return flow is concentrated by strong evapotranspiration and by flushing of fertilisers the concentration increase induces cation exchange whereby ca on the soil exchanger is replaced by na the mixing in the aquifer allows application of a mixing cell model which may then be used to calculate transmissivities from the cl mass balance the calculations are complicated by the time variant behaviour of cl and the method is adjusted to calculate the change of chloride in time results from the calculations appear to be in good agreement with hydrochemical observations

onset of mantle plumes in the presence of preexisting convection the onset of plumes where broad scale convection already exists is investigated along with the thermal history of the earth geological evidence relating to the onset time of mantle plumes is not definitive no well defined hotspot tracks older than mesozoic have been identified but it is difficult to tell whether this absence is a reliable indication of a late onset of mantle plumes since only continental crust of this age exists using the hotspot track distribution for the pacific the absence of pre mesozoic hotspot tracks on the canadian shield cannot easily be attributed to chance however young continental hotspot tracks are controversial and it is not clear whether tracks such as the monteregian hills white mountain magma series would be recognized as such without considering the contributing evidence of the adjacent oceanic crust and its well defined seamount chains our simple models yield potentially testable predictions the instability of plume formation in a system where broad scale convection already exists depends on horizontal conduit flow in the basal boundary layer toward the plume source geographically plumes are most likely to form where the boundary layer is thickest and where broad scale ascending flow already exists parameterized convection indicates that the heat transport by conduit flow in the boundary layer toward plumes depends linearly on the ratio of effective viscosity for broad scale convection to effective boundary layer viscosity in terms of material properties this ratio is the ratio of viscosity across the boundary layer to about the 0 45 power for the case where the broad scale convection is heated from below the viscosity contrast across the boundary layer depends on the temperature contrast across the layer a late onset of mantle plumes is implied if the temperature contrast at the base of the mantle has increased with time that is if the core has cooled more slowly than the interior of the mantel in one scenario before about 3 b y b p the mantle temperature varied slowly because surface heat flow was nearly balanced by internal radioactivity during this time the mantle was hotter than present and its internal viscosity lower hie temperature contrast at the base of the mantel was thus small after 3 b y internal radioactivity no longer balanced surface heat flow and the mantle cooled about a few hundred degrees celsius as indicated by petrological observations the core cooled slower than the mantle and the temperature contrast across the bottom boundary layer increased with time leading to a late onset of plumes phenomenological inferences are obtained as venus may have cooled more rapidly than the earth its mantle may be too cool for rapid plate tectonics and the gravity field and the convective pattern are dominated by hot upwelling regions

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

new evidence for global tectonic zones on venus venera 15 and 16 spacecraft images show clear evidence of major crustal disruptions on venus which have been interpreted to indicate crustal divergence complementary to the divergent zones are mountain belts that border the continent like high terrains the requisite transcurrent motions appear to be manifested as diffuse shear zones the rift zones form an interconnected transpolar system which ties in with previously recognized equatorial disruption zones suggesting a global tectonic network several independent lines of evidence suggest that the tectonism may be geologically young

a damped decadal oscillation in the north atlantic climate system a simple stochastic atmosphere model is coupled to a realistic model of the north atlantic ocean a north south sst dipole with its zero line centered along the subpolar front influences the atmosphere model which in turn forces the ocean model by surface fluxes related to the north atlantic oscillation the coupled system exhibits a damped decadal oscillation associated with the adjustment through the ocean model to the changing surface forcing the oscillation consists of a fast wind driven positive feedback of the ocean and a delayed negative feedback orchestrated by overturning circulation anomalies the positive feedback turns out to be necessary to distinguish the coupled oscillation from that in a model without any influence from the ocean to the atmosphere using a novel diagnosing technique it is possible to rule out the importance of baroclinic wave processes for determining the period of the oscillation and to show the important role played by anomalous geostrophic advection in sustaining the oscillation

cleopatra crater on venus venera 15 16 data and impact volcanic origin controversy cleopatra structure is a 100 km diameter feature having a morphology similar to that of double ring basins of the moon and mercury and dissimilar to that of volcanic calderas on mars earth and venus the 2 4 km depth of cleopatra is anomalously large compared to venusian and terrestrial impact craters of equivalent diameters an impartial summary of the situation is as follows if cleopatra is a volcanic caldera it is a strange caldera if cleopatra is an impact crater it is a strange crater

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

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

interdecadal variations of the thermohaline circulation in a coupled ocean atmosphere model abstract a fully coupled ocean atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the north atlantic ocean with a time scale of approximately 50 years the irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation approximately 52 n to 72 n combined with much smaller density anomalies of opposite sign in the broad rising region the spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the north atlantic the anomalies of sea surface temperature induce model surface air temperature anomalies over the northern north atlantic arctic and northwestern europe

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

an ocean model s response to north atlantic oscillation like wind forcing the response of the atlantic ocean to north atlantic oscillation nao like wind forcing was investigated using an ocean only general circulation model coupled to an atmospheric boundary layer model a series of idealized experiments was performed to investigate the interannual to multi decadal frequency response of the ocean to a winter wind anomaly pattern overall the strength of the sst response increased slightly with longer forcing periods in the subpolar gyre however the model showed a broad response maximum in the decadal band 12 16 years

the influence of mantle water and heterogeneity on the viscous relaxation of venus impact craters empirical flow laws for peridotite and diabase were used to model the viscous relaxation of large impact craters on venus the models were then compared to the depths of the three largest impact craters 60 km diameter measured by venera 15 16 radar altimetry the maximum crater retention times predicted by models that assumed 0 03 wt water in the upper mantle were inconsistent with the observed depths of these craters and a mean minimum age of 0 5 ga models that assumed an upper mantle water content of 0 003 wt were consistent with the observations and a maximum crater retention time of 1 0 ga this result was insensitive to up to 60 by volume diabase mixed into the upper mantle as inclusions at the several kilometer scale and to the presence of a basalt eclogite or melting transition at a depth of 100 km

combined time series resistivity and geochemical tracer techniques to examine submarine groundwater discharge at dor beach israel 1 a high resolution stationary geophysical and geochemical survey was conducted at dor beach israel to examine the shallow coastal hydrogeology and its control on the exchange of submarine groundwater with the shallow mediterranean sea time series resistivity profiles using a new 56 electrode 112 m long marine cable produced detailed profiles of the fresh water salt water interface and the subtle response of this interface to tidal excursions and other forcing factors such information when ground truthed with representative pore water salinities and formation resistivity values can provide unique information of the extent and rates of submarine groundwater discharge sgd time series 222rn measurements of the adjacent coastal water column complemented these geophysical techniques and were modeled to yield integrated advective flow rates across the sediment water interface which at dor beach ranged from about 0 to 30 cm day 1 mean 7 1 cm d 1 depending on the tidal range such results suggest that the underlying hydrogeologic framework at dor is favorable for substantial sgd extrapolating these sgd estimates across a 100 m wide coastal zone implies that the rn derived sgd rate would equal 7 1 m3 d 1 per m of shoreline and that the source of this discharging groundwater is a complex mixture of fresh groundwater derived from the upland kurkar deposits as well as locally recycled seawater

