Small domes on Venus: Characteristics and origin

Approximately 22,000 small domes have been identified on the 25% of the surface of Venus imaged by Venera 15/16. The word dome is used to imply a broad, lens-shaped, positive topographic feature. The domes: (1) are generally circular in planimetric outline; (2) range in diameter from the effective limit of Venera resolution (2 km) to 20 km; (3) show flank slopes generally 10 ° and possibly 5 °; and (4) occur in association with mottled plains units. Associated features include summit pits, radar bright surfaces, and basal topographic platforms. There are two significant areas of major dome concentrations approximately 180 ° in longitude apart: (1) the largest concentration occurs in the Akkruva Colles area of Niobe Planitia, centered at approximately 45 ° N/120 ° E, just north of the flanks of the Thetis Regio rise; and (2) another concentration occurs in northwestern Guinevere Planitia, centered at approximately 35 ° N/300 ° E, on the north flank of the Beta Regio rise. In addition to these major areas of concentrations, domes occur in smaller concenrations throughout the imaged area of Venus, in association with coronae, arachnoids, intermediate sized hills interpreted to be volcanic constructs, large volcanic centers and calderas. The characteristics and geologic associations of small domes are consistent with an interpretation of their origin as volcanic, and on the basis of their low slopes, individual characteristics, and geologic associations they are interpreted to represent dominantly effusive low shield volcanoes. The large number of small domes implies a large number of multiple centralized eruptions, each one of which represents a discrete, relatively small, volume of material available to build an edifice over a finite time period. Calculated modal volume is 0.73 km³ for individual edifices. Based on the number identified by Venera, the total number of small domes estimated for the entire planet 4.4 × 10⁶ and total edifice volume over the entire planet represents a minimum volume equivalent to a layer approximately 7 m thick over the planet and representing 0.03% of the estimated crustal volume of Venus. In absolute number, size range, and distribution they appear to be similar to terrestrial oceanic seamounts. The global abundance and distribution, size frequency distribution, minimum size, and changes in these characteristics with latitude for the domes will be particularly important in understanding the way in which the domes form and their relationship to global models of tectonism and heat flow on Venus. Increased spatial resolution and coverage from Magellan data will enable a more thorough assessment of these features and associated questions, particularly where radar incidence angles are 15 °.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Ridge belt tectonics of Ganiki Planitia on Venus

The main major ridge belts of Ganiki Planitia on Venus (Lama, Ahsonnutli and Pandrosos Dorsa) are part of the fan-shaped ridge belt complex along the 200 parallel of longitude. These ridge belts with evidence of crustal shortening support the idea of a large-scale E-W compression. The ridge belt patterns indicate a N-S shear component. These forces are explained by a triangular planitia area which compressed by surrounding terrains. The crustal shortening and ridge belt formation indicates compressional plate movement stresses in the uppermost lithosphere.Three sizes of ridge belt structure are to be found within Ganiki Planitia. (1) The ridge belt spacing of 200–400 km can be used to estimate the depth of the major uppermost homogeneous layer of Venus. There are numerous volcanic coronae, paterae and montes located along the main ridge belts or at their junctions. (2) Mid-size ridge groups or subbelts are to be found within the major ridge belts. These are formed by more local responses to tectonic stresses in the stratified uppermost crust. A wavelength of 40–70 km can be seen as a result of bending of the crustal strata and may relate to its thickness. (3) Small individual ridges are connected with most local stresses, defining places where the surface layers broke along the crests of large ridge belts or mid-scale subbelts. Radial and concentric mare ridge-like structures around coronae indicate that corona formation was effective at a sufficiently close vicinity to fault the surface.     

