was the mangala valles system incised by volcanic flows 1 the mangala valles system of channels heads at one of the graben that comprise memnonia fossae extends northward nearly 1000 km across noachian and hesperian highlands and terminates at basins contiguous with amazonis planitia the mangala valles system has previously been interpreted to have formed through one or more catastrophic aqueous floods on the basis of similarities between the characteristics of its channel landforms and those of terrestrial systems including the channeled scabland of washington although aqueous mechanisms for formation of mangala valles are broadly congruous with known characteristics of the channel system an alternative volcanic hypothesis for formation of the system appears to be worthy of consideration on the basis of 1 its consistency with the volcanotectonic nature of the system and 2 commonalities between the basic nature of the system and that of large volcanic channels of the inner solar system estimates based on thermal considerations suggest that formation of mangala valles could conceivably have taken place through eruption of a lava volume of 2 105 km3 or roughly the total volume of the terrestrial columbia river basalt group the volcanic hypothesis for formation of mangala valles and a hybrid hypothesis involving formation of mangala valles through aqueous processes followed by or in concert with substantial modification of the system by volcanic erosion and deposition appears viable and worthy of future consideration

on observing the compositional variability of the surface of venus using nightside near infrared thermal radiation 1 there are several windows in the near infrared wavelength range 1 m in which thermal radiation emitted from the planetary surface penetrates through the thick venus atmosphere and clouds in this study we develop an improved method to estimate the surface emissivity on the basis of near infrared thermal emission from the nightside venus which is observed through these windows a simple radiative transfer model demonstrates that multiple reflection of thermal radiation between the atmosphere including clouds and the solid surface has a significant influence on the observed radiance under the condition of venus where reflectivity of overlying atmosphere and clouds is high thus it is necessary to take the effect of the reflection by the planetary surface into account in order to estimate accurately the variation in the surface emissivity on the basis of near infrared observation of venus the net effect of multiple reflection of surface thermal radiation between the atmosphere and the surface is to significantly reduce the spatial contrast in thermal radiation due to surface compositional variation the model calculation demonstrates that despite this effect detection of granitic rocks on the venus surface using near infrared windows is feasible since granitic and basaltic rocks have dramatically different 1 m emissivities granitic rocks are distinguishable from basaltic rocks by ground based telescopic observation a venus orbiter that measures both the near infrared thermal radiation and the surface altitude with great accuracy will provide us with a reliable surface emissivity map of venus which is a very valuable tool to detect granitic i e earth like continental rocks on venus

steep sided domes on venus characteristics geologic setting and eruption conditions from magellan data a survey of more than 95 percent of the venus surface reveals 145 steep sided domes which can be subdivided into a variety of morphologic forms the most common being shaped like inverted bowls or flat topped domes results of a preliminary analysis of the distribution and geologic setting of the domes are presented the relation of the domes to analogous terrestrial features is examined and possible models for their mode of emplacement are outlined

how does venus lose heat the tectonics and volcanism of the terrestrial planets are controlled by the loss of heat from the planetary interior on the earth about 70 of the heat flow through the mantle is attributed to the subduction of cold lithosphere in order to understand the tectonic and volcanic processes on venus it is necessary to understand how heat is transported through its mantle in this paper three alternative end member hypotheses are considered the first is the steady state loss of heat through the mantle to the surface in analogy to the earth however without plate tectonics and subduction on venus a steady state requires either a very high plume flux or very rapid rates of lithospheric delamination the required plume flux would be equivalent to about 80 plumes with the strength of the hawaiian plume the required delamination flux implies a 50 delamination of the entire venus lithosphere every 10 m y neither appears possible so that it is concluded that venus cannot transport heat through its mantle to its surface on a steady state basis the second hypothesis is that there has been a strong upward concentration of the heat producing elements into the crust of venus the heat generated is then lost by conduction surface measurements of the concentrations of the heat producing elements place constraints on this model if everything is favorable this hypothesis might be marginally acceptable but it is considered to be highly unlikely the third hypothesis is that heat is lost by episodic global subduction events followed by long periods of surface quiescence the near random distribution of craters suggests that the last subduction event occurred about 500 ma this model implies a thick thermal lithosphere 300 km at the present time which is consistent with a variety of surface observations lava lakes on the earth are considered as analogies to plate tectonics they also exhibit episodic subduction events