bunte breccia of the ries continuous deposits of large impact craters the ries crater an impact structure of 26 km diameter in south germany is the largest terrestrial crater where substantial amounts of ejecta are preserved on occasion 100 m deep further the target stratigraphy is well known and it is possible to relate specific clasts and breccia lithologies to initial target depth as a consequence the continuous deposits of the ries also known as bunte breccia may be studied with exceptional field control we report field observations and laboratory analyses obtained from 560 m core materials taken at nine different locations that range from 16 to 37 km in radial distance from the impact center the objective is to relate the ries observations to ejection and to emplacement processes of large scale planetary crater deposits the observations regarding the modal stratigraphic characteristics of the bunte breccia may be summarized as follows only 600 m some 0 7 is composed of triassic clasts originating from 300 to 600 m depths lower and middle jurassic horizons approximately 300 150 m constitute some 2 3 and upper jurassic 0 150 m makes up some 31 5 in addition the bunte breccia contains tertiary materials in the form of 1 cm clasts 29 1 and as highly comminuted fine grained matrix 10 gpa constitute 0 1 weight of the entire deposit which indicates that bunte breccia was emplaced at essentially ambient temperatures when possible the above observations are quantified via linear regressions throughout the text all of these observations are consistent with if not predicted by a ballistic emplacement scenario as postulated by oberbeck and co workers primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate a mixture of primary and secondary ejecta results and combines into a highly turbulent ground hugging debris surge as the final phase of ejecta emplacement total emplacement time for the bunte breccia 200 km is estimated to be of the order of 5 min only these findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so called z model addressing material flow during various stages of crater formation qualitative to fair agreement of observations and predictions results an initial crater radius of 6 5 km an excavation depth of 1650 m an excavation volume of 136 km and an associated transient cavity volume of aproximately 230 km appear to be reasonable estimates approximately 170 km of material was involved in slumping and restoration of the transient cavity for the above radius and z 2 7 the modal composition of ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large complex planetary craters are predominantly derived from depths as small as one hundredth the apparent crater diameter a number of implications are addressed regarding remote sensing of planetary surfaces and investigation of lunar and meteoritic impact breccias

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

cleopatra patera on venus venera 15 16 evidence for a volcanic origin the non concentric nature anomalous depth and terraced morphology of the nested craters that compose cleopatra patera are more closely analogous to volcanic craters calderas than multi ring impact structures associated deposits northeast and downslope of the patera first recognized on venera 15 16 radar images are interpreted as volcanic plains related to and perhaps cogenetic with cleopatra the plains lavas probably originated as effusions from a radial rift zone or ring fissures rim deposits surrounding cleopatra are asymmetric along the structural fabric of the region also indicating fissure type eruptions finally a volcanic origin not only is easily reconciled with the tectonic setting of the patera it is almost required by the correlation between the patera and regional structural trend

methodological study of submarine groundwater discharge from a karstic aquifer in the western mediterranean sea summary a multiproxy approach of complementary techniques is applied to localise and quantify submarine groundwater discharge sgd from el maestrazgo jurassic aquifer an example of widespread carbonate coastal aquifer of the western mediterranean sea castellon spain unconventional water resources are critical in this area where highly populated resorts have been developed the aquifer has two main features of particular interest a karstic system with a deep saturated zone reaching 450 m and with no significant surface discharge occurring within the continental area on the other hand sgd can alter the condition of coastal marine environments reducing salinities and providing nutrients that can cause eutrophication and algal blooms the applied combined techniques make it possible to detect groundwater outflows using airborne thermal infrared tir images to monitor physico chemical anomalies in the sea and to quantify the submarine groundwater discharges by means of direct measurements and radium isotopic analyses three main coastal areas affected by sgd and a submarine spring located at 3 km from the coast which seems to be associated with tectonic discontinuities were localised the calculated fresh sgd from ra isotopes 375 mm 3 y 1 is in agreement with the mean aquifer recharge obtained by hydrogeological methods 394 mm 3 y 1

response of the north atlantic subpolar gyre to persistent north atlantic oscillation like forcing the response of the north atlantic subpolar gyre spg to a persistent positive or negative phase of the north atlantic oscillation nao is investigated using an ocean general circulation model forced with idealized atmospheric reanalysis fields the integrations are analyzed with reference to a base line integration for which the model is forced with idealized fields representing a neutral state of the nao in the positive nao case the results suggest that the well known cooling and strengthening of the spg are after about 10 years replaced by a warming and subsequent weakening of the spg the latter changes are caused by the advection of warm water from the subtropical gyre stg region driven by a spin up of the atlantic meridional overturning circulation amoc and the effect of an anomalous wind stress curl in the northeastern north atlantic which counteracts the local buoyancy forcing of the spg in the negative nao case however the spg response does not involve a sign reversal but rather shows a gradual weakening throughout the integration the asymmetric spg response to the sign of persistent nao like forcing and the different time scales involved demonstrate strong non linearity in the north atlantic ocean circulation response to atmospheric forcing the latter finding indicates that analysis based on the arithmetic difference between the two nao states e g nao minus nao may hide important aspects of the ocean response to atmospheric forcing

karst groundwater resources in the countries of eastern mediterranean the example of lebanon the hydrogeological study of some karst systems in lebanon shows important storage capacity up to 27 billions m3 for zarka system the spring of orontes river their geological and morphological settings as well as their hydrodynamic function show structures developed below the level of present springs probably up to several hundreds meters at depth since miocene those carbonate formations were subject to a remarkable instability of their base levels variations of sea level of which the messinian salinity crisis is the major phase and effects of the alpine orogenesis combined with climate variations to develop complex multiphased karst systems karstification occurs frequently in the whole formation often deeply below sea level or underneath sediment filling of continental basins the surface karst landscape is often intensely eroded those multiple superimposed karst phases give the carbonate aquifers their large extension and storage capacity these karst aquifers show the typical easy restoration of storage they have the ability to bear large interannual recharge variations and support exploitation under high pumping rates however these aquifers have some disadvantages for sustainable management in some regions successful boreholes which allow high pumping rates also induce uncontrolled exploitation another consequence is natural seawater intrusion as well as the submarine discharge of fresh groundwater in coastal aquifers those carbonate aquifers are subject to important economical stress which jeopardizes their durability the systematic study and understanding of carbonate aquifers in eastern mediterranean is a preliminary condition to any integrated and sustainable management of water resources studies in progress in lebanon may serve as examples to the whole region

