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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]

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]

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]

Spectroscopic solar occultation measurements by the Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus/Solar Occultation at Infrared instrument (SPICAV/SOIR) onboard the Venus Express orbiter gave new data about upper haze aerosol properties, its vertical distribution and spatial and temporal variations. Early study with three channels of SPICAV/SOIR instrument using a few selected orbits indicated presence of two aerosol modes in particle size distribution (Wilquet et al., 2009). Analysis of aerosol properties from the SPICAV−IR spectrometer for the whole Venus Express data set obtained from May 2006 till November 2014 has proved it for some occultations (Luginin et al., 2016). In this work, we report retrieval of the upper haze (81–100 km) aerosol properties from 101 simultaneous SPICAV−UV and –IR solar occultation sessions acquired between March 2007 and January 2013. A joint analysis of the data from two spectrometers allowed us to characterize the size distribution ∼10 km higher in the atmosphere compared to previous analysis and to detect bimodal distribution in ∼50% of observations previously believed to be unimodal. At altitudes 81–92 km bimodality is observed in >50% of cases. Mode 2 particles are detected up to 98 km and mode 1 up to 100 km. Mean radius equals 0.14 ± 0.03 μm for mode 1 and 0.78 ± 0.18 μ m for mode 2. Number density profiles for both modes of particles exponentially decrease with altitude, starting from 50 cm−3 and 0.3 cm−3 at 82 km for mode 1 and mode 2, respectively, and reaching 3 cm−3 at 98 km for mode 1 and 0.03 cm−3 at 94 km for mode 2. • At altitudes 81–92 km bimodal distribution was observed in >50% of observations. • The mean effective radii are 0.14 ± 0.03 μm for mode 1 and 0.78 ± 0.18 μm for mode 2. • Number density profiles for both modes exponentially decrease with altitude. • Effective variance of mode 1 reaches 0.4 at <84 km and 0.03–0.15 at higher altitudes. • Effective variance of mode 2 is almost constant with height with mean value 0.04 ± 0.02. [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 • 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]

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]

We conducted observing system simulation experiments (OSSEs) for radio occultation measurements (RO) among small satellites, which are expected to be useful for future Venus missions. The effectiveness of the observations based on realistic orbit calculations was evaluated by reproduction of the "cold collar", a unique thermal structure in the polar atmosphere of Venus. Pseudo-temperature observations for the OSSEs were provided from the Venus atmospheric GCM in which the cold collar was reproduced by the thermal forcing. The vertical temperature distributions between 40 and 90 km altitudes at observation points were assimilated. The result showed that the cold collar was most clearly reproduced in the case where the temperature field in high-latitudes was observed twice a day, suggesting that the proposed observation is quite effective to improve the polar atmospheric structure at least. Although the cold collar was also reproduced in the OSSEs for Longwave Infrared Camera (LIR) observations, the result seemed unrealistic and inefficient compared to that obtained in the RO OSSEs. The present study shows that the OSSEs can be used to evaluate observation plans and instruments in terms of reproducibility of specific atmospheric phenomena, and applied to future missions targeting planetary atmospheres. • We conducted observing system simulation experiments (OSSEs) for the Venus atmosphere. • These OSSEs are targeted at radio occultation measurements (RO) in realistic orbits of multiple small satellites. • The effectiveness of the observations was evaluated by reproduction of the "cold collar", a unique thermal structure in the polar atmosphere. • The present study shows that the OSSEs can be used to evaluate observation plans and instruments for the Venus atmosphere. [ABSTRACT FROM AUTHOR]

The 2. 3 μ m spectral window has been used to constrain the composition of the lower atmosphere (in the 30–40 km altitude range) on the night side of Venus for more than thirty years. Here, we present a follow-up study of Marcq et al. (2008), but using the full VIRTIS-H/Venus Express data archive as well as an updated radiative transfer forward model. We are able to confirm a latitudinal increase of CO of about 30% between 0° and 60°N, as well as an anti-correlated vertical shift of OCS profile by about − 1 km in the same latitude range. Both variations are about twice smaller in the southern hemisphere. Correlations of low latitude CO and OCS variations with zonally shifted surface elevation is tentatively found. These results are consistent with CO and OCS variations resulting from the competition between local thermochemistry and a Hadley-cell-like general circulation, albeit influenced by the orography. Finally, no evidence for spatial variations of water vapor (combined H 2 O and HDO) or sulfur dioxide could be evidenced in this data set; better constraining possible variations of these species would require future missions to include infrared spectrometers operating at a spectral resolving power higher than ∼ 1 0 4 , such as VenSpec-H onboard EnVision. Plain Language Summary Remotely measuring the composition of the Venusian atmosphere below the clouds is challenging, yet yields invaluable insights about the atmospheric chemistry, circulation and interaction with the surface and interior of the planet. The VIRTIS-H instrument on board ESA's Venus Express orbiter (2006–2014) provides a rich data set in this regard, thanks to its ability to observe and analyze, on the night side of the planet, the infrared radiation emitted by the deep atmospheric layers. The results of our analyses confirm the previously observed trends for the variations of two trace gases (carbon monoxide and carbonyl sulfide) with latitude, explained by the combined effects of chemical reactions and transport by the atmospheric circulation. Variations of carbon monoxide may also be linked to the variations of ground elevation, confirming the link between surface topography and atmospheric circulation. However, we were unable to separate the signature of heavy water vapor from ordinary water vapor or to detect any variations in sulfur dioxide, both of which require more powerful infrared instruments such as those planned on future Venus orbiters such as ESA's EnVision. • The latitudinal variability of carbon monoxide on Venus is confirmed, as well as its anticorrelation with carbonyl sulfide. • Zonal variations of carbon monoxide near the equator may be correlated with zonally shifted surface elevation. • No variations of water vapor and sulfur dioxide were found; a spectral resolving power nearing 10000 is needed to further investigate. [ABSTRACT FROM AUTHOR]

