Spatially-resolved high-resolution spectroscopy of Venus 2. Variations of HDO, OCS, and SO2 at the cloud tops

While CO, HCl, and HF, that were considered in the first part of this work, have distinct absorption lines in high-resolution spectra and were detected four decades ago, the lines of HDO, OCS, and SO₂ are either very weak or blended by the telluric lines and have not been observed previously by ground-based infrared spectroscopy at the Venus cloud tops. The H₂O abundance above the Venus clouds is typically below the detection limit of ground-based IR spectroscopy. However, the large D/H ratio on Venus facilitates observations of HDO. Converted to H₂O with D/H ≈ 200, our observations at 2722 cm⁻¹ in the Venus afternoon show a H₂O mixing ratio of ∼1.2 ppm at latitudes between ±40° increasing to ±60° by a factor of 2. The observations in the early morning reveal the H₂O mixing ratio that is almost constant at 2.9 ppm within latitudes of ±75°. The measured H₂O mixing ratios refer to 74 km. The observed increase in H₂O is explained by the lack of photochemical production of sulfuric acid in the night time. The recent observations at the P-branch of OCS at 4094 cm⁻¹ confirm our detection of OCS. Four distributions of OCS along the disk of Venus at various latitudes and local times have been retrieved. Both regular and irregular components are present in the variations of OCS. The observed OCS mixing ratio at 65 km varies from ∼0.3 to 9 ppb with the mean value of ∼3 ppb. The OCS scale height is retrieved from the observed limb darkening and varies from 1 to 4 km with a mean value of half the atmospheric scale height. SO₂ at the cloud tops has been detected for the first time by means of ground-based infrared spectroscopy. The SO₂ lines look irregular in the observed spectra at 2476 cm⁻¹. The SO₂ abundances are retrieved by fitting by synthetic spectra, and two methods have been applied to determine uncertainties and detection limits in this fitting. The retrieved mean SO₂ mixing ratio of 350 ± 50 ppb at 72 km favors a significant increase in SO₂ above the clouds since the period of 1980-1995 that was observed by the SOIR occultations at Venus Express. Scale heights of OCS and SO₂ may be similar, and the SO₂/OCS ratio is ∼500 and may be rather stable at 65-70 km under varying conditions on Venus.     

Sulfur chemistry in the Venus mesosphere from SO2 and SO microwave spectra

First measurements of SO₂ and SO in the Venus mesosphere (70-100 km) are reported. This altitude range is distinctly above the ∼60-70 km range to which nadir-sounding IR and UV investigations are sensitive. Since July 2004, use of ground-based sub-mm spectroscopy has yielded multiple discoveries. Abundance of each molecule varies strongly on many timescales over the entire sub-Earth Venus hemisphere. Diurnal behavior is evident, with more SO₂, and less SO, at night than during the day. Non-diurnal variability is also present, with measured SO₂ and SO abundances each changing by up to 2× or more between observations conducted on different dates, but at fixed phase, hence identical sub-Earth Venus local times. Change as large and rapid as a 5σ doubling of SO on a one-week timescale is seen. The sum of SO₂ and SO abundances varies by an order of magnitude or more, indicating at least one additional sulfur reservoir must be present, and that it must function as both a sink and source for these molecules. The ratio SO₂/SO varies by nearly two orders of magnitude, with both diurnal and non-diurnal components. In contrast to the strong time dependence of molecular abundances, their altitude distributions are temporally invariant, with far more SO₂ and SO at 85-100 km than at 70-85 km. The observed increase of SO₂ mixing ratio with altitude requires that the primary SO₂ source be upper mesospheric photochemistry, contrary to atmospheric models which assert upward transport as the only source of above-cloud SO₂. Abundance of upper mesospheric aerosol, with assumption that it is composed primarily of sulfuric acid, is at least sufficient to provide the maximum gas phase (SO + SO₂) sulfur reported in this study. Sulfate aerosol is thus a plausible source of upper mesospheric SO₂.     

