Light scattering in a spherical,exponential atmosphere,with applications to Venus

The problem of the reflection of light from an optically thick, spherical atmosphere in which the scatterers are distributed exponentially with a scale height small compared to the radius of the planet is discussed. Exact formal solutions are obtained for the single scattered component. Useful approximate analytic solutions, which also include multiply scattered light, are given. The results are applied to the analysis of the Mariner 10 limb and terminator images of Venus. The altitude of the “detached” haze layer discovered by Mariner 10 is at 79–85 km, but in places the haze exists above 100 km. This layer apparently is a stable, planetwide feature which forms at the top of the Pioneer Venus upper haze layer. It was similar in location, scale height, and thickness at the times of the two missions, in contrast to the lower, high-altitude haze which changed dramatically. We discuss two possibilities for the nature of the limb hazes. (1) The lower haze is probably the sulfuric acid cloud and the “detached” layer may be a separate water-ice haze. (2)The “detached” haze layer may not be separate at all, but part of the sulfuric acid haze, and the apparent “gap” at 75–80 km may be the source region of a broadband absorber. The spatial distribution of the strong near-UV absorber, which may be elemental sulfur as first suggested by B. Hapke and R. Nelson (1975, J. Atmos. Sci.32, 1212–1218), is examined in light of our results. Several arguments indicate that there is no nonabsorbing, overlying haze and that the UV absorber extends to the top of the haze 8layer.     

Dynamical properties of the Venus mesosphere from the radio-occultation experiment VeRa onboard Venus Express

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.     

Diurnal CO variations in the Venus mesophere from CO microwave spectra

Microwave spectra of carbon monoxide (¹²CO) in the mesosphere of Venus were measured in December 1978, May and December 1980, and January, September, and November 1982. These spectra are analyzed to provide mixing profiles of CO in the Venus mesosphere and best constrain the mixing profile of CO between ～ 100 and 80 km altitude. From the January 1982 measurement (which, of all our spectra, best constrains the abundance of CO below 80 km altitude) we find an upper limit for the CO mixing ratio below 80 km altitude that is two to three times smaller than the stratospheric (～65 km) value of 4.5 ± 1.0 × 10^?5 determined by P. Connes, J. Connes, L.D. Kaplan, and W. S. Benedict (1968, Astrophys. J.152, 731–743) in 1967, indicating a possible long-term change in the lower atmospheric concentration of CO. Intercomparison among the individual CO profiles derived from our spectra indicates considerable short-term temporal and/or spatial variation in the profile of CO mixing in the Venus mesosphere above 80 km. A more complete comparison with previously published CO microwave spectra from a number of authors specifies the basic diurnal nature of mesospheric CO variability. CO abundance above ～ 95 km in the Venus atmosphere shows approximately a factor of 2–4 enhancement on the nightside relative to the dayside of Venus. Peak nightside CO abundance above ～95 km occurs very near to the antisolar point on Venus (local time of peak CO abundance above ～95 km occurs at 0.6_?0.6^+0.7 hr after midnight on Venus), strongly suggesting that retrograde zonal flow is substantially reduced at an altitude of 100 km in the Venus mesosphere. In contrast, CO abundances between 80 and 90 km altitude show a maximum that is shifted from the antisolar point toward the morningside of Venus (local time of peak CO abundance between 80 and 90 km occurs at 8.5 ± 1.0 hr past midnight on Venus). The magnitude of the diurnal variation of CO abundance between 80 and 90 km is again, approximately a factor of 2–4. Disk-averaged spectra of Venus do not determine the exact form for the diurnal distribution of CO in the Venus mesosphere as indicated by comparison of synthetic spectra, based upon model distributions, and the measured spectra. However, the offset in phase for the diurnal variation for the >95 km and 80–90-km-altitude regions requires an asymmetric (in solar zenith angle) distribution.     