Global Climate Models (GCM) are very useful tools to study theoretically the general dynamics and specific phenomena in planetary atmospheres. In the case of Venus, several GCMs succeeded in reproducing the atmosphere’s superrotation and the global temperature field. However, the highly variable polar temperature and the permanent cold collar present at 60 ∘ − 80 ∘ latitude have not been reproduced satisfactorily yet. Here we improve the radiative transfer scheme of the Institut Pierre Simon Laplace Venus GCM in order to numerically simulate the polar thermal features in Venus atmosphere. The main difference with the previous model is that we now take into account the latitudinal variation of the cloud structure. Both solar heating rates and infrared cooling rates have been modified to consider the cloud top’s altitude decrease toward the poles and the variation in latitude of the different particle modes’ abundances. A new structure that closely resembles the observed cold collar appears in the average temperature field at 2 × 10 4 − 4 × 10 3 Pa ( ∼ 62 − 66 km) altitude range and 60 ∘ − 90 ∘ latitude band. It is not isolated from the pole as in the observation-based maps, but the obtained temperature values (220 K) are in good agreement with observed values. Temperature polar maps across this region show an inner warm region where the polar vortex is observed, but the obtained 230 K average value is colder than the observed mean value and the simulated horizontal structure does not show the fine-scale features present within the vortex. The comparison with a simulation that does not take into account the latitudinal variation of the cloud structure in the infrared cooling computation, shows that the cloud structure is essential in the cold collar formation. Although our analysis focuses on the improvement of the radiative forcing and the variations it causes in the thermal structure, polar dynamics is definitely affected by this modified environment and a noteworthy upwelling motion is found in the cold collar area. [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]

Measurements of discharge currents on the Venera 13 and 14 landers during their descent in the lowest 35 km of the Venus atmosphere are interpreted as driven either by an ambient electric field, or by deposition of charge from aerosols. The latter hypothesis is favored (`triboelectric charging' in aeronautical parlance), and would entail an aerosol opacity and charge density somewhat higher than that observed in Saharan dust transported over long distances on Earth. [ABSTRACT FROM AUTHOR]

Long-Infrared camera (LIR) onboard Akatsuki have detected bow-shaped features over mountain regions, suggesting that mountain waves propagate upward to reach the cloud top level. In this study, we investigated how the vertical propagation characteristics of the mountain waves depend on the thickness and static stability of the planetary boundary layer and the zonal wind near the surface by using a two-dimensional non-hydrostatic model. The obtained results were compared with the observations mainly at the cloud top level. The amplitude of waves reproduced in the model is comparable to the observations at the cloud top level when the zonal wind near the surface is relatively fast and the top of the planetary boundary layer is lower than the top of the mountain. The wave amplitude also becomes comparable to the observations when the top of the planetary boundary layer is higher than the top of the mountain, while it depends also on the zonal wind speed near the surface and the static stability in the planetary boundary layer. Our results suggest that the zonal wind speed and the atmospheric structure near the surface vary with local time and might be able to explain why the bow-shaped structure is often seen in the evening time. • A two-dimensional non-hydrostatic model simulates Venus mountain waves. • Vertical propagation depends on static stability and zonal wind near the surface. • Waves consistent with Akatsuki observations are reproduced in several conditions. • Our results suggest atmospheric conditions near the surface vary with local time. • Zonal-mean zonal wind below 70 km is not affected by the mountain waves. [ABSTRACT FROM AUTHOR]

The photochemical model of Venus atmosphere at 47–112 km (Krasnopolsky 2012, Icarus 218, 230–246) is updated using the recent calculations of production and photolysis of the S 2 O 2 isomers by Frandsen et al. (2016, Geophys. Res. Lett. 43, 11146–11155) and improved densities of H 2 O, OCS, and H 2 at 47 km from the chemical kinetic model by Krasnopolsky (2013, Icarus 225, 570–580). SO recycles in the termolecular association of S 2 O 2 followed by its photolysis, while it was lost in these processes in the previous model. Consequences of this change to balances of sulfur species are discussed. The basic model and four versions with small deviations in eddy diffusion and SO 2 abundance at the lower boundary are calculated. The models agree with the observed abundances of CO, H 2 O, SO 2 , SO, OCS and their variations; some differences are discussed. Volcanism is not required to explain variations of sulfur species that may be induced by minor changes in atmospheric dynamics. Three methods are applied to evaluate S 2 O 2 abundance sufficient for the NUV absorption on Venus, and the derived S 2 O 2 exceeds the model value by two orders of magnitude. The SO profile by Na et al. (1994, Icarus 112, 389–395) with 12 ± 5 ppb from 64 to 95 km agrees with the model above 75 km, being significantly greater below this altitude. If SO ≈ 12 ppb were near 64 km, then the calculated S 2 O 2 absorption would contribute to but not completely explain the NUV absorption. [ABSTRACT FROM AUTHOR]