analysis and interpretation of high resolution topography and gravity of ishtar terra venus the apparent depth of compensation adc of the topography of venus determined on a global scale using the airy isostasy model shows that the topography specified by the spherical harmonics of degree lower than 10 is compensated at depths greater than 150 km that of degree higher than about 35 is compensated at a constant depth of about 35 km and the compensation depths of the harmonics of degree 10 35 shoal as the harmonic degree increases based on these characteristics of the globally determined adc three different maps of the topography and gravitational potential of the greater ishtar terra are derived using the harmonics of degree 1 75 lw 10 75 iw and 30 75 sw respectively the lw maps of the topography and potential do not correlate well but the iw and sw maps show good correlation emphasizing that the long wavelength components of the gravitational potential over the ishtar terra have contributions from sources broader than the terra the energy of the gravitational potential of the topography is about an order of magnitude greater than that of the observed potential indicating that the topography is strongly compensated the adc of the topography is estimated by both spectral and space domain analysis in the spectral domain three independent procedures the energy spectrum the statistical method and the wavelength dependent method are employed the first two yield a mean adc value for the entire spectrum whereas the last one calculates the adc for different wavelengths the adc decreases as the longer wavelength components are excluded the mean adc values determined for the lw iw and sw maps by the energy spectrum method are 115 65 and 40 km respectively those calculated through the statistical method are 85 55 and 34 km respectively these values reflect the upper and lower limits because the energy spectrum method overestimates and the statistical method underestimates the adc the wavelength dependent method shows that the adcs of the wavelengths 500 1350 km are similar for all the maps but those of the longer wavelengths significantly differ among the maps the mean adc values also differ from the adc of local topographic features for example the adc of maxwell montes is about 55 km in both the lw and iw maps in the space domain the adc is determined for pratt and airy isostasy models through fitting a linear and a quadratic function to the geoid height versus topography data respectively the adc values determined show good agreement with the above mentioned values in addition the relationship between the high resolution topography and the line of sight acceleration residuals of the western ishtar terra specified by wavelengths of 300 500 km suggests that the high resolution topography is compensated at a depth of about 25 km we also determine the lateral density perturbations inside a surface layer so that together with the topography of the greater ishtar terra they give rise to the observed gravitational potential over the terra three layer thicknesses of 50 100 and 200 km are examined the resulting density perturbations are too large to be interpreted in terms of lateral temperature variations alone this suggests lateral variations in the rock type the crust beneath the mountain belts may contain considerable amounts of low density material

search for active lava flows with virtis on venus express there are several lines of evidence suggesting that effusive volcanism still shapes the surface of venus but the rate is not well constrained the preservation of impact craters is consistent with a rate of 1 km year comparable to earth intraplate volcanism as well as a rate several orders of magnitude less it has been proposed to search images of the venus nightside near 1 m for the excess thermal emission of active lava flows the scattering of photons in the optically thick cloud cover is similar in effect to a gaussian blur with a full width half maximum fwhm of 90 km this reduces the maximum intensity of the anomalies introduced by eruptions making detection more difficult than in a clear atmosphere but also provides larger targets for imaging the virtis instrument on venus express observed surface thermal emission at 1 02 m wavelength between april 2006 and october 2008 at a spatial resolution of typically a few tens of km we have searched the 1000 images for the signatures of active lava flows in form of transient excess emission measured in several adjacent pixels we estimate that eruptions with 1gw m sr total excess specific thermal emission would be clearly visible compared to the instrumental and atmospheric noise but no such anomalies were detected it has been shown that the total thermal emission of eruptions on earth can be related to the lava discharge rate we adjust this model to the venus surface environment by adjusting observed lava surface temperature distributions and estimate that a discharge rate on the order of 1000 m s is required to produce an anomaly clearly identifiable in virtis data this is a relatively high value for effusive volcanism but a few historical eruptions on earth surpassed it adopting a fit to the discharge rates of historical eruptions on hawaii as a model 4 to 10 of the eruptions in the field of view of virtis images would have been detectable in addition to the low detection probability the low surface coverage of on average 1 of the surface per day over a period of 800 days indicates that the probability of an eruption imaged by virtis was low to begin with therefore the virtis data set fails to provide a useful constraint on the rate of volcanism on venus venus express continues to observe surface thermal emission with the venus monitoring camera for future missions near infrared imaging would profit from more frequent images of the same area than is possible from the excentric 24h orbit of venus express the superrotation of the cloud cover allows to reduce atmospheric noise by averaging of images taken hours apart and the detection of eruptions with 100 m s lava or less might then be possible nevertheless searching for lava flows emplaced between two observations of the same surface by high resolution radar imaging altimetry or interferometry generally provides a better constraints than infrared imaging for the same observation duration and surface coverage however high resolution radar and low resolution infrared observations are highly complementary and could be accommodated on the same mission preferably with a low circular polar orbit

