crustal thickness and support of topography on venus united states national aeronautics and space administration nasa planetary geology and geophysics program grant

a global survey of possible subduction sites on venus abstract about 10 000 km of trenches in chasmata and coronae have been identified as possible sites of retrograde subduction on venus all the sites have narrow deep trenches elongate along strike with arcuate planforms ridge trench outer rise topographic profiles typical of terrestrial subduction zones large outer rise curvatures 10 7 m 1 fractures parallel to the strike of the trench on the outer trench wall and outer rise and no cross strike fractures across the trench both the northern and southern margins of latona corona are possible subduction sites identification of a major graben between the two principal outer ridges in southern latona corona is evidence of back arc extension in the corona the amount of extension is estimated to be more than 2 11 km the moment exerted by the ridges of southern latona corona is insufficient to bend the lithosphere into the observed outer rise shape a negatively buoyant subducted or underthrust slab is needed depending on the unknown trench migration rate lithospheric subduction can make a significant contribution to mantle cooling on venus venusian chasmata could have a dual character they may be propagating rifts near major volcanic rises and subduction trenches far from the rises in the lowlands subduction and rifting may occur in close proximity on venus unlike on earth rifting induced by hotspots on venus may be necessary to break the lithosphere and allow subduction to occur such a process could result in gradual lithospheric subduction or global episodic overturn of the lithosphere

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

transient mantle layering and the episodic behaviour of venus due to the basalt barrier mechanism abstract numerical models of mantle convection that include the basalt barrier mechanism are explored for venus the basalt barrier mechanism is due to the positive buoyancy of subducted basaltic crust between the mantle depths of 660 and 750 km the inclusion of this mechanism in models of earth s evolution has been shown to cause episodic mantle layering early in earth history and we explore whether it can also operate on venus the models presented here include a moderately mobile lithosphere which is not representative of the current state of venus but this allows us to exclude the effects of episodic lithosphere mobility and thus to isolate the effect of the basalt barrier this is a step in a systematic approach to models with a mostly static lithosphere we find the basalt barrier does yield episodically layered mantle convection in some venus models the likelihood of episodic layering is increased by venus high surface temperature and by its less mobile or immobile lithosphere surprisingly secondary differences from earth including the lower gravity density and mantle depth also promote episodic layering the models suggest that mantle layering and overturns may still be likely to occur in venus the breakdown of mantle layering and consequent mantle overturns would lead to dramatic episodes of volcanism formation of large amounts of crust and tectonic activity on the planet s surface as has been inferred to have happened on venus around 500 ma ago from surface morphology and cratering these results thus suggest that a transient layering of the mantle by the basalt barrier mechanism and mantle overturns may be part of the explanation for venus s recent resurfacing

simulating the thermochemical magmatic and tectonic evolution of venus s mantle and lithosphere two dimensional models 1 numerical convection models of the thermochemical evolution of venus are compared to present day topography and geoid and recent resurfacing history the models include melting magmatism decaying heat producing elements core cooling realistic temperature dependent viscosity and either stagnant lid or episodic lithospheric overturn in stagnant lid convection the dominant mode of heat loss is magmatic heat pipe which requires massive magmatism and produces very thick crust inconsistent with observations partitioning of heat producing elements into the crust helps but does not help enough episodic lid overturn interspersed by periods of quiescence effectively loses venus s heat while giving lower rates of volcanism and a thinner crust calculations predict 5 8 overturn events over venus s history each lasting 150 myr initiating in one place and then spreading globally during quiescent periods convection keeps the lithosphere thin magmatism keeps the mantle temperature constant over venus s history crustal recycling occurs by entrainment in stagnant lid convection and by lid overturn in episodic mode venus like amplitudes of topography and geoid can be produced in either stagnant or episodic modes with a viscosity profile that is earth like but shifted to higher values the basalt density inversion below the olivine perovskite transition causes compositional stratification around 730 km breakdown of this layering increases episodicity but far less than episodic lid overturn the classical stagnant lid mode with interior temperature rheological temperature scale lower than tcmb is not reached because mantle temperature is controlled by magmatism while the core cools slowly from a superheated start core heat flow decreases with time possibly shutting off the dynamo particularly in episodic cases