From the angular positions of the glory features observed on the upper cloud deck of Venus in three VMC channels (at 0.365, 0.513, and 0.965 µm), the dominating sizes of cloud particles and their refractive indices have been retrieved, and their spatial and temporal variations have been analyzed. For this, the phase profiles of brightness were compared to the single-scattering phase functions of particles of different sizes, since diffuse multiple scattering in the clouds does not move the angular positions of the glory, which is produced by the single scattering by cloud particles, but only makes them less pronounced. We presented the measured phase profiles in two ways: they were built for individual images and for individual small regions observed in series of successive images. The analysis of the data of both types has yielded consistent results. The presently retrieved radii of cloud particle average approximately 1.0–1.2 µm (though some values reach 1.4 µm) and demonstrate a variable pattern versus latitude and local solar time (LST). The decrease of particle sizes at high latitudes (down to 0.6 µm at 60°S) earlier found from the 0.965-µm and partly 0.365-µm data has been definitely confirmed in the analysis of the data of all three channels considered. To obtain the consistent estimates of particle sizes from the UV glory maximum and minimum positions, we have to vary the effective variance of the particle sizes, while it was fixed constant in our previous studies. The twofold increase of this parameter (from 0.07 to 0.14) diminishes the estimates of particle sizes by 10–15%, while the effect on the retrieved refractive index is negligible. The obtained estimates of the refractive index are more or less uniformly distributed over the covered latitude and LST ranges, and most of them are higher than those of concentrated sulfuric acid solution. This confirms our previous result obtained only at 0.965 µm, and now we may state that the cases of a relatively high real part of the refractive index are often observed for the 1-µm mode of cloud particles on Venus. Consequently, an additional component with a high value of the refractive index is required to be present in the cloud droplets. We suggest that this component is in small submicron particles; during the condensation process, they become incorporated into sulfuric acid droplets, which results in forming the complex UV absorbing particles with an increased refractive index. We suppose that this material can be ferric chloride that is one of the candidates for the so-called unknown UV absorber in the upper clouds of Venus. [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]

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 morphology of the dark polar oval seen at the Venus cloud top in visible and ultraviolet wavelengths has been unclear because the portion of the oval on the nightside is invisible. We analyzed in detail the variability of the whole shape of the oval by connecting VMC/Venus Express visible images taken on different days after shifting the images in zonal direction based on the estimated zonal advection speed. The shape of the oval was found to be changing over time between elongated shape and near-circular shape. The dominant period of this variation changes with time in the range of 200–350 Earth days, and does not seem to coincide with the orbital period, the rotation period, and the length of the day. This suggests that the variation of the oval shape is driven by some internal nonlinear process. The mechanism of oval formation is discussed. [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]

HBr (1–0) R2 2605.8/6.2 cm −1 , the strongest line of the strongest band of HBr, was observed when searching for this species on Venus. The observation was conducted using the NASA IRTF and a high-resolution long-slit spectrograph CSHELL with resolving power of 4 × 10 4 . 101 spectra of Venus were analyzed, and the retrieved HBr abundances varied from -8 to + 5 ppb. Their mean value is -1.2 ppb, standard deviation is 2.5 ppb, and uncertainty of the mean is 0.25 ppb. The negative value presumes a systematic error, and the estimated upper limit of the HBr mixing ratio at the cloud tops of Venus is ∼1 ppb. From the simultaneously retrieved CO 2 abundances, this corresponds to an altitude of 78 km for the uniform distribution of HBr. A simplified version of the bromine photochemistry is included into the photochemical model (Krasnopolsky 2012, Icarus 218, 230–246). Photolysis of HBr and its reactions with O and H deplete the HBr mixing ratio at 70–80 km relative to that below 60 km by a factor of ≈300. Reanalysis of the observational data with the calculated profile of HBr gives an upper limit of 20–70 ppb for HBr below 60 km and the aerosol optical depth of 0.7 at 70 km and 3.84 µm. The bromine chemistry may be effective on Venus even under the observed upper limit. However, if a Cl/Br ratio in the Venus atmosphere is similar to that in the Solar System, then HBr is ≈1 ppb in the lower atmosphere and the bromine chemistry is insignificant. Thermodynamic calculations based on the chemical kinetic model (Krasnopolsky 2013, Icarus 225, 570–580) predict HBr as a major bromine species in the lower atmosphere. [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]

First submm spectroscopic observations of the 625.9 GHz H 35 Cl absorption lines of the Venus dayside atmosphere were obtained with the James Clerk Maxwell Telescope (JCMT) on March 2, 2013. These data, which support retrieval of HCl altitude distributions in the Venus mesosphere (70–100 km), are presented here and compared with previously reported JCMT observations of Venus nightside HCl ( Sandor et al., 2012 ). The measured dayside profile agrees with that of the nightside, indicating no diurnal variation is present. More specifically, the nightside spectra revealed a secular decrease of upper mesospheric HCl between observations one month apart, at fixed latitude and local time. The dayside profile reported here presents upper mesospheric abundances that are bracketed by the two previously measured nightside profiles, indicating that if diurnal variation is present, it must be weaker than the secular variations occurring at fixed local time. The previous study, which measured nightside HCl abundances above 85 km to be much smaller than predicted from photochemical modeling, suggested a dynamical explanation for the disagreement wherein nightside downwelling associated with the SubSolar to AntiSolar (SSAS) atmospheric circulation might suppress upper mesospheric abundances predicted purely from photochemistry. However a straightforward prediction from the proposed mechanism is that HCl abundance on the dayside, where the SSAS drives upward rather than downward transport should at least agree with, and perhaps exceed that of the photochemical model. The finding that dayside HCl abundance agrees with that of the nightside, hence also is much smaller than that of the model shows the SSAS hypothesis to be incorrect. [ABSTRACT FROM AUTHOR]

