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Aerosol phase elements such as phosphorus (P), sulfur (S), and metals including iron (Fe) are essential nutrients that could help sustain potential biodiversity in the cloud deck of Venus. While the presence of S and Fe in the venusian cloud deck has been broadly discussed (Zasova

The baseline Venera-D mission, proposed to be launched after 2026, consists of an orbiter and a landing module, which includes a lander and a small long-lived station. In this work, we present the possibility of augmenting the mission with one or two micro-spacecraft in Lagrange point orbits to significantly enhance the science return. Both L 1 and L 2 collinear points, situated at about 1 million km from Venus, are considered for scientific objectives. The paper focuses on trajectory design for the micro-spacecraft to be deployed by the Venera-D mission. The transfer scenario we propose performs the insertion of micro-spacecraft into Lagrange point orbit by making use of the main orbiter’s propulsion, while the propulsion on board micro-spacecraft is used only for trajectory correction and station-keeping manoeuvres. This scenario allows the Venera-D mission to add one or two micro-spacecraft of mass about 50 kg with only a total Δ V budget 30 m/s over 3 years on orbit.

Explaining super-rotation on Venus The solid surface of Venus rotates very slowly, once every 243 days, but its thick atmosphere circles the planet in just 4 days. This phenomenon, known as super-rotation, requires a continuous input of angular momentum, from an unknown source, to overcome friction with the surface. Horinouchi et al. mapped the planet's winds using ultraviolet observations of Venus' clouds from the orbiting Akatsuki spacecraft (see the Perspective by Lebonnois). They incorporated these data into a global model of angular momentum transport in the atmosphere, finding that the super-rotation is maintained through thermal tides driven by solar heating. Science , this issue p. 405 ; see also p. 363

We present a re‐examination of mass spectral data obtained from the Pioneer Venus Large Probe Neutral Mass Spectrometer. Our interpretations of differing trace chemical species are suggestive of redox disequilibria in Venus' middle clouds. Assignments to the data (at 51.3 km) include phosphine, hydrogen sulfide, nitrous acid, nitric acid, carbon monoxide, hydrochloric acid, hydrogen cyanide, ethane, and potentially ammonia, chlorous acid, and several tentative PxOy species. All parent ions were predicated upon assignment of corresponding fragmentation products, isotopologues, and atomic species. The data reveal parent ions at varying oxidation states, implying the presence of reducing power in the clouds, and illuminating the potential for chemistries yet to be discovered. When considering the hypothetical habitability of Venus' clouds, the assignments reveal a potential signature of anaerobic phosphorus metabolism (phosphine), an electron donor for anoxygenic photosynthesis (nitrite), and major constituents of the nitrogen cycle (nitrate, nitrite, ammonia, and N2)., Key Points Mass data from the Pioneer Venus Large Probe Neutral Mass Spectrometer reveals several trace chemical species suggestive of disequilibriaTrace species in the middle clouds include phosphine, hydrogen sulfide, nitrous acid, nitric acid, hydrogen cyanide, and carbon monoxideData reveal chemicals related to anaerobic phosphorus metabolism (phosphine), anoxygenic photosynthesis (nitrite), and the nitrogen cycle

The interest in the possibility of life on Venus is driven not just by curiosity about life originating in another Earth-like environment, but because of the possibility that life may be playing a critical role in the planet’s present, and possibly its past, atmospheric state. The brilliance of Venus in the night sky (as viewed from Earth) is due to its highly reflective cloud cover, about 28 km thick at the equator. Its spectral albedo is about 90% at wavelengths > 500 nm, but it drops gradually to about 40% around 370 nm before rising slightly at shorter wavelengths. This albedo drop is due to the presence of several absorbers in the atmosphere and the cloud cover. A very large fraction of the energy absorbed by Venus is at ultraviolet wavelengths with sulfur dioxide above the clouds contributing to the absorption below 330 nm; however, the identities of the other absorbers remain unknown. The inability to identify the absorbers that are responsible for determining the radiative energy balance of Venus over the last century is a major impediment to understanding how the planet “works”, a major component of NASA’s efforts in planetary exploration. Limaye et al. (Astrobiology 18, 1181-1198, 2018) presented a hypothesis suggesting that cloud-based microbial life could be contributors to the spectral signatures of Venus’ clouds, building upon previous suggestions of the possibility of life in the clouds of Venus.Four interconnected themes for the exploration of Venus as an astrobiology target are: – (i) investigations focused on the likelihood that liquid water existed on the surface in the past leading to the potential for the origin and evolution of life, (ii) investigations into the potential for habitable zones within Venus’ clouds and Venus-like atmospheres, (iii) theoretical investigations into how active aerobiology may impact the radiative energy balance of Venus’ clouds and Venus-like atmospheres, and (iv) application of these investigative themes towards better understanding the atmospheric dynamics and habitability of exoplanets. These themes can serve as a basis for proposed Venus Astrobiology Objectives and suggestions for measurements for future missions, as per the goals and objectives developed by the Venus Exploration Analysis Group (VEXAG), which is sponsored by NASA to plan for the future exploration of Venus. A Venus Collection to be published in Astrobiology journal in 2021 will include papers from the “Habitability of the Venus Cloud Layer”, Moscow (October 2019) workshop.

We show that solar irradiances calculated across Venus' clouds support the potential for Earth-like phototrophy and that treatment of Venus' aerosols containing neutralized sulfuric acid favor a habitable zone. The phototrophic potential of Venus' atmosphere was assessed by calculating irradiances (200-2000 nm, 15° solar zenith angle, local noon) using a radiative transfer model that accounted for absorption and scattering by the major and minor atmospheric constituents. Comparisons to Earth's surface (46 W m

A lightweight, low-power instrument package to measure,in situ,both (1) the local gaseous environment and (2) the composition and microphysical properties of attendant venusian aerosols is presented. This Aerosol-Sampling Instrument Package (ASIP) would be used to explore cloud chemical and possibly biotic processes on future aerial missions such as multiweek balloon missions and on short-duration ( A quadrupole ion-trap mass spectrometer (QITMS; Madzunkov and Nikolić,J Am Soc Mass Spectrom25:1841–1852, 2014) fed alternately by (1) an aerosol separator that injects only aerosols into a vaporizer and mass spectrometer and (2) the pure aerosol-filtered atmosphere, achieves the compositional measurements. Aerosols vaporized 2SO4aerosols, to better than 20% in 2SO4-relative abundances of 2 × 10−9. An integrated lightweight, compact nephelometer/particle-counter determines the number density and particle sizes of the sampled aerosols.