constraints on mantle plumes on venus implications for volatile history abstract the analysis of venus gravity field and topography suggests the presence of a small number of deep mantle plumes 9 this study predicts the number of plumes formed at the core mantle boundary their characteristics and the production of partial melt from adiabatic decompression numerical simulations are performed using a 3d spherical code that includes large viscosity variations and internal heating this study investigates the effect of several parameters including the core mantle boundary temperature the amount of internal heating and the mantle viscosity the smallest number of plumes is achieved when no internal heating is present however scaling earth s radiogenic heating to venus suggests a value of 16 tw cases with internal heating produce more realistic lid thickness and partial melting but produce either too many plumes or no plumes if a high mantle temperature precludes the formation of a hot thermal boundary layer mantle viscosity must be reduced to at least 10 20 pa s in order to include significant internal heating and still produce hot plumes in all cases that predict melting melting occurs throughout the upper mantle only cases with high core temperature 1700 k produce dry melting over time the upper mantle may have lost significant volatiles depending on the water content of the lower mantle deep plumes may contribute to present day atmospheric water via volcanic outgassing assuming 50 ppm water in mantle 10 plumes with a buoyancy flux of 500 kg s continuously erupting for 4 myr will outgas an amount of water on the order of that in the lower atmosphere a higher level of internal heating than achieved to date as well as relatively low mantle viscosity may be required to achieve simulations with 10 plumes and a thinner lid alternatively if the mantle is heating up due to the stagnant lid the effect is equivalent to having lower rates of internal heating a temperature increase of 110 k byr is equivalent to 13 tw this value along with the internal heating of 3 tw used in this study may represent the approximate heat budget of venus mantle

inferences on the mantle viscosity structure and the post overturn evolutionary state of venus abstract venus has similar size density and bulk composition as earth but has tectonically evolved clearly differently and this divergence remains enigmatic surface observations such as gravity topography and surface age constrain venus evolution but interpreting these signals requires understanding of the surface interior coupling and thus insight into the structure and evolution of the venusian mantle and lithosphere here we investigate how such observables may be generated from interior dynamics using numerical forward models of global mantle convection that consistently link the thermochemical magmatic and tectonic evolution of venus venus present surface gravity spectrum and its relation to topography is matched best by our models with a mantle viscosity profile featuring a sublithospheric minimum of 2 1020 pa s and a gradual increase by a factor of 100 down to a depth of 250 km above the core mantle boundary no pronounced viscosity jump around the mantle transition as inferred for earth is favoured for venus which points to a relatively dry venusian upper mantle compared to earth s as previously suggested this holds true for both a pure stagnant lid scenario and in the presence of episodic catastrophic overturns triggered by cumulative crustal growth due to on going magmatism and volcanism overturns strongly perturb the surface gravity spectrum up to 150 myr after overturn cessation material deeply recycled by the resurfacing event annihilates the developed plume pattern which needs much longer than those 150 myr to recover to a state comparable to the pattern suggested by thermal emissivity anomalies observed on venus moreover overturns limit crustal thicknesses to reasonable values and are more capable than stagnant lid evolutions in generating mean surface ages 500 myr these findings seem to confirm previous suggestions that the episodic regime is more applicable to venus than a purely stagnant lid regime yet the relatively long time span required to recycle the entire surface 150 200 myr and the presently on going volcanic resurfacing predicted by our models complicate the formation of a uniform surface age as indicated by venus crater population and may also suggest that the latest overturn has ceased longer ago than indicated by venus present mean surface age

