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Power fluctuations are observed on radio signals propagating through the Venus atmosphere. These scintillations, due to fluctuations in refractive index, are particularly strong between 56 and 72 km altitude where the strong convective stability favors gravity wave propagation. We examine low-latitude X-band radio occultation experiments performed by the Akatsuki spacecraft to assess the statistical variability of these scintillations. The observed root-mean-square scintillation amplitudes at a given altitude vary by a factor of about 3. Part of this range is due to the different geometry of the occultations: the corresponding variations in refractive structure constant at 60-70 km altitude span a factor of ∼2. These propagation characteristics may influence the optimization of the two-way radio link for the DAVINCI mission currently in development. • Examines scintillations in Akatsuki Radio Occultations. • Changes of as much as 15 dB of signal power in 1 s are seen. • Scintillation amplitudes at a given altitude vary by a factor of about 3. [ABSTRACT FROM AUTHOR]

Parga Chasmata is a 10,000 km long rift system on Venus with abundant coronae (enigmatic circular tectonomagmatic features). In order to further investigate the rift-corona relationship, detailed 1:500,000 geological mapping was focused on Onenhste Corona and the surrounding region (16°-24° S, 145°-131° W). More than 46,000 extensional lineaments were mapped and grouped into 50 sets, comprising 17 radiating, 28 circumferential and 5 linear sets (interpreted to overlie mafic dyke swarms). Radiating and circumferential swarms are linked with Onenhste Corona (OC), Momu Coronae (MC), Ulgen-ekhe Coronae (UEC), Rzhanitsa Corona (RzC, centred just outside the study area) and 5 Unnamed Coronae (UC1–5), as well as Malibran Patera (MP) and Fedchenko Patera (FP). Linear swarms are provisionally linked with magmatic centres outside the study area. Crosscutting relationships between the graben sets (dyke swarms) were used to identify the relative ages of magmatic centres. We applied an additional approach to recognize coeval centres, the swinging or deflection of radiating and circumferential dyke swarms to reveal the stress interaction between different coeval centres. The relative ages (oldest to youngest) of the magmatic centres are UC2 > RzC > MC ≥ OC = UC1 = UEC ≥ MP > UC5, and FP > UC3. These new detailed data provide significant information on coronae timing, evolution and relation to rift zones that will serve as a basis for future quantitative studies of lithospheric and mantle evolution and the combined cause and effect of rifting and mantle diapiric upwelling. Several centres, RzC, UC2, MC, UC3 and FP, ordered from NW to SE, are aligned along a trend parallel to Parga Chasmata but offset about 900 km to the south from the main zone of rifting. It is inferred that this alignment is related to a zone of weakness associated with the rift extension. Along this trend the centres do not show an age progression. Coronae MC, OC, UC1 and UEC are aligned along the NNE trending P13 Linea, orthogonal to the main Parga Chasmata rift. The main centre, Onenhste, is coeval with UC1, which in turn is coeval with UEC. Additional age relationships indicate that the late stage of OC was active at the same time as MP, MC and the UC2–9 centre of UC2. Linea P13 is older than Chondi Chasma. This study, and continuing, detailed geological mapping and analysis are increasing our understanding of the relationships between the formation of the main WNW trending Parga Chasmata rift zone, the orthogonal trends of rifting and the numerous magmatic centres (mainly coronae) distributed along both trends. • Dyke swarm history of coronae-rich study area south of Parga Chasmata. • 46,000 lineaments grouped as 17 radiating, 28 circumferential, 5 linear dyke swarms. • Swings in radiating & circumferential swarms– stress interactions between centres. • RzC, UC2, MC, UC3 & FP coronae aligned parallel to Parga Chasmata but offset 900 km. • MC, OC, UC1 and UEC corona aligned orthogonal to the Parga Chasmata trend. [ABSTRACT FROM AUTHOR]

Meteor and bolide phenomena caused by the atmospheric ablation of incoming meteoroids are predicted to occur at the planet Venus. Their systematic observation would allow to measure and compare the sub-mm to m meteoroid flux at different locations in the solar system. Using a physical model of atmospheric ablation, we demonstrate that Venus meteors would be brighter, shorter-lived, and appear higher in the atmosphere than Earth meteors. To investigate the feasibility of meteor detection at Venus from an orbiter, we apply the SWARMS survey simulator tool to sets of plausible meteoroid population parameters, atmospheric models and instrument designs suited to the task, such as the Mini-EUSO camera operational on the ISS since 2019. We find that such instrumentation would detect meteors at Venus with a 1.5 × to 2.5 × higher rate than at Earth. The estimated Venus–Earth detection ratio remains insensitive to variations in the chosen observation orbit and detector characteristics, implying that a meteor survey from Venus orbit is feasible, though contingent on the availability of suitable algorithms and methods for efficient on-board processing and downlinking of the meteor data to Earth. We further show that a hypothetical camera onboard the upcoming EnVision mission to Venus similar to the ISS instrument should detect many times more meteors than needed for an initial characterisation of the large meteoroid population at 0.7 au from the Sun. • We use physics-based modelling to understand the properties of meteors in the upper atmosphere of Venus and to compare meteor survey efficiency from Earth orbit and from Venus orbit. • Venus meteors would be brighter and shorter-lived than Earth meteors, due to different atmospheric density scale heights. • Assuming similar meteoroid populations at the two planets, orbital meteor surveys would detect 1.5 × –2.5 × more meteors per hour at Venus than at Earth. [ABSTRACT FROM AUTHOR]

Coronae are enigmatic, quasi-circular/elliptical tectono-magmatic features, common across the surface of Venus ranging in size from 60 to 2600 km This study undertook a detailed mapping (1:500,000 scale) of Heng-o Corona, the second largest on Venus. 24,600 graben-fissure lineaments were mapped, interpreted to be underlain by mafic dykes, and grouped into radiating, circumferential and linear swarms. A series of 5 nearly-concentric rings were identified from unflooded or partially flooded circumferential swarms. The largest of these has a diameter of 1970 km, which almost doubles Heng-o's known size (previously 1060 km based on its topographic annulus). This new diameter also narrows the gap between it and Artemis Corona, the largest on Venus, to only 630 km. Three major radiating graben swarms were identified, extending >700–1000 km from their sources within the central annulus. From cross-cutting relationships, we interpreted that the major radiating swarms formed first, upon initial plume arrival and flattening against the base of the lithosphere. The circumferential swarms followed, starting with the innermost ring, migrating with the leading edge of the plume head as it spread outward against the base of the lithosphere. The distinctive annular ridge/trough system (at about 500 km radius) is associated with the transition between plume head arrival and spreading. The centers of most of the radiating and circumferential swarms are aligned along a NNW-SSE trend that is interpreted to represent an underlying zone of lithospheric weakness. Additional graben-fissure systems in the study area are not associated with Heng-o Corona; some are linked with other magmatic centers nearly up to 1000 km outside the study area. • Mapping 24,600 lineaments to determine dyke swarm history of Heng-o Corona, Venus. • Circumferential swarm doubles Heng-o size to 1970 km (almost as large as Artemis). • Plume head arrival (radial swarms) then plume spreading (circumferential swarms). • Formation of annular rim/trough during transition from plume arrival to spreading. • Alignment of Heng-o magmatic centers along NNW-SSE zone of lithospheric weakness. [ABSTRACT FROM AUTHOR]

We perform a survey of the Visible and Infrared Thermal Imaging Spectrometer-Mapper (VIRTIS-M) images onboard the Venus Express space mission, at four narrow wavelength bands that target different altitude regions on the cloud deck of Venus' atmosphere (280–320, 365–400, 580–600, 900–920 nm). Our goal was to detect and characterize atmospheric gravity waves, using several processing techniques based on high-pass filtering to enhance features in low-contrast images. The period of our selected dataset was between August 2007–October 2008 on the dayside hemisphere, identical to a previous study of waves located on the nightside lower cloud, so that a temporally overlapped comparison could be established, although over different longitudinal locations. We retrieved the morphological properties of these waves including horizontal wavelength and packet width along with several orientations of the identified waves. We retrieved properties for 69 wave packets across all analysed wavelength ranges, and dynamical parameters for 16 wave packets. Waves observed across the four wavelength ranges examined here have similar properties, with horizontal wavelengths of a few hundred kilometres and full length of wave-trains going up to 1000 km. Although these wave properties do not seem to depend on latitude, we notice an increase in the values of several of these parameters close to the evening terminator. Considering our results and a comparison with previous studies of stationary features interpreted as gravity waves, we argue that forcing from topography is not the main cause of the wave packets observed here, whose properties are better supported by a convective generation scenario. The retrieved properties show a consistent agreement between waves on the upper and lower cloud, suggesting a similar forcing mechanism based on convection from a neutral stability layer between the lower and middle-upper cloud. Despite the similar properties, we find no evidence of any correlation between wave packets propagating in the lower cloud and upper cloud, based on their shape and relative position. • Detection and characterization of gravity waves on Venus's atmosphere at 3 altitudes. • Spatial scales of waves are similar to nightside waves found in the lower cloud. • Some waves were observed simultaneously at multiple altitudes within the upper cloud. • Possible influence of local time on waves' spatial scales to be further investigated. [ABSTRACT FROM AUTHOR]