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

decadal variability of subpolar gyre transport and its reverberation in the north atlantic overturning 1 analyses of sea surface height ssh records based on satellite altimeter data and hydrographic properties have suggested a considerable weakening of the north atlantic subpolar gyre during the 1990s here we report hindcast simulations with high resolution ocean circulation models that demonstrate a close correspondence of the ssh changes with the volume transport of the boundary current system in the labrador sea the 1990s decline of about 15 of the long term mean appears as part of a decadal variability of the gyre transport driven by changes in both heat flux and wind stress associated with the north atlantic oscillation nao the changes in the subpolar gyre as manifested in the deep western boundary current off labrador reverberate in the strength of the meridional overturning circulation moc in the subtropical north atlantic suggesting the potential of a subpolar transport index as an element of a moc monitoring system

venus crater related radar dark parabolas and neighboring terrains a comparison of 1 μm emissivity and microwave properties abstract this work presents a comparative study of microwave properties and 1 m emissivity for five venus crater associated radar dark parabolas along with neighboring plains and some other geologic units craters of interest are adivar bassi bathsheba sitwell and du chatelet with nearby crater caccini having non parabolic radar dark halo microwave properties under study include emissivity fresnel reflectivity decameter scale surface roughness and radar cross section from the magellan mission 1 m emissivity was derived using venus monitoring camera vmc measurements on board of venus express vex results of our analysis show that 1 m emissivity which characterizes the uppermost hundreds microns thick layer of parabola mantles usually relates to microwave properties as following the lower 1 m emissivity the lower fresnel reflectivity the higher microwave emissivity for loose mantles having the same composition lower 1 m emissivity indicates smaller particles differences in bulk properties of parabola units showing the same 1 m emissivity appear to reflect differences in a packing style of mantles consisting of particles having similar sizes properties of radar bright inner parabola parts observed within the three of five studied parabolas possibly indicate more turbulent comparing to radar dark parts deposition environment thinner parabola mantles and or partial coverage of the underlying surface the non parabolic halo of the crater caccini exhibits characteristics close to dark parabolas suggesting that in a process of shrinking of a parabolic like deposit into non parabolic one parabola mantle properties remain mainly unchanged observed differences in microwave emissivity and fresnel reflectivity between parabolas and adjacent plains may indicate highly weathered loose parabola material with oxidation of their iron into hematite that is more easily accessible for atmospheric gases studied properties of tessera terrains confirmed suggestions of earlier works for a possible non basaltic composition of tesserae material the distinctive from plains composition of tesserae also could indicates an effective down slope movement of these material uncovering new portions of pristine tessera material otherwise considered in this work horizontal transportation of the very thin layer of surface material would equalize the observed properties

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

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

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

influence of submarine springs and wastewater on nutrient dynamics of caribbean seagrass meadows abstract the east coast of the yucatan peninsula mexico consists of highly permeable limestone such that surface flow and rivers are absent in this region extensive underground cave systems connect sink holes cenotes to submarine springs ojos de aqua which vent into the seagrass meadows of the adjacent oligotrophic coastal lagoons this study investigated the potential for these submarine springs to influence nutrient processes within seagrass meadows by assessing nutrient status of thalassia testudinum meadows in two contrasting coastal lagoons along the north eastern yucatan peninsula tissue nutrient concentrations as well as 15n values of t testudinum were surveyed in the puerto morelos reef lagoon and the nichupte lagoon system cancun hotel zone during an extended dry period and again following heavy rainfall after a period of heavy rainfall t testudinum near submarine springs in puerto morelos reef lagoon had exceptionally high leaf tissue phosphorus concentrations of 0 38 0 06 these submarine springs may have been a direct source of phosphorus and or a source of iron to this very iron limited carbonate system thalassia testudinum nutrient concentrations suggest that nitrogen loading to the nichupte lagoon system is regionally high and has increased over the past decade mean leaf n 2 04 n in 1991 to 2 71 n in 2002 nitrogen content in leaf tissue of t testudinum was significantly higher within the poorly flushed nichupte lagoon system 2 93 0 12 n than in the well flushed puerto morelos reef lagoon 1 80 0 07 n stable isotope ratios of nitrogen suggest that this high and increasing nitrogen loading within the nichupte lagoon system is a result of wastewater nitrogen 15n 9 06 0 07 in northern nichupte lagoon system vs 1 69 0 07 in puerto morelos reef lagoon

submarine springs and coastal karst aquifers a review this article reports on current knowledge of coastal karst aquifers in which conduit flow is dominant and its aim is to characterise the functioning of these systems which are closely linked to the sea first earlier and recent studies of these aquifers are discussed on the basis of their findings it can be shown that two essential mechanisms are involved in the functioning of these systems i e aquifer discharge through submarine springs and saline intrusion through conduits open to the sea then the conditions that give rise to these aquifers are described and particular emphasis is placed on the influence of deep karstification when the sea level falls the base level variations are attributed to the glaciations or in the specific case of the mediterranean to the salinity crisis in the messinian period it is this inherited structure sometimes containing very deep conduits below sea level that today conditions the aquifer flow the flow in the conduits open to the sea depends on the hydraulic head gradient between the aquifer and the sea and is therefore a function of the water density and head losses in the aquifer this survey of coastal karst aquifers has revealed some common characteristics that show the development and or functional capacity of their karstic drainage networks a classification of such systems into three categories is proposed with the aim of assisting in the decision making concerning potential exploitation of water resources in coastal regions

effects of nao variability on the north atlantic ocean circulation 1 the ocean response to an idealised north atlantic oscillation like wind stress is evaluated as a function of the atmospheric forcing frequency in order to elucidate the relationship between internal and forced variability the nao is modulated with two specific timescales 10 and 50 years which characterise the spectrum of the system s internal variability different timescales of atmospheric variability select distinct sea surface temperature sst and large scale circulation patterns under a 50 year nao forcing period a lagged sst response is excited in the gulf stream extension region which is consistent with the spin up of the gyre circulation the thermohaline circulation varies in phase with the nao and shows a strong sensitivity to the forcing frequency a dipole mode of the overturning is excited by a 10 year modulation of the nao while an enhanced overturning response emerges under a 50 year nao with low enough lateral mixing the ocean exhibits an irregular response to a regular nao like forcing