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

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

deep versus shallow origin of gravity anomalies topography and volcanism on earth venus and mars abstract the relation between gravity anomalies topography and volcanism can yield important insights about the internal dynamics of planets from the power spectra of gravity and topography on earth venus and mars we infer that gravity anomalies have likely predominantly sources below the lithosphere up to about spherical harmonic degree l 30 for earth 40 for venus and 5 for mars to interpret the low degree part of the gravity spectrum in terms of possible sublithospheric density anomalies we derive radial mantle viscosity profiles consistent with mineral physics for these viscosity profiles we then compute gravity and topography kernels which indicate how much gravity anomaly and how much topography is caused by a density anomaly at a given depth with these kernels we firstly compute an expected gravity topography ratio good agreement with the observed ratio indicates that for venus in contrast to earth and mars long wavelength topography is largely dynamically supported from the sublithospheric mantle secondly we combine an empirical power spectrum of density anomalies inferred from seismic tomography in earth s mantle with gravity kernels to model the gravity power spectrum we find a good match between modeled and observed gravity power spectrum for all three planets except for 2 l 4 on venus density anomalies in the venusian mantle for these low degrees thus appear to be very small we combine gravity kernels and the gravity field to derive radially averaged density anomaly models for the martian and venusian mantles gravity kernels for l 5 are very small on venus below 800 km depth thus our inferences on venusian mantle density are basically restricted to the upper 800 km on mars gravity anomalies for 2 l 5 may originate from density anomalies anywhere within its mantle for mars as for earth inferred density anomalies are dominated by l 2 structure but we cannot infer whether there are features in the lowermost mantle of mars that correspond to earth s large low shear velocity provinces llsvps we find that volcanism on mars tends to occur primarily in regions above inferred low mantle density but our model cannot distinguish whether or not there is a martian analog for the finding that earth s large igneous provinces mainly originate above the margins of llsvps

mean age of rifting and volcanism on venus deduced from impact crater densities unlike the extensively cratered highlands of the moon and mars the surface of venus does not preserve a record of heavy bombard ment from the early history of the solar system1 3 those craters that are found on venus appear to be statistically indistinguishable from a random spatial population and rarely show modification by folds faults and lava flows1 3 although the volcanic and tec tonic history of venus is still much debated2 5 there is mounting evidence for near global resurfacing 300 500 myr ago1 2 6 moreover it has recently been noted that the density of impact craters on large volcanic structures is less than the average crater density of the planet suggestive of significant activity after the resurfacing event7 it is not clear however whether these features represent late remnants of the global event or continuing volcanism and tectonism of a still active planet to address this question we have used the regional variations in crater density to date volcan oes rifts and coronae which based on stratigraphic evidence clearly post date the main resurfacing event8 11 the calculated mean ages of 70 125 myr exclude the possibility that the majority of these features represent the final stages of the global event

interior structure of the moon constraints from seismic tomography gravity and topography abstract seismic tomography can be combined with constraints from geoid topography and other surface observations to gain information about mantle structure and dynamics this approach has been taken with much success for the earth mantle and here it is for the first time applied to the moon lunar tomography has much lower resolution as for the earth and is mostly restricted to the near side nevertheless we can assess under what assumptions the fit between predicted geoid based on a tomography model and observed geoid is best among the models tested we find the most similar pattern correlation about 0 5 if we only consider tomography below 225 km depth if density anomalies cause little or no dynamic topography and if we compare to the geoid with the flattening l 2 m 0 term removed this could mean that a like for the earth seismic anomalies shallower than 225 km are caused by a combination of thermal and compositional effects and therefore cannot be simply converted to density anomalies b the lithosphere is sufficiently thick to prevent dynamic topography more than a small fraction of total topography and c flattening is a fossil bulge unrelated to present day mantle anomalies however we have to be cautious with interpreting our results because for models with a comparatively higher correlation and a conversion from seismic velocity to density anomalies similar to the earth s upper mantle the amplitude of the predicted geoid is much lower than observed this could either mean that the tomography model is strongly damped or that the geoid is mostly due to shallow causes such as crustal thickness variations with only a small part coming from the deeper mantle