Tepev Mons on Venus: Morphology and elastic bending models

Tepev Mons is a large volcanic structure of about 250 km in diameter with an elevation of 5 km above the surroundings, located at the southwestern edge of Bell Regio. It is surrounded by a moat with a depth of about 0.5 km. If this moat is considered to be caused by bending of the lithosphere due to the load of the volcano, then elastic bending models give limits for the effective flexural rigidity FR and the effective elastic thickness of the lithosphere L: 2 x 10²³ Nm FR 3 x 10²⁴ Nm and 30 km L 100 km. High flexural rigidities are associated with small depressions and large thicknesses of the lithosphere and vice versa.Contribution No 345, Institut für Geophysik der Universität Kiel, F.R.G.     

Ridge belts: Are they compressional or extensional structures?

Today one of the main questions standing before geology of Venus is comprehension of tectonic style on surface of this planet. The observed ridge belts on the images of the radar surveyors of Venera-15 and -16 are related to one aspect of a solution of this problem. In general there are two views on the origin of the ridge belts on the planet. Some of the investigators consider them as a result of compression while others as extension. These discrepancies are connected with uncertainties of interpretations of the major structures of these belts, i.e., the ridges. This paper represents a review of evidence in favor of extension and compression observed within the ridge belts. Moreover it discusses the major speculative ideas and models of the ridge belts origin. The supporters of a ridge formation by extension suggest that these linear features are results of crust cracking and intruding of magma along these cracks so they propose that the ridge belts on the plains of Venus can represent analogies of the spreading zones on Earth. Other investigators assume that the ridge of belts represent structures of folding and or thrust faulting due to the tectonic compression environment so the ridge belts can be analogies of the orogenic belts and subduction zones on Earth. So the question of the ridge belts' origin remains controversial and for a solution, set one's hope on the Magellan mission.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Lakshmi Planum,Venus: Characteristics and models of origin

Lakshmi Planum is distinctive and unique on the surface of Venus as an expansive (~2 × 10⁶km²), relatively smooth, flat plateau containing two large shield volcanoes and abundant volcanic plains in the midst of a region of extreme relief. It rises 3–5 km above the datum and is surrounded on all sides by bands of mountains interpreted to be of compressional tectonic origin. The major units mapped on Lakshmi are volcanic edifices, smooth, ridged and grooved plains units, and structural units referred to as ridged terrain. Three styles of volcanism are observed to dominate the surface of Lakshmi. Distributed effusive volcanism is associated with extensive plains deposits and many of the small shields, domes and cones mapped within the plateau. Centralized effusive volcanism is primarily associated with the paterae, Colette and Sacajawea, and their circumferential low-shield-forming deposits. The precise origin and evolution of these unusually large and complex structures is not understood, although a catastrophic, explosive origin is unlikely. Pyroclastic volcanism may be represented by a unit referred to as the diffuse halo. The origin and evolution of Lakshmi Planum is closely related to its compressional tectonic environment; volcanism on Lakshmi has occurred synchronously with tectonism in the surrounding orogenic belts. A model for the origin and evolution of Lakshmi Planum consisting of a continuous sequence of convergence and horizontal shortening of crustal segments against a preexisting block of tessera seems best able to account for the elevation, plateau shape and irregular polygonal outline of Lakshmi, as well as the presence of ridged terrain and its resemblance to tessera. Volcanism on Lakshmi is proposed to be the result of basal melting of a thickened crustal root. According to this model, the origin and evolution of Lakshmi Planum has consisted of the following sequence of events: (1) formation of a large, elevated block of tessera surrounded by low-lying plains; (2) convergence and underthrusting of crustal segments to produce peripheral mountain ranges, thickening, and uplift of the plateau; and (3) basal melting of the thickened crust and underthrust material and surface volcanism that occurred synchronously with continued edge deformation.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence). Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Possible origin of radar bright craters on Venus

Radar images of Venus' surface, obtained from the Venera 15 and 16 spacecraft and from the Earth, show many impact craters that are radar bright compared with their surroundings. It is demonstrated in this report that this phenomenon cannot be caused by higher surface roughness of meteorite impact area only. Obviously, enhanced reflectivity is required as well. It is supposed that the necessary enlargement of reflectivity might be caused by raised conductivity of surface material in the impact area, if the meteorite is related by chemical composition to iron or iron-stony types.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Evidence for a low-altitude origin of lightning on Venus

Radio emissions attributed to lightning on Venus have been recorded by Venera 11 and 12 and by the Pioneer Venus Orbiter. The Venera descent records are compared to patterns of radio propagation within the Venusian atmosphere and an explanation is found for some timing trends that, if correct, indicates the lightning was below 33 km in altitude.     

