Evdokimova, Daria Gennadyevna

Evdokimova D., Fedorova A., Zharikova M., Montmessin F., Korablev O., Soret L., Gorinov D., Belyaev D., Bertaux J.-.

Evdokimova D., Fedorova A., Belyaev D., Montmessin F., Korablev O., Bertaux J.

<p><strong>Introduction</strong></p> <p>Infrared O<sub>2</sub> (α<sup>1</sup>Δ<sub>g</sub>) airglow at 1.27 μm on the night side of Venus was for the first time identified during ground-based observations in 1975 (Connes et al., 1979). The airglow reaches its maximal intensity at ~96 km. These altitudes correspond to the transitional region between two regimes of global atmospheric circulation on Venus. Below 70 km, the cloud layer is involved in the zonal super-rotation. At altitudes higher than 110 km, the subsolar to anti-solar (SSAS) circulation transfers atoms and ions produced by photolysis in the sunlit hemisphere to the night side. Here, dowelling oxygen atoms recombine to the exited O<sub>2</sub> (α<sup>1</sup>Δ<sub>g</sub>) molecules which radiative relaxation to the ground state results in the IR emission formation. Thus, the O<sub>2</sub> (α<sup>1</sup>Δ<sub>g</sub>) airglow is a tracer of the dynamical processes occurring in the 90-100 km range on the night side.</p> <p>The maximal emission brightness was observed around the anti-solar point by ground-based and orbital measurements; this result demonstrated a domination of the SSAS circulation in the 90-100 km range. The VIRTIS-M infrared spectrometer on board the Venus Express spacecraft studied in detail the morphological features of the emission in 2006-2009 (Gérard et al., 2008; Piccioni et al., 2009; Shakun et al., 2010; Soret et al., 2012). Gérard et al. (2008) and Piccioni et al. (2009) concluded that intensity of the anti-solar emission maximum is ​​equal to 3 МR and 1.2 МR respectively. The work of Shakun et al. (2010) revealed a slight shift of the nightglow's statistical maximum towards the evening terminator and a latitude of ~10° N. However, a simultaneous independent analysis of VIRTIS-M limb and nadir observations (Soret et al., 2012) confirmed the previous conclusions. </p> <p>Analysis of the SPICAV IR observations contributes to the O<sub>2</sub> (α<sup>1</sup>Δ<sub>g</sub>) airglow study. The instrument dataset extends the long-term and latitudinal coverage of the VIRTIS-M experiment, which poorly observed the Northern Hemisphere of Venus at night. </p> <p><strong>Data analysis</strong></p> <p>The SPICAV IR instrument (0.65-1.7 µm) accumulated a dataset encompassing almost the entire Venus globe by nadir night observations in 2006-2014. The spatial resolution changes in range of 50-1000 km depending on the spacecraft distance to the planet due to the orbit elongation (Korablev et al., 2012). The SPICAV IR spectral range also covers several transparency windows where the thermal emission originating from the Venus deep atmosphere and surface escapes to space. The transparency window at 1.28 μm overlaps the O<sub>2</sub> (α<sup>1</sup>Δ<sub>g</sub>) emission band at 1.27 μm. However, the high resolving power of the spectrometer (~1400) allows a robust algorithm to extract the oxygen emission spectrum. For each measurement Venus thermal emission is optimized by a 1-D radiative transfer model with multiple scattering. The direct model is computed by the SHDOMPP program solving the radiative transfer equation by the method of discrete ordinates and spherical harmonics in a plane-parallel atmosphere. This routine developed by Bézard et al. (2011) and Fedorova et al. (2015) is used in this study with a cloud layer model of Haus et al. (2016). The thermal emission model is computed for three atmospheric windows at 1.1, 1.18 and 1.28 μm to increase the accuracy, and it is set by 3 free parameters: a scaling factor applied to mode 2 and 3 particle distributions of the cloud layer model, the H<sub>2</sub>O mixing ratio in the lower atmosphere of Venus and the surface emissivity.</p> <p><strong>Result</strong></p> <p>In total, 605 sessions of nadir observations (~6000 spectra) with chosen emission angle ≤2° were analysed. Based on these observations, the local time and latitude distribution of the O2 (α<sup>1</sup>Δ<sub>g</sub>) airglow in the night hemisphere was obtained. It has the maximum at the anti-solar point with the intensity value of ~2 MR. An emission tendency to be slightly shifted towards the morning terminator can be suggested. In general, the pattern is fairly symmetrical about the equator. The result is in correspondence with the analysis of VIRTIS data (Shakun et al., 2010; Soret et al., 2012).</p> <p><strong>References</strong></p> <p>Bézard, B., et al., 2011. The 1.10-and 1.18-μm nightside windows of Venus observed by SPICAV-IR aboard Venus Express. Icarus, 216(1), 173- 183.</p> <p>Connes, P. et al., 1979. O<sub>2</sub>(1Δ) emission in the day and night airglow of Venus. The Astrophysical Journal, 233, L29-L32.</p> <p>Fedorova, A., et al., 2015. The CO<sub>2</sub> continuum absorption in the 1.10-and 1.18-μm windows on Venus from Maxwell Montes transits by SPICAV IR onboard Venus express. Planetary and Space Science, 113, 66-77.</p> <p>Gérard, J. C., et al., 2008. Distribution of the O<sub>2</sub> infrared nightglow observed with VIRTIS on board Venus Express. Geophysical research letters, 35(2).</p> <p>Haus, R., et al., 2016. Radiative energy balance of Venus based on improved models of the middle and lower atmosphere. Icarus, 272, 178-205.</p> <p>Piccioni, G., et al., 2009. Near-IR oxygen nightglow observed by VIRTIS in the Venus upper atmosphere. J. Geophys. Res. – Planets, 114.</p> <p>Shakun, A. V., et al., 2010. Investigation of oxygen O<sub>2</sub> (a<sup>1</sup>Δ<sub>g</sub>) emission on the nightside of Venus: Nadir data of the VIRTIS-M experiment of the Venus Express mission. Cosmic Research, 48(3), 232-239.</p> <p>Soret, L., et al., 2012. Atomic oxygen on the Venus nightside: Global distribution deduced from airglow mapping. Icarus, 217(2), 849-855.</p>