mantle downwelling and crustal convergence a model for ishtar terra venus the ishtar terra region contains the highest topography known on venus over 10 km above mean planetary radius as well as abundant tectonic features many of apparently compressional origin these characteristics suggest that ishtar is a crustal convergence zone overlying downwelling mantle in order to explore quantitatively the implications of this hypothesis for ishtar s origin we present models of viscous crustal flow driven by gradients in topography assuming a free slip surface boundary condition we find that if the crustal convergence hypothesis is correct then the crustal thickness in the plains surrounding ishtar can be no more than about 25 km thick this upper bound assumes a cold 10 k km 1 geotherm and the stiffest available diabase flow law if the geothermal gradient is larger or the rheology is weaker the crust must be even thinner for net crustal convergence to be possible this upper bound is in good agreement with several independent estimates of crustal thickness of 15 30 km in the plains of venus based on modeling of the spacing of tectonic features and of impact crater relaxation if the surface layer of venus provides a no slip boundary then our models allow the crustal thickness in the plains to be up to 50 km but the likely existence of faults that cut through the crust makes a no slip surface layer unlikely our upper bound on crustal thickness is much less than that derived from an airy isostasy model of ishtar s gravity anomaly much of the observed gravity anomaly must be due to density anomalies in the mantle beneath ishtar although we treat ishtar as a crustal convergence zone our crustal flow model shows that under some circumstances near surface material may actually flow away from ishtar providing a possible explanation for grabenlike structures in fortuna tessera

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

uranium thorium and potassium in the venusian rocks at the landing sites of vega 1 and 2 concentrations of natural radioactive elements in venusian rocks at the landing sites of vega 1 and vega 2 were determined by gamma ray spectrometers on the landers as follows vega 1 k 0 45 0 22 u 0 64 0 47 10 4 th 1 5 1 2 10 4 vega 2 k 0 40 0 20 u 0 68 0 39 10 4 th 2 0 1 0 10 4 the abundances of natural radioactive elements in the investigated rocks are similar to that of basic rocks tholeiitic basalts and gabbros of the earth s crust the same conclusion about the type of the rocks at the landing site of vega 2 was made from the xrf experiment

directional errors in remanent magnetization of non cubic soft sediment specimens measured with spinner magnetometers summary this paper assesses theoretically and experimentally the errors in palaeomagnetic directions introduced by measuring rectangular but non cubic blocks of soft sediments in slow speed spinner magnetometers plastic boxes commonly used for this purpose hold samples 21 21 15 mm this study models the response of schonstedt and molspin spinners to such specimens and presents the results of experiments in which the remanent magnetization rm of obsidian blocks both as cubes and after subsequent cutting to fit within the boxes was measured to determine actual instrument behaviour these experiments indicate that the error introduced by use of specimens of the above dimensions may be as much as 8 5 for the smallest specimen sensor separation in the schonstedt spinner for the molspin spinner and for the next smallest specimen sensor separation in the schonstedt instrument the error is smaller but nevertheless non negligible