Understanding Venus will benefit from a dedicated effort through an international program of missions that are coordinated and offering opportunities for coooperation. The recent discovery of phosphene in the Venus clouds and possibility of life will increase the exploration missions and cooperation and coordination will be very benefical.

We present a case for the exploration of Venus as an astrobiology target—(1) investigations focused on the likelihood that liquid water existed on the surface in the past, leading to the potential for the origin and evolution of life, (2) investigations into the potential for habitable zones within Venus’ present-day clouds and Venus-like exo atmospheres, (3) theoretical investigations into how active aerobiology may impact the radiative energy balance of Venus’ clouds and Venus-like atmospheres, and (4) application of these investigative approaches toward better understanding the atmospheric dynamics and habitability of exoplanets. The proximity of Venus to Earth, guidance for exoplanet habitability investigations, and access to the potential cloud habitable layer and surface for prolonged in situ extended measurements together make the planet a very attractive target for near term astrobiological exploration.

We present distributions of the zonal-mean temperature and static stability in the Venusian atmosphere obtained from Venus Express and Akatsuki radio occultation profiles penetrating down to an altitude of 40 km. At latitudes equatorward of 75°, static stability derived from the observed temperature profiles is consistent with previous in-situ measurements in that there is a low-stability layer at altitudes of 50–58 km and highly and moderately stratified layers above 58 km and below 50 km, respectively. Meanwhile, at latitudes poleward of 75°, a low-stability layer extends down to 42 km, which has been unreported in analyses of previous measurements. The deep low-stability layer in the polar region cannot be explained by vertical convection in the middle/lower cloud layer, and the present result thus introduces new constraints on the dynamics of the sub-cloud atmosphere. The Venusian atmosphere is in striking contrast to the Earth’s troposphere, which generally has a deeper low-stability layer at low latitudes than at mid- and high latitudes.

Planetary-scale waves are thought to play a role in powering the yet-unexplained atmospheric superrotation of Venus. Puzzlingly, while Kelvin, Rossby and stationary waves manifest at the upper clouds (65--70 km), no planetary-scale waves or stationary patterns have been reported in the intervening level of the lower clouds (48--55 km), although the latter are probably Lee waves. Using observations by the Akatsuki orbiter and ground-based telescopes, we show that the lower clouds follow a regular cycle punctuated between 30$^{\circ}$N--40$^{\circ}$S by a sharp discontinuity or disruption with potential implications to Venus's general circulation and thermal structure. This disruption exhibits a westward rotation period of $\sim$4.9 days faster than winds at this level ($\sim$6-day period), alters clouds' properties and aerosols, and remains coherent during weeks. Past observations reveal its recurrent nature since at least 1983, and numerical simulations show that a nonlinear Kelvin wave reproduces many of its properties., Comment: 21 pages, 10 figures, 2 animated figures and 2 tables

International audience; Hubble Space Telescope Imaging Spectrograph (HST/STIS) observations were obtained on 3 dates in December 2010 and January 2011 recording the cloud top properties over Aphrodite Terra and a low elevation region downwind of Aphrodite through LSTs extending from 7 to 11 a.m. From these data we trace the cloud top sulfur-oxide chemistry and UV albedo sensitivity to LST, latitude and topography. Above regions co-located in LST and latitude, albedo variations observed at 245 nm parallel those observed at 365 nm-following the pattern expected from Hadley cell circulation. However, darkening of the cloud top albedo at LSTs between 9.5 and 11 h beyond that expected from simple Hadley circulation was also observed. Above the plains the albedo darkening intensified rapidly with LST and was observed at latitudes extending as high as ~30 N; however, above the mountains the darkening was either entirely absent or evident only at 0 N at an intensity 2× lower than that observed over the plains. Because the observed 245 nm albedo LST variations were inconsistent with that expected from multiple scattering of the coincidently retrieved SO2 gas abundance, we conclude that the 245 nm albedo is diagnostic of the vertical and spatial distribution, abundance (and potentially the identity) of Venus' unidentified UV absorber—rather than SO2 gas. The LST albedo trends are best explained by the onset of subsolar convective activity that intensifies with LST expanding vertically from the boundary between the middle and upper clouds to the cloud tops and increasing the detectability of the unknown absorbing species at the cloud tops. The terrain dependence in the observed intensity implies the time at which the expansion reaches the cloud tops is later above the mountains than over the plains. Additionally, at the time of observation, the low-latitude large-scale vertical mixing rates that control the latitudinal gradients of the SO2 and unknown absorber abundances above and within the cloud top region were lower over Aphrodite Terra than the plains, to the extent that photochemical processing destroyed the spatial correlation between those absorbing species. These observations show the power of UV spectroscopy to diagnose the distinct influences of deep (Hadley-cell type) and shallow convective mixing processes on the vertical and horizontal distribution of Venus' unknown absorbing species, and the sensitivity of these processes to LST and topography, relative to the sulfur oxide chemistry. These results are essential for accurate climate modeling—and when compared to recent Venus missions motivate a need for additional follow-on observing campaigns that simultaneously trace key cloud top chemistry and dynamic processes including the LST dependent evolution of planetary scale gravity waves (GWs). With the inevitable aging of the Hubble Telescope, follow-on observations providing temporally coincident traces of the cloud top albedo, sulfur-oxide chemistry and GW features will require a new age of space-based telescopes and Venus orbiting mission with sensitivity to UV, visible and IR wavelengths.

