Mesosphere Of Venus Research Articles

Unexpected increase of the deuterium to hydrogen ratio in the Venus mesosphere

This study analyzes H2O and HDO vertical profiles in the Venus mesosphere using Venus Express/Solar Occultation in the InfraRed data. The findings show increasing H2O 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 H2O 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 H2O, 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 H2O, which are then transported downward. Moreover, this cycle may explain the SO2 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 H2O 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.

Ephemeral Ice Clouds in the Upper Mesosphere of Venus

AbstractThe conditions in Venus' upper mesosphere at around 120 km have some similarities to the upper mesosphere of Earth and Mars where ice clouds form. Here we show, using published satellite products and numerical modeling, that the upper mesosphere of Venus can be sufficiently cold that both H2O and CO2 may condense to form particles. We show that amorphous solid water particles (ASW) are likely to nucleate both heterogeneously on meteoric smoke and also homogeneously, resulting in clouds of nano‐scaled particles at around 120 km that will occur globally. The temperatures may then become sufficiently low, below ∼90 K, that CO2 particles can nucleate on ASW particles. Given the uncertainty associated with retrievals of temperature in the upper mesosphere, it is unclear how frequently this occurs, but it could be >30% of the time poleward of 60°. Since the main component of Venus' tenuous atmosphere is CO2, any CO2 crystals that form will grow and sediment on a timescale of 10–20 min. Mie calculations show that these Venusian mesospheric clouds (VMCs) should be observable by contemporary satellite instruments, although their short lifetime means that the probability of detection is small. We suggest that VMCs are important for the redistribution of meteoric smoke and may serve as a cold‐trap, removing some water vapor from the very upper mesosphere of Venus through the growth and sedimentation of cloud particles, and possibly reducing the loss of water to space.

Direct detection of atomic oxygen on the dayside and nightside of Venus

Atomic oxygen is a key species in the mesosphere and thermosphere of Venus. It peaks in the transition region between the two dominant atmospheric circulation patterns, the retrograde super-rotating zonal flow below 70 km and the subsolar to antisolar flow above 120 km altitude. However, past and current detection methods are indirect and based on measurements of other molecules in combination with photochemical models. Here, we show direct detection of atomic oxygen on the dayside as well as on the nightside of Venus by measuring its ground-state transition at 4.74 THz (63.2 µm). The atomic oxygen is concentrated at altitudes around 100 km with a maximum column density on the dayside where it is generated by photolysis of carbon dioxide and carbon monoxide. This method enables detailed investigations of the Venusian atmosphere in the region between the two atmospheric circulation patterns in support of future space missions to Venus.

Update on SO2, detection of OCS, CS, CS2, and SO3, and upper limits of H2S and HOCl in the Venus mesosphere using SOIR on board Venus Express

We report on the update of SO2 and first detections of SO3, CS, and CS2, and detection of OCS above the cloud deck using the SOIR instrument on-board Venus Express, and upper limit profiles of H2S and HOCl. Based on the solar occultation spectra measured by SOIR covering all latitudes over the 2006–2014 period, we find an average SO2 volume mixing ratio of ∼0.02 ppm below 90 km which increases to 5 ppm at 100 km; average OCS abundance of 1 ppb to 1 ppm between 65 and 100 km increasing with altitude; mean SO3 values of 0.1 ppm at 75 km, 1 ppm at 85 km, and 10 ppm at 95 km; CS volume mixing ratios varying from 0.1 ppm at 65 km to 40 ppm at 100 km; finally we report 0.03 ppm of CS2 at 70 km and 5 ppm at 90 km. It is important to note the detections for all of these species may be biased to higher volume mixing ratios at higher altitudes based on the detection limits for SOIR. Upper-limits for H2S and HOCl are determined. All abundances show large variabilities with time and/or latitude equal to at least one order of magnitude at all altitudes. We also study the detection statistics of all detected species as a function of time, latitude, and side of the terminator.

The Spatial and Temporal Distribution of Nighttime Ozone and Sulfur Dioxide in the Venus Mesosphere as Deduced From SPICAV UV Stellar Occultations

AbstractThe 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.