The structure and microphysical properties of the Venus clouds: Venera 9, 10, and 11 data

Data processing and interpretation of the nephelometer measurements made in the Venus atmosphere aboard the Venera 9, 10 and 11 landers in the sunlit hemisphere near the equator are discussed. These results were used to obtain the aerosol distribution and its microphysical properties from 62 km to the surface. The main aerosol content is found in the altitude range between 62 km (where measurements began) and 48 km, the location of the cloud region. Three prominent layers labeled as I (between 62 and 57 km), II (between 57 and 51 km) and III (between 51 and 48 km), each with different particle characteristics are discovered within the clouds. The measured light-scattering patterns can be intrepreted as having been produced by particles with effective radii from 1 to 2 μm depending on height and indices of refractivity from 1.45 in layer I to 1.42 in layer III. These values do not contradict the idea that the droplets are made of sulfuric acid. In layers II and III the particle size distribution is at least bimodal rather than uni-modal. The index of refraction is found to decrease to 1.33 in the lower part of layer II, suggesting a predominant abundance of larger particles of different chemical origin, and chlorine compounds are assumed to be relevant to this effect. In the entire heightrange of the Venera 9–11 craft descents, the clouds are rather rarefied and are characterized by a mean volume scattering coefficient σ ～ 2 × 10^?5 cm^?1 that corresponds to the mean meteorological range of visibility of about 2 km. The average mass content of condensate is estimated to be equal to 4 × 10^?9 g/cm³, and the total optical depth of clouds to τ ～ 35. Near the bottom of layer III clouds are strongly variable. In the subcloud atmosphere a haze was observed between 48 and 32 km; that haze is mainly made of submicron particles, r_eff ～ 0.1μm. The atmosphere below that is totally transparent but separate (sometimes possibly disappearing) layers may be present up to a height of 8 km above the surface. A model of this region with a very low particle density (N ? 2–3 cm^?3) strongly refractive large particles (r_eff ? 2.5 μm; 1.7 < n < 2.0) provided satisfactory agreement. The optical depth of aerosol in the atmosphere below the subcloud haze does not exceed 2.5.     

The planetary fourier spectrometer (PFS) onboard the European Venus Express mission

The planetary fourier spectrometer (PFS) for the Venus Express mission is an infrared spectrometer optimized for atmospheric studies. This instrument has a short wavelength (SW) channel that covers the spectral range from 1700 to 11400 cm⁻¹ (0.9–5.5 μm) and a long wavelength (LW) channel that covers 250–1700 cm⁻¹ (5.5–45 μm). Both channels have a uniform spectral resolution of 1.3 cm⁻¹. The instrument field of view FOV is about 1.6 ° (FWHM) for the short wavelength channel and 2.8 ° for the LW channel which corresponds to a spatial resolution of 7 and 12 km when Venus is observed from an altitude of 250 km. PFS can provide unique data necessary to improve our knowledge not only of the atmospheric properties but also surface properties (temperature) and the surface-atmosphere interaction (volcanic activity).PFS works primarily around the pericentre of the orbit, only occasionally observing Venus from larger distances. Each measurements takes 4.5 s, with a repetition time of 11.5 s. By working roughly 1.5 h around pericentre, a total of 460 measurements per orbit will be acquired plus 60 for calibrations. PFS is able to take measurements at all local times, enabling the retrieval of atmospheric vertical temperature profiles on both the day and the night side.The PFS measures a host of atmospheric and surface phenomena on Venus. These include the:(1) thermal surface flux at several wavelengths near 1 μm, with concurrent constraints on surface temperature and emissivity (indicative of composition); (2) the abundances of several highly-diagnostic trace molecular species; (3) atmospheric temperatures from 55 to 100 km altitude; (4) cloud opacities and cloud-tracked winds in the lower-level cloud layers near 50-km altitudes; (5) cloud top pressures of the uppermost haze/cloud region near 70–80 km altitude; and (6) oxygen airglow near the 100 km level. All of these will be observed repeatedly during the 500-day nominal mission of Venus Express to yield an increased understanding of meteorological, dynamical, photochemical, and thermo-chemical processes in the Venus atmosphere. Additionally, PFS will search for and characterize current volcanic activity through spatial and temporal anomalies in both the surface thermal flux and the abundances of volcanic trace species in the lower atmosphere.Measurement of the 15 μm CO₂ band is very important. Its profile gives, by means of a complex temperature profile retrieval technique, the vertical pressure-temperature relation, basis of the global atmospheric study.PFS is made of four modules called O, E, P and S being, respectively, the interferometer and proximity electronics, the digital control unit, the power supply and the pointing device.     

