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Mesospheric CO 2 clouds are one of two types of carbon dioxide clouds known on Mars. We present observations of mesospheric CO 2 clouds made by Atmospheric Chemistry Suite (ACS) onboard the ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO). We analyzed 1663 solar occultation sessions of Thermal InfraRed (TIRVIM) and Middle InfraRed (MIR) channels of ACS covering more than two Martian years that contain spectra of 2.7 μm carbon dioxide ice absorption band. That allowed us to unambiguously discriminate carbon dioxide ice aerosols from mineral dust and water ice aerosols, not relying on the information of atmospheric thermal conditions. CO 2 clouds were detected in eleven solar occultation observations at altitudes from 39 km to 90 km. In five cases, there were two or three layers of CO 2 clouds that were vertically separated by 5–15 km gaps. Effective radius of CO 2 aerosol particles is in the range of 0.1–2.2 μm. Spectra produced by the smallest particles indicate a need for a better resolved CO 2 ice refractive index. Nadir optical depth of CO 2 clouds is in the range 5 × 10−4–4 × 10−2 at both 2.7 μm and 0.8 μm. Asymmetrical diurnal distribution of detections observed by ACS is potentially due to local time variations of temperature induced by thermal tides. Two out of five cases of carbon dioxide cloud detections made by the TIRVIM instrument reveal the simultaneous presence of CO 2 ice and H 2 O ice aerosols. Temperature profiles measured by the Near InfraRed (NIR) channel of ACS are used to calculate CO 2 saturation ratio S at locations of carbon dioxide clouds. Supersaturation S > 1 is detected in only 6 out of 19 cases of CO 2 cloud layers; extremely low values of S < 0.1 are found in 9 out of 19 cases. • Detection of CO 2 mesospheric clouds relies on observing the 2.7 μm absorption band. • CO 2 clouds were detected in 11 solar occultation observations at 39–90 km altitudes. • Effective radius of CO 2 aerosol particles is in range of 0.1–2.2 μm. • Two cloud layers were formed by CO 2 and H 2 O ice aerosols. • CO 2 cloud layers with S > 1 detected in 6/19 cases, S < 0.1 found in 9/19 cases. [ABSTRACT FROM AUTHOR]

Spectroscopic solar occultation measurements by the Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus/Solar Occultation at Infrared instrument (SPICAV/SOIR) onboard the Venus Express orbiter gave new data about upper haze aerosol properties, its vertical distribution and spatial and temporal variations. Early study with three channels of SPICAV/SOIR instrument using a few selected orbits indicated presence of two aerosol modes in particle size distribution (Wilquet et al., 2009). Analysis of aerosol properties from the SPICAV−IR spectrometer for the whole Venus Express data set obtained from May 2006 till November 2014 has proved it for some occultations (Luginin et al., 2016). In this work, we report retrieval of the upper haze (81–100 km) aerosol properties from 101 simultaneous SPICAV−UV and –IR solar occultation sessions acquired between March 2007 and January 2013. A joint analysis of the data from two spectrometers allowed us to characterize the size distribution ∼10 km higher in the atmosphere compared to previous analysis and to detect bimodal distribution in ∼50% of observations previously believed to be unimodal. At altitudes 81–92 km bimodality is observed in >50% of cases. Mode 2 particles are detected up to 98 km and mode 1 up to 100 km. Mean radius equals 0.14 ± 0.03 μm for mode 1 and 0.78 ± 0.18 μ m for mode 2. Number density profiles for both modes of particles exponentially decrease with altitude, starting from 50 cm−3 and 0.3 cm−3 at 82 km for mode 1 and mode 2, respectively, and reaching 3 cm−3 at 98 km for mode 1 and 0.03 cm−3 at 94 km for mode 2. • At altitudes 81–92 km bimodal distribution was observed in >50% of observations. • The mean effective radii are 0.14 ± 0.03 μm for mode 1 and 0.78 ± 0.18 μm for mode 2. • Number density profiles for both modes exponentially decrease with altitude. • Effective variance of mode 1 reaches 0.4 at <84 km and 0.03–0.15 at higher altitudes. • Effective variance of mode 2 is almost constant with height with mean value 0.04 ± 0.02. [ABSTRACT FROM AUTHOR]

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 τ DL ∼ 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. [ABSTRACT FROM AUTHOR]