To study the Venusian ionosphere, a 3D ionospheric model is included in the Venus Planetary Climate Model (Venus PCM), which we present here with a number of recent extensions and improvements. Our ionospheric model of Venus consists of 15 charged species (electron and 14 ions), 13 photo-ionization for 8 species, an ion-neutral chemistry (61 reactions) and an ambipolar diffusion scheme for the ion vertical transport. The electron temperature is assumed independent of the solar activity. Simulation results are compared with observation from the Pioneer Venus (PV) and Venus Express (VEX) for high and low/intermediate solar activity respectively. The model shows that ambipolar diffusion dominates photochemical equilibrium above 180 km and above 130 km altitude on the dayside and the nightside respectively. On the dayside, the electron density predicted by Venus PCM and its variation with solar zenith angle (SZA) are in good agreement with PV observations at high solar activity, even if the secondary electron peak is underestimated by the model, due to the absence of (photo-)electron impact ionization process, in Venus PCM. At higher solar activity, the predicted horizontal and vertical variations in electron and ion densities are both significantly underestimated on the nightside, probably due to the currently predicted weak day-to-night transport and the absence of photo-electron impact ionization process which is the main source of ion production on the nightside. On the dayside at low solar activity, the electron density predicted by Venus PCM are over-estimated by a factor 1.25 at the altitude of the main ionospheric peak and by a factor 2–3 at 250 km, compared to VEX observations. We suggest that this difference between high and low solar activity is linked to the overestimation of the neutral density at low solar activity and the independence of electron temperature with solar activity used in Venus PCM, which should have a significant effect on ion chemistry and the scale height of ion species. [ABSTRACT FROM AUTHOR]

The recently selected missions to Venus have opened a new era for the exploration of this planet. These missions will provide information about the chemistry of the atmosphere, the geomorphology, local-to-regional surface composition, and the rheology of the interior. One key scientific question to be addressed by these future missions is whether Venus remains volcanically active, and if so, how its volcanism is currently evolving. Hence, it is fundamental to analyze appropriate terrestrial analog sites for the study of possibly active volcanism on Venus. To this regard, we propose Mount Etna - one of the most active and monitored volcanoes on Earth - as a suitable terrestrial laboratory for remote and in-situ investigations to be performed by future missions to Venus. Being characterized by both effusive and explosive volcanic products, Mount Etna offers the opportunity to analyze multiple eruptive styles, both monitoring active volcanism and identifying the possible occurrence of pyroclastic activity on Venus. We directly compare Mount Etna with Idunn Mons, one of the most promising potentially active volcanoes of Venus. Despite the two structures show a different topography, they also show some interesting points of comparison, and in particular: a) comparable morpho-structural setting, since both volcanoes interact with a rift zone, and b) morphologically similar volcanic fields around both Mount Etna and Idunn Mons. Given its ease of access, we also propose Mount Etna as an analog site for laboratory spectroscopic studies to identify the signatures of unaltered volcanic deposits on Venus. [ABSTRACT FROM AUTHOR]

Venusian thermal tides are discussed as a three-dimensional response to solar heating, which is strongly modified by both barotropic and baroclinic energy conversions in a fully developed super-rotation. Thermal tides such as gravity waves propagate upward and downward from the cloud top (∼65 km) at low latitudes, while tidal gyres such as Rossby waves appear at high latitudes according to the general circulation model (GCM) developed at Atmosphere and Ocean Research Institute (AORI), The University of Tokyo. Around the cloud-top heating maximum, strong poleward/equatorward tidal flows are formed by both divergent and rotational flows that are in phase, and equatorial super-rotation is accelerated by the equatorward and vertical momentum fluxes of the diurnal tides and the vertical flux of the semidiurnal tides. These thermal tides are a result of solar heating as well as energy conversions from the zonal-mean jet and its related temperature gradient. The gravity wave is thermally forced in the cloud-top heating maximum where diabatic heating produces eddy available potential energy. The Rossby waves are formed in mid- and high-latitudinal regions where diurnal barotropic energy conversion occurs around the zonal-mean jet core and semidiurnal baroclinic energy conversion occurs below the cloud layer far from the solar heating region. • Flow structures and energetic of thermal tides are examined in a Venus GCM. • The tides composed of the divergences/convergences and gyres propagate globally. • Diabatic heating produces the potential energy of the tides around the cloud top. • The diurnal tide is modified by barotropic energy conversion around the jet core. • The semidiurnal tide is modified by baroclinic energy conversion below the clouds. [ABSTRACT FROM AUTHOR]

Eistla Regio, Venus, is a locus of major plume-generated magmatism and tectonics. We focus on the central portion, termed Central Eistla Regio. More than 19,000 extensional lineaments (grabens, fissures and fractures) were mapped in the area 13°-26°E, 0°-10°N, at 1:500,000 scale using full resolution Magellan SAR (radar) images. These structures are interpreted to represent the surface expression of underlying mafic dykes and have been grouped, on the basis of trend, into radiating, circumferential and linear dyke swarms. Thirty-two swarms are identified and linked to 20 magmatic centres, most of which are located within or near two coronae, Belet-ili and Gaia (Gaya) coronae, while others are linked to magmatic centres outside the study area. These magmatic centres are interpreted to be linked to underlying mantle plumes, and the sizes of the underlying flattened plume heads are estimated from the interpreted dyke swarms: for radiating swarms, the radius of the transition from proximal radiating to more distal linear pattern, and for circumferential dykes, the radial distance to the outer boundary of the circumferential pattern. Plume heads for the largest centres, Belet-ili and Gaia coronae are estimated at about 230 km and 275 km in radius, respectively. These sizes are less than the 1200 km radii predicted for a plume arriving from the deep mantle, and would suggest an origin at a shallower level (likely a mid-mantle boundary, i.e. between upper and lower mantle). The NW alignment of Belet-ili and Gaia corona and other magmatic centres in our study area potentially represents structural control by a rift zone, which can be traced further north to Anala Mons (one of two main plume centres of Central Eistla Regio). [Display omitted] • Detailed mapping of graben-fissure systems (dyke swarms) in Central Eista Regio. • Grouped into 32 swarms: 14 radiating, 6 circumferential and 12 linear. • Plume head diameters from swarms: 227 km for Belet-ili & 274 km for Gaia Corona. • Magmatic centres of Belet ili and Gaia mostly falling along a NNW-SSE trend. • This trend may mark a rift zone associated with Anala Mons. [ABSTRACT FROM AUTHOR]

This study investigates the impact of model input parameters on the characteristic features of the dayside V 2 layer in the Venus ionosphere using an in-house developed one-dimensional photochemical model (1D-PCM) and observations from Venus Express radio science experiments (VeRa). 1D-PCM is simulated for different combinations of neutral density models (VTS3, a global empirical model of the Venus Thermosphere, and VenusGRAM, the Venus Global Reference Atmospheric Model), solar flux models (Solar 2000 (S2K), and observations by SEE (Solar EUV Experiment) onboard the NASA's TIMED, the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics, satellite). The hmV 2 (height of the V 2 layer plasma density) is found to be better reproduced in VTS3-SEE and VTS3-S2K pairs of combination, indicating that VTS3 is a more representative model for Venusian neutral density. The nmV 2 (peak plasma density of the V 2 layer) remains consistently higher when using SEE data for solar fluxes, but the model deviates significantly for Solar Zenith Angles (SZA) between 65 ∘ and 85 ∘ , possibly due to differences in neutral density at high SZA where the temperature is lower than predicted by the neutral density model. Quantitative analysis reveals that errors in the 1D-PCM, with deviations of 1 to 2 km in hmV 2 and 5%–10% difference in nmV 2 between the model and observations, could be due to uncertainties in the neutral density and solar flux models. Comparison with the Transplanet model suggests that accounting for input uncertainties can improve the model's ability to reproduce observations. This study provides insights into the influence of input parameters on the Venus ionosphere and highlights the importance of considering uncertainties in modeling studies. • The electron density profiles are derived from the frequency residuals of VeRa observations. • The 1D-PCM reproduces features of the V 2 within the uncertainty limit. • The deviation between the observation and the model is maximum at large solar zenith angles. • The VTS3 neutral density model is a better representative of the Venusian atmosphere. [ABSTRACT FROM AUTHOR]

During the years 2016 to 2018, the instruments Akatsuki/IR2 (JAXA) and IRTF/SpeX (NASA) acquired a large set of images at 1.74, 2.26 and 2.32 μm to study the nightside mid-to-lower clouds (48–60 km) of Venus. Here we summarize the rich variety of cloud morphologies apparent in these images: from frequent wave packets and billows caused by shear instabilities, to features reported decades ago like the circum-equatorial belts, bright blotches and equatorial troughs, and previously unseen features like dark spots, sharp dark streaks at mid latitudes and fully-developed vortices. • Summary of the main morphologies found on the nightside lower clouds of Venus. • The full dataset of images from Akatsuki/IR2 and images from IRTF/SpeX were explored. • New morphologies found: vortices, sharp dark streak and black spots. • The bright trough, blotch and the circum-equatorial belts seen again on Venus after decades. [ABSTRACT FROM AUTHOR]

We measured the length of the Venus sidereal day (LOD) from Earth-based radar observations collected from 1988 to 2017, using offsets in surface feature longitudes from a prediction based on a 243.0185d period derived from analysis of Magellan mission images over a 487-day interval. We derive a mean LOD over 29 years of 243.0212 ± 0.0006d. Our result is consistent with earlier estimates (but with smaller uncertainties), including those based on offsets between Venus Express infrared mapping data and Magellan topography that suggest a mean LOD of 243.0228 ± 0.002d over a 16-year interval. We cannot detect subtle, short-term oscillations in rate, but the derived value provides an excellent fit to observational data over a 29-year period that can be used for future landing-site planning. • The rotation period of Venus is measured from changes in surface feature positions over time. • We use Earth-based radar images from 1988 to 2017 to map surface feature locations. • The average length of day over that 29-year period is 243.0212 ± 0.0006d. [ABSTRACT FROM AUTHOR]