gravity anomalies compensation mechanisms and the geodynamics of western ishtar terra venus pioneer venus line of sight orbital accelerations have been used to calculate the geoid and vertical gravity anomalies for western ishtar terra on various planes of altitude z0 the apparent depth of isostatic compensation at z0 1400 km is 180 20 km based on the usual method of minimum variance in the isostatic anomaly we seek to explain this observation as well as the regional elevation peripheral mountain belts and inferred age of western ishtar terra in terms of one of three broad geodynamic models under local mantle upwelling western ishtar is the surface expression of a hotspot which produces topography both dynamically and by volcanic construction this model holds that the mountain belts form as a result of incipient downward mantle return flow an upwelling with its centroid at 160 km depth and buoyancy stress 100 mpa can satisfy the observed long wavelength geoid and topography provided that the crust mantle density contrast does not control the response of the crust so that late time subsidence does not occur and that the mantle viscosity does not increase markedly with depth mantle upwelling produces a combination of strike slip motion and azimuthal extension on the uplift itself radial compression is restricted to the surrounding plains under local mantle downwelling cold sinking mantle beneath western ishtar induces shear tractions on the crust which thicken and elevate it volcanism is potentially the result of basal crustal remelting the formation of mountain belts is assumed to be a by product of crustal thickening a deeply seated downwelling 300 700 km buoyancy stress 200 300 mpa can satisfy the geoid and central topography provided that the crust can respond separately to mantle flow and that a more viscous lower mantle exists the predicted surface strains during intermediate stages of the uplift are in qualitative agreement with observations azimuthally oriented thrusts at the uplift margin incorporation of the effective viscosities of diabase and olivine suggests that uplift above a plume will be rapid but the thermal gradient over downgoing flow must be at least as high as the expected mean value for venus 20 k km in order to attain substantial uplift within several hundred million years a third scenario regional compression may be distinguished from the second in that western ishtar is simply a locus of strain accumulation and crustal thickening from regional stress fields neither broad downward mantle flow nor lithospheric subduction need occur however both the source of stress and geoid anomaly are problematic if the topography of western ishtar is assumed to be airy compensated then the geoid signature must arise from unrelated mantle density anomalies in this case the lithosphere must be decoupled from mantle flow or else the mantle must be effectively rigid in order to suppress dynamic topography a more complex model than those analyzed in detail here is needed to produce both the correct surface strains and apparent compensation depth we favor upwelling models in which a mantle plume has flattened out and the surface has been subsequently deformed western ishtar terra must be distinguished from other paradigm hotspots on venus by different environmental conditions or by being in a unique stage of hotspot evolution further quantitative investigations of the interaction of mantle shear tractions with a laterally heterogeneous lithosphere are required

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

tessera terrain venus characterization and models for origin and evolution tessera terrain is the dominant tectonic landform in the northern high latitudes of venus mapped by the venera 15 and 16 orbiters and is concentrated in the region between the mountain ranges of western ishtar terra and aphrodite terra tesserae are characterized by regionally high topography a high degree of small scale surface roughness and sets of intersecting tectonic features available pioneer venus line of sight gravity data suggest that tessera terrain is compensated at shallow depths relative to many topographic highs on venus and may be supported by crustal thickness variations three types of tessera terrain can be defined on the basis of structural patterns subparallel ridged terrains t sub sr trough and ridge terrain t sub tr and disrupted terrain t sub ds observed characteristics of tessera terrain are compared to predictions of models in order to begin to address the question of its origin and evolution formational models in which high topography is created along with surface deformation include 1 horizontal convergence 2 mantle upwelling 3 crustal underplating and 4 a seafloor spreading analogy modification models in which deformation occurs as a response to the presence of elevated regions consist of 1 gravity sliding and 2 gravitational relaxation the authorsmore find that horizontal convergence and late stage gravitational relaxation are the most consistent with basic observations for subparallel ridged terrain and disrupted terrain understanding of the basic structural characteristics of trough and ridge terrain is more tentative and models involving a spreading process or convergence and relaxation merit further study less

comment on impact craters of venus a continuation of the analysis of data from the venera 15 and 16 spacecraft by a t basilevsky et al

the geology of venus mental questions about the origin and evolution of the terrestrial planets and is of cri tical significance to comparative planetology the last 25 years of solar system exploration have provided unprecedented views of the earth moon mars and mercury these views have shown that the smaller terrestrial planetary bodies those one half the radius of the earth or less the moon mercury and mars are characterized by globally continuous unsegmented litho spheres that stabilized very early in the history of the solar system and by ancient surfaces that preserve the several billion year 0 1d record of early heavy bombardment and early heating and volcanism head solomon 1981 in contrast the earth is characterized by a globally segmented laterally moving lithosphere which is created at divergent plate boundaries and destroyed at convergent plate boundaries movement is measured in centimeters per year and the average age of the surface of the planet is less than 2 b y plate tectonics and plate recycling are fundamental mechanisms of heat loss for the earth in contrast to conduction which is the dominant mechanism for the smaller one plate planetary bodies solomon head 1982 what are the reasons for these differences is size or perhaps position in the solar system and initial conditions the key venus which is approximately the same size and density as earth and is the closest planet to earth offers an opportunity to test these ideas

venus summary and review of science research activities 1983 87 in the past four years the first high resolution synoptic images of the surface of venus have been obtained and we have learned more about the nature of its surface and atmosphere the major accomplishments of the u s s r in venus exploration have been observed with great interest by u s investigators some of whom have begun joint research efforts with soviet scientists venera 15 and 16 radar imaging experiments and new data obtained at the arecibo observatory have provided answers to some important questions conjectured volcanic features have been resolved into unmistakable calderas or distinct lava flow morphologies the question of whether venus like the earth may have high erosion rates is answered and from the observed number of presumed impact craters it appears that resurfacing rates on venus are much smaller than on earth no evidence of water or wind activity has been seen

styles of tectonic deformations on venus analysis of venera 15 and 16 data analysis of the radar images obtained by veneras 15 and 16 leads to the conclusion that the ridge and groove structures on the surface of venus are the result of tectonic deformation although the mechanism of such deformation cannot yet be unequivocally deduced several styles of deformation can be described areal deformation occurs where horizontal stresses have operated over large areas shear deformation appears in bands showing differential longitudinal deformation transversal stresses operating over long and relatively narrow areas have produced belt deformation circular deformation is related to a specific locus of the stresses the absence of densely cratered areas indicates that the terrains were deformed after the period of heavy bombardment the origin of the stresses could be drag of the lithosphere by asthenospheric currents or gravity induced spreading of surface material over upwellings a general conclusion is that in the surveyed area of venus neither terrestrial plate tectonics nor lunar highland type terrain exists