Polarization studies of the Venus uv contrasts: Cloud height and haze variability

Polarimetry is able to show direct evidence for compositional differences in the Venus clouds. We present observations (collected during $\text{2}\text{1}\text{2}$ Venus years by the Pioneer Venus Orbiter) of the polarization in four colors of the bright and dark ultraviolet features. We find that the polarization is significantly different between the bright and dark areas. The data show that the “null” model of L. W. Esposito (1980, J. Geophys. Res.85, 8151–8157) and the “overlying haze” model of J. B. Pollack et al. (1980, J. Geophys. Res.85, 8223–8231) are insufficient. Exact calculations of the polarization, including multiple scattering and vertical inhomogeneity near the Venus cloud tops, are able to match the observations. Our results give a straightforward interpretation of the polarization differences in terms of known constituents of the Venus atmosphere. The submicron haze and uv absorbers are anticorrelated: for haze properties as given by K. Kawabata et al. (1980, J. Geophys. Res.85, 8129–8140) the excess haze depth at 9350 Å over the bright regions is Δτ_h = 0.03 ± 0.02. The cloud top is slightly lower in the dark features: the extra optical depth at 2700 Å in Rayleigh scattering above the darker areas is Δτ_R = 0.010 ± 0.005. This corresponds to a height difference of 1.2 ± 0.6 km at the cloud tops. The calculated polarization which matches our data also explains the relative polarization of bright and dark features observed by Mariner 10. The observed differential polarization cannot be explained by differential distribution of haze, if the haze aerosols have an effective size of 0.49 μm, as determined by K. Kawabata et al. (1982, submitted) for the aerosols overlying the Venus equator. We propose two models for the uv contrasts consistent with our results. In a physical model, the dark uv regions are locations of vertical convergence and horizontal divergence. In a chemical model, we propose that the photochemistry is limited by local variations in water vapor and molecular oxygen. The portions of the atmosphere where these constituents are depleted at the cloud tops are the dark uv features. Strong support for this chemical explanation is the observation that the number of sulfur atoms above the cloud tops is equal over both the bright and dark areas. The mass budget of sulfur at these altitudes is balanced between excess sulfuric acid haze over the bright regions and excess SO₂ in the dark regions.     

A model of Titan's haze of fractal aerosols constrained by multiple observations

We use Titan's geometric albedo to constrain the vertical distribution of the haze. Microphysical models incorporating fractal aggregates do not readily fit the methane features at 0.62 μm band and the dark 0.88 μm of the albedo spectrum simultaneously. We take advantage of this apparent discrepancy to constrain the haze vertical profile.We used the geometric albedo and several results and constraints from other works to better constrain the vertical haze extinction profile, especially in the low stratosphere. The objective of this model is to give a solution that simultaneously fits the main constraints known to apply to the haze.We find that the haze extinction increases with decreasing altitude with a scale height about equal to the atmospheric scale height down to 100 km. Below this altitude, extinction must decrease down to 30 km. This is necessary in order to have enough haze to sustain a relatively high albedo (0.076) in the dark 0.88 μm methane band and to show the 0.62 μm band in the haze continuum. We set the haze production rate around 7×10⁻¹⁴ kgm⁻² s⁻¹, and the aerosols production altitude around 400 km (or at pressure 1.5 Pa).The physical processes which generate such a profile are not clear. However, purely one-dimensional effects such as condensation, sedimentation, and rainout can be ruled out, and we believe that this relative clearing in Titan's troposphere and lower stratosphere is due to particle horizontal transport by the mean circulation.     