thermal convection in a cylindrical annulus with a non newtonian outer surface this study presents the results of numerical simulations of a model for lithospheremantle coupling in a terrestrial type planet to first order a geologically active terrestrial type planet may consist of a metallic core silicate mantle and lithosphere with the lithosphere being rheologically different from the mantle therefore we have developed a numerical model consisting of a thin non newtonian fluid hoop that is dynamically coupled to a thick newtonian fluid cylindrical annulus thus the rheological dichotomy between mantle and lithosphere is built into the model time dependent calculations show the existence of at least two regimes of behaviors in one regime the behavior of the hoop switches between periods characterized by low or high speeds in response to changes in convective vigor and planform this regime may apply to the planet venus where the available evidence indicates that prior to 500 myr ago the planet was resurfaced on a time scale of 100 myr since that time large scale tectonic activity on venus has been sharply curtailed in the other regime which is more like plate tectonics on earth the hoop speeds rise and fall on short time scales

internal crater modification on venus recognizing crater centered volcanism by changes in floor morphometry and floor brightness bright floored and dark floored craters on venus show systematic differences in their size distribution and apparent modification bright floored craters exhibit the following characteristics an interior radar brightness comparable to the youngest craters on venus a tendency toward smaller crater diameters and a broad range of crater elevations in contrast the dark floored craters are darker than pristine craters on venus are typically much larger and preferentially occur at lower elevations they also have larger floors than pristine craters of the same size and are similar in many respects to floor fractured craters on venus of the four proposed origins for dark crater floors these observations are most consistent with crater centered volcanism surface weathering or eolian deposition may contribute to floor darkening in some cases but neither of these mechanisms nor impact melts can independently explain the full range of observed modifications based on the measurements of this study volcanism has affected at least 6 13 of the impact craters on venus the estimated lava fill thicknesses are of the order of 100 500 m thus if these flows are representative of eruptions elsewhere in the venusian lowlands the implied average flux of plains forming volcanism on venus for the last 300 m y is 0 01 0 02 km3 yr

why does venus lack a magnetic field venus and earth have similar radii and estimated bulk compositions and both have an iron core that is at least partially liquid however despite these similarities venus lacks an appreciable dipolar magnetic field here i examine the hypothesis that this absence is due to venus9s also lacking plate tectonics for the past 0 5 b y the generation of a global magnetic field requires core convection which in turn requires extraction of heat from the core into the overlying mantle plate tectonics cools earth9s mantle on the basis of elastic thickness estimates and convection models it is argued here that the mantle temperature on venus is currently increasing this heating will reduce the heat flux out of the core to zero over 1 b y halting core convection and magnetic field generation if plate tectonics was operating on venus prior to ca 0 5 ga a magnetic field may also have existed on earth the geodynamo may be a consequence of plate tectonics this connection between near surface processes and core magnetism may also be relevant to the generation of magnetic fields on mars mercury and ganymede

venus rock composition at the vega 2 landing site the vega 2 lander determined the composition of rock in the area of the northern slope of the high mountain massif aphrodite terra which had never previously been analyzed the rock was analyzed by x ray fluorescence analysis employing instrumentation similar to that on venera 13 and 14 however our experience with those probes made it possible to introduce some changes in the design of the device and in the program of its operation as a result the reliability of the device was improved at the same time the analytical characteristics of the instrument were improved the range of the elements determined was expanded and the sensitivity was increased the x ray fluorescence spectrometer operated on the venusian surface in a preprogrammed mode the abundances of the major rock forming elements from mg to fe inclusively and also upper limits for some heavier rare elements were determined comparison of these data with analyses of terrestrial rocks enables us with certain reservations to suggest that the analyzed venusian sample is olivine gabbro norite