plume induced crustal convection 3d thermomechanical model and implications for the origin of novae and coronae on venus novae and coronae are large volcanotectonic features on venus with a contentious and possibly non unique origin and enigmatic relationship their formation is most commonly explained by flexure and fracturing of the strong rigid venus lithosphere atop mantle upwelling and or downwelling here we present new 3d high resolution thermomechanical model of mantle plume diapir impingement into warm and thin lithosphere with venus like surface temperature numerical results suggests that nova like and corona like structures can result from magma assisted convection of weak ductile crust induced by decompression melting of the hot rising mantle plume during the initial stage nova forms by stellate fracturing of a topographic rise forming atop the growing crustal convection cell few million years later nova can convert to coronae by inward dipping concentric fracturing of the nova rise margins and subsequent outward thrusting of partially molten crustal rocks over the surface an outer annulus of concentric normal faults forms in the outer rise region of the downbending brittle upper crust whereas an inner annulus of concentric thrust faults forms in front of the outward thrusting crustal wedge a trench like depression forms between these two annuli resembling retreating subduction the rudimentary concentric upper crustal slab warms up rapidly and recycles into the convection cell the convection cell remains active for up to 15 million years fueled by heat and magma from the plume predicted surface topography and fracturing patterns agree with some small to moderate size novae and coronae on venus

recent hotspot volcanism on venus from virtis emissivity data the questions of whether venus is geologically active and how the planet has resurfaced over the past billion years have major implications for interior dynamics and climate change nine hotspots areas analogous to hawaii with volcanism broad topographic rises and large positive gravity anomalies suggesting mantle plumes at depth have been identified as possibly active this study used variations in the thermal emissivity of the surface observed by the visible and infrared thermal imaging spectrometer on the european space agency s venus express spacecraft to identify compositional differences in lava flows at three hotspots the anomalies are interpreted as a lack of surface weathering we estimate the flows to be younger than 2 5 million years and probably much younger about 250 000 years or less indicating that venus is actively resurfacing

the effects of internal heating and large scale climate variations on tectonic bi stability in terrestrial planets abstract we use 3d mantle convection and planetary tectonics models to explore the links between tectonic regimes and the level of internal heating within the mantle of a planet a proxy for thermal age planetary surface temperature and lithosphere strength at both high and low values of internal heating for moderate to high lithospheric yield strength hot and cold stagnant lid single plate planet states prevail for intermediate values of internal heating multiple stable tectonic states can exist in these regions of parameter space the specific evolutionary path of the system has a dominant role in determining its tectonic state for low to moderate lithospheric yield strength mobile lid behavior a plate tectonic like mode of convection is attainable for high degrees of internal heating i e early in a planet s thermal evolution however this state is sensitive to climate driven changes in surface temperatures relatively small increases in surface temperature can be sufficient to usher in a transition from a mobile to a stagnant lid regime once a stagnant lid mode is initiated a return to mobile lid is not attainable by a reduction of surface temperatures alone for lower levels of internal heating the tectonic regime becomes less sensitive to surface temperature changes collectively our results indicate that terrestrial planets can alternate between multiple tectonic states over giga year timescales within parameter space regions that allow for bi stable behavior any model based prediction as to the current mode of tectonics is inherently non unique in the absence of constraints on the geologic and climatic histories of a planet