An exciting and novel science mission concept called Seismic and Atmospheric Exploration of Venus (SAEVe) has been developed which uses high-temperature electronics to enable a three-order magnitude increase in expected surface life (120 Earth days) over what has been achieved to date. This enables study of long-term, variable phenomena such as the seismicity of Venus and near surface weather, near surface energy balance, and atmospheric chemical composition. SAEVe also serves as a critical pathfinder for more sophisticated landers in the future. For example, first order seismic measurements by SAEVe will allow future missions to deliver better seismometers and systems to support the yet unknown frequency and magnitude of Venus events. SAEVe is focused on science that can be realized with low data volume instruments and will most benefit from temporal operations. The entire mission architecture and operations maximize science while minimizing energy usage and physical size and mass. The entire SAEVe system including its protective entry system is estimated to be around 45 ​kg and approximately 0.6 ​m diameter. These features allow SAEVe to be relatively cost effective and be easily integrated onto a Venus orbiter mission. The technologies needed to implement SAEVe are currently in development by several funded activities. Component and system level work is ongoing under NASA’s Long Lived Insitu Solar System Explorer (LLISSE) project and the HOTTech program. . LLISSE, is a NASA project to develop a small Venus lander that will operate on the surface of Venus for 60 days and measure variations in meteorology, radiance, and atmospheric chemistry. LLISSE is developing a full-function engineering model of a Venus lander that contains essentially all the core capabilities of SAEVe thus greatly reducing the technology risk to SAEVe. The SAEVe long duration Venus lander promises exciting new science and is an ideal complimentary element to many future Venus orbiter missions being proposed or planned today.

Orbits around the two close collinear Sun-Earth Lagrange points have been utilized in recent decades for many solar and astronomical missions to exploit the observational advantages. DSCOVR is the first satellite to observe Earth continuously from the vicinity of the L1 point and is filing a crucial gap in Earth observations with its vast collection of polar, geosynchronous and some retrograde low inclination orbits. Spacecraft orbiting Venus in the last four decades have provided us with a wealth of information and many unanswered questions, which cannot be addressed adequately by observing from polar or near equatorial orbits around the planet. The Sun-Venus collinear Lagrange points L1 (sunward) and L2 (behind the planet from the Sun) points are key vantage points located about a million km away from the planet along the direction to the Sun which enable continuous monitoring of the planet's day and night hemispheres. As spacecraft positions at L1 and L2 points are unstable, they can be inserted in orbits around them to observe Venus over a small range of phase angles unlike any Venus orbiter observations which cover 0–180° solar phase angle twice each orbit for a very long time with minimal station keeping costs. To help better understand Venus, we propose that monitoring Venus continuously at nearly the same phase angle from the vicinity of L1 and L2 Lagrange points is critical. Such Lagrange orbiters around Sun-Venus L1 and L2 points can provide crucial information continuously about the evolution and variability of: (i) reflectance of the global cloud cover, (ii) the night side cloud cover opacity, (iii) surface activity, and (iv) interaction of planet's atmosphere with the solar wind, and loss of atmosphere. In addition, the two Lagrange orbiters can provide a crucial continuous communications capability for relaying data from in-situ atmospheric or surface platforms. Well instrumented missions to L1 and L2 points of Venus would significantly improve our understanding Earth's perplexing neighbor by obtaining continuous record of data on the day and night hemispheres not available from Venus orbiting missions.

International audience; Recent observations of sulfur containing species (SO2, SO, OCS, and H2SO4) 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 SO2 and SO variability in the Venus atmosphere. This first part of the study will focus on the vertical distribution of SO2, 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 SO2 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 SO2 and SO) must be present throughout the 70–100 km altitude region to explain the inversion in the SO2 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 latitudesThe current study has been carried out within the frame of the International Space Science Institute (ISSI) International Team entitled ‘SO2 variability in the Venus atmosphere’.

The lower cloud layer of Venus (47.5-50.5 km) is an exceptional target for exploration due to the favorable conditions for microbial life, including moderate temperatures and pressures (∼60°C and 1 atm), and the presence of micron-sized sulfuric acid aerosols. Nearly a century after the ultraviolet (UV) contrasts of Venus' cloud layer were discovered with Earth-based photographs, the substances and mechanisms responsible for the changes in Venus' contrasts and albedo are still unknown. While current models include sulfur dioxide and iron chloride as the UV absorbers, the temporal and spatial changes in contrasts, and albedo, between 330 and 500 nm, remain to be fully explained. Within this context, we present a discussion regarding the potential for microorganisms to survive in Venus' lower clouds and contribute to the observed bulk spectra. In this article, we provide an overview of relevant Venus observations, compare the spectral and physical properties of Venus' clouds to terrestrial biological materials, review the potential for an iron- and sulfur-centered metabolism in the clouds, discuss conceivable mechanisms of transport from the surface toward a more habitable zone in the clouds, and identify spectral and biological experiments that could measure the habitability of Venus' clouds and terrestrial analogues. Together, our lines of reasoning suggest that particles in Venus' lower clouds contain sufficient mass balance to harbor microorganisms, water, and solutes, and potentially sufficient biomass to be detected by optical methods. As such, the comparisons presented in this article warrant further investigations into the prospect of biosignatures in Venus' clouds.

From the discovery that Venus has an atmosphere during the 1761 transit by M. Lomonosov to the current exploration of the planet by the Akatsuki orbiter, we continue to learn about the planet’s extreme climate and weather. This chapter attempts to provide a comprehensive but by no means exhaustive review of the results of the atmospheric thermal structure and radiative balance since the earlier works published in Venus and Venus II books from recent spacecraft and Earth based investigations and summarizes the gaps in our current knowledge. There have been no in-situ measurements of the deep Venus atmosphere since the flights of the two VeGa balloons and landers in 1985 (Sagdeev et al., Science 231:1411–1414, 1986). Thus, most of the new information about the atmospheric thermal structure has come from different remote sensing (Earth based and spacecraft) techniques using occultations (solar infrared, stellar ultraviolet and orbiter radio occultations), spectroscopy and microwave, short wave and thermal infrared emissions. The results are restricted to altitudes higher than about 40 km, except for one investigation of the near surface static stability inferred by Meadows and Crisp (J. Geophys. Res. 101:4595–4622, 1996) from 1 $\upmu$ m observations from Earth. Little information about the lower atmospheric structure is possible below about 40 km altitude from radio occultations due to large bending angles. The gaps in our knowledge include spectral albedo variations over time, vertical variation of the bulk composition of the atmosphere (mean molecular weight), the identity, properties and abundances of absorbers of incident solar radiation in the clouds. The causes of opacity variations in the nightside cloud cover and vertical gradients in the deep atmosphere bulk composition and its impact on static stability are also in need of critical studies. The knowledge gaps and questions about Venus and its atmosphere provide the incentive for obtaining the necessary measurements to understand the planet, which can provide some clues to learn about terrestrial exoplanets.