bilateral topographic symmetry patterns across 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 aphrodite terra highlands the broad characteristics of the cross strike discontinuities csds are similar to both strike slip fault zones and terrestrial oceanic fracture zones in an effort to distinguish between these two hypotheses topographic profiles were taken across aphrodite terra to test for bilateral symmetry of the type associated with thermal boundary layer topography at divergent plate boundaries on earth in addition to a broad bilateral symmetry at a range of angles across aphrodite terra detailed bilateral symmetry is observed within domains between linear discontinuities in directions generally parallel to the strike of the discontinuities in addition within a domain the centers of symmetry of several profiles define a linear rise crest that is oriented normal to the bounding csds and terminates against them furthermore linear rise crests are offset in a right lateral sense along the csds between domains by 200 1200 km if the discontinuities represent strike slip movement then retrodeformation of the axes of symmetry along the csds to form a straight line should form a broadly symmetrical topographic high instead when the topography is retrodeformed the edges of the highlands are characterized by distinctive misfits of topography at the edge of the rise where it intersects the csds the nature and configuration of the csds the characteristics of the topography and topographic patterns including bilateral topographic symmetry generally parallel to csds within domains the existence of linear rise crests defined by the axes of bilateral symmetry within domains and the offsets of these linear rise crests between domains all suggest that western aphrodite terra on venus is similar to terrestrial oceanic divergent plate boundary environments and that the cross strike discontinuities are analogous to oceanic fracture zones rather than strike slip faults

formation and propagation of great salinity anomalies north atlantic arctic ocean and sea ice variability for the period 1948 2001 is studied using a global ocean general circulation model coupled to a dynamic thermodynamic sea ice model forced by daily ncep ncar reanalysis data kalnay et al 1996 variability of arctic sea ice properties is analysed in particular the formation and propagation of sea ice thickness anomalies that are communicated via fram strait into the north atlantic these export events led to the great salinity anomalies gsa of the 1970s 1980s and 1990s in the labrador sea ls all gsas were found to be remotely excited in the arctic rather than by local atmospheric forcing over the ls sea ice and fresh water exports through the canadian archipelago caa are found to be only of minor importance except for the 1990s gsa part of the anomalies are tracked to the newfoundland basin where they enter the north atlantic current the experiments indicate only a minor impact of a single gsa event on the strength of the north atlantic thermohaline circulation thc

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

spatial and temporal relations between coronae and extensional belts northern lada terra venus preliminary studies of the distribution of coronae and volcanic rises on venus show that many of these features tend to cluster along zones of rifting and extension the plains north of lada terra are crossed by two such extensional belts each belt is composed of grabens ridges faults volcanic flows coronae and coronalike features the longer and more prominent belt is the nw trending alpha lada extensional belt which is over 6000 km long and 50 200 km wide and includes the coronae eve tamfana carpo selu derceto otygen and an unnamed corona south of otygen the second belt is the nne trending derceto quetzalpetlatl extensional belt which is about 2000 km long and in places over 300 km wide and includes the coronae sarpanitum eithinoha and quetzalpetlatl the two belts intersect at the 1600 600 km wide derceto volcanic plateau it is apparent that deformation along the two belts overlapped in time though deformation along the alpha lada extensional belt probably continued after the deformation along the derceto quetzalpetlatl extensional belt terminated in certain areas volcanism originated in grabens within the extensional belts whereas in other areas such as in eve selu derceto and quetzalpetlatl volcanism originated in the coronae and flowed into the lower parts of the extensional belts regional extension has affected the evolution of all the coronae at some stage of their development regional deformation occurred before the initiation of derceto and eithinoha and after the initiation of carpo tamfana otygen and sarpanitum it is thus unlikely that coronae formation along the belts is solely a consequence of the regional extension and it is also unlikely that regional extension has been caused solely by the coronae no corona along the belts was formed subsequent to the cessation of the regional extension we therefore suggest that the regional extension and the coronae are interrelated some of the coronae may have determined the location of the surface expression of the regional extension whereas the locations of other coronae may have been influenced by the concentration of regional extensional stresses

characterization of melt and ejecta deposits of kepler crater from remote sensing data we used moon mineralogy mapper m3 arecibo and mini rf radar and diviner radiometer data with lunar reconnaissance orbiter lro camera and kaguya terrain camera images to characterize the target ejecta and impact melt rich lithologies in and around lunar central peak crater kepler m3 data indicate the impact melt rocks of crater floor to be high ca pyroxene dominated distinct from the low ca pyroxene dominated crater wall the central uplift is high ca pyroxene dominated and has higher albedo these observations are consistent with thin mare basalts underlain by noritic imbrium ejecta underlain by gabbroic crustal material m3 data reveal an enigmatic splash like feature of melt rich material on the southeastern uprange crater wall and flank m3 data also highlight halos around kepler in detail the halos are slightly variable but in broad terms they define a consistent feature offset to the inferred downrange direction and interpreted to reflect the distribution of glass bearing impact breccia the radar data sets show most of the proximal ejecta to be radar bright however diviner rock abundance data do not indicate the presence of blocks on the surface nor can they be seen using lro narrow angle camera images thus the blocks giving rise to the enhanced radar signal are buried beyond the radar bright zone a subtle radar dark halo emerges coincident with a region of very low rock abundance in diviner data this multidisciplinary approach provides a robust analysis of the main characteristics of a lunar complex crater and reveals previously unidentified features related to the distribution of impact melt

atmospheric effects on cratering on venus a paraboloidal bow shock model is developed in order to estimate the surface distribution of gas shock induced modifications surrounding venusian impact craters we apply two dimensional oblique shock dynamics to describe a three dimensional paraboloidal shaped bow shock impinging upon an assumed incompressible venusian surface the effects of the hypersonic atmospheric shock acting on the venusian surface are considered in terms of induced maximum gas pressure density particle velocity and temperature for varying angles and velocities of impact the maximum boulder size that can be saltated by the shock wave induced gas flow and the degree of mutual collision of the surface materials are also considered the present calculations quantitatively predict the areal extent of the gas shock perturbed surface for normal and oblique impact as a function of impact angle and velocity and radii of impactors for a 1 km radius stony meteorite impacting normally at 20 km s the radius of the disturbed area extends 10 17 times the 3 5 km crater radius the perturbed surface affects the surface radar properties and the present results can provide an explanation of the wide dark bright halos surrounding some of the venusian impact craters observed via magellan imagery for example a 50 km radius bright halo surrounding a 20 km dark halo is observed around the 3 1 km radius crater located at 16 5 north latitude and 334 4 longitude the average value of the radar backscatter cross section of the 20 km radius dark halo indicates that 50 cm thick layer of porous lithologic material is superimposed upon an assumed undisturbed basement rock surface the bright halo indicates that the surface roughness in this region is 30 greater than that of the surrounding original surface these features can be induced by atmospheric shock waves the present model can relate the observed crater halo radii to the impact parameters such as projectile radius and density and the impact velocity and angle