Karpovich E., Kombaev T., Gueraiche D., Evdokimova D., Alexandrov K.

The paper presents specifications for the Long-Endurance Mars Exploration Flying Vehicle (LEMFEV), which will be used as future design input data. The specifications are based on the analysis of previous Mars missions and scientific data collected by the operating Martian probes. The design specifications include the requirements related to the airplane’s delivery to the Martian surface; the requirements related to the Martian conditions (atmosphere and climate); and the requirements related to the scientific payload parameters and the mission flight profile.

Evdokimova D., Belyaev D., Montmessin F., Korablev O., Bertaux J.‐., Verdier L., Lefèvre F., Marcq E.

The nighttime ozone and sulfur dioxide distributions were analyzed using the entire Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus (SPICAV) UV/Venus Express stellar occultation data set. After the discovery of an ozone layer at 100 km in the mesosphere reported by Montmessin, Bertaux, et al. (2011, https://doi.org/10.1016/j.icarus.2011.08.010), 132 other detections were made during the entire 8 years long observing period of the SPICAV UV instrument. In the rare detections, the peak abundances of O3 accumulating in the mesosphere are observed with densities from 107 to 108 molecules⋅cm−3 at 85–110 km. The ozone layer is estimated to vary from 1 to 30 ppbv at 85–95 km while at 95–105 km the volume mixing ratio is expected within an interval from 6 to 120 ppbv. Below 93 km, a puzzling decrease of mixing ratio is observed towards midnight at 30°N. Our work also provides an improved sequel to the analysis of the sulfur dioxide survey previously made in the upper mesosphere by Belyaev, Evdokimova, et al. (2017, https://doi.org/10.1016/j.icarus.2017.05.002). On average, the SO2 content is found to remain constant throughout the vertical profile at a value of around 135 ± 21 ppbv between 85 and 100 km. Rapid and large variations prevent to conclude firmly on any time or space pattern of SO2.

Pinto J.P., Li J., Mills F.P., Marcq E., Evdokimova D., Belyaev D., Yung Y.L.

The abundance of SO dimers (SO)2 in the upper atmosphere of Venus and their implications for the enigmatic ultraviolet absorption has been investigated in several studies over the past few years. However, the photochemistry of sulfur species in the upper atmosphere of Venus is still not well understood and the identity of the missing ultraviolet absorber(s) remains unknown. Here we update an existing photochemical model of Venus’ upper atmosphere by including the photochemistry of SO dimers. Although the spectral absorption profile of SO dimers fits the unknown absorber, their abundance is found to be too low for them to contribute significantly to the absorption. It is more likely that their photolysis and/or reaction products could contribute more substantively. Reactions of SO dimers are found to be important sources of S2O, and possibly higher order SnO species and polysulfur, Sn. All of these species absorb in the critical ultraviolet region and are expected to be found in both the aerosol and gas phase. indicating that in-situ high resolution aerosol mass spectrometry might be a useful technique for identifying the ultraviolet absorber on Venus. Photochemistry of sulfur species in the upper Venus atmosphere is not well understood and the identity of ultraviolet (UV) absorber(s) remain unknown. Here, the authors show that sulfur monoxide dimer chemistry is a possible source of polysulfur, which could be responsible for the UV absorption.

Evdokimova D., Belyaev D., Montmessin F., Bertaux J., Korablev O.