Observational definition of the Venus mesopause: vertical structure, diurnal variation, and temporal instability

Submillimeter line observations of CO in the Venus middle atmosphere (mesosphere) were observed with the James Clerk Maxwell Telescope (JCMT, Mauna Kea) about the May 2000, February 2002 superior and July 1999, March 2001 inferior conjunctions of Venus. Combined ¹²CO and ¹³CO isotope spectral line measurements at 345 and 330 gHz frequencies, respectively, provided enhanced sensitivity and vertical coverage for simultaneous retrievals of atmospheric temperatures and CO mixing ratios over the altitude region 75-105 km with vertical resolution 4-5 km. Supporting millimeter ¹²CO spectral line observations with the Kitt Peak 12-m telescope (Steward Observatories) provide enhanced temporal coverage and CO mixing sensitivity. Implementation of CO/temperature profile retrievals for the 2000, 2002 dayside (superior conjunction) and 1999, 2001 nightside (inferior conjunction) periods yields a first-time definition of the vertical structure and diurnal variation of a low-to-mid-latitude mesopause within the Venus atmosphere. At the times of these 1999-2002 observations, the Venus mesopause was located at a slightly lower level in the nightside (0.5 mbar, ∼87 km) versus the dayside (0.2 mbar, ∼91 km) atmosphere. Average diurnal variation of Venus mesospheric temperatures appears to be ≤ 5 K at and below the mesopause. Diurnal variation of Venus thermospheric temperatures increases abruptly just above the mesopause, reaching 50 K by the 0.01-mbar pressure level (∼102 km). Atmospheric temperatures above and below the Venus mesopause exhibited global-scale (≥4000 km horizontal) variations of large amplitude (7-15 K) on surprisingly short timescales (daily to monthly) during the 2001 nightside and 2002 dayside observing periods. Venus dayside mesospheric temperatures observed during the 2002 superior conjunction were also 10-15 K warmer than observed during the 2000 superior conjunction. A characteristic timescale for these global temperature variations is not defined, but their magnitude is comparable to previous determinations of secular variability in nightside mesospheric temperatures (Clancy and Muhleman, 1991).     

Vertical structure of the Venus cloud top from the VeRa and VIRTIS observations onboard Venus Express

We investigate the Venus cloud top structure by joint analysis of the data from Visual and Thermal Infrared Imaging Spectrometer (VIRTIS) and the atmospheric temperature sounding by the Radio Science experiment (VeRa) onboard Venus Express. The cloud top altitude and aerosol scale height are derived by fitting VIRTIS spectra at 4–5 μm with temperature profiles taken from the VeRa radio occultation. Our study shows gradual descent of the cloud top from 67.2 ± 1.9 km in low latitudes to 62.8 ± 4.1 km at the pole and decrease of the aerosol scale height from 3.8 ± 1.6 km to 1.7 ± 2.4 km. These changes correlate with the mesospheric temperature field. In the cold collar and high latitudes the cloud top position remarkably coincides with the sharp minima in temperature inversions suggesting importance of radiative cooling in their maintenance. This behaviour is consistent with the earlier observations. Spectral trend of the cloud top altitude derived from a comparison with the earlier observations in 1.6–27 μm wavelength range is qualitatively consistent with sulphuric acid composition of the upper cloud and suggests that particle size increases from equator to pole.     

Infrared photometry of Venus: Variation during 10 years?