International audience; 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 SO2 on Venus. In this second part, we focus our attention on the spatial (horizontal) and temporal variability exhibited by SO2. 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 SO2 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 SO2 and possibly to variations in the abundance and production and loss of H2O, H2SO4, and Sx.

Context. Radio occultation is a technique used to study planetary atmospheres by means of the refraction and absorption of a spacecraft carrier signal through the atmosphere of the celestial body of interest, as detected from a ground station on Earth. This technique is usually employed by the deep space tracking and communication facilities (e.g., NASA’s Deep Space Network (DSN), ESA’s Estrack). Aims. We want to characterize the capabilities of the Planetary Radio Interferometry and Doppler Experiment (PRIDE) technique for radio occultation experiments, using radio telescopes equipped with Very Long Baseline Interferometry (VLBI) instrumentation. Methods. We conducted a test with ESA’s Venus Express (VEX), to evaluate the performance of the PRIDE technique for this particular application. We explain in detail the data processing pipeline of radio occultation experiments with PRIDE, based on the collection of so-called open-loop Doppler data with VLBI stations, and perform an error propagation analysis of the technique. Results. With the VEX test case and the corresponding error analysis, we have demonstrated that the PRIDE setup and processing pipeline is suited for radio occultation experiments of planetary bodies. The noise budget of the open-loop Doppler data collected with PRIDE indicated that the uncertainties in the derived density and temperature profiles remain within the range of uncertainties reported in previous Venus’ studies. Open-loop Doppler data can probe deeper layers of thick atmospheres, such as that of Venus, when compared to closed-loop Doppler data. Furthermore, PRIDE through the VLBI networks around the world, provides a wide coverage and range of large antenna dishes, that can be used for this type of experiments.

Six years of continuous monitoring of Venus by European Space Agency’s Venus Express orbiter provides an opportunity to study dynamics of the atmosphere our neighbor planet. Venus Monitoring Camera (VMC) on-board the orbiter has acquired the longest and the most complete so far set of ultra violet images of Venus. These images enable a study the cloud level circulation by tracking motion of the cloud features. The highly elliptical polar orbit of Venus Express provides optimal conditions for observations of the Southern hemisphere at varying spatial resolution. Out of the 2300 orbits of Venus Express over which the images used in the study cover about 10 Venus years. Out of these, we tracked cloud features in images obtained in 127 orbits by a manual cloud tracking technique and by a digital correlation method in 576 orbits. Total number of wind vectors derived in this work is 45,600 for the manual tracking and 391,600 for the digital method. This allowed us to determine the mean circulation, its long-term and diurnal trends, orbit-to-orbit variations and periodicities. We also present the first results of tracking features in the VMC near-IR images. In low latitudes the mean zonal wind at cloud tops (67 ± 2 km following: Rossow, W.B., Del Genio, A.T., Eichler, T. [1990]. J. Atmos. Sci. 47, 2053–2084) is about 90 m/s with a maximum of about 100 m/s at 40–50°S. Poleward of 50°S the average zonal wind speed decreases with latitude. The corresponding atmospheric rotation period at cloud tops has a maximum of about 5 days at equator, decreases to approximately 3 days in middle latitudes and stays almost constant poleward from 50°S. The mean poleward meridional wind slowly increases from zero value at the equator to about 10 m/s at 50°S and then decreases to zero at the pole. The error of an individual measurement is 7.5–30 m/s. Wind speeds of 70–80 m/s were derived from near-IR images at low latitudes. The VMC observations indicate a long term trend for the zonal wind speed at low latitudes to increase from 85 m/s in the beginning of the mission to 110 m/s by the middle of 2012. VMC UV observations also showed significant short term variations of the mean flow. The velocity difference between consecutive orbits in the region of mid-latitude jet could reach 30 m/s that likely indicates vacillation of the mean flow between jet-like regime and quasi-solid body rotation at mid-latitudes. Fourier analysis revealed periodicities in the zonal circulation at low latitudes. Within the equatorial region, up to 35°S, the zonal wind show an oscillation with a period of 4.1–5 days (4.83 days on average) that is close to the super-rotation period at the equator. The wave amplitude is 4–17 m/s and decreases with latitude, a feature of the Kelvin wave. The VMC observations showed a clear diurnal signature. A minimum in the zonal speed was found close to the noon (11–14 h) and maxima in the morning (8–9 h) and in the evening (16–17 h). The meridional component peaks in the early afternoon (13–15 h) at around 50°S latitude. The minimum of the meridional component is located at low latitudes in the morning (8–11 h). The horizontal divergence of the mean cloud motions associated with the diurnal pattern suggests upwelling motions in the morning at low latitudes and downwelling flow in the afternoon in the cold collar region.