Abstract Equatorial Kelvin waves, which have been observed in planetary fluids, produce superrotation and climatological variability. Although the meridional flow component is zero on Earth's beta-plane in the absence of background basic flow, it cannot be neglected on spheres in some planetary fluids when the Lamb parameter is small. For a vertically propagating Kelvin-like wave in the absence of background wind shear, meridional flows produce equatorward heat transport, i.e., a downward Eliassen-Palm flux. The vertical momentum transport caused by the equatorward heat flux has a theoretical upper limit, and the normalized flux is analytically expressed as a simple function that is independent of the Lamb parameter. On a slowly rotating and/or small-sized planet, we need to consider the meridional flow component of the equatorial Kelvin-like wave and its related heat flux on the sphere, and further consider their modification owing to background flow shear through coupling with high-latitude Rossby waves. Highlights • Vertically propagating, equatorial Kelvin-like wave on the sphere is derived. • The meridional flow of the Kelvin-like wave produces equatorward heat flux. • The equatorward heat flux of the Kelvin-like wave transports momentum vertically. • The momentum flux due to the Kelvin-like wave heat flux has an upper limit. • Differences with Kelvin-like waves modified/forced by wind shear are discussed. [ABSTRACT FROM AUTHOR]

Abstract Two recent papers, by Karimi et al. (2016, Icarus 272, 102–113) and Karimi and Dombard (2017, Icarus 282, 34–39), tried to deduce paleo-heat flows for, respectively, Mars and Venus from modeling the viscoelastic relaxation of large impact craters. Indeed, crater relaxation would be consequence of the flow of the lower crust and uppermost mantle. This flow is dependent on temperature, permitting the link with the calculation of thermal profiles and heat flows. Both papers used conductive thermal profiles and constant thermal conductivities for both the crust and upper mantle (equivalent to using linear thermal gradients given there were no mention to crustal or mantle heat sources), and appropriate rheological laws. In the present discussion, we show that the background heat flows contemporaneous to impact craters formation and relaxation obtained by Karimi and co-workers, when used along with the assumptions made by these authors, lead to temperatures that produce massive (even total) lower crust melting in all and at least a substantial part of the cases for, respectively, Venus and Mars. It is clear that the heat flow results presented by Karimi and co-workers suffer of inconsistency between model requirements explicitly indicated (a crust free of melting) and the implications of the obtained basal heat flows (a lowermost crust partially or totally molten). Thus, we consider that the papers by Karimi and co-workers do not give reliable information on the thermal history of Mars and Venus. [ABSTRACT FROM AUTHOR]

Highlights • Over the entire mission of Venus Express, O+ ions mainly escape from the induced magnetosphere of Venus. • O+ escape rates from the magnetosheath are controlled by the solar wind and the solar EUV flux. • O+ escape rates from the induced magnetosphere are controlled not only by the solar wind and the solar EUV flux but also by return flows. • Return flows become dominant in the solar maximum and reduces the escape rates in the induced magnetosphere. • Return flows may be produced by a fine-structure of the current sheet in the Venusian magnetotail. Abstract We investigate dependences of O+ escape rates from Venus both on the solar wind and the solar extreme ultraviolet (EUV) flux by using the 8.5-year dataset (May 2006 to December 2014) of the Ion Mass Analyzer and the magnetometer aboard Venus Express. We examine the O+ escape rates for 8 different conditions depending on the solar wind's dynamic pressure, the magnitude of the motional electric field, and the solar F10.7 index which is a good indicator for the solar EUV flux. We find that the O+ escape rates are mainly controlled by the motional electric field and the solar EUV flux in the magnetosheath. We suggest that the ion pickup by the motional electric field is the main ion escape process in the magnetosheath. On the other hand, in the induced magnetosphere, the O+ escape rates are controlled by not only the solar wind and the solar EUV flux but also return flows. We find that the return flows become dominant in the magnetotail in the solar maximum period, which results in reducing the net escape rates in the induced magnetosphere. As a result, net escape rates tend to become larger in the solar minimum period than those in the solar maximum period. Further analysis of the magnetic field shows that the return flows are preferably observed when the magnetic field component along the Venus-Sun line reverses multiple times in the magnetotail. We suggest that ionospheric irregular structures or IMF sector boundary crossings form the fine scale anti-parallel draping pattern of the interplanetary magnetic field (IMF), and this causes ion acceleration towards Venus due to the magnetic tension force or magnetic reconnections. Another possibility is precipitations of O+ pickup ions on the nightside of Venus under the fine structure of IMF. Since the return flows significantly affect the O+ escape rates from Venus, it is important to study not only long-term transition of ion outflow from Venus but also return flows to estimate the net ion loss from Venus over the long history of the solar system. [ABSTRACT FROM AUTHOR]

Highlights • Solar-locked and geographical atmospheric structures on Venus are investigated. • The present study evaluates differences between zonal and dayside averages. • Indirect circulation is formed at high latitudes by thermal tide and baroclinic waves. • Stationary modification of middle-atmospheric structure is forced by topography. • Solar-locked wind fields are compared with the Akatsuki UV cloud tracked ones. Abstract Solar-locked and geographical atmospheric structures of daily averaged wind and temperature on Venus were investigated using an atmospheric general circulation model with Venusian topography and a two-stream radiative code and were compared with wind fields obtained from the Akatsuki ultraviolet imager. The horizontal wind fields simulated around the subsolar region are similar to the observed ones at the cloud top. Mid-latitude jets of ∼120 m s–1 and an equatorial fast flow of ∼90 m s–1 are formed around the cloud top. A poleward flow of >8 m s–1 is formed above the cloud layer, where solar heating is strong. Around the cloud top, a poleward flow of ∼1 m s–1 is confined within the equatorward flank of the jet core, whereas an indirect circulation is formed in the jet core by the eddy heat fluxes owing to the thermal tide and baroclinic waves. In solar-fixed coordinates, the subsolar-to-antisolar circulation is predominant around the cloud top. Thus, differences are significant between the zonal and dayside averages of the meridional wind and its related fluxes within the cloud layer. This suggests the zonal mean meridional wind of the Hadley circulation, eddy momentum, and heat fluxes from the one-side hemisphere must be estimated carefully. In the experiment including topography, a near-surface subrotation is formed in latitudinal zones over high land and mountains, a weakly stable layer is formed at 10–20 km at low latitudes, and the zonal wind is weakened at the cloud top over the Aphrodite Terra. Regional stationary modification of the atmospheric structure due to topographical waves appears in the cloud layer. [ABSTRACT FROM AUTHOR]

Highlights • The rock analyzed at the Venera 8 landing site is thought to be silicic. • Fractional crystallization of Venusian basalt can produce the theoretical Venera 8 rock. • It is possible that sialic crust was encountered at the Venera 8 landing site. Abstract The surface rock composition measured by gamma (γ)-ray spectrometry at the Venera 8 landing site has anomalously high Th (6.5 ± 2.2 ppm) and U (2.2 ± 0.7 ppm) concentrations with respect to the material analyzed at other landing sites (Vega 1, Vega 2, Venera 9, Venera 10). A calculated bulk rock composition of Venera 8, constrained by the measured Th, U and K 2 O (4.0 ± 1.2 wt%) contents, is similar to silicic to intermediate rocks (diorite/granodiorite) that are typical of terrestrial convergent margins (magnesian, calc-alkalic). In this study, major and trace elemental modeling is applied in order to determine if the calculated whole rock composition of Venera 8 can be derived from a parental magma composition similar to Venusian basalt. The modeling results indicate that polybaric fractional crystallization of a hydrous (H 2 O = 0.4 wt%) and relatively oxidizing (ΔFMQ + 0.7) parental composition similar to Venera 14 basalt can yield residual silicic liquids that match the calculated Venera 8 whole rock composition. The measured Th and U concentrations can also be reproduced within the data uncertainty. Although Venus lacks modern Earth-style plate tectonics, magnesian, calc-alkalic compositions are common within Archean greenstone belts and some rift settings (Haida Gwaii). Consequently, it is possible that the Venera 8 probe encountered a fragment of crust that resembles a terrestrial greenstone belt. [ABSTRACT FROM AUTHOR]

Highlights • The displacement of plasma channels measured in the Venus wake has an appreciable deflection in the Y+ direction. • The displacement of those features is in the same sense as that of the O+ ions reported from measurements in many VEX orbits across the Venus wake. • The plasma channels are mostly measured in the vicinity of the midnight plane rather than by the sides of the Venus wake. • An account is made of differences between measurements of the plasma channels by the midnight plane and those that refer to ionospheric holes in the Collinson et al. study. • The momentum flux of the O+ ions in the plasma channels is in agreement with the deficiency of momentum flux of the solar wind H+ ions measured within them. Abstract Plasma channels have been suggested to account for the observation of ionospheric holes in the Venus nightside ionosphere. They are produced by the erosion of the solar wind on the magnetic polar regions of the Venus ionosphere, and are related to a sharp plasma transition that extends along the flanks of the wake and that is located downstream from the bow shock. The sharp plasma (intermediate) transition is a feature that has been detected with instruments onboard all the main spacecraft that have probed the Venus wake. From the early flyby transit of the Mariner 5 spacecraft to the later orbit crossings of the Venera 10, the Pioneer Venus and more recently the Venus Express there has been accumulated evidence of the presence of that transition by the flanks of the Venus wake. We report here measurements made with the ASPERA-4 instrument of the Venus Express that were conducted in orbit crossings by the midnight plane over a 4 year period (between 2006 and 2009), and that show different positions of the intermediate transition in the Venus wake at locations closer to the sun-Venus axis near solar minimum conditions by 2009. Measurements conducted in this latter time period also indicate that the intermediate transition by the midnight plane derives from the plasma channels and that is replaced by a broad velocity layer in orbits that probed farther away from that plane. It is argued that changes in the position of the intermediate transition with respect to the plasma channels are related to processes produced by the transport of solar wind momentum to the Venus polar ionosphere. The energy spectra of planetary ions measured in VEX orbits that probed by the midnight plane show momentum transport from the solar wind protons. This result is contrary to the conclusions reported by Collinson et al. (2014) in the sense that there is ¨no difference in the peak energy of the ionospheric outflow¨ when measured inside and outside an ionospheric hole. [ABSTRACT FROM AUTHOR]