The understanding of spatial and temporal variations in tropospheric abundances of the trace gas carbon monoxide (CO) is key to understanding the deep atmospheric circulation on Venus. CO is entrained in the global circulation, as well as being key ingredients in the multi-reaction chemical cycle that creates and destroys the sulfuric acid that is a primary constituent of the clouds. Long-term temporal variations of CO across Venus’ disc would provide critical insights and constraints into the large-scale circulation and cloud forming processes in the troposphere. Here, we present an in-depth look at the CO as a function of latitude, longitude and local time as seen by the VIRTIS-M-IR instrument onboard the Venus Express spacecraft during its three years of operation. We find that CO is slightly enhanced on the dusk hemisphere near the poles (by ∼2 ppmv) and the equatorial concentrations from 22:00 – 03:00 are also elevated. Longitudinal variations of CO are largely absent, except for a potential correlation of anomalous CO around Themis Regio. These observations provide the most stringent constraints yet on global dynamics and CO chemistry of the deep troposphere on Venus. [ABSTRACT FROM AUTHOR]

We present here the thermal structure of the upper atmosphere of Venus predicted by a full self-consistent Venus General Circulation Model (VGCM) developed at Laboratoire de Météorologie Dynamique (LMD) and extended up to the thermosphere of the planet. Physical and photochemical processes relevant at those altitudes, plus a non-orographic GW parameterisation, have been added. All those improvements make the LMD-VGCM the only existing ground-to-thermosphere 3D model for Venus: a unique tool to investigate the atmosphere of Venus and to support the exploration of the planet by remote sounding. The aim of this paper is to present the model reference results, to describe the role of radiative, photochemical and dynamical effects in the observed thermal structure in the upper mesosphere/lower thermosphere of the planet. The predicted thermal structure shows a succession of warm and cold layers, as recently observed. A cooling trend with increasing latitudes is found during daytime at all altitudes, while at nighttime the trend is inverse above about 110 km, with an atmosphere up to 15 K warmer towards the pole. The latitudinal variation is even smaller at the terminator, in agreement with observations. Below about 110 km, a nighttime warm layer whose intensity decreases with increasing latitudes is predicted by our GCM. A comparison of model results with a selection of recent measurements shows an overall good agreement in terms of trends and order of magnitude. Significant data-model discrepancies may be also discerned. Among them, thermospheric temperatures are about 40–50 K colder and up to 30 K warmer than measured at terminator and at nighttime, respectively. The altitude layer of the predicted mesospheric local maximum (between 100 and 120 km) is also higher than observed. Possible interpretations are discussed and several sensitivity tests performed to understand the data-model discrepancies and to propose future model improvements. [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]

A new simulation of Venus atmospheric circulation obtained with the LMD Venus GCM is described and the simulated wave activity is analyzed. Agreement with observed features of the temperature structure, static stability and zonal wind field is good, such as the presence of a cold polar collar, diurnal and semi-diurnal tides. At the resolution used (96 longitudes × 96 latitudes), a fully developed superrotation is obtained both when the simulation is initialized from rest and from an atmosphere already in superrotation, though winds are still weak below the clouds (roughly half the observed values). The atmospheric waves play a crucial role in the angular momentum budget of the Venus’s atmospheric circulation. In the upper cloud, the vertical angular momentum is transported by the diurnal and semi-diurnal tides. Above the cloud base (approximately 1 bar), equatorward transport of angular momentum is done by polar barotropic and mid- to high-latitude baroclinic waves present in the cloud region, with frequencies between 5 and 20 cycles per Venus day (periods between 6 and 23 Earth days). In the middle cloud, just above the convective layer, a Kelvin type wave (period around 7.3 Ed) is present at the equator, as well as a low-latitude Rossby-gravity type wave (period around 16 Ed). Below the clouds, large-scale mid- to high-latitude gravity waves develop and play a significant role in the angular momentum balance. [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]

SPICAV IR, a channel of the SPICAV/SOIR suite of instruments onboard Venus Express spacecraft measured spectra in nadir and solar occultation modes in the range of 0.65–1.7 μm. We report results from 222 solar occultations observed from May 2006 to November 2014. The vertical resolution of measurements varies from 1 to 25 km depending on the distance of the spacecraft to the limb of Venus. The vertical profiles of atmospheric extinction were obtained at 10 near-IR wavelengths in the altitude range from 70 to 95 km. This allowed us to derive microphysical properties of the mesospheric haze. The aerosol haze top is higher near the equator than near the pole. In the upper haze, the aerosol scale height is found to be 3.3 ± 0.7 km. Detached haze layers were detected at altitudes from 70 to 90 km. Particle size and number density profiles are retrieved from extinction coefficients using Mie scattering theory adopting H 2 SO 4 refractive indices. Bimodal distribution of particles is consistent with data for some orbits with mean radius for mode 1 r eff1 = 0.12 ± 0.03 μm and r eff2 = 0.84 ± 0.16 μm for mode 2. Particle radii tend to cluster within occultation campaign and vary on the time scale of several months. The radius for the single mode case equals R eff = 0.54 ± 0.25 μm, and they are also 1.5–2 times smaller in the polar region (60°N–90°N) than in nonpolar regions (60°S–60°N). In bimodal case the number density profiles decrease smoothly for both modes, from ∼500 cm −3 at 75 km to ∼50 cm −3 at 90 km for mode 1, and from ∼1 cm −3 at 75 km to ∼0.1 cm −3 at 90 km for mode 2. [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]