isostatic compensation of ishtar terra venus we have used spherical harmonic representations of the venus topography and geopotential obtained from magellan data to evaluate isostatic support in several areas within the ishtar terra highlands including the lakshmi plateau its surrounding mountain belts namely akna and freyja and maxwell montes and the fortuna tessera province we find that topography in ishtar is largely isostatically compensated 80 regional geoid topography variations in the subregions can be explained by a combination of airy crustal thickening and thermal lithospheric thinning mechanisms provided venus has a thick reference thermal lithosphere 300 400 km with the exception of eastern fortuna low elevation areas h 4 km above mpr with small gtrs are almost certainly airy compensated via thickened crust relatively large 60 km total airy crustal thicknesses obtained in the western ishtar mountain belts together with a probable basalt eclogite phase change suggest a possible silicic composition for these structures provided they are older than 25 50 ma lakshmi planum seems essentially thermally supported with the thermal lithosphere thinned to 100 km we suggest as one possibility that the lithospheric thinning process under lakshmi is delamination of a dense eclogite lower lithosphere layer into the mantle the decrease in gtr observed in ishtar between lakshmi to the west gtr 20 m km maxwell and west fortuna gtr 8 m km and eastern fortuna gtr 4 m km may correspond to a decay in thermal compensation attributed to lithospheric delamination which would be fairly recent 100 ma in lakshmi partially decayed in west fortuna and absent in east fortuna where a mostly airy supported topography is essentially relaxed with no thermal uplift alternatively if surficial concentrations in radiogenic elements were prevalent throughout the crust partial melting of a thickened crust could account for the thermal uplift in lakshmi and west fortuna the zero elevation basaltic crustal thickness h 24 km obtained for the east fortuna tessera region may be representative of the ambient crustal thickness in the venus lowlands our findings support multicomponent models for tectonic and volcanic activity in ishtar the thick ambient crust and thermal lithosphere implied by this study agree with observational constraints such as support of extreme elevations large topographic slopes unrelaxed craters and the thick elastic lithosphere suggested by flexure studies if the ambient thermal lithosphere on venus were to be relatively thin 100 200 km with a cold mantle and radiogenic elements concentrated in the crust then thermal evolution on venus may be in quasi steady state with the geodynamic evolution in monotonic decline however if the ambient thermal lithosphere is very thick 300 400 km as suggested by our thermal model fits then it is consistent with the predictions of strongly unsteady state thermal evolution models and an interior which is currently heating up this would support the view that catastrophic resurfacing on venus might be episodic

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

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

felsic highland crust on venus suggested by galileo near infrared mapping spectrometer data received 2 march 2008 revised 29 july 2008 accepted 18 september 2008 published 31 december 2008 1 we evaluated the spatial variation of venusian surface emissivity at 1 18 mm wavelength and that of near surface atmospheric temperature using multispectral images obtained by the near infrared mapping spectrometer nims on board the galileo spacecraft the galileo nims observed the nightside thermal emission from the surface and the deep atmosphere of venus which is attenuated by scattering from the overlying clouds to analyze the nims data we used a radiative transfer model based on the adding method although there is still an uncertainty in the results owing to the not well known parameters of the atmosphere our analysis revealed that the horizontal temperature variation in the near surface atmosphere is no more than 2 k on the venusian nightside and also suggests that the majority of lowlands likely has higher emissivity compared to the majority of highlands one interpretation for the latter result is that highland materials are generally composed of felsic rocks since formation of a large body of granitic magmas requires water the presence of granitic terrains would imply that venus may have had an ocean and a mechanism to recycle water into the mantle in the past

magellan and comparative planetology magellan data have been widely used to provide explanations for the tectonics and volcanism on venus however it is also important to compare our understanding of these processes on venus with the earth moon and mars one essential question concerning the behavior of these bodies is how does the heat get out internal heat generation and planetary cooling are the energy sources responsible for planetary tectonics and volcanism on the earth the plates of plate tectonics are the thermal boundary layers of mantle convection cells the subduction of these cold plates at ocean trenches contributes about 75 of the mantle heat flux it is now quite clear that active plate tectonics is not occurring on venus the surface of venus is a single rigid plate as is the case for the moon mars and mercury prior to magellan the general consensus was that venus was in a near uniformatarian state with conductive heat losses through the lithosphere balancing internal heat generation and secular cooling of the planet however crater studies from magellan images provide direct evidence for a catastrophic or episodic evolution of venus one explanation is that venus died tectonically 500 m y ago plate tectonics occurred during the early evolution of the planet but ceased as the radioactive heat production decreased an alternative explanation is that venus experiences episodic global subduction events two major questions concerning the earth remain largely unanswered particularly when compared with venus 1 why does the earth have continents 2 why does the earth have plate tectonics a major difference between the earth and venus is the presence of liquid water oceans during much of the evolution of the earth it has been hypothesized that water is an essential ingredient for the silicic volcanism that produces continental rocks on the earth and the absence of well defined continental masses on venus supports this hypothesis it has long been recognized that plate tectonics cannot evolve in time without internal deformation of plates plate evolution in time results in geometrical incompatibilities that require overlaps or gaps on the earth these incompatibilities lead to broad zones of continental deformation i e the western united states and china the absence of plate tectonics on venus suggests that rheologically soft continents are necessary for plate tectonics a reasonable conclusion is that the earth is the remarkable planet in terms of tectonics and volcanism not venus