Stellar occultation is a powerful method to study vertical structure of the Venus night mesosphere. The UV channel of SPICAV spectrometer, operated in 2006–2014 on board ESA’s Venus Express orbiter, allowed retrieval profiles of atmospheric gases (CO 2 , SO 2 , and O 3 ) and aerosols. It was also able to register different UV emissions around Venus (nitric oxide airglow, Lyman-α) overlapping the absorption features at 120–300 ​nm. Several calibration steps convert the raw data to atmospheric transmission spectra used for the retrievals. The systematic errors of resulted gaseous concentrations mainly relate to: (i) an uncertainty of the wavelength to pixel assignment; (ii) a portion of emitting light contaminating the analyzed transmission spectra. In the present paper, we have tested a new method of the wavelength-to-pixel assignment based on the spectral features of measured stars. Secondly, using imaging capabilities of the instrument, we have demonstrated an accurate separation between different kinds of registered signal: extended UV nightglow, light from a point star, transmitted through the atmosphere, and, sometimes, solar light, scattered by Venus dusk. The efficiency of two approaches performing the separation was studied. As a result, corrected transmission spectra provided retrievals of gaseous concentrations with 20–40% higher precision respectively to those processed in previous SPICAV stellar occultation studies (Montmessin et al., 2011, Icarus 216, 82; Piccialli et al., 2015, Planet. Space Sci. 113–114, 321; Belyaev et al., 2017, Icarus 294, 58). • Systematic errors in retrievals of the SPICAV UV stellar occultation data were analyzed. • Errors relate to a wavelength to pixel assignment and an overlap of point and extended UV sources spectra. • A new method of the wavelength assignment is based on the stellar spectral features. • Two methods separating spectra of a star and extended light sources were studied. • Corrected spectra provide a 20–40% higher precision of stellar occultation retrievals.

Vandaele A.C., Korablev O., Belyaev D., Chamberlain S., Evdokimova D., Encrenaz T., Esposito L., Jessup K.L., Lefèvre F., Limaye S., Mahieux A., Marcq E., Mills F.P., Montmessin F., Parkinson C.D., et. al.

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 latitudes The 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’.

Vandaele A.C., Korablev O., Belyaev D., Chamberlain S., Evdokimova D., Encrenaz T., Esposito L., Jessup K.L., Lefèvre F., Limaye S., Mahieux A., Marcq E., Mills F.P., Montmessin F., Parkinson C.D., et. al.

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.

Belyaev D.A., Evdokimova D.G., Montmessin F., Bertaux J.-., Korablev O.I., Fedorova A.A., Marcq E., Soret L., Luginin M.S.

In this paper we present the first night side distribution of SO 2 content in Venus’ upper mesosphere (altitudes from 85 to 105 km). The dataset is based on the SPICAV UV stellar occultation experiment which took place onboard ESA's Venus Express (VEX) orbiter in 2006-2014. The UV channel of SPICAV spectrometer detected absorption bands of SO 2 and CO 2 in the spectral range 180-300 nm with a resolution of 1-2 nm while stellar light was occulted by the mesosphere. Altitude profiles of sulfur dioxide's volume mixing ratio (VMR) could be retrieved in the upper part of the mesosphere covering the whole night side on Venus. In parallel, we have reprocessed the terminator UV solar occultations dataset (Belyaev et al. (2012). Icarus 217, 740-751) in the same altitude range and extended its statistics until 2014. On average the SO 2 VMR increases with altitude from 10-30 ppb at 85 km to 100-300 ppb at 100 km in both regimes of occultation. The midnight SO 2 abundance appears to be 3-4 times higher than in the terminator region: 150-200 ppbv versus 50 pppv at altitude around 95 km. These new results were compared with the distribution of oxygen atoms, which are tracers of the global subsolar-antisolar (SS-AS) circulation at ∼100 km (the data provided by Soret et al. (2012). Icarus, 217, 849–855). The night time behavior looks similar for SO 2 molecules and O atoms with a correlation coefficient Rcorr = 0.73. Moreover, the retrieved SO 2 enrichment above 85 km appears to correlate with the density of H 2 SO 4 droplets (Luginin et al., 2016; Icarus 277, 154–170).

Xue S., Broerman M.J., Goobie G.C., Kass D.J., Fabisiak J.P., Wenzel S.E., Nouraie S.M.

Encrenaz T., Sicardy B., Roques F., Coustenis A.

The observation of Earth-based stellar occultations by solar system planets and satellites has been used for decades to retrieve information on the physical properties of their atmospheres. From the variations of the stellar flux during ingress and egress and, in some favourable cases, from the central flash, one can infer the vertical density, pressure and temperature profiles around the half-light level (typically in the range of a few μbars), as well as zonal wind regimes and the presence of hazes. Earth-based occultations have been successfully applied to all planets and satellites surrounded by an atmosphere, and have delivered unique and significant information that are often complementary to the results obtained by planetary space missions. The great improvement of the stellar catalogues provided by the Gaia astrometric space mission has drastically enlarged the capabilities of the stellar occultation method, which appears especially promising for probing the tenuous atmospheres of distant objects of the solar system. This article is part of the theme issue ‘Major advances in planetary sciences thanks to stellar occultations’.

Egan J.V., Feng W., James A.D., Manners J., Marsh D.R., Lebonnois S., Lefèvre F., Stolzenbach A., Plane J.M.

