Mesosphere Of Venus Research Articles

The distributions of the OH Meinel and [formula omitted] nightglow emissions in the Venus mesosphere based on VIRTIS observations

O 2 ( a 1 Δ ) and recently discovered OH Meinel nightglow emissions have been observed at the limb with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-M) instrument on board the Venus Express satellite. Hydroxyl bands belonging to Δ v = 1 sequence between 2.60 and 3.14 μ m and to Δ v = 2 sequence at 1.40 – 1.46 μ m have been unambiguously identified. In this study, we analyze the statistical distribution of the Δ v = 1 OH Meinel band sequence and the a 1 Δ g - X 3 Σ (0–0) band of the O 2 Infrared Atmospheric bands at 1.27 μm. We also present an analysis of the correlation between the two emissions. From a statistical point of view, we find that the limb intensity of both emissions reach their maximum value near the antisolar point, while they are significantly dimmer in the vicinity of the terminator. The average altitude of the limb emissions peaks are 95.3 ± 3 km and 96 ± 2.7 km, respectively for the OH Δ v = 1 sequence and O 2 ( a 1 Δ ) emissions. The average intensities are 0.41 ± 0.37 MR and 28 ± 22 MR, respectively, corresponding to a mean ratio of about 70. The altitude of the OH nightglow layer is closely related to that of the O 2 ( a 1 Δ ) emission and some level of co-variation of the maximum intensity along the line of sight is observed. It is suggested that the global subsolar to antisolar circulation plays a key in the control of both airglows by carrying oxygen atoms from the day to the night side of the planet. The O atoms recombine to produce O 2 ( a 1 Δ ) molecules and they also act as precursors of ozone whose reaction with H produces excited hydroxyl.

Preliminary characterization of the upper haze by SPICAV/SOIR solar occultation in UV to mid‐IR onboard Venus Express

The Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus/Solar Occultation at Infrared (SPICAV/SOIR) suite of instruments onboard the Venus Express spacecraft comprises three spectrometers covering a wavelength range from ultraviolet to midinfrared and an altitude range from 70 to >100 km. However, it is only recently (more than 1 year after the beginning of the mission) that the three spectrometers can operate simultaneously in the solar occultation mode. These observations have enabled the study of the properties of the Venusian mesosphere over a broad spectral range. In this manuscript, we briefly describe the instrument characteristics and the method used to infer haze microphysical properties from a data set of three selected orbits. Discussion focuses on the wavelength dependence of the continuum, which is primarily shaped by the extinction caused by the aerosol particles of the upper haze. This wavelength dependence is directly related to the effective particle radius (cross section weighted mean radius) of the particles. Through independent analyses for the three channels, we demonstrate the potential to characterize the aerosols in the mesosphere of Venus. The classical assumption that the upper haze is only composed of submicron particles is not sufficient to explain the observations. We find that at high northern latitudes, two types of particles coexist in the upper haze of Venus: mode 1 of mean radius 0.1 ≤ rg ≤ 0.3 μm and mode 2 of 0.4 ≤ rg ≤ 1.0 μm. An additional population of micron‐sized aerosols seems, therefore, needed to reconcile the data of the three spectrometers. Moreover, we observe substantial temporal variations of aerosol extinction over a time scale of 24 h.

Limb observations of CO2 and CO non‐LTE emissions in the Venus atmosphere by VIRTIS/Venus Express

We report and analyze here observations of strong infrared emissions from the limb of the Venus upper atmosphere during daytime, taken by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) aboard Venus Express. We focus on the measurements taken during the first 4 months of nominal operations. The emissions observed at 4.3 μm and at 2.7 μm are attributed to CO2 fluorescence of solar radiation and are detected up to about 160 km and 130 km, respectively, while the CO fluorescence at 4.7 μm is observed up to about 120 km. The emissions are detected in both the channels of VIRTIS, at different spatial and spectral resolutions (resolving powers about 1800 and 400), for the periapsis and the apoapsis of the Venus Express orbit. From these data sets we built up 2‐D maps of the emissions as well as vertical profiles, which are then studied in order to characterize their variations with geophysical parameters, like solar illumination and emission altitude. Several analyses are performed in order to understand the VIRTIS behavior, to determine systematic effects in the data, and to propose appropriate corrections. We also present comparisons with a theoretical nonlocal thermodynamic equilibrium (non‐LTE) model of the Venus upper atmosphere. The agreement is very encouraging, in general, and the main variability observed in the data, with solar zenith angle and altitude, can be understood with the model. We conclude that the present data set opens brilliant perspectives for deriving densities and rotational temperatures in the upper mesosphere and lower thermosphere of Venus.