SPICAV IR, a channel of the SPICAV/SOIR suite of instruments onboard Venus Express spacecraft measured spectra in nadir and solar occultation modes in the range of 0.65–1.7 μm. We report results from 222 solar occultations observed from May 2006 to November 2014. The vertical resolution of measurements varies from 1 to 25 km depending on the distance of the spacecraft to the limb of Venus. The vertical profiles of atmospheric extinction were obtained at 10 near-IR wavelengths in the altitude range from 70 to 95 km. This allowed us to derive microphysical properties of the mesospheric haze. The aerosol haze top is higher near the equator than near the pole. In the upper haze, the aerosol scale height is found to be 3.3 ± 0.7 km. Detached haze layers were detected at altitudes from 70 to 90 km. Particle size and number density profiles are retrieved from extinction coefficients using Mie scattering theory adopting H 2 SO 4 refractive indices. Bimodal distribution of particles is consistent with data for some orbits with mean radius for mode 1 r eff1 = 0.12 ± 0.03 μm and r eff2 = 0.84 ± 0.16 μm for mode 2. Particle radii tend to cluster within occultation campaign and vary on the time scale of several months. The radius for the single mode case equals R eff = 0.54 ± 0.25 μm, and they are also 1.5–2 times smaller in the polar region (60°N–90°N) than in nonpolar regions (60°S–60°N). In bimodal case the number density profiles decrease smoothly for both modes, from ∼500 cm −3 at 75 km to ∼50 cm −3 at 90 km for mode 1, and from ∼1 cm −3 at 75 km to ∼0.1 cm −3 at 90 km for mode 2. [ABSTRACT FROM AUTHOR]

SPICAV VIS-IR spectrometer on-board the Venus Express mission measured the H 2 O abundance above Venus’ clouds in the 1.38 µm band, and provided an estimation of the cloud top altitude based on CO 2 bands in the range of 1.4–1.6 µm. The H 2 O content and the cloud top altitude have been retrieved for the complete Venus Express dataset from 2006 to 2014 taking into account multiple scattering in the cloudy atmosphere. The cloud top altitude, corresponding to unit nadir aerosol optical depth at 1.48 µm, varies from 68 to 73 km at latitudes from 40ºS to 40ºN with an average of 70.2 ± 0.8 km assuming the aerosol scale height of 4 km. In high northern latitudes, the cloud top decreases to 62–68 km. The altitude of formation of water lines ranges from 59 to 66 km. The H 2 O mixing ratio at low latitudes (20ºS-20ºN) is equal to 6.1 ± 1.2 ppm with variations from 4 to 11 ppm and the effective altitude of 61.9 ± 0.5 km. Between 30º and 50º of latitude in both hemispheres, a local minimum was observed with a value of 5.4 ± 1 ppm corresponding to the effective altitude of 62.1 ± 0.6 km and variations from 3 to 8 ppm. At high latitudes in both hemispheres, the water content varies from 4 to 12 ppm with an average of 7.2 ± 1.4 ppm which corresponds to 60.6 ± 0.5 km. Observed variations of water vapor within a factor of 2-3 on the short timescale appreciably exceed individual measurement errors and could be explained as a real variation of the mixing ratio or/and possible variations of the cloud opacity within the clouds. The maximum of water at lower latitudes supports a possible convection and injection of water from lower atmospheric layers. The vertical gradient of water vapor inside the clouds explains well the increase of water near the poles correlating with the decrease of the cloud top altitude and the H 2 O effective altitude. On the contrary, the depletion of water in middle latitudes does not correlate with the H 2 O effective altitude and cannot be completely explained by the vertical gradient of water vapor within the clouds. Retrieved H 2 O mixing ratio is higher than those obtained in 2.56 µm from VIRTIS-H data (Cottini et al., [2015] Planet. Space Sci., 113, 219–225 ) at altitudes of 68–70 km which is well consistent with the lower altitudes of water mixing ratio from the 1.38 µm band. Observations for different solar and emission angles allowed to constrain also the average vertical distribution of H 2 O mixing ratio in the clouds with 2 ppm at 66 km and 7–7.5 ppm at 59–61 km. The water vapor latitudinal-longitudinal distribution does not show any direct correlation with the cloud tops. Yet a strong asymmetry of H 2 O longitudinal distribution has been observed with a maximum of 7–7.5 ppm from −120º to 30º of longitude and shifted to the southern hemisphere (20ºS-10ºN). To the east, the minimum is observed with values not in excess of 6 ppm and over a wide range of longitudes from 30º to 160º. Bertaux et al. (2015) announced a correlation between the zonal wind pattern in the equatorial region and underlying topography of Aphrodite Terra as the result of stationary gravity waves produced at the ground level near the mountains. The water minimum corresponds to the Aphrodite Terra highlands and can be also associated with the influence of Venus topography. No prominent long-term on the time scale of 8.5 years nor local time variations of water vapor and the cloud top altitude were detected. [ABSTRACT FROM AUTHOR]