AbstractIt has been proposed that two isomers of the SO dimer (cis‐ and trans‐OSSO) are candidates for the unknown UV absorber in Venus' atmosphere because they have a good spectral match with the absorber, despite the low concentrations predicted by 1D photochemical models. Here OSSO chemistry (production from SO and loss by photolysis, thermal decomposition, and reaction with O and Cl) has been included in the photochemistry scheme of a 3D planetary climate model (PCM‐Venus) along with sulfur injection due to meteoric ablation. 1D multiple scattering radiative transfer modeling is then used to predict the resulting top‐of‐the‐atmosphere reflectance produced by OSSO. The modeled OSSO concentrations are shown to be ∼3 orders of magnitude too low to explain the observed absorbance levels, and the predicted ratio of the OSSO isomers provides an unsatisfactory match to the spectral shape of the unknown absorber.

Navarrete-Miguel M., Cuéllar-Zuquin J., Carmona-García J., Abdelgawwad A.M., Soriano-Díaz I., Roldao J.C., Halder D., Borrego-Sánchez A., Francés-Monerris A., Giussani A., Segarra-Martí J., Roca-Sanjuán D.

Computational photochemistry provides a description of light-induced chemical phenomena reaching the molecular size-scale and femtosecond time-scale resolutions. In the period 2022–2023, we can find a significant increase in the use of tools of computational photochemistry in materials science, as compared to previous years, maintaining the relative number of works in the areas of biology, medicine, nanotechnology and atmospheric chemistry. To illustrate such advances in this field, we have chosen here representative applied studies focused on the non-radiative decay paths of DNA nucleobases, the photoreductive repair of thymine dimers, photosensitisers generating singlet oxygen and oxygen-independent photoactivated therapies, conjugated organic oligomers of interest in optoelectronic devices, ionic transition metal complexes for light emitting electrochemical cells, and sulphur chemistry in planetary atmospheres. On this occasion, we also describe the new features implemented in one of the quantum-chemistry packages of software specialised in photochemistry, the OpenMolcas program.

Trabelsi T., Francisco J.S.

Abstract The atmosphere of Venus exhibits absorption in the 300–500 nm wavelength range, which is driven by unknown chemical processes. In our study, we explore electronic transitions in molecules that may exist in the Venusian atmosphere, specifically focusing on the photoabsorption cross sections and the lowest singlet and triplet electronic states of the OCS2, SSCO, and OSCS isomers using highly accurate ab initio methods. Our analysis suggests that the SSCO isomer is a strong candidate for explaining the unknown UV absorption. Furthermore, these isomers may serve as significant astrochemical reservoirs in the atmosphere of Venus, where photodissociation could produce atomic sulfur in both its ground and excited states along with OCS and CS2, offering a plausible mechanism for the sulfur cycle dynamics and the formation of S x species. This study provides valuable insights into the complex sulfur chemistry within the atmosphere of Venus.

Wu G., Tong Y., Zhang J., Zhao Z.

Conan L., Marcq E., Lustrement B., Rouanet N., Parc L., Bertran S., Robert S., Helbert J., Alemanno G.

Cohen M., Holmes J., Lewis S., Patel M.

Abstract The deep atmosphere of Venus remains mysterious because of the planet’s high, optically thick cloud decks. While phenomena such as the observed decadal fluctuations in sulfur dioxide abundance above the clouds could shed light on conditions below, poor understanding of vertical and horizontal transport limits such an approach. Nightside spectral windows permit observation of trace gas species in the lower atmosphere, but incomplete understanding of the circulation makes the distribution of these species challenging to interpret. We performed two simulations with the Venus Planetary Climate Model including an age of air calculation to investigate tracer transport (a) between the surface and the stagnant lower haze layer and (b) between the cloud deck and the observable upper atmosphere. We find a timescale on the order of many decades for surface-to-lower haze layer transport and ∼1.4 yr from the lowest cloud deck to 101 km. The extreme slowness of transport from the surface to the clouds makes it unlikely that compositional variability at the surface could affect the upper atmosphere sulfur dioxide abundance on observed timescales. Planetary-scale Rossby waves with a zonal wavenumber of 1 in both hemispheres are found to circumnavigate the planet in the deep atmosphere in 36 Earth days. These waves are associated with gyres that collect tracers and areas of upwelling that transport them to higher altitudes, leading to significantly younger air at polar latitudes in the altitude range of 25–45 km. The existence of chemically enhanced traveling Rossby gyres could explain the observed deep atmosphere carbon monoxide variability.

Encrenaz T., Greathouse T.K., Giles R., Widemann T., Bezard B., Lefèvre F., Lefèvre M., Shao W., Sagawa H., Marcq E., Arredondo A.

Aims. Following several reports announcing the detection or non-detection of minor species above the clouds of Venus, we have searched for other possible signatures of PH3, HCN, and NH3 in the infrared range. Methods. Since 2012, we have performed ground-based observations of Venus in the thermal infrared at various wavelengths to monitor the behavior of SO2 and H2O at the cloud top. We have identified spectral intervals where transitions of PH3 (around 955 cm−1), HCN (around 747 cm−1), and NH3 (around 951cm−1) are present. Results. From the absence of any feature at these frequencies, we derive, on the disk-integrated spectrum, a 3-σ upper limit of 3 ppbv for the PH3 mixing ratio, 0.5 ppbv for HCN, and 0.3 ppbv for NH3, assuming that these species have a constant mixing ratio throughout the atmosphere. Maps of the Venus disk recorded at the center position of the lines show that there is no evidence for local detection anywhere over the Venus disk. Conclusions. Our results bring new constraints on the maximum abundance of these species at the cloud top and in the lower mesosphere of Venus.