The Phase of Water Ice Which Forms in Cold Clouds in the Mesospheres of Mars, Venus, and Earth

AbstractWater ice clouds form in the mesospheres of terrestrial planets in the solar system (and most likely elsewhere) by vapor deposition at low pressures and temperatures. Under these conditions a range of crystalline and amorphous phases of ice might form. The phase is important because it influences nucleation kinetics, density, vapor pressure over the solid, growth rates and particle shape. In the past, the temperature range over which these different phases exist has been defined on the basis of depositing ice at low temperature and warming it while observing phase changes. However, the direct deposition of ice at a range of temperatures relevant for the terrestrial planets has not been systematically investigated. Here we present X‐ray Diffraction (XRD) measurements of water ice deposited at temperature intervals between 88 and 145 K in a vacuum chamber. XRD patterns showed that low density amorphous ice was formed at ≤120 K, stacking disordered ice I formed from 121 to 135 K and hexagonal ice I formed at 140 and 145 K. Direct deposition results in the stable hexagonal phase at much lower temperatures than when warming stacking disordered ice. All three phases of water ice observed here are possible in clouds in the mesospheres of Earth and Mars, while on Venus only amorphous ice is likely to form.

Determination of the Venus eddy diffusion profile from CO and CO2 profiles using SOIR/Venus Express observations

We present a study of the SOIR/Venus Express CO2 and CO profiles in order to constrain the eddy diffusion profile in the Venus mesosphere (80 to 130 km of altitude). We compute the mean number density profile of both species, and the hydrostatic temperature based on the CO2 mean density profile. We use a 1D-photochemical/diffusion model to adjust the eddy diffusion profile such that the 1D-model and SOIR CO profiles best agree. We compare our results with eddy diffusion profiles found in the literature. We use our approach to revisit the He and CO2 profiles from Pioneer Venus probe mass spectrometer in order to derive an eddy diffusion profile extending from 80 to 200 km of altitude. Our eddy diffusion profile is different in shape and in magnitude compared to the Pioneer Venus profile: it is equal to 3.6 × 106 cm2/s at 110 km, 7.5 × 106 cm2/s at 120 km, 0.4 × 106 cm2/s at 130 km, and 201 × 106 cm2/s at 140 km, and constant below 110 km and above 140 km. This new eddy diffusion profile can be used in Venus Global Circulation Models as well as 1D-photochemical models.

Claimed Detection of PH3 in the Clouds of Venus Is Consistent with Mesospheric SO2

Abstract The observation of a 266.94 GHz feature in the Venus spectrum has been attributed to phosphine (PH3) in the Venus clouds, suggesting unexpected geological, chemical, or even biological processes. Since both PH3 and sulfur dioxide (SO2) are spectrally active near 266.94 GHz, the contribution to this line from SO2 must be determined before it can be attributed, in whole or part, to PH3. An undetected SO2 reference line, interpreted as an unexpectedly low SO2 abundance, suggested that the 266.94 GHz feature could be attributed primarily to PH3. However, the low SO2 and the inference that PH3 was in the cloud deck posed an apparent contradiction. Here we use a radiative transfer model to analyze the PH3 discovery, and explore the detectability of different vertical distributions of PH3 and SO2. We find that the 266.94 GHz line does not originate in the clouds, but above 80 km in the Venus mesosphere. This level of line formation is inconsistent with chemical modeling that assumes generation of PH3 in the Venus clouds. Given the extremely short chemical lifetime of PH3 in the Venus mesosphere, an implausibly high source flux would be needed to maintain the observed value of 20 ± 10 ppb. We find that typical Venus SO2 vertical distributions and abundances fit the JCMT 266.94 GHz feature, and the resulting SO2 reference line at 267.54 GHz would have remained undetectable in the ALMA data due to line dilution. We conclude that nominal mesospheric SO2 is a more plausible explanation for the JCMT and ALMA data than PH3.