seasonal enhancement of submarine groundwater discharge sgd derived nitrate loading into the ria formosa coastal lagoon assessed by 1 d modeling of benthic no3 profiles the role of benthic sandy ecosystems in mitigating no3 loads carried by submarine groundwater discharge sgd to coastal marine ecosystems is uncertain benthic biogeochemical mediation of no3 rich submarine groundwater discharge was studied at the seepage face of a barrier island site in the ria formosa coastal lagoon southern portugal preliminary analysis of no3 porewater distributions at the seepage face during discharge indicated that benthic biogeochemical processes could significantly affect the fluxes of groundwater borne no3 into the lagoon in order to discriminate between the relative contribution of transport and reaction processes to shape and concentration range evidenced by in situ porewater no3 gradients an advection dispersion reaction adr model of no3 diagenesis was applied to describe no3 porewater profiles obtained in march june september and december 2006 good agreement between modeled and measured profiles was obtained model derived apparent benthic nitrification and no3 reduction rates ranged from 0 01 to 5 2 mmol m 2 h 1 sufficient to explain gross observed changes in no3 fluxes arriving at the seepage face up to 70 within the surficial 20 cm depth layer results of the analysis indicated that the upper limit of the seepage face promoted mitigation of no3 fluxes to the lagoon throughout the year in contrast the lower limit of the seepage area promoted net amplification of the no3 fluxes into the lagoon in june and september these results will help constrain further work aiming to clarify the role of permeable sediments in mitigating nitrogen loading of coastal ecosystems

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

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

influence of the atlantic subpolar gyre on the thermohaline circulation during the past decade record high salinities have been observed in the atlantic inflow to the nordic seas and the arctic ocean which feeds the north atlantic thermohaline circulation thc this may counteract the observed long term increase in freshwater supply to the area and tend to stabilize the north atlantic thc here we show that the salinity of the atlantic inflow is tightly linked to the dynamics of the north atlantic subpolar gyre circulation therefore when assessing the future of the north atlantic thc it is essential that the dynamics of the subpolar gyre and its influence on the salinity are taken into account

viscous relaxation of impact crater relief on venus constraints on crustal thickness and thermal gradient because of the high surface temperature on venus and the strong temperature dependence of strain rate in silicates solid state creep may be an important mechanism for the reduction of topographic relief on that planet venera 15 16 radar altimetric profiles of impact craters 30 140 km in diameter however indicate crater depths of several hundred meters suggesting that substantial relaxation has not occurred for these features on the basis of models for the viscous relaxation of impact crater topography we derive constraints on the thickness of the crust and the mean lithospheric thermal gradient beneath the craters from the mean and variance of observed crater depth as a function of crater diameter on venus and from estimates of the initial depths of fresh impact craters on venus obtained by scaling from lunar observations we formulate a statistical test for the maximum amount of viscous relaxation that is consistent with the observations at a specified confidence level we develop a general formulation for gravity driven flow in a linearly viscous fluid incorporating the densities and temperature dependent effective viscosities of distinct crust and mantle layers the statistical comparison of predicted and observed depths yields linked upper limits to crustal thickness h and thermal gradient dt dz since dt dz can be estimated from global heat loss arguments or thermal models an upper bound to h may be derived the preserved relief of the largest craters constrain h to be less than or equal to 10 20 km because the craters with measured depths appear to be representative of a larger population of impact structures in terms of regional elevation geological unit and degree of preservation of morphological detail extrapolation of this result to lowlands and rolling plains regions on a global basis is not unreasonable such an extrapolation together with isostatic considerations yields an upper limit to the crustal volume of venus of 1010 km3 this value is an order of magnitude less than the time integrated volume of crust produced on earth and implies either that the average rate of crustal generation has been much smaller on venus than on earth or that some form of crustal recycling has occurred on venus

gravitational spreading of high terrain in ishtar terra venus magellan altimetry measurements indicate that the mountain belts and plateau margins of ishtar terra have topographic slopes in the range 2 30 magellan radar images reveal that numerous sets of narrow closely spaced troughs and lineations which we interpret as graben and normal faults are located in many of these regions of high slope the orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading in several of the mountain belts the extensional structures are parallel to the apparent direction of shortening consistent with the interpretation that gravitational spreading occurred while mountain building was still active to explore the implications of this spreading for the tectonic evolution of ishtar terra we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range flow in the crust is assumed to be governed by a nonlinear depth dependent rheology we predict brittle failure and relaxation of topographic relief as functions of time for ranges of crustal thickness 10 30 km and thermal gradient 5 25 k km and for observed ranges in topographic elevation 1 6 km and slope 1 30 this parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief for a crustal thickness greater than 10 km a thermal gradient of 15 k km or more and average values of relief 3 km and slope 3 the topography relaxes to 25 of its original height in less than 10 m y although initial failure in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands is predicted to occur much earlier values of horizontal surface strain that are likely to be observable 1 do not accumulate until significant relaxation of relief begins given the predicted rate of topographic relaxation either the crust of ishtar terra must be anomalous in some way either much stronger than predicted by standard flow laws or very thin or cold or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for venus of 500 ma the large apparent depth of compensation the abundance of volcanism and the uncertainty in the local age favor the view that mountain building in ishtar terra has occurred until times at least as recent as 10 ma

the shape and appearance of craters formed by oblique impact on the moon and venus we surveyed the impact crater populations of venus and the moon dry targets with and without an atmosphere to characterize how the 3 dimensional shape of a crater and the appearance of the ejecta blanket varies with impact angle an empirical estimate of the impact angle below which particular phenomena occur was inferred from the cumulative percentage of impact craters exhibiting different traits the results of the surveys were mostly consistent with predictions from experimental work assuming a sin2 dependence for the cumulative fraction of craters forming below angle on the moon the following transitions occur 45 degrees the ejecta blanket becomes asymmetric 25 degrees a forbidden zone develops in the uprange portion of the ejecta blanket and the crater rim is depressed in that direction 15 degrees the rim becomes saddle shaped 10 degrees the rim becomes elongated in the direction of impact and the ejecta forms a butterfly pattern on venus the atmosphere causes asymmetries in the ejecta blanket to occur at higher impact angles the transitions on venus are 55 degrees the ejecta becomes heavily concentrated downrange 40 degrees a notch in the ejecta that extends to the rim appears and as impact angle decreases the notch develops into a larger forbidden zone 10 degrees a fly wing pattern develops where material is ejected in the crossrange direction but gets swept downrange no relationship between location or shape of the central structure and impact angle was observed on either planet no uprange steepening and no variation in internal slope or crater depth could be associated with impact angle on the moon for both planets as the impact angle decreases from vertical first the uprange and then the downrange rim decreases in elevation while the remainder of the rim stays at a constant elevation for craters on venus 15 km in diameter a variety of crater shapes are observed because meteoroid fragment dispersal is a significant fraction of crater diameter the longer path length for oblique impacts causes a correlation of clustered impact effects with oblique impact effects one consequence of this correlation is a shallowing of the crater with decreasing impact angle for small craters