Atomic oxygen distribution in the Venus mesosphere from observations of O 2 infrared airglow by VIRTIS-Venus Express

This VIRTIS instrument on board Venus Express has collected spectrally resolved images of the Venus nightside limb that show the presence of the (0,0) band of the Δ g 1 → Σ g 3 infrared atmospheric system of O 2 at 1.27 μm. The emission is produced by three-body recombination of oxygen atoms created by photodissociation of CO 2 on the dayside. It is consistently bright so that emission limb profiles can be extracted from the images. The vertical distribution of O 2( Δ g 1 ) may be derived following Abel inversion of the radiance limb profiles. Assuming photochemical equilibrium, it is combined with the CO 2 vertical distribution to determine the atomic oxygen density. The uncertainties on the O density caused by the Abel inversion reach a few percent at the peak, increasing to about 50% near 120 km. We first analyze a case when the CO 2 density was derived from a stellar occultation observed with the SPICAV spectrometer simultaneously with an image of the O 2 limb airglow. In other cases, an average CO 2 profile deduced from a series of ultraviolet stellar occultations is used to derive the O profile, leading to uncertainties on the O density less than 30%. It is found that the maximum O density is generally located between 94 and 115 km with a mean value of 104 km. It ranges from less than 1 × 10 11 to about 5 × 10 11 cm −3 with a global mean of 2.2 × 10 11 cm −3 . These values are in reasonable agreement with the VIRA midnight oxygen profile. The vertical O distribution is generally in good agreement with the oxygen profile calculated with a one-dimensional chemical-diffusive model. No statistical latitudinal dependence of the altitude of the oxygen peak is observed, but the maximum O density tends to decrease with increasing northern latitudes. The latitudinal distribution at a given time exhibits large variations in the O density profile and its vertical structure. The vertical oxygen distribution frequently shows multiple peaks possibly caused by waves or variations in the structure of turbulent transport. It is concluded that the O 2 infrared night airglow is a powerful tool to map the distribution of atomic oxygen in the mesosphere between 90 and 115 km and improve future Venus reference atmosphere models.

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.

Venus upper atmospheric CO, temperature, and winds across the afternoon/evening terminator from June 2007 JCMT sub-millimeter line observations