Highlights • First detection of cloud top ozone in Venus atmosphere (about 1000 times less than Earth). • Ozone presence is restricted to high latitudes (polewards of 50°) in both hemispheres. • Measurements are qualitatively supported by our 3D photochemical model. Abstract After the first sporadic detections of an ozone (O 3) nighttime layer in the 90–100 km altitude range (Montmessin et al., 2011), we report here the discovery of another, permanent ozone layer on Venus, restricted to high latitudes (polewards of 50° both N and S) and located at the upper cloud level near 70 km. This detection was performed during a reanalysis of the whole SPICAV-UV nadir dataset through UV absorption near 250 nm in the backscattered solar light. The O 3 volume mixing ratio peaks in the 10–20 ppbv range, yielding observable column densities in the 0.1–0.5 Dobson units (DU), comparable to nominal values on Mars but much smaller than for Earth (∼ 300 DU). These measurements are supported by our 3D-photochemical model coupled with the LMD-IPSL GCM (Lebonnois et al., 2010), which indicates that the ozone layer identified by SPICAV results from downward transport of O 2 (∼ 50 ppmv) molecules over the poles by the mean meridional circulation. Our findings do not contradict previous upper limits (< 2 ppmv) based on O 2 measurements (Mills, 1999), since they were restricted to lower latitudes only. [ABSTRACT FROM AUTHOR]

Few constraints are available to characterize the deep atmosphere of Venus, though this region is crucial to understand the interactions between surface and atmosphere on Venus. Based on simulations performed with the IPSL Venus Global Climate Model, the possible structure and characteristics of Venus’ planetary boundary layer (PBL) are investigated. The vertical profile of the potential temperature in the deepest 10 km above the surface and its diurnal variations are controlled by radiative and dynamical processes. The model predicts a diurnal cycle for the PBL activity, with a stable nocturnal PBL while convective activity develops during daytime. The diurnal convective PBL is strongly correlated with surface solar flux and is maximum around noon and in low latitude regions. It typically reaches less than 2 km above the surface, but its vertical extension is much higher over high elevations, and more precisely over the western flanks of elevated terrains. This correlation is explained by the impact of surface winds, which undergo a diurnal cycle with downward katabatic winds at night and upward anabatic winds during the day along the slopes of high-elevation terrains. The convergence of these daytime anabatic winds induces upward vertical winds, that are responsible for the correlation between height of the convective boundary layer and topography. [ABSTRACT FROM AUTHOR]

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 × 10 20  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. [ABSTRACT FROM AUTHOR]

The comparison of the phase profiles of glories observed on the cloud top of Venus by the Venus Monitoring Camera ( Venus Express ) and the light-scattering characteristics of sulfuric acid droplets, containing admixtures with a high refractive index, makes it easier to choose between some candidates for the so-called unknown UV absorber in the Venus clouds. Since among the candidates there are materials wetted and not wetted by sulfuric acid, we analyze whether small submicron particles adhered to or embedded into the 1-µm H 2 SO 4 droplets may actually change the glory pattern normally produced by homogeneous spherical particles and what the conditions are, under which the composite particles formed in heterogeneous nucleation may still produce a glory feature. We have found that one of the most frequently considered candidates, sulfur, can hardly be responsible for the contrasts observed at 0.365 µm on the upper clouds, since it is not wetted by sulfuric acid and submicron sulfur particles, serving as condensation nuclei for sulfuric acid, can only adhere to the H 2 SO 4 droplets rather than be enveloped by them. Such droplets decorated by sulfur blobs substantially distort the glory feature characteristic of the scattering by spherical particles or even smooth it at all, while a glory pattern is practically always seen in the images of Venus taken at small phase angles. At the same time, the grains of the other UV absorbers that can be embedded in H 2 SO 4 droplets, e.g., the widely discussed ferric chloride, pose no problem in terms of interpretation of the observations of glory. [ABSTRACT FROM AUTHOR]

We derive simple dynamical relationships between wind speed magnitude and meridional temperature contrast. The relationship explains scatter plot distributions of time series of three variables (maximum zonal wind speed U MAX , meridional wind speed V MAX , and equator−pole temperature contrast dT MAX ), which are obtained from a Venus general circulation model with equatorial Kelvin-wave forcing. Along with V MAX and dT MAX , U MAX likely increases with the phase velocity and amplitude of a forced wave. In the scatter diagram of U MAX versus dT MAX , points are plotted along a linear equation obtained from a thermal-wind relationship in the cloud layer. In the scatter diagram of V MAX versus U MAX , the apparent slope is somewhat steep in the high U MAX regime, compared with the low U MAX regime. The scatter plot distributions are qualitatively consistent with a quadratic equation obtained from a diagnostic equation of the stream function above the cloud top. The plotted points in the scatter diagrams form a linear cluster for weak wave forcing, whereas they form a small cluster for strong wave forcing. An interannual oscillation of the general circulation forming the linear cluster in the scatter diagram is apparent in the experiment of weak 5.5-day wave forcing. Although a pair of equatorial Kelvin and high-latitude Rossby waves with a same period (Kelvin–Rossby wave) produces equatorward heat and momentum fluxes in the region below 60 km, the equatorial wave does not contribute to the long-period oscillation. The interannual fluctuation of the high-latitude jet core leading to the time variation of U MAX is produced by growth and decay of a polar mixed Rossby-gravity wave with a 14-day period. [ABSTRACT FROM AUTHOR]

This paper contains a comprehensive dataset of long-term observations between 2009 and 2015 at the upper mesosphere/lower thermosphere providing temperature values at different locations of the morning and evening side of the terminator of Venus. Temperature information is obtained by line-resolved spectroscopy of Doppler broadened CO 2 transitions features. Results are restricted to a pressure level of 1 µbar, ∼110  km altitude due the nature of the addressed non-LTE CO 2 emission line at 10 µm. The required high spectral resolution of the instrumentation is provided by the ground-based spectrometers THIS (University of Cologne) and HIPWAC (NASA GSFC). For the first time upper mesosphere/lower thermosphere temperatures at the Venusian terminator derived from IR-het spectroscopy between 2009 and 2015 are investigated in order to clarify the local-time dependences, latitudinal dependences and the long-term trend. Measured temperatures were distributed in the range between 140 K and 240 K, with mean values equal to 199 K ± 17 K for the morning side of the terminator and 195 K ± 19 K for the evening side of the terminator. Within the uncertainty no difference between the averaged morning and evening terminator side temperature is found. In addition, no strong latitudinal dependency is observed at these near terminator local times. In contrast IR-het data from 2009 show a strong latitudinal dependency at noon, with a temperature difference of around 60 K between the equatorial and polar region (Sonnabend et al., 2012). Accord with the instruments of the Venus Express mission a northern-southern hemispherical symmetry is observed (Mahieux et al., 2012; Piccialli et al., 2015). The data shows no consistent long-term temperature trend throughout the six years of observation, but a variability in the order of tens of Kelvin for the different observing runs representing a time step of few month to two years. This is about the same order of magnitude as the variability within a single run with a typically time range of 2–10 days. This variation is not connected to the solar cycle. Sub-millimeter observations by Clancy et al. found a relation between temperatures and long-term variation in mesospheric water vapor, SO 2 , and sulfate aerosols (Clancy and Muhleman, 1991; Clancy et al., 2012). SO 2 column densities observed by SOIR at the terminator are fairly stable over the time period of 2006–2011 (Mahieux et al., 2015), supporting the hypothesis of a relation between SO 2 and temperature variations. The temperatures derived from the infrared heterodyne spectroscopy (IR-het) are compared to results from the Venus Express space mission (VEx). A consistence with the temperatures from the VEx instruments SOIR, VIRTIS and SPICAV is found. As the instruments probe different local time, SPICAV probes the pure nightside, SOIR across the terminator and IR-het the pure dayside atmosphere it is not surprising that the IR-het temperatures are mostly on the warmer side compared to results from SPICAV and SOIR. [ABSTRACT FROM AUTHOR]

We report on the update of SO 2 and first detections of SO 3 , CS, and CS 2 , and detection of OCS above the cloud deck using the SOIR instrument on-board Venus Express, and upper limit profiles of H 2 S and HOCl. Based on the solar occultation spectra measured by SOIR covering all latitudes over the 2006–2014 period, we find an average SO 2 volume mixing ratio of ∼0.02 ppm below 90 km which increases to 5 ppm at 100 km; average OCS abundance of 1 ppb to 1 ppm between 65 and 100 km increasing with altitude; mean SO 3 values of 0.1 ppm at 75 km, 1 ppm at 85 km, and 10 ppm at 95 km; CS volume mixing ratios varying from 0.1 ppm at 65 km to 40 ppm at 100 km; finally we report 0.03 ppm of CS 2 at 70 km and 5 ppm at 90 km. It is important to note the detections for all of these species may be biased to higher volume mixing ratios at higher altitudes based on the detection limits for SOIR. Upper-limits for H 2 S and HOCl are determined. All abundances show large variabilities with time and/or latitude equal to at least one order of magnitude at all altitudes. We also study the detection statistics of all detected species as a function of time, latitude, and side of the terminator. • First detection in the Venus mesosphere of OCS, SO3, CS, and CS2. • Update of the SO2 concentrations. • Impact on Venus mesospheric chemistry. [ABSTRACT FROM AUTHOR]