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

impact crater degradation on venusian plains in venusian plains impact craters without extensive low backscatter ejecta deposits are more likely to have low backscatter floors be embayed by volcanic deposits and exhibit fractures as compared to craters with extensive low backscatter ejecta we interpret these trends as evidence of ongoing degradation of low backscatter ejecta by aeolian activity weathering and volcanism using a crater age sequence based on extent of preservation of low backscatter ejecta together with monte carlo simulations we find that tectonic activity has extended over a longer time period than volcanism

salinity driven thermocline transients in a wind and thermohaline forced isopycnic coordinate model of the north atlantic abstract an isopycnic coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation the model performance is illustrated by a 30 year spinup run with coarse horizontal resolution 2 mesh in a domain with north atlantic topography extending from 10 to 60 n latitude the vertical structure encompasses 10 isopycnic layers in steps of 0 2 units capped by a thermodynamically active mixed layer from an initially isohaline state with isopycnals prescribed by zonally averaged climatology the model is forced by seasonally varying wind stress radiative and freshwater fluxes and by a thermal relaxation process at the surface after a mechanical spinup time of about 15 years a quasi stationary pattern of mean circulation and annual variability ensues characterized by pronounced subtropical mode water formation

simulated variability of the atlantic meridional overturning circulation to examine the multi annual to decadal scale variability of the atlantic meridional overturning circulation amoc we conducted a four member ensemble with a daily reanalysis forced medium resolution global version of the isopycnic coordinate ocean model micom and a 300 years integration with the fully coupled bergen climate model bcm the simulations of the amoc with both model systems yield a long term mean value of 18 sv and decadal variability with an amplitude of 1 3 sv the power spectrum of the inter annual to decadal scale variability of the amoc in bcm generally follows the theoretical red noise spectrum with indications of increased power near the 20 years period comparison with observational proxy indices for the amoc e g the thickness of the labrador sea water the strength of the baroclinic gyre circulation in the north atlantic ocean and the surface temperature anomalies along the mean path of the gulf stream shows similar trends and phasing of the variability indicating that the simulated amoc variability is robust and real mixing indices have been constructed for the labrador the irminger and the greenland iceland norwegian gin seas while convective mixing in the labrador and the gin seas are in opposite phase and linked to the nao as observations suggest the convective mixing in the irminger sea is in phase with or leads the labrador sea newly formed deep water is seen as a slow anomalous cold and fresh plume flowing southward along the western continental slope of the atlantic ocean with a return flow of warm and saline water on the surface in addition fast travelling topographically trapped waves propagate southward along the continental slope towards equator where they go east and continue along the eastern rim of the atlantic for both types of experiments the northern hemisphere sea level pressure and 2 m temperature anomaly patterns computed based on the difference between climate states with strong and weak amoc yields a nao like pattern with intensified icelandic low and azores high and a warming of 0 25 0 5 c of the central north atlantic sea surface temperature sst the reanalysis forced simulations indicate a coupling between the labrador sea water production rate and an equatorial atlantic sst index in accordance with observations this coupling is not identified in the coupled simulation

corona structures on venus models of origin coronae on venus are circular to elongate structures with maximum widths of 150 1000 km characterized by annuli of concentric ridges surrounding complex interiors the features have raised topography relative to the surroundings they are associated with volcanic activity and most are partially surrounded by a peripheral trough variations in morphology between individual coronae are due to differences in their stage of evolution and or differences in the relative significance of the geologic processes that occur in each stage we examine models for three processes that may be involved in corona origin and evolution 1 a hotspot or rising mantle diapir model 2 a sinking mantle diapir model and 3 gravitational relaxation of topography rising mantle diapirs are caused by heating at depth e g hotspot while sinking mantle diapirs may result from cooling or a phase change causing increased density and negative buoyancy at the base of the lithosphere the hotspot model is most consistent with the major characteristics of coronae with gravitational relaxation occurring as a modificational process the sinking mantle diapir would produce dominant central compression that has not been observed at coronae however higher resolution image and altimetry data from magellan can be used to distinguish more fully between the two models coronae in various states of formation and degradation can be identified in the venera 15 16 data suggesting that the process may be continuing today

construction and analysis of simulated venera and magellan images of venus abstract one look seasat radar images covering the gran desierto dune complex sonora mexico the appalachian valley and ridge province and accreted terranes in the central interior of alaska were digitally processed to simulate both venera 15 16 images 1 to 3 km resolution of venus and image data expected from the magellan mission 120 to 300 m resolution sinc filters were used to simulate the appropriate range and azimuth resolutions speckle was introduced as multiplicative noise and additive gaussian noise was included to simulate expected signal to thermal noise ratios the gran desierto dunes the largest complex in north america are not discernable in the venera simulation whereas the higher resolution magellan simulation shows the dominant dune patterns and specular reflections from dune faces oriented perpendicular to the incident radar anticlinal and synclinal structures are evident in both simulations over the appalachians mainly because differential weathering and erosion left resistant units as topographic highs that delineate the folds the magellan simulation also shows that fluvial processes have dominated erosion and exposure of the folded structures mountainous terrains and their degree of erosion are discernable in both simulations over alaska although only the magellan simulation shows that fluvial glacial and aeolian processes have all been active in shaping the landscape neither simulation provides evidence that diverse lithotectonic terranes in alaska were juxtaposed i e accreted since the primary evidence needed is lithological whereas radar returns are dominated by topography and surface roughness parameters only weakly indicative of lithology venera data show clear evidence for volcanic and tectonic terrains on venus i e endogenic landforms v l barsukov et al 1986 proc lunar planet sci conf xvii part 2 j geophys res 91 d378 d398 on the other hand the simulations suggest that determination of the nature and extent of terrain modification by exogenic processes e g atmosphere surface weathering erosion deposition on venus will remain uncertain since the length scales of features diagnostic of such processes may be too small to be discerned from venera data substantial differential weathering erosion and deposition may have occurred on venus enhancing the appearance of both volcanic and tectonic features alternatively rates of resurfacing by volcanic and tectonic processes may have been much higher than rates of atmosphere surface interactions producing a surface dominated by volcanic burial and tectonic disruption the simulations suggest that magellan data may provide the critical fine scale morphological information needed to test between these two alternative resurfacing scenarios