Egorov O., Rey M.

Dai L., Shao W., Sheng Z.

Atmospheric chemistry plays a crucial role in the evolution of climate habitability on Venus. It has been widely explored by chemistry-transport models, but some characteristics are still poorly interpreted. This study is devoted to developing an open-access chemistry-transport model spanning both the middle and lower atmospheres of Venus. It provides a scheme for the structure of the chemistry, especially for the sulfur and oxygen, and investigates the influence of the cloud diffusivity and the SO2 dissolution that are adopted in the clouds. The developed model is based on the VULCAN framework and was updated with the state-of-the-art Venusian atmospheric chemistry. It includes vertical eddy diffusion retrieved recently with the Venus Express observations, and it resolves radiative transfer containing gas absorption and scattering, Mie scattering of the cloud droplets, and absorption of the unknown UV absorber. The obtained abundance profiles of SO, SO2, CO, COS, O, O2, O3, HCl, and NO are in overall agreement with the observations. The results show that the increase in cloud diffusivity has slight effects on the chemical structure. The SO2 mainly dissolves in 50–90 km and evaporates below the clouds. The rapid dissolution-release cycle is responsible for the large upward flux of SO2 at 58 km. At around 70 km, SO has a significant peak that is larger than that of previous studies by an order of magnitude, and S and SO2 also show slight increases. They are attributed to the buffering effects of liquid SO2 in the clouds. O2 is significantly eliminated by SO in this layer. We emphasize the superior regulation of the sulfur cycle on O2 at 70 km and its potential contributions to the long-standing problem of the overestimated O2 abundance.

Mahieux A., Viscardy S., Yelle R.V., Karyu H., Chamberlain S., Robert S., Piccialli A., Trompet L., Erwin J.T., Ubukata S., Nakagawa H., Koyama S., Maggiolo R., Pereira N., Cessateur G., et. al.

This study analyzes H 2 O and HDO vertical profiles in the Venus mesosphere using Venus Express/Solar Occultation in the InfraRed data. The findings show increasing H 2 O and HDO volume mixing ratios with altitude, with the D/H ratio rising significantly from 0.025 at ~70 km to 0.24 at ~108 km. This indicates an increase from 162 to 1,519 times the Earth’s ratio within 40 km. The study explores two hypotheses for these results: isotopic fractionation from photolysis of H 2 O over HDO or from phase change processes. The latter, involving condensation and evaporation of sulfuric acid aerosols, as suggested by previous authors [X. Zhang et al. , Nat. Geosci. 3, 834–837 (2010)], aligns more closely with the rapid changes observed. Vertical transport computations for H 2 O, HDO, and aerosols show water vapor downwelling and aerosols upwelling. We propose a mechanism where aerosols form in the lower mesosphere due to temperatures below the water condensation threshold, leading to deuterium-enriched aerosols. These aerosols ascend, evaporate at higher temperatures, and release more HDO than H 2 O, which are then transported downward. Moreover, this cycle may explain the SO 2 increase in the upper mesosphere observed above 80 km. The study highlights two crucial implications. First, altitude variation is critical to determining the Venus deuterium and hydrogen reservoirs. Second, the altitude-dependent increase of the D/H ratio affects H and D escape rates. The photolysis of H 2 O and HDO at higher altitudes releases more D, influencing long-term D/H evolution. These findings suggest that evolutionary models should incorporate altitude-dependent processes for accurate D/H fractionation predictions.

Shao W.D., Mendonça J.M., Dai L.

AbstractThe clouds have a great impact on Venus's energy budget and climate evolution, but its three‐dimensional structure is still not well understood. Here we incorporate a simple Venus cloud physics scheme into a flexible GCM to investigate the three‐dimensional cloud spatial variability. Our simulations show good agreement with observations in terms of the vertical profiles of clouds and H2SO4 vapor. H2O vapor is overestimated above the clouds due to efficient transport in the cloud region. The cloud top decreases as latitude increases, qualitatively consistent with Venus Express observations. The underlying mechanism is the combination of H2SO4 chemical production and meridional circulation. The mixing ratios of H2SO4 at 50–60 km and H2O vapors in the main cloud deck basically exhibit maxima around the equator, due to the effect of temperature's control on the saturation vapor mixing ratios of the two species. The cloud mass distribution is subject to both H2SO4 chemical production and dynamical transport and shows a pattern that peaks around the equator in the upper cloud while peaks at mid‐high latitudes in the middle cloud. At low latitudes, H2SO4 and H2O vapors, cloud mass loading and acidity show semidiurnal variations at different altitude ranges, which can be validated against future missions. Our model emphasizes the complexity of the Venus climate system and the great need for more observations and simulations to unravel its spatial variability and underlying atmospheric and/or geological processes.

Trabelsi T., Lipson J., Francisco J.S.