The infrared flux of Venus has been observed with a narrowband filter (λ = 3.6 μm, Δλ = 0.08 μm) from 1982 through 1984, covering a range of the phase angle α from 27 to 94°. Normalized values of flux at the Venus-Earth distance of 1 AU were (4.0–5.4) × 10^?17W/cm²/cm^?1 and the α dependence of the data is rather weak. Furthermore, when the evening terminator of Venus was seen, lower values of flux were obtained in contrast with higher values at the morning terminator. The α dependence is quite different from that of J.V. Martonchik and R. Beer (1975, J. Atmos. Sci.32, 1151–1156). Since we cannot find any significant problem in the two observational methods, the difference might suggest an intrinsic time variation of haze particles during these 10 years in the upper haze layer of the Venus cloud.     

Preliminary results on the Venus atmosphere from the Venera 8 descent module

The Venera 8 descent module measured pressure, temperature, winds and illumination as a function of altitude in its landing on July 22, 1972, just beyond the terminator in the illuminated hemisphere of Venus. The surface temperature and pressure is 741 ± 7°K and 93 ± 1.5kgcm^?2, consistent with early Venera observations and showing either no diurnal variation or insignificant diurnal variation in temperature and pressure in the vicinity of the morning terminator. The atmosphere is adiabatic down to the surface. The horizontal wind speed is low near the surface, about 35m/sec between 20 and 40km altitude, and increasing rapidly above 48km altitude to 100–140m/sec, consistent with the 4-day retrograde rotation of the ultraviolet clouds. The illumination at the center of the day hemisphere of Venus is calculated to be about 1% of the solar flux at the top of the atmosphere, consistent with greenhouse models and high enough to permit photography of the Venus surface by future missions. The attenuation below 35km altitude is explained by Rayleigh scattering with no atmospheric aerosols; above 35km there must be substantial extinction of incident light.     

Vertical distribution of scattering hazes in Titan's upper atmosphere

Radial intensity scars of a Voyager 2 high phase angle image of Titan have been inverted to yield vertical extinction profiles at 1° intervals around the limb. A detached haze layer with peak particle number densities ～0.2 cm^?3 exists at all latitudes south of ～45°N, and at an altitude of 300–350 km. The optical depth 0.01 level lies at a radius of 2932 ± 5 km at the equator and at a radius of 2915 ± 10 km over the poles (altitudes of 357 ± 5 and 340 ± 10 km, respectively). In addition to the haze layer at 300–350 km, there is a small enhancement in the extinction at ～450 km which exists at all latitudes between 75°S and ～60°N.     

Structure of the Venus mesosphere and lower thermosphere from measurements during entry of the pioneer Venus probes

Data on the thermal structure of the nightside middle atmosphere of Venus, from 84 to 137 km altitude, have been obtained from analysis of deceleration measurements from the third Pioneer Venus small probe, the night probe, which entered the atmosphere near the midnight meridian at 27°S latitude. Comparison of the midnight sounding with the morning sounding at 31°S latitude indicates that the temperature structure is essentially diurnally invariant up to 100 km, above which the nightside structure diverges sharply from the dayside toward lower temperatures. Very large diurnal pressure differences develop above 100 km with dayside pressure ten times that on the nightside at 126 km altitude. This has major implications for upper atmospheric dynamics. The data are compared with the measurements of G. M. Keating, J. Y. Nicholson, and L. R. Lake (1980, J. Geophys. Res., 85, 7941–7956) above 140 km with theoretical thermal structure models of Dickinson, and with data obtained by Russian Venera spacecraft below 100 km. Midnight temperatures are ～ 130°K, somewhat warmer than those reported by Keating et al.     