The Venus mesosphere constitutes a highly variable transition region between the zonal rotation of the lower atmosphere and the diurnal circulation of the upper atmosphere. It further serves as the primary photochemical region of the Venus atmosphere. We obtained James Clerk Maxwell Telescope (JCMT, Mauna Kea Hawaii) sub-millimeter line observations of mesospheric 12CO and 13CO during coordinated space (MESSENGER and Venus Express) and ground-based observations of Venus in June of 2007. Such CO spectra line measurements support temperature, CO mixing ratio, and wind retrievals over the 80–110 km altitude range, encompassing the upper mesosphere and lower thermosphere of Venus. Five-point beam integrations were obtained across the observed Venus disk, allowing distinction of afternoon (noon to 6 p.m.) versus evening (6 p.m. to midnight) local times and northern (0–60N) versus southern (0–60S) latitudes. Distinctive diurnal variations (noon to midnight) are retrieved for both temperatures above 95 km and CO mixing ratios above 85 km altitudes. Separate CO line maps obtained on (UT) June 2, 3, 6, and 11 indicate moderate daily variability in afternoon and evening CO mixing ratios (20–50%) and temperatures (5–10 K). Average Venus mesospheric temperatures over this period were 10 K warmer than returned from 1978 to 1979 Pioneer Venus or 2000–01 sub-millimeter measurements, without evidence for the very large temperature inversions indicated by Venus Express SPICAV measurements at 90–100 km altitudes (Bertaux, J.L., Vandaele, A.-C., Korablev, O., Villard, E., Fedorova, A., Fussen, D., Quémerais, E., Belyaev, D., Mahieux, A., Montmessin, F., Muller, C., Neefs, E., Nevejans, D., Wilquet, V., Dubois, J.P. Hauchecorne, A., Stepanov, A., Vinogradov, I., Rodin, A., Bertaux, J.-L., Nevejans, D., Korablev, O., Montmessin, F., Vandaele, A.-C., Fedorova, A., Cabane, M., Chassefière, E., Chaufray, J.Y., Dimarellis, E., Dubois, J.P., Hauchecorne, A., Leblanc, F., Lefèvre, F., Rannou, P., Quémerais, E., Villard, E., Fussen, D., Muller, C., Neefs, E., Van Ransbeeck, E., Wilquet, V., Rodin, A., Stepanov, A., Vinogradov, I., Zasova, L., Forget, F., Lebonnois, S., Titov, D., Rafkin, S., Durry, G., Gérard, J.C., Sandel, B., 2007. A warm layer in Venus’ cryosphere and high-altitude measurements of HF, HCl, H 2O and HDO. Nature 450, 646–649). Measured Doppler shifts associated with June 2 and 11 12CO line center absorptions indicate nearly supersonic (200 m/s, Mach 1) afternoon-to-evening (retrograde) circulation; composed of additive subsolar-to-antisolar (SSAS) and zonal retrograde wind components, which are not separable due to the particular observational geometry.

The temperature of the Venus mesosphere from O2 (aΔg1) airglow observations

We have used near-infrared spectroscopic observations of the Venus nightside taken with the Infrared Imager and Spectrograph 2 (IRIS2) on the Anglo–Australian Telescope to derive temperature maps for the Venus mesosphere at an altitude of ∼95 km. The temperatures are derived from the distribution of rotational line intensities in the O2 (aΔg1) airglow band at 1.27 μm. To obtain reliable temperatures at the relatively low spectral resolution of IRIS2, we have developed a forward modeling approach to handle the blending of individual O2 lines and the telluric absorption in the same O2 band. The technique provides temperature retrievals with accuracy comparable to, or better than that of previous high-spectral resolution determinations. The resulting temperature maps show spatial temperature structure that varies from night to night, as does the intensity distribution. Intensity weighted mean temperatures range from about 181 to 196 K. The temperatures are typically 15–30 K higher than those expected from the Venus International Reference Atmosphere (VIRA) profile. The temperatures fall in regions of low O2 emission rate to values closer to the VIRA levels. Our temperatures are similar to, but slightly lower than those obtained from stellar occultation measurements with SPICAV on Venus Express. We suggest that we are seeing a region of locally enhanced temperature caused by compressional heating in the downwelling gas around the antisolar point.

HDO and H2O vertical distributions and isotopic ratio in the Venus mesosphere by Solar Occultation at Infrared spectrometer on board Venus Express

Vertical distributions of the molecular density and mixing ratios of H2O and HDO in the Venus mesosphere have been obtained using Solar Occultation at Infrared (SOIR), a high‐resolution (with λ/δλ ∼ 20,000) echelle spectrometer on Venus Express. The atmosphere is sounded in solar occultation in the range of altitudes from 65 to 130 km. Simultaneous measurements of water vapor lines in the spectral range around 2.61 μm (3830 cm−1) at altitudes between 70 and 110 km and HDO lines around 3.58 μm (2715cm−1) at altitudes 70–95 km have been performed. During 1 1/2 years, from April 2006 to August 2007, 54 such measurements have been carried out at different locations of Venus from the north pole to middle south latitudes. Most of the observations at morning and evening terminator correspond to high northern latitudes. We report values of mixing ratio and isotopic ratio obtained for 22 of those measurements occurring in the northern polar area. The average value of the volume mixing ratio of H2O is 1.16 ± 0.24 ppm and that of HDO is 0.086 ± 0.020 ppm. A depletion in the mixing ratio for both H2O and HDO is observed at 85 km, which can be related to a depletion of CO2 density above (∼95 km) and a possible temperature inversion at these altitudes. The vertical variation of HDO and H2O mixing ratio is within a factor of 2–3 for the analyzed set of observations. The temporal variations have been investigated, and no noticeable variability of H2O is reported at high northern altitudes. The average ratio of HDO/H2O obtained in this work, 240 ± 25 times the terrestrial ratio, is higher (≈1.5 times) than the value of 157 ± 30 times terrestrial reported for the lower atmosphere. This could be explained by a lower photodissociation of HDO and/or a lower escape rate of D atoms versus H atoms.