We investigate the effect of a single large impact either during the Late Veneer or Late Heavy Bombardment on the evolution of the mantle and atmosphere of Venus. We use a coupled interior/exterior numerical code based on StagYY developed in Gillmann and Tackley (Gillmann, C., Tackley, P.J. [2014]. J. Geophys. Res. 119, 1189–1217). Single vertical impacts are simulated as instantaneous events affecting both the atmosphere and mantle of the planet by (i) eroding the atmosphere, causing atmospheric escape and (ii) depositing energy in the crust and mantle of the planet. The main impactor parameters include timing, size/mass, velocity and efficiency of energy deposition. We observe that impact erosion of the atmosphere is a minor effect compared to melting and degassing triggered by energy deposition in the mantle and crust. We are able to produce viable pathways that are consistent with present-day Venus, especially considering large Late Veneer Impacts. Small collisions (<100 km radius) have only local and transient effects. Medium-sized impactors (100–400 km) do not have much more consequence unless the energy deposition is enhanced, for example by a fast collision. In that case, they have comparable effects to the largest category of impacts (400–800 km): a strong thermal anomaly affecting both crust and mantle and triggering melting and a change in mantle dynamics patterns. Such an impact is a global event and can be responsible for volcanic events focused at the impact location and near the antipode. Depending on the timing of the impact, it can also have major consequences for the long-term evolution of the planet and its surface conditions by either (i) efficiently depleting the upper mantle of the planet, leading to the early loss of its water or (ii) imposing a volatile-rich and hot atmosphere for billions of years. [ABSTRACT FROM AUTHOR]

High spectral resolution observations of Venus were obtained with the TEXES instrument at NASA's Infrared Telescope Facility. These observations focus on a CO 2 absorption feature at 791.4 cm-1 as the shape of this absorption feature can be used to retrieve the vertical temperature profile in Venus' mesosphere. By scan-mapping the planet, we are able to build up three-dimensional temperature maps of Venus' atmosphere, covering one Earth-facing hemisphere and an altitude range of 60–83 km. A temperature map from February 12, 2019 clearly shows the three-dimensional structure of a planetary-scale thermal wave. This wave pattern appears strongest in the mid-latitudes of Venus, has a zonal wavenumber of 2–4 and the wave fronts tilt eastward with altitude at an angle of 8–15 degrees per km. This is consistent with a thermal tide propagating upwards from Venus' upper cloud decks. Ground-based observations provide the opportunity to study Venus' temperature structure on an ongoing basis. • Ground-based observations were used to map Venus' three-dimensional thermal structure. • A temperature offset map from February 2019 shows a planetary-scale wave. • The wave pattern is consistent with an upward propagating thermal tide. [ABSTRACT FROM AUTHOR]

Few spacecraft have studied the dynamics of Venus' deep atmosphere, which is needed to understand the interactions between the surface and atmosphere. Recent global simulations suggest a strong effect of the diurnal cycle of surface winds on the depth of the planetary boundary layer. We propose to use a turbulent-resolving model to characterize the Venus boundary layer and the impact of surface winds for the first time. Simulations were performed in the low plain and high terrain at the Equator and noon and midnight. A strong diurnal cycle is resolved in the high terrain, with a convective layer reaching 7 km above the local surface and vertical wind of 1.3 m/s. The boundary layer depth in the low plain is consistent with the observed wavelength of the dune fields. At noon, the resolved surface wind field for both locations is strong enough to lift dust particles and engender micro-dunes. Convective vortices are resolved for the first time on Venus. • First study of the Venus boundary layer with radiative and large-scale wind advection processes. • Strong impact of the diurnal cycle and the topography on the convective layer. • Noon surface winds strong enough to lift dust and engender micro-dunes. • First convective vortices resolved in the Venus boundary layer. [ABSTRACT FROM AUTHOR]

A surface wind specification is needed for future landed missions to Venus. While sparse, there exist enough data from the limited surface and near-surface measurements to date to define a probability density function that guides expectations of winds for rational design of landing systems. Following a review of all available data (mostly from the Venera missions), a Weibull function, used previously for Mars and Titan, and widely used in terrestrial engineering applications, is proposed. Best-estimate wind measurements are reasonably described by P (> V ) = exp[−( V / c ) k ], with c = 0.8 m/s, k = 1.9: this function yields a 95% chance of winds <1.4 m/s and 99% <1.8 m/s. A worst-case function, allowing the high end of Venera measurement uncertainties to force the fit, has slightly higher values ( c = 0.9 m/s, k = 1.7; 95% wind 1.7 m/s; 99%, 2.2 m/s). The data suggest that winds strong enough to move dust and sand on Venus are rather common (more so than is typical for Mars, Earth or Titan), a prediction testable with radar interferometry on future orbital missions and/or from landed observations. More elaborate analyses should take site-specific factors such as slope or time of day into account, but cannot be meaningfully constrained by present data. [ABSTRACT FROM AUTHOR]