The electronic structure and spectroscopy of ClSOx (x = 1 and 2) isomers were investigated using coupled cluster theory and multireference interaction methods. In this study, the equilibrium geometry and harmonic vibrational frequencies of these isomers in their ground electronic state were shown. Our analysis of the vertical excitation energy and potential energy surface showed the photochemical instability of ClSO for wavelengths below 280 nm. Furthermore, the photodissociation of ClSO was unlikely to cause the formation of diatomic ClS. At the same time, ClSO could form atomic chlorine and SO as a result of photodissociation through the repulsive states. In the case of ClSO2, a novel weakly bound Cl–SO2 isomer was identified, indicating the potential influences on the chlorine and SO2 reactions. The potential energy surface of the most stable ClSO2 isomer also indicated the potential production of SO2 in both its ground and excited states. In addition, the electronic spectrum of ClSO2 was predicted to be broad, with numerous significant peaks in the near-UV‒Vis range. Valuable new insights into the chemical role of chlorine and sulfur in Venus's atmosphere were provided, along with a discussion of a potential mechanism contributing to the H2O and SO2 depletion in Venus's atmosphere.

Lefèvre M., Lefèvre F., Marcq E., Määttänen A., Stolzenbach A., Streel N.

AbstractThe Venusian atmosphere hosts a 10 km deep convective layer that has been studied by various spacecrafts. However, the impact of the strong vertical mixing on the chemistry of this region is still unknown. This study presents the first realistic coupling between resolved small‐scale turbulence and a chemical network. The resulting vertical mixing is different for each species: those with longer chemical timescales will tend to be well‐mixed. Vertical eddy diffusion due to resolved convection motions was estimated, ranging from 102 to 104 m2/s for the 48–55 km convective layer, several orders of magnitude above the typically used value. In the 48–55 km convective layer, the impact of the small‐scale turbulence on the cloud layer boundaries was between 200 m and 1 km. The impact of turbulence on cloud chemistry is consistent with Venus Express/Visible and Infrared Thermal Imaging Spectrometer observations. The observability at the cloud‐top of small‐scale turbulence by VenSpec‐U spectrometer would be challenging.

Shakun A.V., Zasova L.V., Gorinov D.A., Khatuntsev I.V., Ignatiev N.I., Patsaeva M.V., Turin A.V.

This research studies the O2 (a1Δg) nightglow distribution in 1.27 μm to understand the dynamics of the atmosphere of Venus. Several factors were considered in the retrieval process, such as thermal emission of the lower atmosphere, reflection by the clouds. Results show deviation from SS-AS circulation mode: the area where horizontal flows from the dayside converge and where oxygen recombines and emits shifts from the midnight to 22–23 hours local time. This shift is caused by solar-induced thermal tide on Venus nightside. Some conclusions about the upper mesosphere dynamics are also presented.

Ayele W., Maldonado V.

This paper presents the technical barriers and an analysis to advance the conceptual development of novel robotic ground-aerial vehicles (RGAVs) for exploration missions to Mars prior to human arrival and the establishment of a base. The concept for RGAVs for Mars planetary exploration is novel, and will require innovations that are at various stages of development or use by the aerospace community. The RGAV concept will utilize inflatable wing technology, which increases the flexibility of the wing, and thus the possibility of structural dynamic instabilities that must be studied in the context of the Martian atmosphere. An aeroelastic model for wing bending is proposed, which considers wind gusts where the change in wind direction is up to ±6° from the mean, and a turbulence intensity of up to 20%. Their effect on the bending displacement of a semi-elastic wing is quantified, resulting in a maximum wing tip displacement of 16.2 cm. Low-fidelity computational aerodynamic analysis is performed using OpenVSP (3.31.1, NASA, Washington, DC, USA) to compute mean aerodynamic loads during cruise conditions at a cruise Mach number of 0.70. Finally, a non-linear adaptive control system is proposed for the longitudinal aerial dynamics and a proportional integral derivative (PID) controller is outlined for the ground roving lateral dynamics.

Martinez A., Lebonnois S., Millour E., Pierron T., Moisan E., Gilli G., Lefèvre F.

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

Khatuntsev I.V., Patsaeva M.V., Titov D.V., Zasova L.V., Ignatiev N.I., Gorinov D.A.

We present joint analysis of the UV (365 nm) images captured by the cameras on board ESA’s Venus Express and JAXA’s Akatsuki spacecraft. These observations enabled almost continuous characterization of the cloud top circulation over the longest period of time so far (2006–2021). More than 46,000 wind vectors were derived from tracking the UV cloud features and revealed changes in the atmospheric circulation with the period of 12.5 ± 0.5 years. The zonal wind component is characterized by an annual mean of −98.6 ± 1.3 m/s and an amplitude of 10.0 ± 1.6 m/s. The mean meridional wind velocity is −2.3 ± 0.2 m/s and has an amplitude of 3.4 ± 0.3 m/s. Plausible physical explanations of the periodicity include both internal processes and external forcing. Both missions observed periodical changes in the UV albedo correlated with the circulation variability. This could result in acceleration or deceleration of the winds due to modulation of the deposition of the radiative energy in the clouds. The circulation can be also affected by the solar cycle that has a period of approximately 11 years with a large degree of deviation from the mean. The solar cycle correlated with the wind observations can probably influence both the radiative balance and chemistry of the mesosphere. The discovered periodicity in the cloud top circulation of Venus, and especially its similarity with the solar cycle, is strongly relevant to the study of exoplanets in systems with variable “suns”.