To explain Venus' young surface age and lack of plate tectonics, Venus' tectonic regime has often been proposed to be either an episodic-lid regime with global lithospheric overturns, or an equilibrium resurfacing regime with numerous volcanic and tectonic activities. Stratigraphic analysis suggests that Venus' surface tectonics could be a combination of these two end-member models with a global resurfacing event that created most of the crust followed by tectonic and volcanic activities until now. Recent analyses of Venus' satellite images also suggest widespread lithospheric mobility in the lowland basins that does not fit the episodic-lid tectonic regime. Here, we use global 2-D thermochemical convection models with realistic parameters, including rheology (dislocation creep, diffusion creep, and plastic yielding), an experiment-based plagioclase (An 75) crustal rheology, and intrusive magmatism, to investigate the tectonics and mantle evolution of Venus. We find that surface tectonics is strongly affected by crustal rheology. With a "weak" plagioclase-rheology crust, models exhibit episodic overturns but with continuously high surface mobility and high distributed surface strain rates between overturns, leading to a new tectonic regime that we name "deformable episodic lid". On the other hand, olivine-crustal-rheology models exhibit either standard episodic-lid tectonics, i.e. with mobility that is high during overturns and near zero between overturns, or stagnant-lid tectonics, i.e. with near-zero mobility over the entire model time. In models with plagioclase-rheology crust, magmatic resurfacings are short-lived and randomly located, leading to a rather uniform resurfacing rate. Also, a combination of plagioclase crustal rheology and dislocation creep can weaken the lithosphere sufficiently to facilitate lithospheric overturns without applying plastic yielding. Internally, the composition-dependent density profile results in a "basalt barrier" at the mantle transition zone, which strongly affects Venus' mantle evolution. Only strong plumes can penetrate this basalt barrier and cause global lithospheric overturns. This basalt barrier also causes global internal episodic overturns that generate global volcanic resurfacing in stagnant-lid models, which suggests a new resurfacing mechanism that does not involve lithospheric overturns. We name this regime "stagnant episodic-volcanic-resurfacing". Results in models with plagioclase crustal rheology and low eruption efficiencies best fit current estimates of crustal thickness, surface age, and tectonic structures based on observations of Venus. The crustal thickness in models with an olivine-rheology crust is limited by the basalt–eclogite phase transition and is higher than observation-based estimates. Higher intrusion rates better fit these estimates, as models with high eruption efficiency predict very low surface age (< 100 Myr). • Realistic rheology and intrusive magmatism influence the modelled tectonics of Venus. • A weak crustal rheology leads to thinner crust and more uniform resurfacing rates. • Dislocation creep and intrusions can cause global overturn without plasticity. • A "basalt barrier" below the MTZ leads to global internal episodic overturns. • Internal overturns can cause episodic global resurfacing in a stagnant-lid regime. [ABSTRACT FROM AUTHOR]

On terrestrial planets, flow-like deposits of impact melt are commonly found exterior to fresh impact craters. Often, these deposits are asymmetric about the crater rim, and the direction of flow may relate to either the pre-impact topography or the impact azimuth. In this work, we seek to determine the primary mechanism responsible for impact melt emplacement on two very different terrestrial worlds: the Moon and Venus. Newly derived stereo topography data allows us to investigate the role of pre-impact topography in melt emplacement on Venus for the first time. We determine that pre-impact topography plays an important role in melt emplacement around complex craters on the Moon but not on Venus. This difference may relate to the differences in gravity (and hence, crater depth and melt volume) on the two worlds. Melt trapped in the relatively deep lunar craters may require additional momentum to be pushed over the crater rim, and therefore emerges preferentially from the region of rim crest low. This added momentum might come from uplift during the crater modification stage. For the shallower craters on Venus, the downrange momentum imparted in oblique impacts may be sufficient to push the melt up and over the crater rim, explaining the correlation between melt direction and impact azimuth. Understanding the emplacement of impact melt on terrestrial planets of different sizes and geologic histories provides added insight into the impact cratering process. [ABSTRACT FROM AUTHOR]

The Venusian surface has been studied by measuring radar reflections and thermal radio emission over a wide spectral region of several centimeters to meter wavelengths from the Earth-based as well as orbiter platforms. The radiometric observations, in the decimeter (dcm) wavelength regime showed a decreasing trend in the observed brightness temperature ( T b ) with increasing wavelength. The thermal emission models available at present have not been able to explain the radiometric observations at longer wavelength (dcm) to a satisfactory level. This paper reports the first interferometric imaging observations of Venus below 620 MHz. They were carried out at 606, 332.9 and 239.9 MHz using the Giant Meterwave Radio Telescope (GMRT). The T b values derived at the respective frequencies are 526 K, 409 K and <426 K, with errors of ∼7% which are generally consistent with the reported T b values at 608 MHz and 430 MHz by previous investigators, but are much lower than those derived from high-frequency observations at 1.38–22.46 GHz using the VLA. [ABSTRACT FROM AUTHOR]

Recent observations of sulfur containing species (SO 2 , SO, OCS, and H 2 SO 4 ) in Venus’ mesosphere have generated controversy and great interest in the scientific community. These observations revealed unexpected spatial patterns and spatial/temporal variability that have not been satisfactorily explained by models. Sulfur oxide chemistry on Venus is closely linked to the global-scale cloud and haze layers, which are composed primarily of concentrated sulfuric acid. Sulfur oxide observations provide therefore important insight into the on-going chemical evolution of Venus’ atmosphere, atmospheric dynamics, and possible volcanism. This paper is the first of a series of two investigating the SO 2 and SO variability in the Venus atmosphere. This first part of the study will focus on the vertical distribution of SO 2 , considering mostly observations performed by instruments and techniques providing accurate vertical information. This comprises instruments in space (SPICAV/SOIR suite on board Venus Express) and Earth-based instruments (JCMT). The most noticeable feature of the vertical profile of the SO 2 abundance in the Venus atmosphere is the presence of an inversion layer located at about 70–75 km, with VMRs increasing above. The observations presented in this compilation indicate that at least one other significant sulfur reservoir (in addition to SO 2 and SO) must be present throughout the 70–100 km altitude region to explain the inversion in the SO 2 vertical profile. No photochemical model has an explanation for this behaviour. GCM modelling indicates that dynamics may play an important role in generating an inflection point at 75 km altitude but does not provide a definitive explanation of the source of the inflection at all local times or latitudes The current study has been carried out within the frame of the International Space Science Institute (ISSI) International Team entitled ‘SO 2 variability in the Venus atmosphere’. [ABSTRACT FROM AUTHOR]

The vertical distribution of sulfur species in the Venus atmosphere has been investigated and discussed in Part I of this series of papers dealing with the variability of SO 2 on Venus. In this second part, we focus our attention on the spatial (horizontal) and temporal variability exhibited by SO 2 . Appropriate data sets – SPICAV/UV nadir observations from Venus Express, ground-based ALMA and TEXES, as well as UV observation on the Hubble Space Telescope – have been considered for this analysis. High variability both on short-term and short-scale are observed. The long-term trend observed by these instruments shows a succession of rapid increases followed by slow decreases in the SO 2 abundance at the cloud top level, implying that the transport of air from lower altitudes plays an important role. The origins of the larger amplitude short-scale, short-term variability observed at the cloud tops are not yet known but are likely also connected to variations in vertical transport of SO 2 and possibly to variations in the abundance and production and loss of H 2 O, H 2 SO 4 , and S x . [ABSTRACT FROM AUTHOR]

We report a dawn-dusk difference of periodic variations of oxygen EUV dayglow (OII 83.4 nm, OI 130.4 nm and OI 135.6 nm) in the upper atmosphere of Venus observed by the Hisaki spacecraft in 2015. Observations show that the periodic dayglow variations are mainly controlled by the solar EUV flux. Additionally, we observed characteristic ∼1 day and ∼4 day periodicities in the OI 135.6 nm brightness. The ∼1 day periodicity was dominant on the duskside while the ∼4 day periodicity was dominant on the dawnside. Although the driver of the ∼1 day periodicity is still uncertain, we suggest that the ∼4 day periodicity is caused by gravity waves that propagate from the middle atmosphere. The thermospheric subsolar-antisolar flow and the gravity waves dominantly enhance eddy diffusion on the dawnside, and the eddy diffusion coefficient changes every ∼4 days due to large periodic modulations of wind velocity of the super-rotating atmosphere. Since the ∼4 day modulations on the dawnside are not continuously observed, it is possible that there is an intermittent coupling between the thermosphere and middle atmosphere due to variations of wave source altitudes. Moreover, if there are variations of the wind velocity in the mesosphere or lower thermosphere, it is possible that gravity waves occasionally propagate to the thermosphere even on the duskside due to periodic disappearance of the critical level and the ∼4 day periodic O atomic modulations occur. Thus, our observations imply that the ∼4 day periodicity of the EUV dayglow may reflect the dynamics of the middle atmosphere of Venus. We also examined the effects of the solar wind on the dayglow variations by shifting the solar wind measurements from earth to Venus. We did not find clear correlations between them. However, since there are no local measurements of the solar wind at Venus, the effect of the solar wind on the dayglow is still uncertain. [ABSTRACT FROM AUTHOR]