Eastern Aphrodite Terra on Venus: Characteristics,structure, and mode of origin

Eastern Aphrodite Terra and Western Aphrodite form an altimetrically prominent 14,000 km long part of the equatorial highlands on Venus. Several parallel linear discontinuities striking northwest across the general east-west regional strike of the highlands are mapped in the altimetric and radar image data of Eastern Aphrodite and identified on the basis of abrupt termination of rift-like central chasma, offset and segmentation of the center of the highlands, and radar image discontinuities in the lowlands to the north. These characteristics are similar to those of linear discontinuities previously mapped in Western Aphrodite in terms of length, orientation, and influence on the central highlands and adjacent lowlands.Altimetric profiles in directions parallel to the discontinuities are regionally symmetric, more ridge-like in Eastern Aphrodite compared to the plateau-dominated form of topography in Western Aphrodite, and are characterized by alternating paired ridge-and-trough forms near their crests and on their flanks. By mapping the center of symmetry in multiple profiles, the prominent segmentation of the highland is shown to be imparted by an offset of the regional symmetry along the mapped discontinuities. These characteristics are morphologically similar to several of the large-scale characteristics of divergent plate boundaries of Earth, including mid-ocean rise crests and rifts, offset at fracture zones and transform faults, and symmetric thermal boundary layer topography.The altitude of the surface in profiles parallel to the discontinuities decreases as the square root of distance from the symmetry axes and with a slope similar to that predicted for thermal boundary layer topography associated with rates of divergence on Venus of ~ 1 ± 0.5 cm/yr. In order to test the hypothesis that the linear discontinuities are analogous to fracture zones, the predicted altitude of the surface at great distance from the centers of symmetry of the central highland and in directions across the discontinuities was calculated on the basis of a thermal boundary layer topography model with offset of altimetric symmetry at each discontinuity. Similarity of observed Arecibo high-resolution altimetric profiles across the discontinuities with that calculated for thermal boundary layer topography offset by transform faults reveals that in terms of the sense and magnitude of regional steps in altimetry across discontinuities and the altitude of the surface, Eastern Aphrodite is similar to the known characteristics of crustal spreading at divergent boundaries. The plateau-like form of Western Aphrodite and the ridge-like form of Eastern Aphrodite are analogous respectively to the difference between areas of anomalous (Iceland) and normal crustal production along rise crests on Earth. Estimates of volumetric differences in crustal production in the environment of Venus and as it would be influenced by differences in mantle temperature beneath Western and Eastern Aphrodite imply that Eastern Aphrodite represents normal crustal production. On this basis, Western Aphrodite may be characterized by a mantle temperature that is warmer than the mantle beneath Eastern Aphrodite Terra, perhaps in association with deep convective mantle upwelling.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Solar wind origin of 36Ar on Venus

A hypothesis is considered in which the ³⁶Ar found on Venus is of solar origin. This possibility is quantitatively discussed within the framework of present theories of planetary accumulation by sweep up of planetesimals under gas-free conditions. Solar wind implantation of ³⁶Ar would take place by irradiation of accumulating material during the first ≈10⁵ years of planetary growth, provided that the flux of solar wind was enhanced by a factor of ≈100 at that time. Enrichment of Venus in implanted gas would be a consequence of the irradiated material being initially confined to the innermost edge of the radially opaque circusolar planetesimal disk predicted by these theories. The observed atmospheric data require a Ne/Ar fractionation by a factor of ≈100 during the planetesimal stage. It is also necessary that there be very little mixing of irradiated planetesimals from the inner edge of disk to the distance (≈1 AU) at which the Earth formed. The hypothesis can be tested by measurement of the abundance of Kr and Xe in the Venus atmosphere. Venera data indicate a terrestrial ³⁶Ar/Kr ratio, in disagreement with the solar wind hypothesis. In contrast, the Pioneer experiments find a lower limit to this ratio, well above the terrestrial value, that is compatible with the hypothesis. These experiments also show that Venus' ³⁶Ar/Xe ratio does not correspond to the so-called “planetary” trapped inert gas composition. The inert of Venus could be related to result of admixture of gas with solar composition. The inert gas on Venus could be related to that found in enstatite chondrites.     