Spatially-resolved high-resolution spectroscopy of Venus 1. Variations of CO2, CO, HF, and HCl at the cloud tops

Variations of the upper cloud boundary and the CO, HF, and HCl mixing ratios were observed using the CSHELL spectrograph at NASA IRTF. The observations were made in three sessions (October 2007, January 2009, and June 2009) at early morning and late afternoon on Venus in the latitude range of ±60°. CO₂ lines at 2.25 μm reveal variations of the cloud aerosol density (∼25%) and scale height near 65 km. The measured reflectivity of Venus at low latitudes is 0.7 at 2.25 μm and 0.028 at 3.66 μm, and the effective CO₂ column density is smaller at 3.66 μm than those at 2.25 μm by a factor of 4. This agrees with the almost conservative multiple scattering at 2.25 μm and single scattering in the almost black aerosol at 3.66 μm. The expected difference is just a factor of (1 − g)⁻¹ = 4, where g = 0.75 is the scattering asymmetry factor for Venus’ clouds. The observed CO mixing ratio is 52 ± 4 ppm near 08:00 and 40 ± 4 ppm near 16:30 at 68 km, and the higher ratio in the morning may be caused by extension of the CO morningside bulge to the cloud tops. The observed weak limb brightening in CO indicates an increase of the CO mixing ratio with altitude. HF is constant at 3.5 ± 0.2 ppb at 68 km in both morningside and afternoon observations and in the latitude range ±60°. Therefore the observations do not favor a bulge of HF, though HF is lighter than CO. Probably a source in the upper atmosphere facilitates the bulge formation. The recent measurements of HCl near 70 km are controversial (0.1 and 0.74 ppm) and require either a strong sink or a strong source of HCl in the clouds. The HCl lines of the (2-0) band are blended by the solar and telluric lines. Therefore we observed the P8 lines of the (1-0) band at 3.44 μm. These lines are spectrally clean and result in the HCl mixing ratio of 0.40 ± 0.03 ppm at 74 km. HCl does not vary with latitude within ±60°. Our observations support a uniformly mixed HCl throughout the Venus atmosphere.     

Vertical profile of H2SO4 vapor at 70-110 km on Venus and some related problems

The vertical profile of H₂SO₄ vapor is calculated using current atmospheric and thermodynamic data. The atmospheric data include the H₂O profiles observed at 70-112 km by the SOIR solar occultations, the SPICAV-UV profiles of the haze extinction at 220 nm, the VeRa temperature profiles, and a typical profile of eddy diffusion. The thermodynamic data are the saturated vapor pressures of H₂O and H₂SO₄ and chemical potentials of these species in sulfuric acid solutions. The calculated concentration of sulfuric acid in the cloud droplets varies from 85% at 70 km to a minimum of 70% at 90 km and then gradually increasing to 90-100% at 110 km. The H₂SO₄ vapor mixing ratio is ∼10⁻¹² at 70 and 110 km with a deep minimum of 3 × 10⁻¹⁸ at 88 km. The H₂O-H₂SO₄ system matches the local thermodynamic equilibrium conditions up to 87 km. The column photolysis rate of H₂SO₄ is 1.6 × 10⁵ cm⁻² s⁻¹ at 70 km and 23 cm⁻² s⁻¹ at 90 km. The calculated abundance of H₂SO₄ vapor at 90-110 km and its photolysis rate are smaller than those presented in the recent model by Zhang et al. (Zhang, X., Liang, M.C., Montmessin, F., Bertaux, J.L., Parkinson, C., Yung, Y.L. [2010]. Nat. Geosci. 3, 834-837) by factors of 10⁶ and 10⁹, respectively. Assumptions of 100% sulfuric acid, local thermodynamic equilibrium, too warm atmosphere, supersaturation of H₂SO₄ (impossible for a source of SO_X), and cross sections for H₂SO₄·H₂O (impossible above the pure H₂SO₄) are the main reasons of this huge difference. Significant differences and contradictions between the SPICAV-UV, SOIR, and ground-based submillimeter observations of SO_X at 70-110 km are briefly discussed and some weaknesses are outlined. The possible source of high altitude SO_X on Venus remains unclear and probably does not exist.     