We have analyzed the long-term regional and global variation of emitted radiance in 1.74 µ m and 2.30 µ m near infrared spectral windows in the Venus atmosphere using the medium resolution, infrared channel of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-M-IR) on the Venus Express spacecraft. We find a periodic variation in the 1.74 µ m radiance that is most pronounced at mid-latitudes ( 30 ∘ − 60 ∘ latitude). This oscillation has a period of approximately 150 days, and is shown to be unlikely to be driven by variations in viewing geometry, nor by variations in instrumental characteristics such as spectrometer temperature. The oscillation has an amplitude at least as large as that of the typical day-to-day variations observed. The decay and recovery timescales are consistent with response of the cloud vertical structure to the radiative-dynamical feedback. It remains unclear why this variation would be seen only at mid-latitudes, but some hypotheses to guide future modeling testing are suggested. [ABSTRACT FROM AUTHOR]

Iron chloride in the Venus clouds is under discussion for three decades, and the saturated vapor pressure of this species is of crucial importance for its modeling. There is a great scatter in the published data, and the preferable results are by Rustad and Gregory (1983, J. Chem. Eng. Data 28, 151–155) and those based on thermodynamic parameters by Chase (1998, J. Phys. Chem. Ref. Data Monograph 9). Using these data, loss by coagulation with sulfuric acid, transport by eddy diffusion, and the Stokes precipitation, the model confirms conclusions of our early study (Krasnopolsky 1985, Planet. Space Sci. 33, 109–117) that FeCl 3 in the Venus clouds (1) agrees with the near UV and blue reflectivity of Venus (Zasova et al. 1981, Adv. Space Res. 1, #9, 13–16), (2) was observed by the direct X-ray fluorescent spectroscopy, (3) explains the altitude profiles of the mode 1 aerosol in the middle and lower cloud layers and (4) the decrease in the NUV absorption below 60 km. Here we add to these conclusions that (5) the delivery of FeCl 3 into the upper cloud layer and the production of sulfuric acid are just in proportion of 1: 100 by mass that is required to fit the observed NUV albedo. Furthermore, (6) the mode 1 and 2 particle sizes fit this proportion as well. Finally, (7) the required Fe 2 Cl 6 mixing ratio is 17 ppbv in the atmosphere and the FeCl 3 mole fraction is 19 ppbv in the Venus surface rocks. [ABSTRACT FROM AUTHOR]

We present new coordinated observations and measurements of the wind amplitude and its variability at Venus cloud-tops based on the two complementary techniques of Ground-based Doppler velocimetry (Widemann et al., 2007, 2008; Machado et al., 2012, 2014) and cloud-tracked winds using ESA Venus Express/ VIRTIS-M imaging at 0.38 µm (Drossart et al., 2007; Sanchez-Lavega et al., 2008; Hueso et al., 2012; Machado et al., 2014). Cloud-tracked winds trace the true atmospheric motion also responsible for the Doppler-Fizeau shift of the solar radiation on the dayside by super-rotating moving cloud-tops with respect to both the Sun and the observer (Machado et al., 2014), and based on this complementarity, we performed a new coordinated campaign in April 2014 combining both Venus Express observations and ground-based Doppler wind measurements on the dayside of Venus’ cloud tops at Canada-France-Hawaii telescope at a phase angle Φ = ( 76 ± 0.3 ) ∘ . The analysis and results show (1) additional confirmation of the coherence, and complementarity, in the results provided by these techniques, on both spatial and temporal time scales of the two methods; (2) first-time estimation of the meridional component of the wind in another planet using the Doppler velocimetry technique, with evidence of a symmetrical, poleward meridional Hadley flow in both hemispheres of v ¯ m = 22± 15.5 m s − 1 ; (3) spatial and temporal variability of the zonal flow with latitude and local time, with a significant increase of wind amplitude near the morning terminator previously reported in Feb. 2011 observations. [ABSTRACT FROM AUTHOR]

Thermal cooling rates Q C and solar heating rates Q H in the atmosphere of Venus at altitudes between 0 and 100 km are investigated using the radiative transfer and radiative balance simulation techniques described by Haus et al. (2015b, 2016). Q C strongly responds to temperature profile and cloud parameter changes, while Q H is less sensitive to these parameters. The latter mainly depends on solar insolation conditions and the unknown UV absorber distribution. A parameterization approach is developed that permits a fast and reliable calculation of temperature change rates Q for different atmospheric model parameters and that can be applied in General Circulation Models to investigate atmospheric dynamics. A separation of temperature, cloud parameter, and unknown UV absorber influences is performed. The temperature response parameterization relies on a specific altitude and latitude-dependent cloud model. It is based on an algorithm that characterizes Q responses to a broad range of temperature perturbations at each level of the atmosphere using the Venus International Reference Atmosphere (VIRA) as basis temperature model. The cloud response parameterization considers different temperature conditions and a range of individual cloud mode factors that additionally change cloud optical depths as determined by the initial latitude-dependent model. A Q H response parameterization for abundance changes of the unknown UV absorber is also included. Deviations between accurate calculation and parameterization results are in the order of a few tenths of K/day at altitudes below 90 km. The parameterization approach is used to investigate atmospheric radiative equilibrium (RE) conditions. Polar mesospheric RE temperatures above the cloud top are up to 70 K lower and equatorial temperatures up to 10 K higher than observed values. This radiative forcing field is balanced by dynamical processes that maintain the observed thermal structure. [ABSTRACT FROM AUTHOR]

Mead, the largest crater on Venus, has low topographic relief at the surface and at the crust-mantle boundary. Due to high surface temperatures, viscous deformation could play an important role in crustal structure. Using the finite element method, we simulate the long-term viscoelastic deformation of Mead crater and investigate the role of lower crustal flow in the evolution of the surface and subsurface topography. We examine the thermal states that allow this evolution to occur and determine the background heat flux. Our study constrains the background heat flux in the vicinity of Mead basin to 55–90 mW m −2 . This surface heat flow is generally higher than the average Venusian global values suggested by recent thermal models. In addition by applying hydrous and anhydrous creep rheological parameters, we demonstrate that the Venus's interior is rheologically dry and that the crust near Mead could be relatively high in plagioclase. [ABSTRACT FROM AUTHOR]

We present here three-dimensional simulations of the Venus photochemistry and clouds from the ground to the bottom of the thermosphere. For that purpose, we have implemented a state-of-the-art photochemical and equilibrium cloud model in the Venus Planetary Climate Model (Venus PCM). The interactive coupling between dynamics, radiation, chemistry and clouds allows a comprehensive description of the CO 2 , CO, sulfur, chlorine, oxygen, and hydrogen species, with tracking of the condensed phase. Regarding the clouds, the Venus PCM calculates the composition, number density, and sedimentation rates of the binary H 2 SO 4 -H 2 O liquid aerosols, based on observed altitude-dependent size distributions. The article describes in detail the new components implemented in the Venus PCM. It then presents an overview of the results concerning clouds and atmospheric chemistry, which are compared with a wide range of observations. The modeled cloud characteristics and vertical profiles of minor species are found to be in broad agreement with most of the measurements available between 30 and 100 km. In particular, the Venus PCM reproduces the steep decrease of H 2 O and SO 2 mixing ratio inside the cloud layer, as well as the observed vertical distribution of species well identified above the clouds, such as CO and O 3. The model also agrees with the ground-based measurements of HCl, but not with the conflicting HCl vertical profiles derived from Venus Express. On the quasi-horizontal plane, latitudinal contrasts in the modeled trace species mostly result from the Hadley-type mean meridional circulation. Large-scale longitudinal variations are essentially created by the diurnal thermal tide above the clouds, and by photolysis above 80 km. • The Venus PCM is the first 3D model of Venus photochemistry and clouds. • SO 2 and H 2 O steep decrease in the clouds is reproduced. • The model computes the composition and number density of the H 2 SO 4 -H 2 O droplets. • The Hadley-type circulation drives the distribution of long-lived species. [ABSTRACT FROM AUTHOR]

Radar images from the Magellan mission reveal compressional ridge belts transecting several low-lying plains on Venus, including Atalanta, Vinmara, Lavinia, and Rusalka/Llorona/Vellamo Planitiae. Vedma Dorsa is an ∼1800 km long ridge belt with 0.5–1 km of relief in Llorona/Vellamo Planitiae. We analyze sections of the ridge belt that have an asymmetric topographic cross section, indicative of deep thrust faulting and shallow folding as the dominant formation mechanisms. We perform elastic dislocation and Monte Carlo tectonic modeling of thrust faulting at several locations along Vedma Dorsa to constrain the fault displacement (D), depth of faulting (z), and fault dip (θ). We find that the models are consistent with blind thrust faulting and folding as the dominant mechanism for producing Vedma, with preferred fault parameters of D ∼ 1–2 km, z ∼ 10–25 km, and θ ∼ 25–30°. The results support an upper fault tip >3 km below the surface (blind fault), and a change in dip at depth. Our preferred models and the additional fault complexities supported at each site suggest that this section of Vedma is an intermediate structure between lobate scraps and fold and thrust belts, incorporating morphometric aspects from both classifications. Observations of terrestrial fault depths and the best-fitting depth of faulting from our models lead to an estimated heat flux of 16–39 mW m−2 at the time of Vedma Dorsa's formation, consistent with formation over a cold, downwelling region of the mantle. • Vedma Dorsa ridge belt topography is consistent with blind thrust faulting at depth. • Modeling suggests faults have dips of 25–30°, depths 10–25 km, and offsets of 1–2 km. • Faulted Layer thickness range leads to heat flux estimate of 16–39 mW m−2. • Vedma Dorsa may be an intermediate tectonic structure similar to a fold-and-thrust belt. [ABSTRACT FROM AUTHOR]