Morphology of the Tessera Terrain on Venus: Implications for the Composition of Tessera Material

The main goal of this paper is to estimate the possible composition of the tessera material on the basis of an interpretation of the morphology of the tessera precursor terrain. The results of detailed photogeologic analysis of tessera are presented. For the study, 56 randomly chosen areas that characterize the surface of large and small tessera massifs were selected. Each area represents a portion of the F-MAP photomosaics acquired at a 75 m/px resolution. The results of this study show that the tessera precursor terrain appears everywhere as plains. In its morphology, these plains are similar to the plains outside the tessera massifs. An overview of all possible mechanisms of the formation of plains on Venus and comparison of these mechanisms with the data of the chemical measurements on the surface of Venus suggests that the Venusian plains were formed as a result of the emplacement of low-viscous basaltic lava. This rather well-known conclusion is made here for the first time in order to estimate the possible composition of the tessera material. Thus, it is likely that the composition of the tessera precursor plains is similar to the composition of the basaltic plains on Venus. The products of posttessera volcanism in the form of morphologically smooth plains commonly occur within the tessera terrains. Morphologically, these plains are similar to the regional Venusian plains, which strongly suggests a basaltic composition of such plains. There are only two volcanic flows within the whole tessera terrain on Venus whose morphology permits one to interpret them as a manifestation of nonbasaltic, more siliceous volcanism. This means that the material of the regional tessera-bearing highlands very rarely responded to the thermal influence from below by siliceous volcanism. If some hypothetical granitelike material makes up the main portion of the tessera highlands, this material remains hidden. Therefore, the hypothesis of the granitelike bulk composition of the tessera highlands has little support from observations. At the current stage of the study of Venus, a model in which tessera highlands are composed predominantly of basalt with a possible, but insignificant component of more siliceous material is thought to be correct.     

Venusian geological features and origin of the Archean Earth greenstone belts

Magellan imagery of Venus reveals a variety of geological features (e.g., coronae and impact craters) which may have been present on the early Earth and preceded the formation of Archean greenstone belts.     

Morphology and origin of palimpsests on Ganymede based on Galileo observations

Palimpsests are large, circular, low-relief impact scars on Ganymede and Callisto. These structures were poorly understood based on Voyager-era analysis, but high-resolution Galileo images allow more detailed inspection. We analyze images of four Ganymedean palimpsests targeted by Galileo: Memphis and Buto Faculae, Epigeus, and Zakar. Ganymedean craters and Europan ring structures are used as tools to help better understand palimpsests, based on morphologic similarities. From analysis of Galileo images, palimpsests consist of four surface units: central plains, an unoriented massif facies, a concentric massif facies, and outer deposits. Using as a tie point the location in these structures where secondary craters begin to appear, outer deposits of palimpsests are analogous to the outer ejecta facies of craters; the concentric massif facies of palimpsests are analogous to the pedestal facies of craters; and the unoriented massif facies and central plains are analogous to crater interiors. These analogies are supported by the presence of buried preexisting structure beneath the outer two and absence of buried structure beneath the inner two units. Our observations indicate that palimpsest deposits represent fluidized impact ejecta, rather than cryovolcanic deposits or ancient crater interiors.     