reconciling channel formation processes with the nature of elevated outflow systems at ophir and aurorae plana mars 1 many hesperian outflow channels head at elevations compatible with aquifer recharge beneath the martian south polar cap and such channels are widely interpreted as the products of this recharge some outflow channels head at greater elevations that are inconsistent with southern recharge including three systems located in aurorae planum and eastern ophir planum these three systems have previously been interpreted as having formed through aqueous outbursts from local aquifers recharged by the meltwaters of ancient upland glaciers to the west however the viability of this interpretation is weakened by the lack of a geomorphic record supportive of past meltwater flow from western uplands inconsistencies between hypothesized processes and the nature of regional topography and mineralogy and the absence of satisfactory analog processes for catastrophic aqueous flow from the subsurface in contrast a volcanic origin for the ophir and aurorae systems appears to be in accord with the basic characteristics of constituent channels and associated landforms the volcanotectonic nature of the valles marineris system and adjacent upland plains and the nature of available analog landforms and processes though numerous uncertainties remain a volcanic interpretation of these outflow systems can more simply account for the existence of component channels the attributes of other elevated outflow systems on mars similarly appear to be most consistent with volcanic origins if the ophir and aurorae systems formed volcanically thermal considerations imply minimum erupted lava volumes of 6 4 103 km3 for allegheny vallis and 6 2 104 km3 for elaver vallis

the gabbro eclogite phase transition and the elevation of mountain belts on venus maxwell montes standing up to 7 km above the adjacent highland plateaus constitute the highest mountain belt on venus because the thickness of the crust is likely to be limited by the gabbro garnet granulite eclogite phase transitions this relief is difficult to reconcile with the assumption of thermodynamic equilibrium and a standard airy isostatic model we explore the hypothesis that the crust mantle boundary is not in phase equilibrium but rather is rate limited by the temperature dependent volume diffusion of the slowest ionic species under the simplifying assumption that the mountains formed by uniform horizontal shortening of the crust and lithospheric mantle at a constant rate we solve the one dimensional thermal evolution problem the time dependent density structure and surface elevation are calculated by assuming a temperature dependent reaction rate and local airy isostatic compensation for a rate of horizontal strain of 10 15 s 1 or greater the rise in temperature at the base of the crust during mountain formation is modest to negligible the deepening lower crust is metastable and surface elevation increases as the crust is thickened for strain rates less than 10 16 s 1 in contrast crustal temperature increases with time because of internal heat production and the lower crust is more readily transformed to the dense eclogite assemblage for such models a maximum elevation is reached during crustal shortening while this maximum relief is 7 km or more for some models a smaller density contrast between crust and mantle than assumed here 500 kg m 3 and incorporation of horizontal heat transport would lessen this value we therefore favor formation of the mountain belt at a strain rate at least of order 10 15 s 1 by this reasoning maxwell montes must be comparatively young of order 50 ma

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 surface thermal emission at 1 μm in virtis imaging observations evidence for variation of crust and mantle differentiation conditions 1 the venus express spacecraft images the nightside thermal emissions using the visible and infrared thermal imaging spectrometer virtis at 1 02 1 10 and 1 18 m thermal emission from the surface is observed the signal is attenuated by scattering and absorption in the dense atmosphere the measured flux at the top of the atmosphere is positively correlated with surface temperature and surface emissivity the surface temperature of venus is relatively well constrained as being mainly a function of altitude with a gradient lesser or equal to the adiabatic lapse rate this study examines the correlation of virtis images showing a signal of the surface at 1 02 m with viewing geometry stray sunlight cloud opacity and topography and applies semiempirical relations to remove their influence the remaining contrast can be either ascribed to surface emissivity or unexpected temperature variations temperature variations due to active volcanism are unlikely to be persistent over the time of observations therefore the mosaic of all processed images is here interpreted in terms of surface emissivity variation the emissivity variation found is correlated with geomorphological features established from magellan synthetic aperture radar images it is generally lower at tessera terrain some but not all volcanic edifices show increased emissivity large lava flows in the lada terra lavinia planitia region also show an increased thermal emission this might indicate a more felsic surface composition of tessera highlands and large scale extrusive volcanism of ultramafic composition