Cyclostrophic winds from the Visible and Infrared Thermal Imaging Spectrometer temperature sounding: A preliminary analysis

Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) onboard the Venus Express spacecraft has been operating since April 2006 providing new observations of the temperature structure of Venus mesosphere (60–95 km). Zonal winds in the middle atmosphere of Venus have been retrieved from VIRTIS temperature profiles using the cyclostrophic approximation. The wind field is characterized by three main features: (1) a midlatitude jet connected to the thermal feature known as the cold collar; the jet reaches a maximum speed of 80–90 ± 10 m/s near the cloud top (∼70 km altitude) at 50°S latitude; (2) a strong decrease of wind from 60°S toward the pole reaching zero velocity at ∼70°S; and (3) the decrease of the wind above the jet with increasing altitude. Local time dependence of the temperature and wind field has been analyzed. Temperatures show at cloud tops a cooling of ∼15 K during the night which propagates also on the zonal wind field. Comparison with cloud‐tracked winds from Venus Monitoring Camera images indicates a first‐order agreement.

Composition of the Venus mesosphere measured by Solar Occultation at Infrared on board Venus Express

Solar Occultation at Infrared (SOIR), which is a part of the Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) instrument on board Venus Express, combines an echelle‐grating spectrometer with an acoustooptical tunable filter. It performs solar occultation measurements in the IR region at a high spectral resolution better than all previously flown planetary spectrometers. The wavelength range probed allows for a detailed chemical inventory of the Venus atmosphere above the cloud layer, with an emphasis on the vertical distribution of the gases. A general description of the retrieval technique is given and is illustrated by some results obtained for CO2 and for a series of minor constituents, such as H2O, HDO, CO, HCl, and HF. Detection limits for previously undetected species will also be discussed.

WITHDRAWN: SWAS Observations of Water Vapor in the Venus Mesosphere

The publisher regrets that this article is an accidental duplication of an article that has already been published in Icarus, volume 188/2 (2007) 288–304 doi: 10.1016/j.icarus.2006.12.004 . The duplicate article has therefore been withdrawn.

SWAS observations of water vapor in the Venus mesosphere

We present the first detections of the ground-state H 2 16O ( 1 10 – 1 01 ) rotational transition (at 556.9 GHz) and the 13CO (5–4) rotational transition from the atmosphere of Venus, measured with the Submillimeter Wave Astronomy Satellite (SWAS). The observed spectral features of these submillimeter transitions originate primarily from the 70–100 km altitude range, within the Venus mesosphere. Observations were obtained in December 2002, and January, March, and July 2004, coarsely sampling one Venus diurnal period as seen from Earth. The measured water vapor absorption line depth shows large variability among the four observing periods, with strong detections of the line in December 2002 and July 2004, and no detections in January and March 2004. Retrieval of atmospheric parameters was performed using a multi-transition inversion algorithm, combining simultaneous retrievals of temperature, carbon monoxide, and water profiles under imposed constraints. Analysis of the SWAS spectra resulted in measurements or upper limits for the globally averaged mesospheric water vapor abundance for each of the four observation periods, finding variability over at least two orders of magnitude. The results are consistent with both temporal and diurnal variability, but with short-term fluctuations clearly dominating. These results are fully consistent with the long-term study of mesospheric water vapor from millimeter and submillimeter observations of HDO [Sandor, B.J., Clancy, R.T., 2005. Icarus 177, 129–143]. The December 2002 observations detected very rapid change in the mesospheric water abundance. Over five days, a deep water absorption feature consistent with a water vapor abundance of 4.5 ± 1.5 parts per million suddenly gave way to a significantly shallower absorption, implying a decrease in the water vapor abundance by a factor of nearly 50 in less that 48 h. In 2004, similar changes in the water vapor abundance were measured between the March and July SWAS observing periods, but variability on time scales of less than a week was not detected. The mesospheric water vapor is expected to be in equilibrium with aerosol particles, primarily composed of concentrated sulfuric acid, in the upper haze layers of the Venus atmosphere. If true, moderate amplitude (10–15 K) variability in mesospheric temperature, previously noted in millimeter spectroscopy observations of Venus, can explain the rapid water vapor variability detected by SWAS.