Landing on Venus: Past and future

We briefly describe the history of landings on Venus, the acquired geochemical data and their potential petrologic interpretations. We suggest a new approach to Venus landing site selection that would avoid the potential contamination by ejecta from upwind impact craters. We also describe candidate units to be sampled in both in situ measurement and sample return missions. For the in situ measurements, the “true” tessera terrain (tt) material is considered as the highest priority goal with the second priority given to transitional tessera terrain (ttt), shield plains (psh) and lobate plains (pl) materials. For the sample return mission, the material of regional plains with wrinkle ridges (pwr) is considered as the highest priority goal with the second priority given to tessera terrain (tt) material. Combining the desire to study materials of specific geologic units with the problem of avoiding potential contamination by ejecta from upwind impact craters, we have suggested several candidate landing sites for each of the geologic units. Although spacecraft ballistics and other constraints of specific mission profiles (VEP or others) may lead to the selection of different candidate sites, we believe that the approaches outlined in this paper can be helpful approach in optimizing mission science return.     

Topographic comparisons of uplift features on Venus and Earth: Implications for Venus tectonics

Venus and Earth display different hypsography. We use topographic profiles to search for well-understood terrestrial analogs to venusian features. Specifically, by using cross-correlation, we correlate average profiles for terrestrial rifts (slow and fast, “ultra-slow,” incipient and inactive) and also hotspots (oceanic and continental) with those for venusian chasmata and regiones, to draw inferences as to the processes responsible for shaping Venus’ surface. Correlations tend to improve with faster spreading rates; Venus’ correlations rank considerably lower than terrestrial ones, suggesting that if chasmata are analogous to terrestrial spreading centers, then spreading on Venus barely attains ultra-slow rates. Individual features’ normalized average profiles are correlated with profiles of other such features to establish the degree of similarity, which in turn allows for the construction of a covariance matrix. Principal component analysis of this covariance matrix shows that Yellowstone more strongly resembles Atla, Beta and W. Eistla regiones than it does the terrestrial oceanic hotspots, and that venusian chasmata, especially Ganis, most closely resemble the ultra-slow spreading Arctic ridge.     

Venus gravity field: Pioneer Venus orbiter navigation results

The gravity field of Venus has been modeled by a spherical harmonic expansion of the potential to degree and order seven. The estimates of these coefficients were obtained by combining information from 43 short arcs (4 hr) of line-of-sight Doppler data centered at periapsis. The data arcs were distributed in longitude and time over more than two circulations of Venus by the Pioneer Venus Orbiter subperiapsis point which was confined to the band of latitudes from 14°N to 17°N. Convergence of the solution has been assured by iterating upon the initial estimate. All estimates were performed with zero a priori information on the gravity coefficients. Since the altitude of periapsis for most of the orbits was within the sensible Venusian atmosphere, drag effects on the estimated harmonics have been removed using an exponential atmosphere density model. Estimates of the mass parameter (GM) of Venus using this dataset are also evaluated.     

Deep versus shallow origin of gravity anomalies, topography and volcanism on Earth, Venus and Mars

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.     

Morphology and movements of polarizations features on Venus as seen in the pioneer Orbiter Cloud Photopolarimeter data

Polarization observations obtained from the Orbiter Cloud Photopolarimeter (OCPP) show local, organized features whose morphology is similar to that of the ultraviolet clouds. No obvious correlation between the observed amount of polarization and relative brightness was found, suggesting that the polarization features are not due to variations in the unpolarized intensity alone, but rather to other causes such are the existence of a haze. Many of the features that can be seen even at 935 nm wavelength are believed to be signatures of local variations in the haze of submicron-size particles that have been detected from the OCCP data (K. Kawabata, D. L/ Cooffeen, J. E. Hansen, W. A. Lane, M. Sato, and L. D. Travis (1980). J. Geophys. Res.85, 8129–8140). Substantial variations in the structure and visibility of the polarization features that are observed suggest that the haze amount mixed with and above the main cloud layer may not be constant by varies with time. Some of these features last for least a few days thus allowing measurements of their apparent motions. The small number of measurements possible from the available data shows movements similar to those of the ultraviolet clouds in both direction and speed. According to Kawabata et al. the haze extends well above the main cloud layer to about 5 mb pressure level while the optical depth unity at 365 nm (corresponding to the level of the ultraviolet clouds tracked to infer the cloud-top level circulation) occurs at about 30 mb pressure level. Thus, the rapid retrograde circulation suggested by the movements of haze features in the polarimetry data would indicate that the layer in which such rapid circulation exists is fairly deep.     