Small-scale temperature fluctuations seen by the VeRa Radio Science Experiment on Venus Express

The Venus Express Radio Science Experiment VeRa retrieves atmospheric profiles in the mesosphere and troposphere of Venus in the approximate altitude range of 40–90 km. A data set of more than 500 profiles was retrieved between the orbit insertion of Venus Express in 2006 and the end of occultation season No. 11 in July 2011. The atmospheric profiles cover a wide range of latitudes and local times, enabling us to study the dependence of vertical small-scale temperature perturbations on local time and latitude.Temperature fluctuations with vertical wavelengths of 4 km or less are extracted from the measured temperature profiles in order to study small-scale gravity waves. Significant wave amplitudes are found in the stable atmosphere above the tropopause at roughly 60 km as compared with the only shallow temperature perturbations in the nearly adiabatic region of the adjacent middle cloud layer, below.Gravity wave activity shows a strong latitudinal dependence with the smallest wave amplitudes located in the low-latitude range, and an increase of wave activity with increasing latitude in both hemispheres; the greatest wave activity is found in the high-northern latitude range in the vicinity of Ishtar Terra, the highest topographical feature on Venus.We find evidence for a local time dependence of gravity wave activity in the low latitude range within ±30° of the equator. Gravity wave amplitudes are at their maximum beginning at noon and continuing into the early afternoon, indicating that convection in the lower atmosphere is a possible wave source.The comparison of the measured vertical wave structures with standard linear-wave theory allows us to derive rough estimates of the wave intrinsic frequency and horizontal wavelengths, assuming that the observed wave structures are the result of pure internal gravity waves. Horizontal wavelengths of the waves at 65 km altitude are on the order of ≈300–450 km with horizontal phase speeds of roughly 5–10 m/s.     

Atomic oxygen distributions in the Venus thermosphere: Comparisons between Venus Express observations and global model simulations

Nightglow emissions provide insight into the global thermospheric circulation, specifically in the transition region (～70–120 km). The O₂ IR nightglow statistical map created from Venus Express (VEx) Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) observations has been used to deduce a three-dimensional atomic oxygen density map. In this study, the National Center of Atmospheric Research (NCAR) Venus Thermospheric General Circulation Model (VTGCM) is utilized to provide a self-consistent global view of the atomic oxygen density distribution. More specifically, the VTGCM reproduces a 2D nightside atomic oxygen density map and vertical profiles across the nightside, which are compared to the VEx atomic oxygen density map. Both the simulated map and vertical profiles are in close agreement with VEx observations within a ～30° contour of the anti-solar point. The quality of agreement decreases past ～30°. This discrepancy implies the employment of Rayleigh friction within the VTGCM may be an over-simplification for representing wave drag effects on the local time variation of global winds. Nevertheless, the simulated atomic oxygen vertical profiles are comparable with the VEx profiles above 90 km, which is consistent with similar O₂ (¹Δ) IR nightglow intensities. The VTGCM simulations demonstrate the importance of low altitude trace species as a loss for atomic oxygen below 95 km. The agreement between simulations and observations provides confidence in the validity of the simulated mean global thermospheric circulation pattern in the lower thermosphere.     

Mesospheric vertical thermal structure and winds on Venus from HHSMT CO spectral-line observations

We report vertical thermal structure and wind velocities in the Venusian mesosphere retrieved from carbon monoxide (¹²CO J=2-1 and ¹³CO J=2-1) spectral line observations obtained with the Heinrich Hertz Submillimeter Telescope (HHSMT). We observed the mesosphere of Venus from two days after the second Messenger flyby of Venus (on 5 June 2007 at 23:10 UTC) during five days. Day-to-day and day-to-night temperature variations and short-term fluctuations of the mesospheric zonal flow were evident in our data. The extensive layer of warm air detected recently by SPICAV at 90-100 km altitude is also detected in the temperature profiles reported here.These data were part of a coordinated ground-based Venus observational campaign in support of the ESA Venus Express mission. Furthermore, this study attempts to cross-calibrate space- and ground-based observations, to constrain radiative transfer and retrieval algorithms for planetary atmospheres, and to contribute to a more thorough understanding of the global patterns of circulation of the Venusian atmosphere.     