A stringent upper limit of the PH3abundance at the cloud top of Venus

Aims.Following the announcement of the detection of phosphine (PH3) in the cloud deck of Venus at millimeter wavelengths, we searched for other possible signatures of this molecule in the infrared range.Methods.Since 2012, we have been observing Venus in the thermal infrared at various wavelengths to monitor the behavior of SO2and H2O at the cloud top. We identified a spectral interval recorded in March 2015 around 950 cm−1where a PH3transition is present.Results.From the absence of any feature at this frequency, we derive, on the disk-integrated spectrum, a 3-σupper limit of 5 ppbv for the PH3mixing ratio, which is assumed to be constant throughout the atmosphere. This limit is four times lower than the disk-integrated mixing ratio derived at millimeter wavelengths.Conclusions.Our result places a strong constraint on the maximum PH3abundance at the cloud top and in the lower mesosphere of Venus.

SOIR/VEx observations of water vapor at the terminator in the Venus mesosphere

The Solar Occultation in the InfraRed (SOIR) instrument onboard Venus Express sounded the Venus mesosphere and lower thermosphere using solar occultation geometry between April 2006 and December 2014. The observations were all taken at the terminator. This paper reports on the water vapor vertical distribution above the clouds and geo-temporal variations, observed across the whole Venus Express mission. Water vapor profiles are sampled between 80 and 125 km from both the morning and evening side of the terminator. Calculations of the water vapor volume mixing ratio agree with those from previous studies, having an average atmospheric profile that varies between 0.56 ppmv and 2.45 ppmv. No significant variations were observed in observations taken between the morning and evening terminator. Short term variations of several Earth days dominate the data set, varying by up to a factor 18 over a 24 h period, from ~10 ppmv to ~0.55 ppmv at an altitude of ~111 km. Similar to previous trace gas observations by SOIR (HCl, HF and SO2), no significant spatial or long term trends are observed.

Millimeter‐Wave Radiometric Information Content Analysis for Venus Atmospheric Constituents

AbstractVarious gaseous constituents present in Venus atmosphere have been detected and measured using in situ and remote sensing instruments. However, the measurements of mesospheric constituents above the cloud level are still scarce and inadequate. This is because constituents above the cloud level have a low abundance and a high level of variability. However, millimeter and submillimeter (mm and sub‐mm) frequencies can be used for Venus mesosphere measurements during prolonged Venusian day and night because several minor species in the upper atmosphere have emission/absorption bands in these frequencies owing to the rotational transitions. This study presents brightness temperatures and the corresponding sensitivity at the mm and sub‐mm wave frequencies for a limb sounding radiometer. This will be useful in potential payload design, that is, instrument design, frequency, bandwidth selection, and satellite altitude as well as pointing accuracy determination for measurement above Venus clouds. For this study, a simulation based experiment has been performed and presented to determine the mm and sub‐mm wave frequencies, optimum for estimation of concentration of water vapor, CO, CO2, SO2, HDO, and HCl present in the mesosphere. The simulations have been performed using a radiative transfer model called Atmospheric Radiative Transfer Simulator. The mm and sub‐mm wave measurement frequencies have been determined for both day and night time of Venus. It has been determined that 183, 380, 448, and 556 GHz frequencies are most suitable for water vapor measurements, 115, 230, and 345 GHz are optimum for CO measurements, and 600, 621, and 690 GHz frequency measurements are sensitive for HCl detection.

Vertical temperature profiles in the Venus mesosphere obtained by two retrieval methods from the VIRTIS-VEX observations

We present vertical temperature profiles derived by two different retrieval methods from nighttime radiation measurements performed by VIRTIS(M)-VEx (Visible and Infrared Thermal Imaging Spectrometer, M channel—Venus Express). The Bayesian approach to the optimal estimation method and the relaxation method are applied in this study. This is a first attempt to present and compare results obtained from two independent methods. It allows us to be more convinced of our interpretation.After comparison of temperature profiles we conclude that both retrieval methods are able to sound the atmospheric layers higher than 59 km (< 150 mb) with a single-standard deviation error of less than 5 K at the bottom and even deeper for some measurements down to 56 km but with error larger than 10 K. The best consistency between temperature profiles obtained from the two algorithms with the standard deviation less than 2.5 K is observed for atmospheric layers between 70 km–77 km (15–3.5 mb).In our conclusion we have no preference for any approach. Two methodologies are of equivalent value. Both methods resolve temperature inversions at high altitudes (∼84 km), the quality of fits for all observations is equally well. Only for the Bayesian approach, the retrieval uncertainty above 62 km up to 95 km is less than 2 K. A disadvantage of this method is the time-consuming calculation of weighting functions.The atmospheric temperatures over the “cold collar” region located at 60°S–75°S (60–70 km) are ∼10 K smaller than for latitudes poleward of 75°S (polar region). The cold collar region is seen very clearly in our results for both methods.