Sulfuric acid aerosols in the atmospheres of the terrestrial planets

Clouds and hazes composed of sulfuric acid are observed to exist or postulated to have once existed on each of the terrestrial planets with atmospheres in our solar system. Venus today maintains a global cover of clouds composed of a sulfuric acid/water solution that extends in altitude from roughly 50 km to roughly 80 km. Terrestrial polar stratospheric clouds (PSCs) form on stratospheric sulfuric acid aerosols, and both PSCs and stratospheric aerosols play a critical role in the formation of the ozone hole. Stratospheric aerosols can modify the climate when they are enhanced following volcanic eruptions, and are a current focus for geoengineering studies. Rain is made more acidic by sulfuric acid originating from sulfur dioxide generated by industry on Earth. Analysis of the sulfur content of Martian rocks has led to the hypothesis that an early Martian atmosphere, rich in SO₂ and H₂O, could support a sulfur-infused hydrological cycle. Here we consider the plausibility of frozen sulfuric acid in the upper clouds of Venus, which could lead to lightning generation, with implications for observations by the European Space Agency's Venus Express and the Japan Aerospace Exploration Agency's Venus Climate Orbiter (also known as Akatsuki). We also present simulations of a sulfur-rich early Martian atmosphere. We find that about 40 cm/yr of precipitation having a pH of about 2.0 could fall in an early Martian atmosphere, assuming a surface temperature of 273 K, and SO₂ generation rates consistent with the formation of Tharsis. This modeled acid rain is a powerful sink for SO₂, quickly removing it and preventing it from having a significant greenhouse effect.     

Temporal variations of Titan’s middle-atmospheric temperatures from 2004 to 2009 observed by Cassini/CIRS

We use five and one-half years of limb- and nadir-viewing temperature mapping observations by the Composite Infrared Radiometer-Spectrometer (CIRS) on the Cassini Saturn orbiter, taken between July 2004 and December 2009 (L_S from 293° to 4°; northern mid-winter to just after northern spring equinox), to monitor temperature changes in the upper stratosphere and lower mesosphere of Titan. The largest changes are in the northern (winter) polar stratopause, which has declined in temperature by over 20 K between 2005 and 2009. Throughout the rest of the mid to upper stratosphere and lower mesosphere, temperature changes are less than 5 K. In the southern hemisphere, temperatures in the middle stratosphere near 1 mbar increased by 1-2 K from 2004 through early 2007, then declined by 2-4 K throughout 2008 and 2009, with the changes being larger at more polar latitudes. Middle stratospheric temperatures at mid-northern latitudes show a small 1-2 K increase from 2005 through 2009. At north polar latitudes within the polar vortex, temperatures in the middle stratosphere show a ∼4 K increase during 2007, followed by a comparable decrease in temperatures in 2008 and into early 2009. The observed temperature changes in the north polar region are consistent with a weakening of the subsidence within the descending branch of the middle atmosphere meridional circulation.     

Simultaneous mapping of SO2, SO, NaCl in Io’s atmosphere with the Submillimeter Array

Many of the key properties of Io’s atmosphere, such as its spatial distribution, temperature, column density and composition, are still not fully assessed despite decades of extensive observations. The contribution of the possible gas sources to the atmospheric replenishment are then still unclear.This paper presents disk-resolved observations performed with the Submillimeter Array (SMA) at 345 GHz of atmospheric rotational lines of the main atmospheric species SO₂, and, for the first time, of the minor species SO and NaCl. All these species appear concentrated on the anti-jovian hemisphere, but do not share the same spatial distribution. The obtained maps and line-averaged fluxes are compared to realistic models simulating gas sources including volcanic plume outgassing, SO₂ frost sublimation and photolysis. Arguments in favor of each sources are examined and compared to observations, putting constraints on their relative roles for each species.While sublimation clearly appears as the favored major source for SO₂, SO₂ photolysis may account for most of the production of SO. Using constraints on the volcanic plumes distribution from Galileo results, we find that direct volcanic input can only contribute for a minor fraction of atmospheric SO₂, but represent a more significant source for SO atmosphere, and is likely to be the only source for NaCl. Temperature and column densities findings are also presented for SO₂, and compare well to previously published observations and atmospheric models.     