Morphology of the cloud tops as observed by the Venus Express Monitoring Camera

Since the discovery of ultraviolet markings on Venus, their observations have been a powerful tool to study the morphology, motions and dynamical state at the cloud top level. Here we present the results of investigation of the cloud top morphology performed by the Venus Monitoring Camera (VMC) during more than 3 years of the Venus Express mission. The camera acquires images in four narrow-band filters centered at 365, 513, 965 and 1010 nm with spatial resolution from 50 km at apocentre to a few hundred of meters at pericentre. The VMC experiment provides a significant improvement in the Venus imaging as compared to the capabilities of the earlier missions. The camera discovered new cloud features like bright “lace clouds” and cloud columns at the low latitudes, dark polar oval and narrow circular and spiral “grooves” in the polar regions, different types of waves at the high latitudes. The VMC observations revealed detailed structure of the sub-solar region and the afternoon convective wake, the bow-shape features and convective cells, the mid-latitude transition region and the “polar cap”. The polar orbit of the satellite enables for the first time nadir viewing of the Southern polar regions and an opportunity to zoom in on the planet. The experiment returned numerous images of the Venus limb and documented global and local brightening events. VMC provided almost continuous monitoring of the planet with high temporal resolution that allowed one to follow changes in the cloud morphology at various scales.We present the in-flight performance of the instrument and focus in particular on the data from the ultraviolet channel, centered at the characteristic wavelength of the unknown UV absorber that yields the highest contrasts on the cloud top. Low latitudes are dominated by relatively dark clouds that have mottled and fragmented appearance clearly indicating convective activity in the sub-solar region. At ～50° latitude this pattern gives way to streaky clouds suggesting that horizontal, almost laminar, flow prevails here. Poleward from about 60°S the planet is covered by almost featureless bright polar hood sometimes crossed by dark narrow (～300 km) spiral or circular structures. This global cloud pattern can change on time scales of a few days resulting in global and local “brightening events” when the bright haze can extend far into low latitudes and/or increase its brightness by 30%. Close-up snapshots reveal plenty of morphological details like convective cells, cloud streaks, cumulus-like columns, wave trains. Different kinds of small scale waves are frequently observed at the cloud top. The wave activity is mainly observed in the 65–80° latitude band and is in particular concentrated in the region of Ishtar Terra that suggests their possible orographic origin. The VMC observations have important implications for the problems of the unknown UV absorber, microphysical processes, dynamics and radiative energy balance at the cloud tops. They are only briefly discussed in the paper, but each of them will be the subject of a dedicated study.     

Resurfacing on Venus

The resurfacing evolution of Venus has been evaluated through Monte Carlo simulations. For the first time, the sizes of volcanic flows in the models were generated using the frequency-size distribution of volcanic units measured on Venus. A non-homogeneous spatial generation of volcanic units was included in the models reproducing the Beta-Alta-Themis volcanic anomaly. Crater modification is simulated using a 3D approach. The final number of modified craters and randomness of the crater population were used to evaluate the success of the models, comparing the results from our simulations with Venus observations. The randomness of the crater population is evaluated using pair-correlation statistics. On the one hand, a catastrophic resurfacing event followed by moderate volcanic activity covering ≈40% of the planetary surface can reproduce the number of modified craters and the pair-correlation statistics do not reject randomness. On the other hand, the pair-correlation test for equilibrium steady-state resurfacing models rejects the randomness of the crater population when reproducing the observed frequency-size distribution of the volcanic units with a non-homogeneous spatial generation of volcanic units.