Scale heights and detached haze layers in the mesosphere of Venus from SPICAV IR data

SPICAV IR, one channel of SPICAV/SOIR instrument suite onboard Venus Express, performed solar occultation measurements of the atmosphere at terminators in 0.65–1.7 µm spectral range. We analyze the properties of the upper part of the Venus aerosol layer (upper haze, 70 − 95 km altitude) from 798 observations performed from May 2006 through November 2014. Vertical profiles of slant optical depth, extinction coefficient, effective radius, and number density of haze particles from 222 orbits were analyzed in a previous publication (Luginin et al., 2016); their diurnal, latitudinal, and interannual variabilities were investigated. The present paper is devoted to analysis of scale heights and properties of detached haze layers from 147 orbits at mid-to-high northern latitudes, where the best spatial resolution was obtained. Scale heights retrieved from 43 orbits were equal to 4 − 5.5 km at the North Pole (82°N-90°N) decreasing to 2 − 4 km at 60°N − 80°N latitudes. As an explanation of such latitudinal variations, we propose a mechanism based on vertical transport driven by winds that are directed upward at the North Pole and downward at 60°N − 80°N latitudes. Detached layers were detected in 93 occultations at 58°N − 90°N. The detached layers are presumably formed through condensation of water vapor on droplets of sulfuric acid water solution; they were mostly seen at 80 − 88 km at the morning terminator, and at 84 − 90 km at the evening one. This difference in altitude of the detached layers can be explained by diurnal variations in thermal structure of Venusian mesosphere. The vertical optical depth of detached layers varies broadly around the mean τDL ∼ 0.8 − 3·10−3; no difference between the morning and the evening terminators was observed. The effective radius and number density of aerosol particles in the detached layers group around a very wide maximum at the morning terminator (0.65 ± 0.25 µm and 0.6 ± 0.4 cm−3) and two maxima at the evening terminator (0.4 ± 0.1 µm and 0.85 ± 0.15 µm; 0.3 ± 0.2 cm−3 and 4.5 ± 2.5 cm−3). This could be explained by differences in initial altitudes at which condensation of particles occurs.

Circulation of Venusian Atmosphere at 90–110 km Based on Apparent Motions of the O2 1.27 μm Nightglow From VIRTIS‐M (Venus Express) Data

AbstractThe paper is devoted to the investigation of Venus mesosphere circulation at 90–110 km altitudes, where tracking of the O2(a1Δg) 1.27 μm nightglow is practically the only method of studying the circulation. The images of the nightglow were obtained by VIRTIS‐M on Venus Express over the course of more than 2 years. The resulting global mean velocity vector field covers the nightside between latitudes 75°S–20°N and local time 19–5 h. The main observed mode of circulation is two opposite flows from terminators to midnight; however, the wind speed in the eastward direction from the morning side exceeds the westward (evening) by 20–30 m/s, and the streams “meet” at 22.5 ± 0.5 h. The influence of underlying topography was suggested in some cases: Above mountain regions, flows behave as if they encounter an “obstacle” and “wrap around” highlands. Instances of circular motion were discovered, encompassing areas of 1,500–4,000 km.

Mapping the thermal structure and minor species of Venus mesosphere with ALMA submillimeter observations