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

The dynamics of Venus’ mesosphere (60–100 km altitude) was investigated using data acquired by the radio-occultation experiment VeRa on board Venus Express. VeRa provides vertical profiles of density, temperature and pressure between 40 and 90 km of altitude with a vertical resolution of few hundred meters of both the Northern and Southern hemisphere. Pressure and temperature vertical profiles were used to derive zonal winds by applying an approximation of the Navier–Stokes equation, the cyclostrophic balance, which applies well on slowly rotating planets with fast zonal winds, like Venus and Titan. The main features of the retrieved winds are a midlatitude jet with a maximum speed up to 140 ± 15 m s −1 which extends between 20°S and 50°S latitude at 70 km altitude and a decrease of wind speed with increasing height above the jet. Cyclostrophic winds show satisfactory agreement with the cloud-tracked winds derived from the Venus Monitoring Camera (VMC/VEx) UV images, although a disagreement is observed at the equator and near the pole due to the breakdown of the cyclostrophic approximation. Knowledge of both temperature and wind fields allowed us to study the stability of the atmosphere with respect to convection and turbulence. The Richardson number Ri was evaluated from zonal field of measured temperatures and thermal winds. The atmosphere is characterised by a low value of Richardson number from ∼45 km up to ∼60 km altitude at all latitudes that corresponds to the lower and middle cloud layer indicating an almost adiabatic atmosphere. A high value of Richardson number was found in the region of the midlatitude jet indicating a highly stable atmosphere. The necessary condition for barotropic instability was verified: it is satisfied on the poleward side of the midlatitude jet, indicating the possible presence of wave instability.

The Venus Express mission will focus on a global investigation of the Venus atmosphere and plasma environment, while additionally measuring some surface properties from orbit. The instruments PFS and SPICAV inherited from the Mars Express mission and VIRTIS from Rosetta form a powerful spectrometric and spectro-imaging payload suite. Venus Monitoring Camera (VMC)—a miniature wide-angle camera with 17.5° field of view—was specifically designed and built to complement these experiments and provide imaging context for the whole mission. VMC will take images of Venus in four narrow band filters (365, 513, 965, and 1000 nm) all sharing one CCD. Spatial resolution on the cloud tops will range from 0.2 km/px at pericentre to 45 km/px at apocentre when the full Venus disc will be in the field of view. VMC will fulfill the following science goals: (1) study of the distribution and nature of the unknown UV absorber; (2) determination of the wind field at the cloud tops (70 km) by tracking the UV features; (3) thermal mapping of the surface in the 1 μm transparency “window” on the night side; (4) determination of the global wind field in the main cloud deck (50 km) by tracking near-IR features; (5) study of the lapse rate and H2O content in the lower 6–10 km; (6) mapping O2 night-glow and its variability.

The Venus Monitoring Camera (VMC) carried by European Space Agency’s Venus Express orbiter (Svedhem et al., 2007) consists of four optical units, each with a separate filter casting an image on a single CCD ( Markiewicz et al., 2007a , 2007b ). The desire to capture as much of the planet in a single frame during the spacecraft’s 24 h, 0.84 eccentricity orbit led to optics with 18° field of view. Analysis of Venus images obtained by the VMC indicated that the computed limb radius and altitude of haze layers were somewhat inconsistent with prior knowledge and expectations. Possible causes include errors in the knowledge of image geometry, misalignment of the optic axis from the pointing direction, and optical distortion. These were explored and eliminated, leaving only deviations from the ground and pre-solar damage estimate of the focal length lengths as the most likely reason. We use the location of planet’s limb to estimate the focal length of each camera using images of the planet when the orbiter was more than 20,000 km from planet center. The method relies on the limb radius to be constant at least over a small range of solar zenith angles. We were able to achieve better estimates for the focal lengths for all four cameras and also estimate small offsets to the boresight alignment. An outcome of this analysis is the finding that the slant unit optical depth varies more rapidly with solar zenith angle in the afternoon as compared to morning, with lowest values at local noon. A variation of this level is also observed with latitude. Both are indicative of the presence of overlying haze above the clouds, and the morning afternoon asymmetry suggests different photochemical processes in destruction and production of the haze.

Understanding how the solar system evolved has been one of the driving reasons for exploring the solar system until recently. Now the focus includes planets around other stars and particularly terrestrial planets and habitability. Mercury and Venus are two extreme members in our solar system but are poorly understood. Our closest planetary neighbors, with their proximity to the sun made it challenging to learn much about them from telescopes, as they are accessible for only a short time before sunrise or after sunset for large telescopes (due to scattered light and sensitive detectors). Venus with it global cloud cover made it impossible to learn about its surface, until new advances in radar and microwave techniques. Only partly surveyed by Mariner 10 from three fly-bys during 1974–1976, Mercury remained enigmatic until MESSENGER. By contrast, Venus has been explored from fly-by spacecraft, orbiters, entry probes and landers and even balloons, yet the major science questions have only become sharper. MESSENGER and Venus Express, the two current spacecraft visitors from Earth to the innermost, entered the final phase of their mission lives in summer 2014 as the fuel required for orbit maintenance was depleted. Orbiting Venus since 15 April 2006, Venus Express conducted an aerobraking experiment in June 2014. It collected its last observations on 27 November 2014 when the fuel was exhausted during orbit raise maneuvers, and the spacecraft entered the atmosphere on 18 January 2015. Over more than eight years of observing Venus from its 24 h, polar elliptic orbit, it collected a large amount of data from its operating instruments which have provided new insights into the atmosphere of Venus and to a limited extent, its surface. The MESSENGER spacecraft also observed Venus on its third fly-by of Venus which also yielded some new results.

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 (0.27nm) and spatial (40-60km/pixel) resolution HST/STIS data provide the first direct and simultaneous record of the latitude and local time distribution of Venus' 70-80km SO and SO2 (SOx) 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 SO2 and dayside SOx vertical profiles between ~75 and 100km; and (b) 0.36μm VEx/VMC images of Venus' cloud-tops. Updating the (Marcq, E. et al. [2011]. Icarus 211, 58-69) radiative transfer model SO2 gas column densities of ~2-10μm-atm and ~0.4-1.8μm-atm are retrieved from the December 2010 and January 2011 HST observations, respectively on Venus' dayside (i.e., at solar zenith angles (SZA)x gas densities. On December 28, 2010 SO2 VMR values ~280-290ppb are retrieved between 74 and 81km 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 SO2 gas abundance declined at all local times between the three HST observing dates. On all dates the average dayside SO2/SO ratio inferred from HST between 70 and 80km is higher than that inferred from the sub-mm the JCMT data above 84km confirming that SOx photolysis is more efficient at higher altitudes. The direct correlation of the SOx gases provides the first clear evidence that SOx photolysis is not the only source for Venus' 70-80km sulfur reservoir. The cloud top SO2 gas density is dependent in part on the vertical transport of the gas from the lower atmosphere; and the 0.24μm 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, H2SO4 cloud formation rate and climate evolution. Additionally, we present the first photochemical model that uniquely tracks the transition of the SO2 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 SOx 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.