Vertical extent of zonal winds on Venus

Possible interrelationships of different observations have been studied to clear up some obvious inconsistencies and develop a coherent picture of the kinematics of the Venus atmosphere. There is a wind shear in the vicinity of 60 km with vertical dimensions on the order of a scale height. The kinematical model has negligible surface winds, speeds increasing with altitude to approximately 45 km, a layer of high-speed retrograde zonal winds extending from approximately 45 to 60 km, a wind shear between 60 and 65 km, and slow atmospheric motions above this. Spacecraft data show that the region of high-speed winds is thicker on the day side of the planet than on the night side.     

New wind measurements in Venus’ lower mesosphere from visible spectroscopy

The dynamics of Venus’ mesosphere (70–110 km) is characterized by the superposition of two different wind regimes: (1) Venus’ retrograde superrotation; (2) a sub-solar to anti-solar (SS–AS) flow pattern, driven by solar EUV heating on the sunlit hemisphere. Here, we report on new ground-based velocity measurements in the lower part of the mesosphere. We took advantage of two essentially symmetric Venus elongations in 2001 and 2002 to perform high-resolution Doppler spectroscopy (R=120,000) in ¹²C¹⁶O₂ visible lines of the 5ν₃ band and in a few solar Fraunhofer lines near 8700 Å. These measurements, mapped over several points on Venus’ illuminated hemisphere, probe the region of cloud tops. More precisely, the solar Fraunhofer lines sample levels a few kilometers below the UV features (i.e. near ∼67 km), while the CO₂ lines probe an altitude higher by about 7 km. The wind field over Venus’ disk is retrieved with an rms uncertainty of 15–25 m s⁻¹ on individual measurements. Kinematical fit to a one- or two-component circulation model indicates the dominance of the zonal retrograde flow with a mean equatorial velocity of ∼75 m s⁻¹, exhibiting very strong day-to-day variations (±65 m s⁻¹). Results are very consistent for the two kinds of lines, suggesting a negligible vertical wind shear over 67–74 km. The SS–AS flow is not detected in single-day observations, but combining the results from all data suggests that this component may invade the lower mesosphere with a ∼40 m s⁻¹ velocity.     

Water vapor near the cloud tops of Venus from Venus Express/VIRTIS dayside data

Observations of the dayside of Venus performed by the high spectral resolution channel (–H) of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the ESA Venus Express mission have been used to measure the altitude of the cloud tops and the water vapor abundance around this level with a spatial resolution ranging from 100 to 10 km. CO₂ and H₂O bands between 2.48 and 2.60 μm are analyzed to determine the cloud top altitude and water vapor abundance near this level. At low latitudes (±40°) mean water vapor abundance is equal to 3 ± 1 ppm and the corresponding cloud top altitude at 2.5 μm is equal to 69.5 ± 2 km. Poleward from middle latitudes the cloud top altitude gradually decreases down to 64 km, while the average H₂O abundance reaches its maximum of 5 ppm at 80° of latitude with a large scatter from 1 to 15 ppm. The calculated mass percentage of the sulfuric acid solution in cloud droplets of mode 2 (～1 μm) particles is in the range 75–83%, being in even more narrow interval of 80–83% in low latitudes. No systematic correlation of the dark UV markings with the cloud top altitude or water vapor has been observed.     

Deuterium on Venus: Model comparisons with pioneer Venus observations of the predawn bulge ionosphere

A model of the predawn bulge ionosphere composition and structure is constructed and compared with the ion mass spectrometer measurements from the Pioneer Venus Orbiter during orbits 117 and 120. Particular emphasis is given to the identification of the mass-2 ion which we find unequivocally due to D⁺ (and not H₂⁺). The atmospheric D/H ratio of 1.4% and 2.5% is obtained at the homopause (～ 130 km) for the two orbits. The H₂⁺ contribution to the mass-2 ion density is less than 10%, and the H₂ mixing ratio must be <0.1 ppm at 130 km altitude. The He⁺ data require a downward He⁺ flux of ～2 × 10⁷ cm^?2 sec^?1 in the predawn region which suggest that the light ions also flow across the terminator from day to night along with the observed O⁺ ion flow.