Context. Water vapor and sulfur compounds are key species in the photochemistry of Venus mesosphere. These species, together with mesospheric temperatures, exhibit drastic temporal variations, both on short timescales (diurnal and day-to-day) as well on long timescales, far from being understood. Aims. We targeted CO, SO, HDO and SO 2 transitions in the submillimeter range using the Atacama Large Millimeter Array (ALMA) to study their spatial and temporal variations. Methods. Four sets of observations were acquired on different dates in November 2011 during the first ALMA Early Science observation Cycle 0. Venus angular diameter was about 11′′ with an illumination factor of 92%, so that mostly the day side of the planet was mapped. Assuming a nominal CO abundance profile, we retrieved vertical temperature profiles over the entire disk as a function of latitude and local time. Temperature profiles were later used to retrieve SO, SO 2 , and H 2 O. We used HDO as a tracer for water assuming a D/H enrichment of 200 times the terrestrial value. Results. We derived 3D maps of mesospheric temperatures in the altitude range 70−105 km. SO, SO 2 , and H 2 O are characterized by a negligible abundance below ~ 85 km followed by an increase with altitude in the upper mesosphere. Disk-averaged SO abundances present a maximum mixing ratio of 15.0 ± 3.1 ppb on November 26 followed the next day by a minimum value of 9.9 ± 1.2 ppb. Due to a very low S/N, SO 2 could only be derived from the disk-averaged spectrum on the first day of observation revealing an abundance of 16.5 ± 4.6 ppb. We found a SO 2 /SO ratio of 1.5 ± 0.4. Global maps of SO reveal strong variations both with latitude and local time and from day to day with abundance ranging from 2 O disk-averages retrievals reveal a steady decrease from November 14 to 27, with the abundance varying from 3.6 ± 0.6 ppm on the first day to 2.9 ± 0.7 ppm on the last day. H 2 O maps reveal a slightly higher abundance on the evening side compared to the morning side and a strong depletion between the first and the second day of observation.

Diurnal observations of HCl altitude variation in the 70–100 km mesosphere of Venus

First submm spectroscopic observations of the 625.9 GHz H35Cl absorption lines of the Venus dayside atmosphere were obtained with the James Clerk Maxwell Telescope (JCMT) on March 2, 2013. These data, which support retrieval of HCl altitude distributions in the Venus mesosphere (70–100 km), are presented here and compared with previously reported JCMT observations of Venus nightside HCl (Sandor et al., 2012). The measured dayside profile agrees with that of the nightside, indicating no diurnal variation is present. More specifically, the nightside spectra revealed a secular decrease of upper mesospheric HCl between observations one month apart, at fixed latitude and local time. The dayside profile reported here presents upper mesospheric abundances that are bracketed by the two previously measured nightside profiles, indicating that if diurnal variation is present, it must be weaker than the secular variations occurring at fixed local time. The previous study, which measured nightside HCl abundances above 85 km to be much smaller than predicted from photochemical modeling, suggested a dynamical explanation for the disagreement wherein nightside downwelling associated with the SubSolar to AntiSolar (SSAS) atmospheric circulation might suppress upper mesospheric abundances predicted purely from photochemistry. However a straightforward prediction from the proposed mechanism is that HCl abundance on the dayside, where the SSAS drives upward rather than downward transport should at least agree with, and perhaps exceed that of the photochemical model. The finding that dayside HCl abundance agrees with that of the nightside, hence also is much smaller than that of the model shows the SSAS hypothesis to be incorrect.

Improved algorithm for the transmittance estimation of spectra obtained with SOIR/Venus Express.

The Solar Occultation in the InfraRed (SOIR) instrument onboard the ESA Venus Express spacecraft, an infrared spectrometer sensitive from 2.2 to 4.3 μm, probed the atmosphere of Venus from June 2006 until December 2014. During this time, it performed more than 750 solar occultations of the Venus mesosphere and lower thermosphere. A new procedure has been developed for the estimation of the transmittance in order to decrease the number of rejected spectra, to check that the treated spectra are well calibrated, and to improve the quality of the calibrated spectra by reducing the noise and accurately normalizing it to the solar spectrum.

Multilayer modeling of the aureole photometry during the Venus transit: comparison between SDO/HMI and VEx/SOIR data