We present a significantly updated CO 2 altitude profile for Venus (64.2–0.9 km) and provide support for a potential deep lower atmospheric haze of particles (≤17 km). We extracted this information by developing a new analytical model for mass spectra obtained by the Pioneer Venus Large Probe (PVLP) Neutral Mass Spectrometer (LNMS). Our model accounts for changes in LNMS configuration and output during descent and enables the disentanglement of isobaric species via a data fitting routine that adjusts for mass-dependent changes in peak shape. The model yields CO 2 in units of density (kg m−3), isotope ratios for 13C/12C and 18O/16O, and 14 measures of CO 2 density across 55.4–0.9 km, which represents the most complete altitude profile for CO 2 at ≤60 km to date. The CO 2 density profile is also consistent with the pressure, temperature, and volumetric gas measurements from the PVLP and VeNeRa spacecraft. Nominal and low-noise operations for the LNMS mass analyzer are supported by the behaviors (e.g. , ionization yields, fragmentation yields, and peak shapes) of several internal standards (e.g. , CH 3 +, CH 4 +, 40Ar+, 136Xe2+, and 136Xe+), which were tracked across the descent. Lastly, our review of the CO 2 profile and LNMS spectra reveals hitherto unreported partial and rapidly clearing clogs of the inlet in the lower atmosphere, along with several ensuing data spikes at multiple masses. Together, these observations suggest that atmospheric intake was impacted by particles at ≤17 km and that rapid particle degradation at the inlet yielded a temporary influx of mass signals into the LNMS. • We obtained novel chemical information by re-analysis of archived mass spectra from the Pioneer Venus Large Probe. • Our analytical model accounts for the impacts of descent and enables the disentanglement of isobaric species. • We obtained the first and most complete CO 2 altitude profile in units of density (kg m-3) across ∼55-1 km. • Our values for CO 2 are consistent with associated other spacecraft measures. • Tracking of the CO 2 profile reveals hitherto unreported partial and rapidly clearing clogs of the LNMS at ≤17 km. • The partial clogs are suggestive of a deep lower atmospheric haze at ≤17 km. [ABSTRACT FROM AUTHOR]

Venus’ highlands appear much brighter than its lowland plains in reflected radar, which has been explained by several conflicting hypotheses. We study this transition at higher spatial and elevation resolution than previously possible by combining Magellan synthetic aperture radar (SAR) images with Magellan SAR stereo elevations. We confirm that SAR backscatter over Ovda Regio (5°N to 15°S) grades from low to high as elevation increases (2–4.5 km above the datum), and then drops precipitously above ∼4.5 km (T= ∼702 K). This pattern is consistent with presence of a substance that undergoes a phase transition from ferroelectric to normal dielectric at ∼700 K; the mineral chlorapatite is a likely candidate. This pattern is seen across Ovda, on other near-equatorial highlands, and on some shield volcanoes like the Tepev Montes. We also confirm that Maxwell Montes (60–68°N) shows a different pattern; its surface transitions abruptly from low backscatter to high backscatter at ∼4.5 km above the datum, and remains so to nearly its highest elevations (∼10 km). This pattern is consistent with the presence of a semiconductor material either precipitated from the atmosphere (e.g., a frost) or produced by atmosphere–surface interaction. If a ferroelectric substance were in the rock at Maxwell (as at Ovda), it could be invisible beneath the coating of semiconductor material. However, the absence of a semiconductor material on Ovda requires either that [1] the atmosphere compositions at Maxwell and Ovda are substantially different, or [2] that the semiconductor at Maxwell forms by atmosphere–surface reaction (not as an atmospheric precipitate) and that the surface materials at Ovda and Maxwell are substantially different. [ABSTRACT FROM AUTHOR]

We provided a new constraint on HCl abundance at the cloud top of Venus by infrared spectroscopy using a cross-dispersed high-resolution echelle spectrograph, iSHELL, mounted on the NASA Infrared Telescope Facility (IRTF). This study investigated the inconsistency in HCl abundance between previous ground-based observations and Venus Express solar occultation measurements. Venusian dayside observations at a solar phase angle of ∼90° were conducted during August 6–7, 2018, and August 18–20, 2020 (UT), when the Venusian afternoon and morning sides were visible, respectively. The high spectral resolving power of ∼80,000 and large Doppler shift (∼13 km/s) enabled the measurement of Venusian lines with less contamination by terrestrial lines. We analyzed the H35Cl P(5) and H37Cl P(6) lines at 2775.8 and 2750.1 cm−1, respectively, in the 1–0 band, together with 16O12C18O P- and R-branch lines of the 20001–00001 band, which fell in the same spectral orders as the HCl lines. The 16O12C18O lines were used to derive the cloud top altitude because the upper clouds significantly impacted the retrieval of HCl abundance. The cloud tops had an equatorially symmetric structure. The average altitude was 70.8 ± 0.6 km in the region equatorward of ±30° and decreased toward higher latitudes. The HCl volume mixing ratio was derived as 0.379 ± 0.013 ppm at a probing altitude of 70.6 ± 1.1 km and showed no significant latitudinal dependence within the range of ±70°. A difference of ∼0.02 ppm between 2018 and 2020 would result mainly from temporal variation. The H35Cl/H37Cl abundance ratio was 3.01 ± 0.16, with no prominent latitudinal dependence. The obtained HCl volume mixing ratio agreed with the results of previous ground-based measurements, which were approximately one order of magnitude larger than those derived from Venus Express solar occultation measurements. The systematic uncertainties in our retrieval analysis cannot explain this significant inconsistency. The impact of diffuse light produced from aerosol scattering on the retrieval method for solar occultation measurements should be investigated further to solve this issue. • HCl abundance at the cloud top of Venus was obtained from IRTF/iSHELL spectra. • HCl mixing ratio was 0.379 ± 0.013 ppmv at a probing altitude of 70.6 ± 1.1 km. • HCl mixing ratio showed no marked latitudinal dependence within the range of ±70°. • HCl mixing ratio agreed with the results of previous ground-based measurements. • HCl mixing ratio was about one order of magnitude larger than the VEX/SOIR results. [ABSTRACT FROM AUTHOR]

Recent simulations of the Institut Pierre-Simon Laplace (IPSL) Venus Global Climate Model (VGCM) developed at the Laboratoire de Météorologie Dynamique (LMD) were performed with a model top raised from ∼10−5 (∼150 km) to ∼10−8 Pa (180–250 km; upper boundary). The parameterizations of non-LTE CO 2 near infrared heating rates and of non-orographic gravity waves were improved. In addition, a tuning of atomic oxygen production was introduced to improve related effects (heating and cooling) and resulting thermospheric number densities. This work focusses on validating the modelled thermospheric structure using data from the Pioneer Venus, Magellan and Venus Express missions which cover similar and complementary (equator and pole) regions at different periods of solar activity, typically above altitudes of 130 km. This version of the IPSL VGCM shows good agreement with the diurnal evolution of the exospheric temperature at the equator reconstructed from the atomic oxygen scale height of the Pioneer Venus Orbiter Neutral Mass Spectrometer data. The model is also able to reproduce the sensitivity of the exospheric temperature and species density to the EUV flux of the solar high activity period (from 180 to 230 solar flux unit; s.f.u). However, to fit with the PV-ONMS density observations, it was necessary to increase the photodissociation of CO 2 into CO and O above 135 km by a factor of 10. Indeed, our study points to the importance of an additional source of oxygen and carbon monoxide production above 130 km other than CO 2 photolysis to explain the vertical profiles of CO and O number density in the thermosphere. Moreover, the presence of a GW drag at altitudes above 140 km has a significant impact on the nightside temperature value and its distribution. [ABSTRACT FROM AUTHOR]

A planetary-scale bright and dark UV feature, known as the “Y-feature,” rotates around Venus with a period of 4–5 days and has been long-time interpreted as planetary waves. When assuming this, its rotation period and spatial structure might help to understand the propagation of the planetary-scale waves and find out their role in the acceleration-deceleration of the zonal wind speed, which is essential for understanding the super-rotation of the planet. The rotation period of the UV feature varied over the course of observation by the Pioneer Venus orbiter. However, in previous explorations of Venus such as Pioneer Venus and Venus Express, the spacecraft were operated in nearly fixed inertial space. As a result, the periodicity variations on sub-yearly timescales (one Venusian year is ∼224 Earth days) were obscured by the limitations of continuous dayside observations. We newly conducted six periods of ground-based Venus imaging observations at 365 nm from mid-August 2013 to the end of June 2014. Each observation period spanned over half or one month, enabling long-term monitoring of Venus’ atmosphere above the equator region. Distributions of the relative brightness were obtained from the equatorial (EQ) to mid-latitudinal regions in both hemispheres, and from the cyclical variations of these distributions we deduced the rotation periods of the UV features of the cloud tops brightness. The relative brightness exhibited periods of 5.2 and 3.5 days above 90% of significance. The relative intensities of these two significant components also seemed subject to temporal variations. Although the 3.5-day component considered persists throughout the observation periods, its dominance over the longer period varied in a cyclic fashion. The prevailing first significant mode seems to change from 5.2-day waves to 3.5-day waves in about nine months, which is clearly inconsistent with the Venusian year. Clear periodic perturbations, indicating stability of the planetary-scale UV-feature, were observed only in the presence of single longer or shorter periodic waves. During the transition periods of dominant-wave changing, the amplitude of the relative brightness was largely changed. This can be explained by the deformation of the Y-shaped UV feature as observed by Pioneer Venus in 1979. [ABSTRACT FROM AUTHOR]