Water vapor variations in the Venus mesosphere from microwave spectra

Mm-wave spectra of HDO in the Venus mesosphere (65–100 km) were obtained over the period March 1998 to June 2004. Each spectrum is a measurement of the hemispheric-average H 2O vapor mixing ratio in the Venus mesosphere. Observations were conducted for wide ranges of Venus solar elongations (46° W to 47° E), and fractional disk illuminations ( f = 0 % to 99%), yielding water vapor abundances on 17 dates and over a full range of local solar time (LST) at the sub-Earth point on Venus. Our mesopheric H 2O values are more numerous and far more precise than the earliest mm-derived H 2O measurements [Encrenaz, Th., Lellouch, E., Paubert, G., Gulkis, S., 1991. First detection of HDO in the atmosphere of Venus at radio wavelengths: An estimate of the H 2O vertical distribution. Astron. Astrophys. 246, L63–L66; Encrenaz, Th., Lellouch, E., Cernicharo, J., Paubert, G., Gulkis, S., Spilker, T., 1995. The thermal profile and water abundance in the Venus mesosphere from H 2O and HDO millimeter observations. Icarus 117, 162–172], allowing an analysis of variability that was previously impossible. Measured 65–100 km H 2O ranged from 0.0 ± 0.06 to 3.5 ± 0.3 ppmv , with significantly different variability than found in previous infrared (lower altitude, cloudtop) studies. Strong global variability on a 1–2 month timescale is clear and unambiguous. A limited number of excellent s/n measurements tentatively indicate the 1–2 month variability manifests most rapidly as change in the lower mesosphere, and more slowly as change in the upper mesosphere. Neither long term (1998–2004) nor diurnal variability in 65–100 km H 2O is evident. While six-year and/or diurnal variabilities are not ruled out, they are weaker than the 1–2 month timescale variation. These conclusions are supported by initial (2004) sub-mm measurements.

Temporal and spatial variations in the Venus mesosphere retrieved from Pioneer Venus OIR

Measurements of the Venus mesosphere made in 1978/79 by the Pioneer Venus Orbiter Infrared Radiometer (OIR) have been reanalysed. An optimal estimation retrieval technique has been applied to data from individual orbits. These retrievals reveal the structure of transient features such as the polar dipole and polar collar and short term variations in water vapour abundance and cloud top height. High abundances of water vapour are observed at equatorial latitudes in the early afternoon with a spatial structure which appears consistent with the ultraviolet ‘Y’ shaped structure. Additionally a two to four day period is observed in both water vapour abundance and cloud top height which is the characteristic period of this ultraviolet feature.

Ground‐based near‐infrared observations of the Venus nightside: 1.27‐μm O2(a1Δg) airglow from the upper atmosphere