Woods T.N., Harder J.W., Kopp G., Snow M.

The Solar Radiation and Climate Experiment (SORCE) was a NASA mission that operated from 2003 to 2020 to provide key climate-monitoring measurements of total solar irradiance (TSI) and solar spectral irradiance (SSI). This 17-year mission made TSI and SSI observations during the declining phase of Solar Cycle 23, during all of Solar Cycle 24, and at the very beginning of Solar Cycle 25. The SORCE solar-variability results include comparisons of the solar irradiance observed during Solar Cycles 23 and 24 and the solar-cycle minima levels in 2008 – 2009 and 2019 – 2020. The differences between these two minima are very small and are not significantly above the estimate of instrument stability over the 11-year period. There are differences in the SSI variability for Solar Cycles 23 and 24, notably for wavelengths longer than 250 nm. Consistency comparisons with SORCE variability on solar-rotation timescales and solar-irradiance model predictions suggest that the SORCE Solar Cycle 24 SSI results might be more accurate than the SORCE Solar Cycle 23 results. The SORCE solar-variability results have been useful for many Sun–climate studies and will continue to serve as a reference for comparisons with future missions studying solar variability.

Takagi M., Ando H., Sugimoto N., Matsuda Y.

TAYLOR F.W., CRISP D., BEZARD B.

LELLOUCH E., CLANCY T., CRISP D., KLIORE A.J., TITOV D., BOUGHER S.W.

ESPOSITO L.W., KNOLLENBERG R.G., MAROV M.Y., TOON O.B., TURCO R.P.

Navarro T., Gilli G., Schubert G., Lebonnois S., Lefèvre F., Quirino D.

A numerical simulation of the upper atmosphere of Venus is carried out with an improved version of the Institut Pierre-Simon Laplace (IPSL) full-physics Venus General Circulation Model (GCM). This simulation reveals the organization of the atmospheric circulation at an altitude above 80 km in unprecedented detail. Converging flow towards the antisolar point results in supersonic wind speeds and generates a shock-like feature past the terminator at altitudes above 110 km. This shock-like feature greatly decreases nightside thermospheric wind speeds, favoring atmospheric variability on a hourly timescale in the nightside of the thermosphere. A ∼ 5-day period Kelvin wave originating in the cloud deck is found to substantially impact the Venusian upper atmosphere circulation. As the Kelvin wave impacts the nightside, the poleward meridional circulation is enhanced. Consequently, recombined molecular oxygen is periodically ejected to high latitudes, explaining the characteristics of the various observations of oxygen nightglow at 1 . 27 μ m . An analysis of the simulated 1 . 27 μ m oxygen nightglow shows that it is not necessarily a good tracer of the upper atmospheric dynamics, since contributions from chemical processes and vertical transport often prevail over horizontal transport. Moreover, dayside atomic oxygen abundances also vary periodically as the Kelvin wave momentarily decreases horizontal wind speeds and enhances atomic oxygen abundances, explaining the observations of EUV oxygen dayglow. Despite the nitrogen chemistry not being currently included in the IPSL Venus GCM, the apparent maximum NO nightglow shifted towards the morning terminator might be explained by the simulated structure of winds. • An improved numerical simulation of the upper atmosphere of Venus is carried out. • The nightside atmospheric flow is highly variable, the dayside is steady. • The simulation suggests a planetary-size nightside shock may exist. • A cloud deck Kelvin wave impacts the whole upper atmosphere. • Observed variable patterns of airglow are reproduced and explained.

Brecht A.S., Bougher S.W., Shields D., Liu H.‐., Lee C.

Bertaux J., Hauchecorne A., Lefèvre F., Bréon F., Blanot L., Jouglet D., Lafrique P., Akaev P.