Corrigendum: Equatorial jet in the lower to middle cloud layer of Venus revealed by Akatsuki

We present an overview analysis of an airplane concept for a Venus in situ exploration mission. Venus possesses a hostile environment with strong zonal winds and large scale vertical drafts. Further exploration of this mysterious, yet comparable terrestrial planet is of immense scientific interest and potential benefit for the improved understanding of Earth's future climate. Previous probes have not been able to sample the cloud top region of Venus and thus the desire for exploring the feasibility of an unmanned flying platform in the altitude range of 60–70 km is crucial. The airplane is powered by advanced solar panels installed on its top and bottom surfaces, which fully exploits the abundant sunlight available above the clouds as well as the reflected sunlight from the cloud below the flight altitude. The primary focus of this effort is the computational fluid dynamics analysis of the airplane in the atmosphere of Venus, at desired altitudes and various angles of attack. Design optimization is carried out using various wing planform such as straight and elliptic, as compared to a curved wing planform design selected for less induced drag and larger volume for inflation. The airplane sustains its weight by utilizing the ambient winds to gain lift along with the buoyancy generated lift by the helium-filled structure of the airplane. This hybrid airplane concept ensures it's round the clock operation due to the fact that even in the event of airplane mishaps or wing failure, it would lose altitude and rest on the lower, much denser clouds as a fully buoyant configuration. Constraints as well as results for this conceptual design are also presented.

We report on the implications of the observations of the glory phenomenon made recently by Venus Express orbiter. Glory is an optical phenomenon that poses stringent constraints on the cloud properties. These observations thus enable us to constrain two properties of the particles at the cloud tops (about 70 km altitude) which are responsible for a large fraction of the solar energy absorbed by Venus. Firstly we obtain a very accurate estimate of the cloud particles size to be 1.2 μm with a very narrow size distribution. We also find that for the two observations presented here the clouds are homogenous, as far as cloud particles sizes are concerned, on scale of at least 1200 km. This is in contrast to previous estimates that were either local, from entry probes data, or averaged over space and time from polarization data. Secondly we find that the refractive index for the data discussed here is higher than that of sulfuric acid previously proposed for the clouds composition (Hansen, J.E., Hovenier, J.W. [1974]. J. Atmos. Sci. 31, 1137–1160; Ragent, B. et al. [1985]. Adv. Space Res. 5, 85–115). Assuming that the species contributing to the increase of the refractive index is the same as the unknown UV absorber, we are able to constrain the list of candidates. We investigated several possibilities and argue that either small ferric chloride (FeCl 3 ) cores inside sulfuric acid particles or elemental sulfur coating their surface are good explanations of the observation. Both ferric chloride and elemental sulfur have been suggested in the past as candidates for the as yet unknown UV absorber (Krasnopolsky, V.A. [2006]. Planet. Space Sci. 54, 1352–1359; Mills, F.P. et al. [2007]. In: Esposito, L.W., Stofan, E.R., Cravens, T.E. (Eds.), Exploring Venus as a Terrestrial Planet, vol. 176. AGU Monogr. Ser., Washington, DC, pp. 73–100).

In the context of an International Space Science Institute (ISSI) working group, we have conducted a project to compare the most recent General Circulation Models (GCMs) of the Venus atmospheric circulation. A common configuration has been decided, with simple physical parametrization for the solar forcing and the boundary layer scheme.

International audience; This issue of Icarus presents papers on the planet Venus based principally on presentations at two international conferences during the summer of 2010. Under the sponsorship of the European Space Agency, the International Venus Conference (Aussois, France, 20–26 June 2010) focused on the results from the Venus Express Mission. Venus Express is expected to continue operations through December 2014 and beyond. The second conference, “Venus Our Closest Earth-like Planet: From Surface to Thermosphere – How does it work?”, was sponsored by the Venus Exploration Analysis Group (VEXAG) chartered by NASA in Madison, Wisconsin (29 August–1 September, 2010). The work presented at these conferences illustrates the resurgence in Venus research since the arrival at Venus of the European Space Agency’s Venus Express orbiter in April 2006. The issue also includes papers that were inspired by JAXA’s launch of Venus Climate Orbiter (also known as Akatsuki) in May 2010.The papers reflect the international interest in Venus and cover many different aspects of the planet, ranging from interior and surface to the upper atmosphere, with many results focusing on the coupling between different layers. Papers on surface and interior process include electromagnetic sounding of the lithosphere, models of surface atmosphere-interaction, constraints on the process of resurfacing, and links between convective processes and topography and between convection and resurfacing and outgassing. The issue contains 39 papers. Additional papers will be published in later regular issues of the Journal.The Guest Editors are pleased to acknowledge the service provided by numerous dedicated reviewers whose critical comments greatly improved the manuscripts. The Venus Express Project is supported not only by the European Space Agency directly but also by many other countries through support of the respective instrument teams and National Space Agencies. VEXAG is chartered by the National Aeronautics and Space Administration (www.lpi.usra.edu/vexag). We would like to thank Dr. Adriana Ocampo (NASA/HQ) for her consistent interest and support of VEXAG activities and Dr. Håkan Svedhem, Venus Express project Scientist for his support for the two conferences. Without the support of the Icarus Editorial Office (particularly Ms. Cheryl Hall) the Guest Editors would have had a formidable challenge in producing this issue on time.