Hubble Space Telescope Imaging Spectrograph (HST/STIS) UV observations of Venus’ upper cloud tops were obtained between 20N and 40S latitude on December 28, 2010; January 22, 2011 and January 27, 2011 in coordination with the Venus Express (VEx) mission. The high spectral (0.27 nm) and spatial (40–60 km/pixel) resolution HST/STIS data provide the first direct and simultaneous record of the latitude and local time distribution of Venus’ 70–80 km SO and SO 2 (SO x ) gas density on Venus’ morning quadrant. These data were obtained simultaneously with (a) VEx/SOIR occultation and/or ground-based James Clerk Maxwell Telescope sub-mm observations that record respectively, Venus’ near-terminator SO 2 and dayside SO x vertical profiles between ∼75 and 100 km; and (b) 0.36 μm VEx/VMC images of Venus’ cloud-tops. Updating the (Marcq, E. et al. [2011]. Icarus 211, 58–69) radiative transfer model SO 2 gas column densities of ∼2–10 μm-atm and ∼0.4–1.8 μm-atm are retrieved from the December 2010 and January 2011 HST observations, respectively on Venus’ dayside (i.e., at solar zenith angles (SZA) < 60°); SO gas column densities of 0.1–0.11 μm-atm, 0.03–0.31 μm-atm and 0.01–0.13 μm-atm are also retrieved from the respective December 28, 2010, January 22, 2011 and January 27, 2011 HST observations. A decline in the observed low-latitude 0.24 and 0.36 μm cloud top brightness paralleled the declining SO x gas densities. On December 28, 2010 SO 2 VMR values ∼280–290 ppb are retrieved between 74 and 81 km from the HST and SOIR data obtained near Venus’ morning terminator (at SZAs equal to 70° and 90°, respectively); these values are 10× higher than the HST-retrieved January 2011 near terminator values. Thus, the cloud top SO 2 gas abundance declined at all local times between the three HST observing dates. On all dates the average dayside SO 2 /SO ratio inferred from HST between 70 and 80 km is higher than that inferred from the sub-mm the JCMT data above 84 km confirming that SO x photolysis is more efficient at higher altitudes. The direct correlation of the SO x gases provides the first clear evidence that SO x photolysis is not the only source for Venus’ 70–80 km sulfur reservoir. The cloud top SO 2 gas density is dependent in part on the vertical transport of the gas from the lower atmosphere; and the 0.24 μm cloud top brightness levels are linked to the density of the sub-micron haze. Thus, the new results may suggest a correlation between Venus’ cloud-top sub-micron haze density and the vertical transport rate. These new results must be considered in models designed to simulate and explore the relationship between Venus’ sulfur chemistry cycle, H 2 SO 4 cloud formation rate and climate evolution. Additionally, we present the first photochemical model that uniquely tracks the transition of the SO 2 atmosphere from steady to non-steady state with increasing SZA, as function of altitude within Venus’ mesosphere, showing the photochemical and dynamical basis for the factor of ∼2 enhancements in the SO x gas densities observed by HST near the terminator above that observed at smaller SZA. These results must also be considered when modeling the long-term evolution of Venus’ atmospheric chemistry and dynamics. [ABSTRACT FROM AUTHOR]

While the present-day surface of Venus is certainly incompatible with terrestrial biology, the planet may have possessed oceans in the past and provided conditions suitable for the origin of life. Venusian life may persist today high in the atmosphere where the temperature and pH regime is tolerable to terrestrial extremophile microbes: an aerial habitable zone. Here we argue that on the basis of the combined biological hazard of high temperature and high acidity this habitable zone lies between 51 km (65 °C) and 62 km (−20 °C) altitude. Compared to Earth, this potential venusian biosphere may be exposed to substantially more comic ionising radiation: Venus has no protective magnetic field, orbits closer to the Sun, and the entire habitable region lies high in the atmosphere – if this narrow band is sterilised there is no reservoir of deeper life that can recolonise afterwards. Here we model the propagation of particle radiation through the venusian atmosphere, considering both the background flux of high-energy galactic cosmic rays and the transient but exceptionally high-fluence bursts of extreme solar particle events (SPE), such as the Carrington Event of 1859 and that inferred for AD 775. We calculate the altitude profiles of both energy deposition into the atmosphere and the absorbed radiation dose to assess this astrophysical threat to the potential high-altitude venusian biosphere. We find that at the top of the habitable zone (62 km altitude; 190 g/cm 2 shielding depth) the radiation dose from the modelled Carrington Event with a hard spectrum (matched to the February 1956 SPE) is over 18,000 times higher than the background from GCR, and 50,000 times higher for the modelled 775 AD event. However, even though the flux of ionising radiation can be sterilizing high in the atmosphere, the total dose delivered at the top of the habitable zone by a worst-case SPE like the 775 AD event is 0.09 Gy, which is not likely to present a significant survival challenge. Nonetheless, the extreme ionisation could force atmospheric chemistry that may perturb a venusian biosphere in other ways. The energy deposition profiles presented here are also applicable to modelling efforts to understand how fundamental planetary atmospheric processes such as atmospheric chemistry, cloud microphysics and atmospheric electrical systems are affected by extreme solar particle events. The companion paper to this study, Constraints on a potential aerial biosphere on Venus: II. Solar ultraviolet radiation (Patel et al., in preparation), considers the threat posed by penetration of solar UV radiation. The results of these twin studies are based on Venus but are also applicable to extrasolar terrestrial planets near the inner edge of the circumstellar habitable zone. [ABSTRACT FROM AUTHOR]