Near‐infrared spectroscopic observations of Venus taken in 1975 revealed O2(a1Δg) airglow from both the dayside and nightside of the planet with emission rates exceeding 1 mega‐Rayleigh (1 MR = 1012 photons cm−2 s−1 into 4π sr). These large emission rates indicated that most of the atomic oxygen produced through the photolysis of CO2 on the dayside of Venus eventually recombined to produce O2 in the excited (a1Δg) state. This result was initially surprising because available laboratory measurements indicated O2(a1Δg) yields from atomic oxygen recombination reactions that were no larger than a few percent. More recent observations reveal even larger O2(a1Δg) airglow intensities as well as dramatic spatial and temporal variations in this airglow. High‐resolution (0.3 cm−1) spectra of the Venus nightside taken with the Canada France Hawaii Telescope/Fourier transform spectrometer in 1991 show spectrally integrated O2(a1Δg) intensities as large as 1.1 mW m−2 sr−1. Once these values are corrected for viewing angle and reflection from the underlying clouds, they indicate emission rates near 3 MR. These spectra also yield rotational temperatures of 186 ± 6 K in the emitting layer (90 to 115 km). Spectral image cubes taken with the Anglo‐Australian Telescope/infrared imaging spectrometer and the Canada France Hawaii Telescope/imaging Fourier transform spectrometer during 1991, 1993, and 1994 provide a more complete description of the spatial and temporal variability in this emission. Images extracted at wavelengths within the O2(a1Δg) Q‐branch (1.269 μm) often show contrasts larger than 10 to 1 across the nightside. Even though the disk‐averaged intensities are comparable to those seen in 1975, some localized regions have airglow emission rates larger than 5 MR. The brightest emission is often confined to 1000‐ to 2000‐km‐diameter regions. These bright regions have been detected over a broad range of latitudes and local times, but they are most often seen at low latitudes and at local times between midnight and 0300 on Venus. The intensity of the brightest spots can change by 20% in less than 1 hour, and they can vanish entirely in less than 1 day. These new observations are providing improved constraints on atmospheric chemical and dynamical models of the upper mesosphere and lower thermosphere of Venus.

The Thermal Profile and Water Abundance in the Venus Mesosphere from H 2O and HDO Millimeter Observations

The 183.310-GHz water vapor transition has been observed in the Venus atmosphere for the first time. The detection was made in January 1991, using the 30-m IRAM radiotelescope. A spectrum of Saturn, recorded during the same night, was used to remove the contribution due to terrestrial mesospheric water. Using the depth and the FWHM of the observed absorption line, we derive a water mixing ratio of 1.0 (+ 1.0, -0.5) ppm above the clouds, and a temperature of 140 (±10) K at the level z = 95 km ( P = 0.06 mb). In July 1993, we reobserved the HDO transition at 225.987 GHz, which we first detected in August 1990 (T. Encrenaz et al. 1991, Astron. Astrophys. 246, L63-L66). The new set of data was obtained at the Caltech Submillimeter Observatory (CSO), with improved signal-to-noise and spectral resolution with respect to previous measurements. From the 1993 data we derive a water mixing ratio of 7.0 (+5.0, -4.0) ppm above the clouds, and a temperature higher than 145 K at the z = 95-km level. The water abundance is derived assuming a deuterium enrichment factor of 120 with respect to the terrestrial value (C. de Bergh et al. 1991, Science 251, 547-549). If the deuterium enrichment factor is close to 160, as measured by G. L. Bjoraker et al. (1992, Bull. Am. Astron. Soc. 24, 995-995), the difference between the H 2O measurements is reduced, but the data might still suggest some possible fluctuation in the water vapor content above the clouds between 1991 and 1993.

Observations of the CO Bulge on Venus and Implications for Mesospheric Winds

Observations of CO at 2.6 mm (115.27 GHz) were made with the OVRO millimeter interferometer in 1986 and 1988, yielding high-quality disk-resolved spectra which were inverted to determine the CO mixing ratio profile from distinct regions on the disk, allowing us to map the distribution of CO in the upper mesosphere of Venus both horizontally and vertically. The 1986 observations were of the morning terminator and were particularly useful in searching for a suspected CO maximum ("bulge") on the nightside. The resulting CO mixing ratio profiles were mapped for various altitudes as functions of latitude and local time, and we report that we have resolved the previously inferred CO bulge. The bulge increases in magnitude from a small day-night variation at 90 km to an extensive nightside peak at 100 km, the upper limit of our observations. The peak bulge-to-dayside ratio approached 20-30 at 100 km in 1986 and may have been as large as 50-100, assuming late-afternoon CO abundances found in 1988 were similar to those in 1986. Three-dimensional mapping shows that in the upper mesosphere the bulge was displaced from local midnight toward the morning equator, centered at 3:30 AM local time. Using the qualitative model of mesospheric circulation on Venus proposed by Clancy and Muhleman (1985, Icarus 64, 183-204; 1991, Icarus 89, 129-146), we explain this shift in terms of strong retrograde zonal winds throughout the mesosphere, matching the directly detected mesospheric circulation (Shah, K., D. O. Muhleman, and G. L. Berge 1991, Icarus 93, 96-121) observed with the same dataset in 1988.