The European Venus Explorer (EVE) mission described in this paper was proposed in December 2010 to ESA as an 'M-class' mission under the Cosmic Vision programme. It consists of a single balloon platform floating in the middle of the main convective cloud layer of Venus at an altitude of 55 km, where temperatures and pressures are benign (~25°C and ~0. 5 bar). The balloon float lifetime would be at least 10 Earth days, long enough to guarantee at least one full circumnavigation of the planet. This offers an ideal platform for the two main science goals of the mission: study of the current climate through detailed characterization of cloud-level atmosphere, and investigation of the formation and evolution of Venus, through careful measurement of noble gas isotopic abundances. These investigations would provide key data for comparative planetology of terrestrial planets in our solar system and beyond. © 2011 Springer Science+Business Media B.V.

[1] Some dynamical and morphological similarities exist between the vortex organization of the atmosphere in the northern and southern hemispheres of Venus and the tropical cyclones/hurricanes on Earth. An S-shape feature detected in the center of the vortices on Venus from Pioneer Venus Orbiter and Venus Express observations has also been seen in tropical cyclones. This feature can be simulated with an idealized nonlinear and non-divergent barotropic model and, like in the vortices on Venus and in tropical cyclones, it is found to be transient. Given the challenges in measuring the deep, atmospheric circulation of Venus, the morphological similarities provide clues toward understanding the processes involved in the maintenance of Venus' atmospheric super rotation.

[1] To date dynamical observations of the Venus clouds have delivered mainly either only short-term or long-term averaged results. With the Venus Monitoring Camera (VMC) it finally became possible to investigate the global dynamics with a relatively high resolution in space and time on a long-term basis. Our findings from manual cloud feature wind tracking in VMC UV image sequences so far show that the details of the mesospheric dynamics of Venus appear to be highly variable. Although the general rotation of the atmosphere remained relatively stable since Mariner 10, more than 30 years ago, by now, there are indications of short-term variations in the general circulation pattern of the Venus atmosphere at cloud top level. In some cases, significant variations in the zonal wind properties occur on a timescale of days. In other cases, we see rather stable conditions over one atmospheric revolution, or longer, at cloud top level. It remains an interesting question whether the irregularly observed midlatitude jets are indeed variable or simply become shielded from view by higher H2SO4 haze layers for varying time intervals. Winds at latitudes higher than 60°S are still difficult to obtain track because of low contrast and scarcity of features but increasing data is being collected. Over all, it was possible to extend latitudinal coverage of the cloud top winds with VMC observations. Thermal tides seem to be present in the data, but final confirmation still depends on synthesis of Visible and Infrared Thermal Imaging Spectrometer and VMC observations on night and dayside. Although poorly resolved, meridional wind speed measurements agree mainly with previous observations and with the presence of a Hadley cell spanning between equatorial region and about 45°S latitude.

[1] After a pause of more than two decades, Venus' atmosphere is being explored again. Since April 2006, European Space Agency's Venus Express has been acquiring data, exploiting the near-infrared windows that allow us to peer into the deep night-side atmosphere. In June 2007, NASA's MESSENGER mission will fly past Venus on its way to Mercury, collecting useful data for a few weeks. Instruments and experiments on Venus Express are expected to provide a more comprehensive set of atmospheric observations over three and perhaps more Venus days. Venus Express observations are already providing clues about the processes that maintain the rapid circulation of Venus' atmosphere. The early observations show not only that the global circulation is organized into two hemispheric vortices centered over respective poles, but also that the vortex organization extends deep into the atmosphere. The limited Galileo NIMS near-infrared observations in 1989 had revealed the deep circulation in the equatorial regions, but because of the spacecraft, flyby trajectory could not observe the high latitudes to elucidate the polar circulation and its organization. The long hiatus in systematic Venus observations has provided an opportunity to perform some new analysis of the previous Pioneer Venus observations. This paper presents a synopsis of the circulation measurements at high latitudes and an analysis of the solar thermal tides seen in the cloud motions and suggests some limitations of previous estimates of transport of angular momentum by eddies.

ESA's Venus Express probe has been in orbit since April 2006. Eight research papers in this issue present new results from the mission, covering the atmosphere, polar features, interactions with the solar wind and the controversial matter of venusian lightning. Hakan Svedham et al. open the section with a review of the similarities and (mostly) differences between Venus and its 'twin', the Earth. Andrew Ingersoll considers the latest results, and also how the project teams plan to make the most of the probe's remaining six years of life. Venus is completely covered by a thick cloud layer, with the cloud tops in fast retrograde rotation. Global and small scale properties of these clouds and their temporal and latitudinal variations are investigated, and the wind velocities are derived. The southern polar region is highly variable and can change dramatically on time scales as short as one day, perhaps arising from the injection of SO2 into the mesosphere. Venus is completely covered by a thick cloud layer, of which the upper part is composed of sulphuric acid and some unknown aerosols1. The cloud tops are in fast retrograde rotation (super-rotation), but the factors responsible for this super-rotation are unknown2. Here we report observations of Venus with the Venus Monitoring Camera3 on board the Venus Express spacecraft. We investigate both global and small-scale properties of the clouds, their temporal and latitudinal variations, and derive wind velocities. The southern polar region is highly variable and can change dramatically on timescales as short as one day, perhaps arising from the injection of SO2 into the mesosphere. The convective cells in the vicinity of the subsolar point are much smaller than previously inferred4,5,6, which we interpret as indicating that they are confined to the upper cloud layer, contrary to previous conclusions7,8, but consistent with more recent study9.

With its comprehensive suite of near-infrared instruments, Venus Express will perform the first detailed global exploration of the depths of the thick Venusian atmosphere. Through the near-daily acquisition of Visible and Infrared maps and spectra, three infraredsensing instruments—the Planetary Fourier Spectrometer (PFS), the Venus Monitoring Camera (VMC), and the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)—will comprehensively investigate the Thermal structure, meteorology, dynamics, chemistry, and stability of the deep Venus atmosphere. For the surface, these instruments will provide clues to the emissivity of surface materials and provide direct evidence of active volcanism. In so doing, ESA’s Venus Express Mission directly addresses numerous high-priority Venus science objectives advanced by America’s National Research Council (2003) decadal survey of planetary science. r 2006 Elsevier Ltd. All rights reserved.