A method developed by Krasnopolsky and Pollack (Krasnopolsky, V.A., Pollack, J.B. [1994]. Icarus 109, 58–78) to model vertical profiles of H 2 O and H 2 SO 4 vapors and sulfuric acid concentration in the Venus cloud layer has been updated with improved thermodynamic parameters for H 2 O and H 2 SO 4 and reduced photochemical production of sulfuric acid. The model is applied to the global-mean conditions and those at the low latitudes and at 60°. Variations in eddy diffusion near the lower cloud boundary are used to simulate variability in the cloud properties and abundances of H 2 O and H 2 SO 4 . The best version of the model for the global-mean condition results in a lower cloud boundary (LCB) at 47.5 km, H 2 SO 4 peak abundance of 7.5 ppm at the LCB, and H 2 O mixing ratios of 7 ppm at 62 km and 3.5 ppm above 67 km. The model for low latitudes gives LCB at 48.5 km, the H 2 SO 4 peak of 5 ppm, H 2 O of 8.5 ppm at 62 km and 3 ppm above 67 km. The model for 60° shows LCB at 46 km, the H 2 SO 4 peak of 8.5 ppm, H 2 O of 9 ppm at 62 km and 4.5 ppm above 67 km. The calculated variability is induced by the proper changes in the production of sulfuric acid (by factors of 1.2 and 0.7 for the low latitudes and 60°, respectively) and reduction of eddy diffusion near 45 km relative to the value at 54 km by factors of 1.1, 3, and 4.5 for the low and middle (global-mean) latitudes and 60°, respectively. Concentration of sulfuric acid at the low and middle latitudes varies from ∼98% near 50 km to ∼80% at 60 km and then is almost constant at 79% at 70 km. Concentration at 60° is 98% at 50 km, 73% at 63 km, and 81% at 70 km. There is a reasonable agreement between the model results and observations except for the sulfuric acid concentration in the lower clouds. Variations of eddy diffusion in the lower cloud layer simulate variations in atmospheric dynamics and may induce strong variations in water vapor near the cloud tops. Variations in temperature may affect abundances of the H 2 O and H 2 SO 4 vapors as well. [ABSTRACT FROM AUTHOR]

Because of atmospheric shielding and endogenic resurfacing, the population of impact craters on Venus is small (about a thousand) and consists of large craters. This population has been used in numerous studies with the goal of deciphering the geologic and geodynamic history of Venus, but the nearly spatially random nature of the crater population has complicated efforts to understand this history. Here we utilize the recent 1:15 M-scale global geological map of Venus (Ivanov, M.A., Head, J.W. [2011]. Planet. Space Sci. 59, 1559–1600) to help address this problem. The global geological map provides a stratigraphic sequence of units, and known areas where each unit is exposed on the planet. For each crater on Venus we identify the specific geological units predating and postdating the crater. We perform a statistical analysis of this set of observations with a buffered crater density approach, which rigorously and consistently takes into account the large size of craters and the fact that many craters are known to predate and/or postdate more than one unit. In this analysis we consider crater emplacement as random and resurfacing history as determined (although unknown). We obtain formal confidence intervals for the mean ages of geological units and the mean age differences between the pairs of units at the unit boundaries. We find that (1) size–frequency distributions of craters superposed on each unit are consistent with each other; (2) regional plains and stratigraphically older units have similar crater retention ages; (3) stratigraphically younger units have a mean crater retention age significantly younger than the regional plains. These findings are readily and consistently explained by global resurfacing scenarios and are difficult to reconcile with equilibrium resurfacing scenarios. Our analysis also shows that the latest recorded part of intensive resurfacing period lasted on the order of 10% of the mean surface age (tens of millions of years). The termination of intensive resurfacing may or may not be synchronous over the planet. Our results also indicate that there are extended deposits associated with large craters that are almost indiscernible in the radar images, but obscure radar contrasts between predating lava flows. We do not see evidence for any significant and prolonged change of atmospheric pressure following the termination of the intensive resurfacing epoch. [ABSTRACT FROM AUTHOR]

We combine Earth-based radar maps of Venus from the 1988 and 2012 inferior conjunctions, which had similar viewing geometries. Processing of both datasets with better image focusing and co-registration techniques, and summing over multiple looks, yields maps with 1–2 km spatial resolution and improved signal to noise ratio, especially in the weaker same-sense circular (SC) polarization. The SC maps are unique to Earth-based observations, and offer a different view of surface properties from orbital mapping using same-sense linear (HH or VV) polarization. Highland or tessera terrains on Venus, which may retain a record of crustal differentiation and processes occurring prior to the loss of water, are of great interest for future spacecraft landings. The Earth-based radar images reveal multiple examples of tessera mantling by impact “parabolas” or “haloes”, and can extend mapping of locally thick material from Magellan data by revealing thinner deposits over much larger areas. Of particular interest is an ejecta deposit from Stuart crater that we infer to mantle much of eastern Alpha Regio. Some radar-dark tessera occurrences may indicate sediments that are trapped for longer periods than in the plains. We suggest that such radar information is important for interpretation of orbital infrared data and selection of future tessera landing sites. [ABSTRACT FROM AUTHOR]

The Venus thermal radiation spectrum exhibits the signature of CO 2 absorption bands. By means of inversion techniques, those bands enable the retrieval of atmospheric temperature profiles. We have analyzed VIRTIS-M-IR night-side data obtaining high-resolution thermal maps of the Venus south polar region between 55 and 85 km altitudes. This analysis is specific to three Venus Express orbits where the vortex presents different dynamical configurations. The cold collar is clearly distinguishable centered at ∼62 km (∼100 mbar) altitude level. On average, the cold collar is more than 15 K colder than the pole, but its specific temperature varies with time. In the three orbits under investigation the South Polar Vortex appears as a vertically extended hot region close to the pole and squeezed by the cold collar between altitudes 55 and 67 km but spreading equatorwards at about 74 km. Both the instantaneous temperature maps and their zonal averages show that the top altitude limit of the thermal signature from the vortex is at ∼80 km altitude, at least on the night-side of the planet. The upper part of the atmosphere (67–85 km) is more homogeneous and has long-scale horizontal temperature differences of about 25 K over horizontal distances of ∼2000 km. The lower part (55–67 km) shows more fine-scale structure, creating the vortex morphology, with thermal differences of up to about 50 K over the same altitude range and ∼500 km horizontal distances. This lower part of the atmosphere is highly affected by the upper cloud deck, leading to stronger local temperature variations and larger uncertainties in the retrieval. From the temperature maps, we also study the vertical stability of different atmospheric layers for the three vortex configurations. The static stability is always positive ( S T > 0) in the considered altitude range (55–85 km) and in the whole polar vortex. The cold collar is the most vertically stable structure at polar latitudes, while the vortex and sub-polar latitudes show lower stability values. Furthermore, the hot filaments present within the vortex exhibit lower stability values than their surroundings. The layer between 62 and 67 km resulted to be the most stable. These results are in good agreement with conclusions from previous radio occultation analyses. [ABSTRACT FROM AUTHOR]