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

large submarine groundwater discharge and benthic eutrophication in bangdu bay on volcanic jeju island korea we estimated the submarine discharge of groundwater sgd and associated nutrients into the semienclosed bangdu bay on a volcanic island jeju korea by analyzing 222 rn ra isotopes 224 ra and 226 ra and nutrients in seawater pore water and coastal groundwater the submarine inputs of groundwater into bangdu bay of 120 180 m 3 m 22 yr 21 on the basis of 222 rn 224 ra 226 ra and si mass balances were much higher than those reported from typical continental margins the nutrient fluxes from sgd were about 90 20 and 80 of the total input except from open ocean waters for dissolved inorganic nitrogen din dissolved inorganic phosphorus dip and dissolved inorganic silicate dsi respectively these excess nutrient inputs from sgd are the major sources of new nutrients in this bay on the basis of photosynthetic pigments and benthic algal distributions we suggest that the large fluxes of excess nutrients from sgd can cause benthic eutrophication in a semienclosed bay on this highly permeable volcanic island

a study of the interaction of the north atlantic oscillation with ocean circulation observed patterns of wind stress curl and air sea heat flux associated with the north atlantic oscillation nao are used to discuss the response of ocean gyres and thermohaline circulation to nao forcing and their possible feedback on the nao the observations motivate and are interpreted in the framework of a simple mathematical model that couples ekman layers ocean gyres and thermohaline circulation to the atmospheric jet stream meridional shifts in the zero wind stress curl line are invoked to drive anomalies in ocean gyres and north south dipoles in air sea flux drive anomalous thermohaline circulation both gyres and thermohaline circulation play a role in modulating sea surface temperature anomalies and hence through air sea interaction the overlying jet stream the model which can be expressed in the form of a delayed oscillator with ocean gyres and or thermohaline circulation providing the delay identifies key nondimensional parameters that control whether the ocean responds passively to nao forcing or actively couples it suggests that both thermohaline circulation and ocean gyres can play a role in coupled interactions on decadal timescales

the resurfacing history of venus constraints from buffered crater densities because of atmospheric shielding and endogenic resurfacing the population of impact craters on venus is small about a thousand and consists of large craters this population has been used in numerous studies with the goal of deciphering the geologic and geodynamic history of venus but the nearly spatially random nature of the crater population has complicated efforts to understand this history here we utilize the recent 1 15 m scale global geological map of venus ivanov m a head j w 2011 planet space sci 59 1559 1600 to help address this problem the global geological map provides a stratigraphic sequence of units and known areas where each unit is exposed on the planet for each crater on venus we identify the specific geological units predating and postdating the crater we perform a statistical analysis of this set of observations with a buffered crater density approach which rigorously and consistently takes into account the large size of craters and the fact that many craters are known to predate and or postdate more than one unit in this analysis we consider crater emplacement as random and resurfacing history as determined although unknown we obtain formal confidence intervals for the mean ages of geological units and the mean age differences between the pairs of units at the unit boundaries we find that 1 size frequency distributions of craters superposed on each unit are consistent with each other 2 regional plains and stratigraphically older units have similar crater retention ages 3 stratigraphically younger units have a mean crater retention age significantly younger than the regional plains these findings are readily and consistently explained by global resurfacing scenarios and are difficult to reconcile with equilibrium resurfacing scenarios our analysis also shows that the latest recorded part of intensive resurfacing period lasted on the order of 10 of the mean surface age tens of millions of years the termination of intensive resurfacing may or may not be synchronous over the planet our results also indicate that there are extended deposits associated with large craters that are almost indiscernible in the radar images but obscure radar contrasts between predating lava flows we do not see evidence for any significant and prolonged change of atmospheric pressure following the termination of the intensive resurfacing epoch

variability of the simulated meridional heat transport in the north atlantic for the period 1951 1993 a 43 year ocean model simulation for the period 1951 1993 is analyzed with a focus on the meridional heat transport mht as a proxy for the strength of meridional overturning cell moc at 25 n the surface heat flux forcing associated with the north atlantic oscillation nao pattern is related to the variability in mht both on interannual and longer timescales the manifestation of the interannual and decadal oceanic response to nao is nonlocal as evidenced by the concentrated heat content and sea level variability at the gulf stream and north atlantic current regions and shown by the comparison of the model sea level variability to the altimeter data between low mht years in mid 1980s and high mht years in early 1990s the same area is singled out by empirical orthogonal function analysis in the leading mode of the sea level mht and time series of the leading sea level mode are highly correlated which reflects the importance of the moc in determining the variability of the heat content of the whole basin also the model suggests that the mht moc has entered a very strong phase since the mid 1980s and this trend has continued up to the end of 1993 this behavior follows the general trend in the nao index

the magellan venus radar mapping mission the nasa magellan venus radar mapper spacecraft was launched into an interplanetary transfer trajectory to venus on may 4 1989 and will be placed into orbit around venus on august 10 1990 the orbiter carries a 12 cm wavelength multimode radar system in the synthetic aperture mode it is capable of imaging most of the venus surface at a resolution of better than 300 m approaching 120 m over more than half the planet in the altimeter mode it will determine topographic relief to a vertical accuracy of better than 50 m averaged over a surface resolution cell approximately 10 km in diameter where the surface relief is not too extreme in the radiometer mode the radar receiver can determine the surface radio emission brightness temperature with an absolute accuracy of 20 k at a resolution of 2 k tracking of the orbiter s coherent radio telemetry transmitter will permit observations of small accelerations related to gravitational inhomogeneities in the planet s interior objectives of the mission include deducing the geological history of the surface and the geophysical state of the interior specific attention focuses on the origin present distribution and activity of four geological processes that modify the surface 1 volcanic and tectonic 2 impact 3 erosional depositional and chemical and 4 isostatic and convective data products resulting from the mission will be made available to members of the scientific community through nasa s planetary data system

gulf stream variability in five oceanic general circulation models five non eddy resolving oceanic general circulation models driven by atmospheric fluxes derived from the ncep reanalysis are used to investigate the link between the gulf stream gs variability the atmospheric circulation and the atlantic meridional overturning circulation amoc despite the limited model resolution the temperature at the 200 m depth along the mean gs axis behaves similarly in most models to that observed and it is also well correlated with the north atlantic oscillation nao indicating that a northward southward gs shift lags a positive negative nao phase by 0 2 yr the northward shift is accompanied by an increase in the gs transport and conversely the southward shift with a decrease in the gs transport two dominant time scales appear in the response of the gs transport to the nao forcing a fast time scale less than 1 month for the barotropic component and a slower one about 2 yr for the baroclinic component in addition the two components are weakly coupled the gs response seems broadly consistent with a linear adjustment to the changes in the wind stress curl and evidence for baroclinic rossby wave propagation is found in the southern part of the subtropical gyre however the gs shifts are also affected by basin scale changes in the oceanic conditions and they are well correlated in most models with the changes in the amoc a larger amoc is found when the gs is stronger and displaced northward and a higher correlation is found when the observed changes of the gs position are used in the comparison the relation between the gs and the amoc could be explained by the inherent coupling between the thermohaline and the wind driven circulation or by the nao variability driving them on similar time scales in the models

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