The regional structure of Venus seen in polarization data acquired during the first 600 orbits of the Orbiter Cloud Polarimeter are described. Local organized features are seen whose morphology is similar to that of the ultraviolet clouds. No obvious correlation between the observed amount of polarization and the relative brightness is found, suggesting that the polarization features are not due to variations in the unpolarized intensity alone. Substantial variations in the structure and visibility of the polarization features suggest that the amount of haze in and above the main cloud layer is not constant. These features show movements similar to those of the ultraviolet clouds in both direction and speed.

Pioneer Venus Orbiter images obtained in 1982 indicate a marked solar-locked dependence of cloud level circulation in both averaged cloud motions and cloud layer UV reflectivity. An apparent relationship is noted between horizontal divergence and UV reflectivity: the highest reflectivities are associated with regions of convergence at high latitudes, while lower values are associated with equatorial latitude regions where the motions are divergent. In solar-locked coordinates, the rms deviation of normalized UV brightness is higher at 45-deg latitudes than in equatorial regions.

Thermal structure data obtained by Pioneer Venus (PV) were analyzed and used to make calculations concerning cyclostrophic circulation around Venus. These indicate a balanced zonal (east to west) circulation, with midlatitude jets of peak velocities in the 110-120 m/s range, located between 50 and 40 mb in each hemisphere of the planet near 45 deg latitude. The calculations indicate breakdown of the balance conditions near the upper and lower boundaries at low latitudes. A slight asymmetry in the balanced zonal circulation arises out of an asymmetry in the thermal field. The PV radio occultation data show evidence of a direct meridional circulation that may be important in sustaining the atmospheric circulation of Venus. The value of continuous radio occultation measurements is stressed for studying shortand long-term variations of the atmospheric circulation.

The first polarization measurements of the orbiter cloud photopolarimeter have detected a planet-wide layer of submicrometer aerosols of substantial visible optical thickness, of the order of 0.05 to 0.1, in the lower stratosphere well above the main visible sulfuric acid cloud layer. Early images show a number of features observed by Mariner 10 in 1974, including planetary scale markings that propagate around the planet in the retrograde sense at roughly 100 meters per second and bright- and dark-rimmed cells suggesting convective activity at low latitudes. The polar regions are covered by bright clouds down to latitudes aproximately 50 degrees, with both caps significantly brighter (relative to low latitudes) than the south polar cloud observed by Mariner 10. The cellular features, often organized into clusters with large horizontal scale, exist also at mid-latitudes, and include at least one case in which a cell cuts across the edge of the bright polar cloud of the northern hemisphere.

Current knowledge of the temperature structure of the atmosphere of Venus is briefly summarized. The principal features to be explained are the high surface temperature, the small horizontal temperature contrasts near the cloud tops in the presence of strong apparent motions, and the low value of the exospheric temperature. In order to understand the role of radiative and dynamical processes in maintaining the thermal balance of the atmosphere, a great deal of additional data on the global temperature structure, solar and thermal radiation fields, structure and optical properties of the clouds, and circulation of the atmosphere are needed. The ability of the Pioneer Venus Orbiter and Multiprobe Missions to provide these data is indicated.

From a critical comparison and synthesis of data from the four Pioneer Venus Probes, the Pioneer Venus Orbiter, and the Venera 10, 12, and 13 landers, models of the lower and middle atmosphere of Venus are derived. The models are consistent with the data sets within the measurement uncertainties and established variability of the atmosphere. The models represent the observed variations of state properties with latitude, and preserve the observed static stability. The rationale and the approach used to derive the models are discussed, and the remaining uncertainties are estimated.

With the possible exception of the lowest one or two scale heights, the dominant mode of circulation of Venus' atmosphere is a rapid, zonal, retrograde motion. Global albedo variations in the ultraviolet may reflect planetary scale waves propagating relative to the zonal winds. Other special phenomena such as cellular convection in the subsolar region and internal gravity waves generated in the interaction of the zonal circulation with the subsolar disturbance may also be revealed in ultraviolet imagery of the atmosphere. We discuss the contributions of experiments on the Orbiter and Entry Probes of Pioneer Venus toward unravelling the mystery of the planet's global circulation and the role played by waves, instabilities and convection therein

Significant mean cloud level circulation changes since 1974, noted in 1982 Venus cloud motion observations, have been validated by independent measurements of cloud motions in nearly-identical sets of images; agreement is obtained not only for the average zonal and meridional components, but for the eddy circulation's meridional transport of momentum. In contrast to 1979 observations, the time latitudinal profile and the longitudinally-averaged zonal component of the cloud motions for 1982 exhibit jets near 45 deg latitude in both the northern and southern hemispheres.

Results on cloud motions on Venus obtained over a period of 3.5 days from Mariner 10 television images are presented. The implied atmosphere flow is almost zonal everywhere on the visible disk, and is in the same retrograde sense as the solid planet. Objective analysis of motions suggests the presence of jet cores (-130 m/s) and organized atmospheric waves. The longitudinal mean meridional profile of the zonal component of motion of the ultraviolet features shows presence of a midlatitude jet stream (-110 m/s). The mean zonal component is -97 m/s at the equator. The mean meridional motion at most latitudes is directed toward the pole in either hemisphere and is at least an order of magnitude smaller so that the flow is nearly zonal. A tentative conclusion from the limited coverage available from Mariner 10 is that at the level of ultraviolet features mean meridional circulation is the dominant mode of poleward angular momentum transfer as opposed to the eddy circulation.

Independent observations are presented of the zonal mean circulation deduced from the Pioneer Venus Orbiter images which substantiate the somewhat different circulation in the Venus atmosphere at the time of Pioneer observations. The most reassuring result from the present investigation is that measurements made by different individuals on the same set of images yield virtually the same zonal mean statistics for the cloud top circulation of Venus. The mean zonal component is about -95 m/sec at the equator, corresponding to a rotational period of about 4.8 days. The period of rotation increases to about 4.2 days for latitude regions 35-45 deg south and 25-40 deg north. The mean meridional motion is toward the northern pole north of 5 deg south latitude and toward the southern pole south of this latitude.