tracing magma mixing in granite genesis in situ u pb dating and hf isotope analysis of zircons in situ zircon u pb and hf isotopic data have been determined for mafic microgranular enclaves and host granitoids from the early cretaceous gudaoling batholith in the liaodong peninsula ne china in order to constrain the sources and petrogenesis of granites the zircon u pb age of the enclaves 120 1 ma is identical to that of the host monzogranite 120 1 ma establishing that the mafic and felsic magmas were coeval the hf isotopic composition of the enclaves ehf t 4 5 to 6 2 is distinct from the host monzogranite ehf t 15 1 to 25 4 indicating that both depleted mantle and crustal sources contributed to their origin the depleted mantle component was not previously revealed by geochemical and nd and sr isotopic studies showing that zircon hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information some wall rock contamination is indicated by inherited zircons with considerably older u pb ages and low initial hf isotopic compositions hafnium isotopic variations in early cretaceous zircons rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead mixing of mantle derived mafic magmas with crustal derived felsic magmas coupled with assimilation of wall rocks is compatible with the data

global mapping of crustal and lithospheric thickness on venus 1 we have produced a high resolution global admittance map of venus using a spatiospectral localization technique calculating 64 800 admittance spectra evenly spaced in latitude and longitude we grouped the admittance spectra into 35 classes that we inverted for elastic crustal and lithospheric thickness using bottom loading top loading and short wavelength top loading models with long wavelength bottom loading hot spot models our best fit models identify 17 classes consistent with top loading 40 of the surface 7 with bottom loading 39 and 4 with a long wavelength bottom loading model 15 the remaining seven classes 6 of the surface generally had large amplitude top loading signatures that could not be fit with our models for the classes that were successfully modeled the estimated range of crustal 0 90 km and elastic thickness 0 100 km is larger than results for most previous studies primarily because of the incorporation of bottom loading nearly half of the planet has elastic thickness 20 km which we cannot distinguish from isostasy suggesting that these regions are tectonically inactive a key finding is that the lithospheric properties typically vary over scales of 1000 km or less except in some plains and crustal plateau regions both the scale and range of crustal and elastic thickness variations indicate that venus remains an active planet and that geologic processes may be more complex than previously recognized in many regions the correlation with surface geology is not obvious and this suggests that deformation of the lower lithosphere and crust may occur with little surface manifestation

plate tectonics damage and inheritance lithospheric damage combined with transient mantle flow and migrating proto subduction is proposed to explain the apparent emergence of plate tectonics three billion years ago modelling confirms that tectonic plate boundaries and fully formed tectonic plates can arise under conditions characteristic of earth but not of venus plate tectonics the division of the earth s surface into rigid plates separated by linear zones of concentrated deformation is unique among known terrestrial bodies as yet there is no consensus view on how the phenomenon arose here david bercovici and yanick ricard outline an explanation that starts with the microscopic properties of deforming minerals when sufficient lithospheric damage accumulates the theory states shear localization occurs and long lived weak zones develop transient mantle flow and migrating proto subduction then lead to the accumulation of plate boundaries and eventually tectonic plates the authors model this process with a grain evolution and damage mechanism coupled to an idealized model of pressure driven lithospheric flow and show that tectonic plate boundaries and fully formed tectonic plates can form under such conditions the initiation of plate tectonics on earth is a critical event in our planet s history the time lag between the first proto subduction about 4 billion years ago and global tectonics approximately 3 billion years ago suggests that plates and plate boundaries became widespread over a period of 1 billion years the reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics here we suggest that when sufficient lithospheric damage which promotes shear localization and long lived weak zones combines with transient mantle flow and migrating proto subduction it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone we simulate this process using a grain evolution and damage mechanism with a composite rheology which is compatible with field and laboratory observations of polycrystalline rocks1 2 coupled to an idealized model of pressure driven lithospheric flow in which a low pressure zone is equivalent to the suction of convective downwellings in the simplest case for earth like conditions a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate with passive spreading and strike slip margins that persist and localize further even though flow is driven only by subduction but for hotter surface conditions such as those on venus accumulation and inheritance of damage is negligible hence only subduction zones survive and plate tectonics does not spread which corresponds to observations after plates have developed continued changes in driving forces combined with inherited damage and weak zones promote increased tectonic complexity such as oblique subduction strike slip boundaries that are subparallel to plate motion and spalling of minor plates