The atmospheres of the terrestrial planets are exposed to solar and galactic cosmic rays, the most energetic of which are capable of affecting deep atmospheric layers through extensive nuclear and electromagnetic particle cascades. In the venusian atmosphere, cosmic rays are expected to be the dominant ionization source below ∼100 km altitude. While previous studies have considered the effect of cosmic ray ionization using approximate transport methods, we have for the first time performed full 3D Monte Carlo modeling of cosmic ray interaction with the venusian atmosphere, including the contribution of high- Z cosmic ray ions ( Z = 1–28). Our predictions are similar to those of previous studies at the ionization peak near 63 km altitude, but are significantly different to these both above and below this altitude. The rate of atmospheric ionization is a fundamental atmospheric property and the results of this study have wide-reaching applications in topics including atmospheric electrical processes, cloud microphysics and atmospheric chemistry. [ABSTRACT FROM AUTHOR]

We have investigated the cloud top structure of Venus by analyzing ground-based images taken at the mid-infrared wavelengths of 8.66 μm and 11.34 μm. Venus at a solar phase angle of ∼90°, with the morning terminator in view, was observed by the Cooled Mid-Infrared Camera and Spectrometer (COMICS), mounted on the 8.2-m Subaru Telescope, during the period October 25–29, 2007. The disk-averaged brightness temperatures for the observation period are ∼230 K and ∼238 K at 8.66 μm and 11.34 μm, respectively. The obtained images with good signal-to-noise ratio and with high spatial resolution (∼200 km at the sub-observer point) provide several important findings. First, we present observational evidence, for the first time, of the possibility that the westward rotation of the polar features (the hot polar spots and the surrounding cold collars) is synchronized between the northern and southern hemispheres. Second, after high-pass filtering, the images reveal that streaks and mottled and patchy patterns are distributed over the entire disk, with typical amplitudes of ∼0.5 K, and vary from day to day. The detected features, some of which are similar to those seen in past UV images, result from inhomogeneities of both the temperature and the cloud top altitude. Third, the equatorial center-to-limb variations of brightness temperatures have a systematic day–night asymmetry, except those on October 25, that the dayside brightness temperatures are higher than the nightside brightness temperatures by 0–4 K under the same viewing geometry. Such asymmetry would be caused by the propagation of the migrating semidiurnal tide. Finally, by applying the lapse rates deduced from previous studies, we demonstrate that the equatorial center-to-limb curves in the two spectral channels give access to two parameters: the cloud scale height H and the cloud top altitude z c . The acceptable models for data on October 25 are obtained at H = 2.4–4.3 km and z c = 66–69 km; this supports previous results determined from spacecraft observations. [ABSTRACT FROM AUTHOR]

We present new results based on ground-based Doppler spectroscopic measurements, obtained with the ESPaDOnS spectrograph at Canada–France–Hawaii telescope (CFHT) and simultaneous observations of velocity fields, obtained from space by the VIRTIS-M instrument on board the Venus Express spacecraft. These measurements are based on high-resolution spectra of Fraunhofer lines in the visible to NIR range (0.37–1.05 μm) acquired on February 19–21, 2011 at a resolution of about 80,000, measuring Venus’ winds at 70 km, using incoming solar radiation scattered by cloud top particles in the observer’s direction (Widemann, T., et al., [2007]. Planet. Space Sci. 55, 1741–1756; Widemann, T., et al., [2008]. Planet. Space Sci. 56, 1320–1334). The zonal wind field has been characterized by latitudinal bands, at a phase angle Φ = ( 68.7 ± 0.3 ) ° , between +10°N and 60°S, by steps of 10°, and from [ ϕ - ϕ E ] = - 50 ° to sub-Earth longitude ϕ E = 0 ° , by steps of 12°. From space, VIRTIS-M UV (0.38 μm) imaging exposures on the dayside were acquired simultaneously in orbit 1786, providing the first simultaneous cloud-tracking measurements with Doppler velocimetry. From the ground, we measured a zonal mean background velocity of v ‾ z = ( 117.3 ± 18.0 ) m s - 1 on February 19, and v ‾ z = ( 117.5 ± 14.5 ) m s - 1 on February 21. We detect an unambiguous poleward meridional flow on the morning dayside hemisphere of (18.8 ± 12.3) m s −1 on February 19/21. Latitudinal variations of the zonal and meridional winds are further compared with the simultaneous VIRTIS data. We discuss temporal variability as well as its statistical significance. [ABSTRACT FROM AUTHOR]

Highlights: [•] The Venus upper atmosphere transition region dynamics is not fully understood. [•] Individual O2(a1Δ) nightglow emission spots were used as tracers. [•] This emission is highly variable both in location and intensity. [•] Animations following spots over time have been generated. [•] Characteristic e-folding times and a map of the wind speeds have been obtained. [Copyright &y& Elsevier]