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

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

new data on the composition structure and properties of venus rock obtained by venera 13 and venera 14 a study of the composition structure and properties of venusian rocks was carried out by venera 13 and venera 14 the main structural morphological types of provinces of the venus surface were chosen for the study the chemical and mineralogical composition of the rock at the venera 13 landing site shows that it belongs to a group of weakly differentiated melanocratic alkaline gabbroids at the venera 14 landing site the rock is similar to oceanic tholeiitic basalts a most interesting property of the surface rocks is the great number of the strata that were visible in panoramas of the landing sites measurements of their physical and mechanical properties show that venusian rocks have friable weakly cemented porous structures analysis of information from venera 8 9 10 13 and 14 led to the conclusion that the most likely global process responsible for the formation of the present rock is volcanic eruption moreover due to the high gas content of basaltic melts eruptions are of an explosive character leading to the formation of pyroclastic rocks summing up data obtained by venera enabled us to compare venus with the earth mars and the moon to point out some regular global features in the evolution of these bodies and to suggest some new problems for investigation

explosive volcanism on venus transient volcanic explosions as a mechanism for localized pyroclast dispersal it is proposed that transient volcanic explosions of the vulcanian type may provide a mechanism for the generation and dispersal of pyroclastic material on venus the influence of the venusian high atmospheric pressure environment implies that continuous discharge plinian eruptive activity is relatively uncommon the tendency for suppression of exsolution and expansion of magmatic gases favors effusive eruptions however it may be possible for explosive activity to occur in a fashion analogous to vulcanian eruptions on earth as a result of the accumulation of hot pressurized gas under a coherent rock lid the explosion may be initiated by the failure of this retaining caprock causing the catastrophic release of the high pressure gas which expands out of the vent driving the fragmented caprock material ahead of it and displacing the surrounding atmosphere on earth the driving gas may originate either from vaporization of groundwater or from degassing of a stalled magma body in the near surface crust whereas on venus where the presence of crustal stores of volatile compounds is uncertain the latter option only is favored prolonged degassing may lead to an accumulation of gas sufficient to initiate an explosion this paper presents the results of a numerical model describing the explosion process under boundary conditions representing the venusian physical environment this involves treatments of the acceleration of the driving gas caprock and displaced atmospheric gas out of the vent and the subsequent motions and aerodynamic interactions between the atmosphere and the ejected blocks of fragmented caprock in this way predictions of the eruption velocities and of the resulting distribution of large solid ejecta can be obtained for likely conditions on venus deposits of large blocky debris are predicted to range up to a maximum distance of the order of 1 km from the vent on venus compared with distances of several kilometers commonly attained by ejecta from transient explosions on earth more typical blocky deposits may extend for only a few hundred meters which implies that they would not be detected in the magellan radar data however the possible presence of associated pyroclastic flow and fine grained ashfall deposits may constitute aids to the identification of sites of vulcanian eruptions on venus

effects of planetary thermal structure on the ascent and cooling of magma on venus magellan radar images of the surface of venus show a spatially broad distribution of volcanic features models of magmatic ascent processes to planetary surfaces indicate that the thermal structure of the interior significantly influences the rate of magmatic cooling and thus the amount of magma that can be transported to the surface before solidification in order to understand which aspects of planetary thermal structure have the greatest influence on the cooling of buoyantly ascending magma we have constructed magma cooling profiles for a plutonic ascent mechanism and evaluated the profiles for variations in the surface and mantle temperature surface temperature gradient and thermal gradient curvature results show that for a wide variety of thermal conditions smaller and slower magma bodies are capable of reaching the surface on venus compared to earth primarily due to the higher surface temperature of venus little to no effect on the cooling and transport of magma are found to result from elevated mantle temperatures elevation dependent surface temperature variations or details of the thermal gradient curvature the enhanced tendency of magma to reach the surface on venus may provide at least a partial explanation for the extensive spatial distribution of observed volcanism on the surface