Abstract. Monitoring CO2 from space is essential to characterize the spatiotemporal distribution of this major greenhouse gas and quantify its sources and sinks. The mixing ratio of CO2 to dry air can be derived from the CO2∕O2 column ratio. The O2 column is usually derived from its absorption signature on the solar reflected spectra over the O2 A band (e.g. Orbiting Carbon Observatory-2 (OCO-2), Thermal And Near infrared Sensor for carbon Observation (TANSO)/Greenhouse Gases Observing Satellite (GOSAT), TanSat). As a result of atmospheric scattering, the atmospheric path length varies with the aerosols' load, their vertical distribution, and their optical properties. The spectral distance between the O2 A band (0.76 µm) and the CO2 absorption band (1.6 µm) results in significant uncertainties due to the varying spectral properties of the aerosols over the globe. There is another O2 absorption band at 1.27 µm with weaker lines than in the A band. As the wavelength is much closer to the CO2 and CH4 bands, there is less uncertainty when using it as a proxy of the atmospheric path length to the CO2 and CH4 bands. This O2 band is used by the Total Carbon Column Observing Network (TCCON) implemented for the validation of space-based greenhouse gas (GHG) observations. However, this absorption band is contaminated by the spontaneous emission of the excited molecule O2*, which is produced by the photo-dissociation of O3 molecules in the stratosphere and mesosphere. From a satellite looking nadir, this emission has a similar shape to the absorption signal that is used. In the frame of the CNES (Centre National d'Études Spatiales – the French National Centre for Space Studies) MicroCarb project, scientific studies have been performed in 2016–2018 to explore the problems associated with this O2* airglow contamination and methods to correct it. A theoretical synthetic spectrum of the emission was derived from an approach based on A21 Einstein coefficient information contained in the line-by-line high-resolution transmission molecular absorption (HITRAN) 2016 database. The shape of our synthetic spectrum is validated when compared to O2* airglow spectra observed by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY)/Envisat in limb viewing. We have designed an inversion scheme of SCIAMACHY limb-viewing spectra, allowing to determine the vertical distribution of the volume emission rate (VER) of the O2* airglow. The VER profiles and corresponding integrated nadir intensities were both compared to a model of the emission based on the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model. The airglow intensities depend mostly on the solar zenith angle (both in model and data), and the model underestimates the observed emission by ∼15 %. This is confirmed with SCIAMACHY nadir-viewing measurements over the oceans: in such conditions, we have disentangled and retrieved the nadir O2* emission in spite of the moderate spectral resolving power (∼860) and found that the nadir SCIAMACHY intensities are mostly dictated by solar zenith angle (SZA) and are larger than the model intensities by a factor of ∼1.13. At a fixed SZA, the model airglow intensities show very little horizontal structure, in spite of ozone variations. It is shown that with the MicroCarb spectral resolution power (25 000) and signal-to-noise ratio (SNR), the contribution of the O2* emission at 1.27 µm to the observed spectral radiance in nadir viewing may be disentangled from the lower atmosphere/ground absorption signature with a great accuracy. Indeed, simulations with 4ARCTIC radiative transfer inversion tool have shown that the CO2 mixing ratio may be retrieved with the accuracy required for quantifying the CO2 natural sources and sinks (pressure-level error ≤1 hPa; XCO2 accuracy better than 0.4 ppmv) with the O2 1.27 µm band only as the air proxy (without the A band). As a result of these studies (at an intermediate phase), it was decided to include this band (B4) in the MicroCarb design, while keeping the O2 A band for reference (B1). Our approach is consistent with the approach of Sun et al. (2018), who also analysed the potential of the O2 1.27 µm band and concluded favourably for GHG monitoring from space. We advocate for the inclusion of this O2 band on other GHG monitoring future space missions, such as GOSAT-3 and EU/European Space Agency (ESA) CO2-M missions, for a better GHG retrieval.

Evdokimova D., Belyaev D., Montmessin F., Bertaux J., Korablev O.

Stellar occultation is a powerful method to study vertical structure of the Venus night mesosphere. The UV channel of SPICAV spectrometer, operated in 2006–2014 on board ESA’s Venus Express orbiter, allowed retrieval profiles of atmospheric gases (CO 2 , SO 2 , and O 3 ) and aerosols. It was also able to register different UV emissions around Venus (nitric oxide airglow, Lyman-α) overlapping the absorption features at 120–300 ​nm. Several calibration steps convert the raw data to atmospheric transmission spectra used for the retrievals. The systematic errors of resulted gaseous concentrations mainly relate to: (i) an uncertainty of the wavelength to pixel assignment; (ii) a portion of emitting light contaminating the analyzed transmission spectra. In the present paper, we have tested a new method of the wavelength-to-pixel assignment based on the spectral features of measured stars. Secondly, using imaging capabilities of the instrument, we have demonstrated an accurate separation between different kinds of registered signal: extended UV nightglow, light from a point star, transmitted through the atmosphere, and, sometimes, solar light, scattered by Venus dusk. The efficiency of two approaches performing the separation was studied. As a result, corrected transmission spectra provided retrievals of gaseous concentrations with 20–40% higher precision respectively to those processed in previous SPICAV stellar occultation studies (Montmessin et al., 2011, Icarus 216, 82; Piccialli et al., 2015, Planet. Space Sci. 113–114, 321; Belyaev et al., 2017, Icarus 294, 58). • Systematic errors in retrievals of the SPICAV UV stellar occultation data were analyzed. • Errors relate to a wavelength to pixel assignment and an overlap of point and extended UV sources spectra. • A new method of the wavelength assignment is based on the stellar spectral features. • Two methods separating spectra of a star and extended light sources were studied. • Corrected spectra provide a 20–40% higher precision of stellar occultation retrievals.

Nara Y., Imamura T., Masunaga K., Lee Y. ., Terada N., Yoshioka K., Yamazaki A., Seki K., Yoshikawa I., Yamada M., Watanabe S.

Jessup K., Marcq E., Bertaux J., Mills F.P., Limaye S., Roman A.

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.