Limits on Venus' SO2 abundance profile from interferometric observations at 3.4 mm wavelength

The role of SO₂ in the chemistry of the clouds of Venus has been investigated by deducing its mixing ratio profile in the atmosphere through millimeter wavelength interferometric measurements of the planet's limb darkening. The first zero crossing of the Venus visibility function was measured to be β₀ = 0.6221 ± 0.0007 at a wavelength of 3.4 mm using a reference radius for Venus of 6100 km. This measurement constrains the amount of limb darkening and shows that the high concentrations of SO₂ found in the lower atmosphere do not persist above an altitude of 42 km. Thus, a sink for SO₂ exists below the level of the lowest cloud deck.     

The microwave absorption of SO2 in the Venus atmosphere

Sulfur dioxide has a strong and complex rotational spectrum in the microwave and far infrared regions. The microwave absorption due to SO₂ in a CO₂ mixture is calculated for conditions applicable to the Venus atmosphere. It is shown that at the concentrations detected by Pioneer-Venus in situ measurements, SO₂ may be expected to contribute significantly to the microwave opacity of the Venus atmosphere. In particular, SO₂ might provide the major source of opacity in the atmospheric region immediately below the main sulfuric acid cloud deck. The spectrum is largely nonresonant at the pressures where SO₂ is expected to occur, however.     

Io's dayside SO2 atmosphere

An extensive set of HI Lyman-α images obtained with the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) from 1997-2001 has been analyzed to provide information about the spatial and temporal character of Io's SO₂ atmosphere. An atmospheric distribution map derived from the observations reveals that the sunlit SO₂ atmosphere is temporally stable on a global scale, with only small local changes. An anti-/sub-jovian asymmetry in the SO₂ distribution is present in all 5 years of the observations. The average daytime atmosphere is densest on the anti-jovian hemisphere in the equatorial regions, with a maximum equatorial column density of 5.0×¹⁰¹⁶ cm−2 at 140° longitude. The SO₂ atmosphere also has greater latitudinal extent on the anti-jovian hemisphere as compared to the sub-jovian. The atmospheric distribution appears to be best correlated with the location of hot spots and known volcanic plumes, although small number statistics for the plumes limits the correlation.     

High-resolution spectroscopy of Venus: Detection of OCS, upper limit to H2S, and latitudinal variations of CO and HF in the upper cloud layer

Venus was observed at 2.4 and 3.7 μm with a resolving power of 4×¹⁰⁴ using the long-slit high-resolution spectrograph CSHELL at NASA IRTF. The observations were made along a chord that covered a latitude range of ± 60° at a local time near 8:00. The continuous reflectivity and limb brightening at 2.4 μm are fitted by the clouds with a single scattering albedo 1−a=0.01 and a pure absorbing layer with τ=0.09 above the clouds. The value of 1−a agrees with the refractive index of H₂SO₄ (85%) and the particle radius of 1 μm. The absorbing layer is similar to that observed by the UV spectrometer at the Pioneer Venus orbiter. However, its nature is puzzling. CO₂ was measured using its R32 and R34 lines. The retrieved product of the CO₂ abundance and airmass is constant at 1.9 km-atm along the instrument slit in the latitude range of ± 60°. The CO mixing ratio (measured using the P21 line) is rather constant at 70 ppm, and its variations of ∼10% may be caused by atmospheric dynamics. The observed value is higher than the 50 ppm retrieved previously from a spectrum of the full disk, possibly, because of some downward extension of the mesospheric morningside bulge of CO. The observations of the HF R3 line reveal a constant HF mixing ratio of 3.5±0.5 ppb within ± 60° of latitude, which is within the scatter in the previous measurements of HF. OCS has been detected for the first time at the cloud tops by summing 17 lines of the P-branch. The previous detections of OCS refer to the lower atmosphere at 30-35 km. The retrieved OCS mixing ratio varies with a scale height of 1 to 3 km. The mean OCS mixing ratio is ∼2 ppb at 70 km and ∼14 ppb at 64 km. Vertical motions in the atmosphere may change the OCS abundance. The detected OCS should significantly affect Venus' photochemistry. A sensitive search for H₂S using its line at 2688.93 cm⁻¹ results in a 3 sigma upper limit of 23 ppb, which is more restrictive than the previous limit of 100 ppb.     