the tectonics of venus solid venus has several differences from solid earth a mild variation in topography with marked departures of some kilometres confined to less than 10 of the surface no interconnected system of ridges such as would be associated with a spreading lithospheric boundary layer a high correlation of gravity with topography and a ratio of gravity to topography implying compensation depths in excess of 100 km for major features this high admittance ratio implies a stiff upper mantle as also indicated by the high depth diameter ratio of craters the details of magellan radar imagery enable some inference of event sequences the morphology generally indicates a more regional scale of change than on earth the principal chronometer is impact crater distribution the number of impact craters identified is 914 which is equivalent to about 500 ma s infall of bodies big enough to penetrate the atmosphere but the number of craters per unit area has but slightly more variations from the mean than random and only a mild negative correlation with topographic elevation only one third of the craters evidence tectonic or volcanic modification hence the rate of resurfacing over the last few 100 ma must be quite slight compared to earth s the main debate is whether venerean tectonic activity is in monotonic decline or whether it is episodic on a 500 ma time scale evidencing some current tectonic and volcanic activity are a few limited regions of marked topographic and geoidal highs exceeding 8 km and 80 m respectively with steep slopes clearly plate tectonics does not exist on venus the underlying cause of this different evolution appears to be the lack of water this dryness makes the upper mantle stiff enough to regionalize the tectonics and inhibit recycling of crust the lack of water also prevents erosion and thus the recycling of secondary differentiates these deficiencies in recycling imply a strong net upward differentiation of heat sources and a thick crust now allowed by new results on dry diabase rheology

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

a mantle plume model for the equatorial highlands of venus the equatorial highlands of venus consist of four main structures atla beta ovda and thetis regiones each has a circular to oval shaped planform and rises 4 5 km above the mean planetary radius these highlands are associated with long wavelength geoid highs with amplitudes ranging from 35 m at ovda to 120 m at atla they also contain topographic valleys interpreted as extensional rift zones and beta is known to contain shield volcanoes these characteristics are all consistent with the equatorial highlands being formed by mantle plumes an alternative model in which ovda and thetis are interpreted as spreading centers analogous to terrestrial mid ocean ridges fails to explain most of the observed geoid anomalies and topography in these regions some smaller highlands such as bell regio eistla regio and the hathor innini ushas region may also be plume related but most coronae are unlikely to be the direct result of plume activity we have modeled plumes using a cylindrical axisymmetric finite element code and a depth dependent newtonian rheology we compare our model results with profiles of geoid and topography across atla beta ovda and thetis our best model fits are for beta and atla assuming whole mantle convection and that earth and venus have similar mantle heat flows venus must lack an earth like low viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features this conclusion is consistent with the long wavelength admittance spectrum of venus and with the observed differences in the slopes of the geoid spectra for the two planets one explanation for the different viscosity structures of the two planets could be that the mantle of venus is drier than earth s mantle

reexamination of fault angles predicted by shear localization theory this paper reexamines orientations of shear bands fault angles predicted by a theory of shear localization as a bifurcation from homogeneous deformation in contrast to the coulomb prediction which does not depend on deviatoric stress state the angle between the band normal and the least most compressive principal stress increases as the deviatoric stress state varies from axisymmetric compression to axisymmetric extension this variation is consistent with the data of mogi 1967 on dunham dolomite for axisymmetric compression extension and biaxial compression but the predicted angles are generally less than observed this discrepancy may be due to anisotropy that develops due to crack growth in preferred orientations results from specialized constitutive relations for axisymmetric compression and plane strain that include this anisotropy indicate that it tends to increase the predicted angles measurements for a weak porous sandstone castlegate indicate that the band angle decreases with increasing inelastic compaction that accompanies increasing mean stress this trend is consistent with the predictions of the theory but for this rock the observed angles are less than predicted