In this study we describe a dedicated analysis of luminous transient events on Venus night side atmosphere with the visible channel of the VIRTIS instrument (280–1100 nm), this being the most comprehensive search of lightning conducted so far with Venus Express data. Our search results in thousands of signal detections, but unfortunately they can be all explained by cosmic rays impinging on the detector, and further statistical analysis shows that all of the events are randomly distributed along the spectral dimension, therefore not showing any clear evidence of signal coming from lightning emission in the Venus atmosphere. This does not exclude the existence of lightning, but imposes some constraints on their occurrence that are important for future research. [ABSTRACT FROM AUTHOR]

The crater density and distribution of Venus indicates the average surface age is younger (≤1 Ga) than most terrestrial planets and satellites in the Solar System. The type and rate (i.e. equilibrium, catastrophic or differential) of volcanism associated with the stagnant lid tectonic system of Venus is a first order problem that has yet to be resolved but is directly related to the thermal conditions of the mantle. The calculated primary melt composition of basalt at the Venera 14 landing site is high-Mg basalt to picrite with a mantle potential temperature estimate similar to terrestrial ambient mantle (1370 ± 70 °C). The calculated accumulated fractional melting curves indicate the olivine compositions from the melt have Mg# of 89–91. The results show that the thermal regime required to generate the primary melt composition of the Venera 14 basalt was not anomalously high (i.e. mantle-plume system) but rather consistent with a lithospheric tensional rift system. The juxtaposition of high thermal regime structures (e.g. Beta Regio) and ‘ambient’ mantle potential temperature estimates of the Venera 14 basalt suggests that the relatively young surface of Venus is the result of volcanism from a combination of thermal systems that resurfaced the planet at variable rates. [ABSTRACT FROM AUTHOR]

The photochemical model for the middle atmosphere of Venus (Krasnopolsky, V.A. [2012] Icarus, 218, 230–246) predicts sulfur aerosol as a product of the OCS photolysis at 55–60 km. The calculated mass loading is much smaller than that of the mode 1 particles in the upper cloud layer. The chemical kinetic model for the lower atmosphere ( Krasnopolsky, V.A. [2013], Icarus, 225, 570–580) results in a constant mixing ratio of 20 ppm for OCS + X S X . This means the S 8 mixing ratio of 2.5 ppm near the model upper boundary at 47 km. Using this abundance, the calculated profile of the sulfur aerosol has a bottom that coincides with the lower boundary of modes 2 and 3 and constitutes ∼10% of the total mass loading in the lower cloud layer. Sulfur aerosol cannot be the near UV absorber because its abundance is too low at the cloud tops and disagrees with the profile of the absorber observed by Venera 14. [ABSTRACT FROM AUTHOR]

Venus is gradually losing some of its atmosphere in the form of ions through its induced magnetotail. Some of these ions have been reported previously to flow back to the planet. Proposed drivers are magnetic reconnection and deflection of pickup ions in the magnetic field. We analyze protons and oxygen ions with eV to keV energies acquired by the ASPERA-4/IMA instrument throughout the entire Venus Express mission. We find that venusward flowing ions are important in the sense that their density and deposition rate into the atmosphere is of the same order of magnitude as the density and escape rate of downtail flowing ions. Our analysis shows that during strong EUV irradiance, which occurs during solar maximum, the flux of venusward flowing protons is weaker and of oxygen ions is stronger than during weak irradiance. Since such a behavior was observed when tracing oxygen ions through a MHD model, the ultimate driver of the venusward flowing ions may simply be the magnetic field configuration around Venus. Although the pure downtail oxygen flux stays mostly unchanged for all observed EUV conditions, the increase in venusward oxygen flux for high irradiance results in a lower net atmospheric escape rate. Venusward bulk flows are mostly found in locations where the magnetic field is weak relative to the interplanetary conditions. Although a weak field is generally an indicator of proximity to the magnetotail current sheet, these flows do not cluster around current sheet crossings, as one may expect if they would be driven by magnetic reconnection. [ABSTRACT FROM AUTHOR]

The distribution of sources and sinks of radiative energy forces the atmospheric dynamics. The radiative transfer simulation model described by Haus et al. (2015b) is applied to calculate fluxes and temperature change rates in the middle and lower atmosphere of Venus (0–100 km) covering the energetic significant spectral range 0.125–1000 µm. The calculations rely on improved models of atmospheric parameters (temperature profiles, cloud parameters, trace gas abundances) retrieved from Venus Express (VEX) data (mainly VIRTIS-M-IR, but also VeRa and SPICAV/SOIR with respect to temperature results). The earlier observed pronounced sensitivity of the radiative energy balance of Venus to atmospheric parameter variations is confirmed, but present detailed comparative analyses of possible influence quantities ensure unprecedented insights into radiative forcing on Venus by contrast with former studies. Thermal radiation induced atmospheric cooling rates strongly depend on temperature structure and cloud composition, while heating rates are mainly sensitive to insolation conditions and UV absorber distribution. Cooling and heating rate responses to trace gas variations and cloud mode 1 abundance changes are small, but observed variations of cloud mode 2 abundances and altitude profiles reduce cooling at altitudes 65–80 km poleward of 50°S by up to 30% compared to the neglect of cloud parameter changes. Cooling rate variations with local time below 80 km are in the same order of magnitude. Radiative effects of the unknown UV absorber are modeled considering a proxy that is based on a suitable parameterization of optical properties, not on a specific chemical composition, and that is independent of the used cloud model. The UV absorber doubles equatorial heating near 68 km. Global average radiative equilibrium at the top of atmosphere (TOA) is characterized by the net flux balance of 156 W/m 2 , the Bond albedo of 0.76, and the effective planetary emission temperature of 228.5 K in accordance with earlier results. TOA radiative equilibrium can be achieved by slight adjustments of either UV absorber or cloud mode abundances. Ratios of synthetic spectral albedo values at 0.36 µm calculated for different abundance factors of the UV absorber are suggested to provide a possible tool to interpret observed VMC/VEX brightness variations with respect to actual absorber abundances. Atmospheric net heating dominates the low and mid latitudes above 82 km, while net cooling prevails at high latitudes at all mesospheric altitudes (60–100 km). This radiative forcing field has to be balanced by dynamical processes to maintain the observed thermal structure. A similar but much smaller meridional gradient is also observed at altitudes between 62 and 72 km where the unknown UV absorber provides additional heating. At these altitudes, equatorial net heating dominates net cooling from about 07:30 h until 16:30 h local time. Intermediate altitudes (72–82 km) are characterized by net cooling at all latitudes in case of VIRTIS temperature data. This planet-wide net cooling region is not observed when calculations are based on VeRa temperatures, and low latitudes are then characterized by small net heating. When a warm atmospheric layer as detected by SPICAV/SOIR around 100 km is considered, strong global average net cooling occurs above 90 km that is far away from radiative equilibrium. A weak net cooling layer (1–2 K/day) exists at altitudes between 55 and 60 km, while very weak net heating (0.1–0.5 K/day) takes place near the cloud base (48 km). Almost zero net heating prevails in the deep atmosphere below 44 km. On global average, the entire atmosphere of Venus at altitudes between 0 and 90 km is not far away from radiative equilibrium (usually within± 2 K/day). Maximum temperature change rate deviations from mean values at each altitude and latitude are defined based on retrieved atmospheric parameter single standard deviations using VIRTIS data. This is an important prerequisite to investigate parameterization approaches for the calculation of atmospheric temperature change rates that can be used in Global Circulation Models. This will be a major topic of future studies on radiative energy balance of Venus’ atmosphere. [ABSTRACT FROM AUTHOR]

The SOIR instrument on board the ESA Venus Express mission has been operational during the complete duration of the mission, from April 2006 up to December 2014. Spectra were recorded in the IR spectral region (2.2–4.3 µm) using the solar occultation geometry, giving access to a vast number of ro-vibrational lines and bands of several key species of the atmosphere of Venus. Here we present the complete set of vertical profiles of carbon monoxide (CO) densities and volume mixing ratios (vmr) obtained during the mission. These profiles are spanning the 65–150 km altitude range. We discuss the variability which is observed on the short term, but also the long term trend as well as variation of CO with solar local time and latitude. Short term variations can reach one order of magnitude on less than one month periods. SOIR does not observe a marked long term trend, except perhaps at the beginning of the mission where an increase of CO density and vmr has been observed. Evening abundances are systematically higher than morning values at altitudes above 105 km, but the reverse is observed at lower altitudes. Higher abundances are observed at the equator than at the poles for altitude higher than 105 km, but again the reverse is seen at altitudes lower than 90 km. This illustrates the complexity of the 90–100 km region of the Venus’ atmosphere where different wind regimes are at play. [ABSTRACT FROM AUTHOR]

Accurate wind observation is a key to study atmospheric dynamics. A new automated cloud tracking method for the dayside of Venus is proposed and evaluated by using the ultraviolet images obtained by the Venus Monitoring Camera onboard the Venus Express orbiter. It uses multiple images obtained successively over a few hours. Cross-correlations are computed from the pair combinations of the images and are superposed to identify cloud advection. It is shown that the superposition improves the accuracy of velocity estimation and significantly reduces false pattern matches that cause large errors. Two methods to evaluate the accuracy of each of the obtained cloud motion vectors are proposed. One relies on the confidence bounds of cross-correlation with consideration of anisotropic cloud morphology. The other relies on the comparison of two independent estimations obtained by separating the successive images into two groups. The two evaluations can be combined to screen the results. It is shown that the accuracy of the screened vectors are very high to the equatorward of 30 degree, while it is relatively low at higher latitudes. Analysis of them supports the previously reported existence of day-to-day large-scale variability at the cloud deck of Venus, and it further suggests smaller-scale features. The product of this study is expected to advance the dynamics of venusian atmosphere. [ABSTRACT FROM AUTHOR]