The mesosphere of Venus is a critical range of altitudes in which complex temperature variability has been extensively studied by the space mission Venus Express (Vex) during its eight-years mission (2006-2014). Data collected at different epochs and latitudes show evidence of short and medium timescale variability as well as latitudinal differences. Spatial and temporal variability is also predicted in mesospheric and thermosphere terminator models with lower boundary conditions at 70 km near cloud tops. The Venus transit on June 5-6 2012 was the first to occur with a spacecraft in orbit around Venus. It has been shown that sunlight refraction in the mesosphere of Venus is able to provide useful constraints on mesospheric temperatures at the time of the transit. The European Space Agency's Venus Express provided space-based observations of Venus during the transit. Simultaneously, the Venus aureole photometry was observed using ground-based facilities and solar telescopes orbiting Earth (NASA Solar Dynamic Observatory, JAXA HINODE). As the properties of spatial and temporal variability of the mesosphere are still debated, the opportunity of observing it at all latitudes at the same time, offered by the transit, is rather unique. In this first paper, we establish new methods for analyzing the photometry of the so-called aureole that is produced by refraction of the solar light, and we investigate the choice of physical models that best reproduce the observations. We obtain an independent constraint of 4.8 +/- 0.5 km for the aerosol scale height in the upper haze region above 80 km. We show that a full multiple-layer approach is required to adequately reproduce the aureole photometry, which appears to be sensitive to several second-order variations in the vertical refractivity.

HDO and SO2thermal mapping on Venus

We present the analysis of a four-year observational campaign using the TEXES high-resolution imaging spectrometer at the NASA Infrared Telescope Facility to map sulfur dioxide and deuterated water over the disk of Venus. Data have been recorded in two spectral ranges around 1345 cm −1 (7.4 µm) and 530 cm −1 (19 µm) in order to probe the cloudtop at an altitude of about 64 km (SO 2 and HDO at 7.4 µm) and a few kilometers below (SO 2 at 19 µm). Observations took place during six runs between January 2012 and January 2016. The diameter of Venus ranged between 12 and 33 arcsec. Data were recorded with a spectral resolving power up to 80 000 and a spatial resolution of about 1 arcsec (at 7.4 µm) and 2.5 arcsec (at 19 µm). Mixing ratios were estimated from HDO/CO 2 and SO 2 /CO 2 line depth ratios, using weak neighboring transitions of comparable depths. The whole dataset demonstrates that the two molecules behave very differently to each other. The HDO maps are uniform over the disk. The disk-integrated H 2 O mixing ratio (estimated assuming a D/H of 200 VSMOW in the mesosphere of Venus) show moderate variations (by less than a factor of 2) over the four-year period. A value of 1.0−1.5 ppmv is obtained in most of the cases. The SO 2 maps, in contrast, show strong variations over the disk of Venus, by a factor as high as 5. Long-term variations of SO 2 show that the disk-integrated SO 2 mixing ratio also varies between 2012 and 2016 by a factor as high as ten, with a minimum value of 30 +/−5 ppbv on February 26, 2014 an a maximum value of 300 +/−50 ppbv on January 14, 2016. The SO 2 maps also show a strong short-term variability. It can be seen that the SO 2 maximum feature usually follows the four-day rotation of the clouds over a timescale of two hours, which corresponds to a rotation of 7.5 deg over the planetary disk, but its morphology also changes, which suggests that the lifetime of this structure is not more than a few hours.

Mesospheric optical signatures of possible lightning on Venus

AbstractA self‐consistent two‐dimensional model is proposed to account for the transient mesospheric nighttime optical emissions associated to possible intracloud (IC) lightning occurring in the Venusian troposphere. The model calculates the mesospheric (between 75 km and 120 km in altitude) quasi‐elestrostatic electric field and electron density produced in response to IC lightning activity located between 40 km and 65 km in the Venusian cloud layer. The optical signatures and the densities of perturbed excited atomic and molecular neutral and ionic species in the mesosphere of Venus are also calculated using a basic kinetic scheme. The calculations were performed for different IC lightning discharge properties. We found that the calculated electric fields in the mesosphere of Venus are above breakdown values and that, consequently, visible transient glows (similar to terrestrial Halos produced by lightning) right above the parent IC lightning are predicted. The transient optical emissions result from radiative deexcitation of excited electronic states of N2 (in the ultraviolet, visible and near‐infrared ranges) and of O(1S) and of O(1D) in, respectively, the green (557 nm) and red (630 nm) wavelengths. The predicted transient lightning‐induced glows from O(1S) can reach an intensity higher than 167 R and, consequently, be above the detection threshold of the Lightning and Airglow Camera instrument aboard the Japanese Akatsuki probe orbiting Venus since December 2015. However, according to our model, successful observations of transient lightning‐induced optical glows could only be possible for sufficiently close (300 km or maximum 1000 km) distances.