Chemical kinetic model for the lower atmosphere of Venus

A self-consistent chemical kinetic model of the Venus atmosphere at 0-47 km has been calculated for the first time. The model involves 82 reactions of 26 species. Chemical processes in the atmosphere below the clouds are initiated by photochemical products from the middle atmosphere (H₂SO₄, CO, S_x), thermochemistry in the lowest 10 km, and photolysis of S₃. The sulfur bonds in OCS and S_x are weaker than the bonds of other elements in the basic atmospheric species on Venus; therefore the chemistry is mostly sulfur-driven. Sulfur chemistry activates some H and Cl atoms and radicals, though their effect on the chemical composition is weak. The lack of kinetic data for many reactions presents a problem that has been solved using some similar reactions and thermodynamic calculations of inverse processes. Column rates of some reactions in the lower atmosphere exceed the highest rates in the middle atmosphere by two orders of magnitude. However, many reactions are balanced by the inverse processes, and their net rates are comparable to those in the middle atmosphere. The calculated profile of CO is in excellent agreement with the Pioneer Venus and Venera 12 gas chromatographic measurements and slightly above the values from the nightside spectroscopy at 2.3 μm. The OCS profile also agrees with the nightside spectroscopy which is the only source of data for this species. The abundance and vertical profile of gaseous H₂SO₄ are similar to those observed by the Mariner 10 and Magellan radio occultations and ground-based microwave telescopes. While the calculated mean S₃ abundance agrees with the Venera 11-14 observations, a steep decrease in S₃ from the surface to 20 km is not expected from the observations. The ClSO₂ and SO₂Cl₂ mixing ratios are ∼10⁻¹¹ in the lowest scale height. The existing concept of the atmospheric sulfur cycles is incompatible with the observations of the OCS profile. A scheme suggested in the current work involves the basic photochemical cycle, that transforms CO₂ and SO₂ into SO₃, CO, and S_x, and a minor photochemical cycle which forms CO and S_x from OCS. The net effect of thermochemistry in the lowest 10 km is formation of OCS from CO and S_x. Chemistry at 30-40 km removes the downward flux of SO₃ and the upward flux of OCS and increases the downward fluxes of CO and S_x. The geological cycle of sulfur remains unchanged.     

Saturn’s tropospheric composition and clouds from Cassini/VIMS 4.6-5.1 μm nightside spectroscopy

The latitudinal variation of Saturn’s tropospheric composition (NH₃, PH₃ and AsH₃) and aerosol properties (cloud altitudes and opacities) are derived from Cassini/VIMS 4.6-5.1 μm thermal emission spectroscopy on the planet’s nightside (April 22, 2006). The gaseous and aerosol distributions are used to trace atmospheric circulation and chemistry within and below Saturn’s cloud decks (in the 1- to 4-bar region). Extensive testing of VIMS spectral models is used to assess and minimise the effects of degeneracies between retrieved variables and sensitivity to the choice of aerosol properties. Best fits indicate cloud opacity in two regimes: (a) a compact cloud deck centred in the 2.5-2.8 bar region, symmetric between the northern and southern hemispheres, with small-scale opacity variations responsible for numerous narrow light/dark axisymmetric lanes; and (b) a hemispherically asymmetric population of aerosols at pressures less than 1.4 bar (whose exact altitude and vertical structure is not constrained by nightside spectra) which is 1.5-2.0× more opaque in the summer hemisphere than in the north and shows an equatorial maximum between ±10° (planetocentric).Saturn’s NH₃ spatial variability shows significant enhancement by vertical advection within ±5° of the equator and in axisymmetric bands at 23-25°S and 42-47°N. The latter is consistent with extratropical upwelling in a dark band on the poleward side of the prograde jet at 41°N (planetocentric). PH₃ dominates the morphology of the VIMS spectrum, and high-altitude PH₃ at p < 1.3 bar has an equatorial maximum and a mid-latitude asymmetry (elevated in the summer hemisphere), whereas deep PH₃ is latitudinally-uniform with off-equatorial maxima near ±10°. The spatial distribution of AsH₃ shows similar off-equatorial maxima at ±7° with a global abundance of 2-3 ppb. VIMS appears to be sensitive to both (i) an upper tropospheric circulation (sensed by NH₃ and upper-tropospheric PH₃ and hazes) and (ii) a lower tropospheric circulation (sensed by deep PH₃, AsH₃ and the lower cloud deck).     