environmental history recorded in aeolian deposits under stone pavements mojave desert usa abstract reconstructing the evolution of arid landscapes is challenged by limited availability of appropriate environmental archives a widespread surface feature stone pavement traps aeolian fines and forms a special accretionary archive seven stone pavement covered sections on basalt flows in the eastern mojave desert are condensed into a composite section comprising five sedimentological units supported by an osl based chronology three of the units are of accretionary nature and each is covered by a stone pavement they were deposited 50 9 36 6 ka

water induced convection in the earth s mantle transition zone 1 water enters the earth s mantle by subduction of oceanic lithosphere most of this water immediately returns to the atmosphere through arc volcanism but a part of it is expected as deep as the mantle transition zone 410 660 km depth there slabs can be deflected and linger before sinking into the lower mantle because it lowers the density and viscosity of the transition zone minerals i e wadsleyite and ringwoodite water is likely to affect the dynamics of the transition zone mantle overlying stagnant slabs the consequences of water exchange between a floating slab and the transition zone are investigated in particular we focus on the possible onset of small scale convection despite the adverse thermal gradient i e mantle is cooled from below by the slab the competition between thermal and hydrous effects on the density and thus on the convective stability of the top layer of the slab is examined numerically including water dependent density and viscosity and temperature dependent water solubility for plausible initial water content in a slab 0 5 wt an episode of convection is likely to occur after a relatively short time delay 5 20 ma after the slab enters the transition zone however water induced rheological weakening is seen to be a controlling parameter for the onset time of convection moreover small scale convection above a stagnant slab greatly enhances the rate of slab dehydration small scale convection also facilitates heating of the slab which in itself may prolong the residence time of the slab in the transition zone

constraints of the topography gravity and volcanism on venusian mantle dynamics and generation of plate tectonics abstract venus s mantle convection model was studied in a three dimensional spherical shell domain with depth and temperature dependent viscosity numerical results show that key observations of venus including the number of major hotspot volcanic systems spectral patterns of the surface topography and geoid at long and intermediate wavelengths can be explained in models that have a spinel to post spinel endothermic phase change of 3 5 mpa k clapeyron slope and averaged mantle viscosity of 2 10 21 pa s i e convective rayleigh number of 1 8 10 7 our models with the endothermic phase change show relatively weak time dependence suggesting that the phase change may not be the primary cause for catastrophic resurfacing on venus our calculations also show that venus cannot have a weak asthenosphere that is similar to that on the earth in order to match the observations thus supporting a key role of asthenosphere in producing plate tectonics

atmosphere mantle coupling and feedbacks on venus we investigate the coupled evolution of the atmosphere and mantle on venus here we focus on mechanisms that deplete or replenish the atmosphere atmospheric escape to space and volcanic degassing of the mantle these processes are linked to obtain a coupled model of mantle convection and atmospheric evolution including feedback of the atmosphere on the mantle via the surface temperature during early atmospheric evolution hydrodynamic escape is dominant while for later evolution we focus on nonthermal escape as observed by the analyzer of space plasma and energetic atoms instrument on the venus express mission the atmosphere is replenished by volcanic degassing from the mantle using mantle convection simulations based on those of armann and tackley 2012 and include episodic lithospheric overturn the evolving surface temperature is calculated from the amount of co2 and water in the atmosphere using a gray radiative convective atmosphere model this surface temperature in turn acts as a boundary condition for the mantle convection model we obtain a venus like behavior episodic lid for the solid planet and an atmospheric evolution leading to the present conditions co2 pressure is unlikely to vary much over the history of the planet with only a 0 25 20 postmagma ocean buildup in contrast atmospheric water vapor pressure is strongly sensitive to volcanic activity leading to variations in surface temperatures of up to 200 k which have an effect on volcanic activity and mantle convection low surface temperatures trigger a mobile lid regime that stops once surface temperatures rise again making way to stagnant lid convection that insulates the mantle