Radiative balance in the mesosphere of Venus from the VENERA-15 infrared spectrometer results

The results of the VENERA-15 Fourier Transform Infrared Spectrometer (FTIR) experiment provided a unique chance to retrieve simultaneously the thermal and aerosol structures of the mesosphere of Venus. These data were used as a basis for the numerical modelling of the radiative transfer. The cooling and heating rate altitude profiles in the range from 90 down to 55 kilometers are presented for different latitudes from 20°N to 87°N. The main components that influence the propagation of solar and thermal radiation at these altitudes are carbon dioxide, H 2SO 4 aerosols and the ultraviolet absorbers. The analisys of the cooling, heating and net cooling-heating latitude-altitude fields leads to the conclusion that the mesosphere of Venus is heated at low latitudes ( ϕ<40°) at the cloud tops (58–65 km) mainly due to the presence of the UV absorbers. The strong cooling (10–15°K/Day) prevails in the high latitude ( ϕ>60°) mesosphere with a sharp maximum at the dense cloud tops in the “cold collar” region where the net cooling could reach 25°K/day.

Measurement of winds in Venus' upper mesosphere based on doppler shifts of the 2.6-mm 12CO line

Before inferior conjuction in 1988, we observed Venus at the first rotational transition of 12CO using the three-element, millimeter-wavelenght array at the Owens Valley Radio Astronomy Observatory and successfully detected winds in the upper mesosphere of Venus. The subearth point occurred at 19.3 hr local Venus time and the planet's disk showed the evening terminator. The 33″.4 diameter disk was resolved by a synthesized beam 2″.8 at full-width-half-maximum. The absorption line was finely sampled in frequency by a 32-channel filterbank with channel widths of 50 kHz. With this spatial and spectral resolution, it was possible to measure doppler shifts of the absorption line across the planet due to strong winds in Venus' upper mesosphere. Weighting functions show sampling of the mesosphere within a 12-km-wide layer centered at 97 km at normal incidence in the evening, 99 km on the dayside limb, and 103 km on the nightside limb. The doppler shifts change in a manner indicative of westward, horizontal winds—being strongly blue on the east/dayside limb, zero near the center, and strongly red on the west/nightside limb of the planet. Measurements of these shifts along the planet limb are closest to direct measurements of the horizontal speeds once corrected for incidence angle. The mean value along the dayside limb is 85 ± 8 m sec −1 and along the nightside limb is 106 ± 10 m sec −1. The radial wind speeds from the doppler shifts were smoothed to reduce noise and then fitted in a least-squares sense to canonical forms of the lower atmosphere's westward zonal flow and the thermosphere's subsolar-to-antisolar flow. The two flows are highly correlated in orientation in these observations of Venus' evening terminator. The smoothed measurements are best fitted for zonal flows in 10° latitude bins from 70°S to 61°N. The mean value of these fitted zonal flows is 132 ± 10 m sec −1. No facet of the data exists to firmly support or rule out the existence of a smaller (≤40 m sec −1) subsolar-to-antisolar flow in the upper mesosphere.

Discovery of a Doppler-limited CO line in the upper mesosphere of Venus - A new dynamical probe

The presence of CO in the upper atmosphere of Venus is a consequence of the photochemistry of CO2, the dominant atmospheric constituent. In December 1989 the J = 1-2 transition of CO was observed at 230 GHz. In addition to the broad absorption line first reported by Kakar et al. (1976), a narrow absorption feature at the center of the line due to upper mesospheric CO, where the temperature profile starts to exhibit diurnal variation with altitude. The narrow feature is approximately 600 kHz wide and is predominantly Doppler-broadened. The Doppler core provides a new means of measuring wind velocities at these altitudes in the atmosphere of Venus. Detection of small Doppler shifts in the line core can in principle be used to measure winds with an accuracy of 10 m/s. Results are presently limited by the 17 kHz uncertainty in the measured rest frequency, corresponding to a systematic error in wind velocity up to 22 m/s, and the absence of laboratory measurements of the pressure shift in CO by CO2.