Solar activity variations of thermospheric temperatures on Mars and a problem of CO in the lower atmosphere

Long-term MGS drag density observations at 390 km reveal variations of the density with season L_S (by a factor of 2) and solar activity index F_10.7 (by a factor of 3 for F_10.7 = 40-100). According to Forbes et al. (Forbes, J.M., Lemoine, F.G., Bruinsma, S.L., Smith, M.D., Zhang, X. [2008]. Geophys. Res. Lett. 35, L01201, doi:10.1029/2007GL031904), the variation with F_10.7 reflects variations of the exospheric temperature from 192 to 284 K. However, the derived temperature range corresponds to variation of the density at 390 km by a factor of 8, far above the observed factor of 3. The recent thermospheric GCMs agree with the derived temperatures but do not prove their adequacy to the MGS densities at 390 km. A model used by Forbes et al. neglects effects of eddy diffusion, chemistry and escape on species densities above 138 km. We have made a 1D-model of neutral and ion composition at 80-400 km that treats selfconsistently chemistry and transport of species with F_10.7, T_∞, and [CO₂]_80 km as input parameters. Applying this model to the MGS densities at 390 km, we find variation of T_∞ from 240 to 280 K for F_10.7 = 40 and 100, respectively. The results are compared with other observations and models. Temperatures from some observations and the latest models disagree with the MGS densities at low and mean solar activity. Linear fits to the exospheric temperatures are T_∞ = 122 + 2.17F_10.7 for the observations, T_∞ = 131 + 1.46F_10.7 for the latest models, and T_∞ = 233 + 0.54F_10.7 for the MGS densities at 390 km. Maybe the observed MGS densities are overestimated near solar minimum when they are low and difficult to measure. Seasonal variations of Mars’ thermosphere corrected for the varying heliocentric distance are mostly due to the density variations in the lower and middle atmosphere and weakly affect thermospheric temperature. Nonthermal escape processes for H, D, H₂, HD, and He are calculated for the solar minimum and maximum conditions.Another problem considered here refers to Mars global photochemistry in the lower and middle atmosphere. The models gave too low abundances of CO, smaller by an order of magnitude than those observed. Our current work shows that modifications in the boundary conditions proposed by Zahnle et al. (Zahnle, K., Haberle, R.M., Catling, D.C., Kasting, J.F. [2008]. J. Geophys. Res. 113, E11004, doi:10.1029/2008JE003160) are reasonable but do not help to solve the problem.     

Eclipse reappearances of Io: Time-resolved spectroscopy (1.9-4.2 μm)

We obtained time-resolved, near-infrared spectra of Io during the 60-90 min following its reappearance from eclipse by Jupiter on five occasions in 2004. The purpose was to search for spectral changes, particularly in the well-known SO₂ frost absorption bands, that would indicate surface-atmosphere exchange of gaseous SO₂ induced by temperature changes during eclipse. These observations were a follow-on to eclipse spectroscopy observations in which Bellucci et al. [Bellucci et al., 2004. Icarus 172, 141-148] reported significant changes in the strengths of two strong SO₂ bands in data acquired with the VIMS instrument aboard the Cassini spacecraft. One of the bands (4.07 μm [ν₁ + ν₃]) observed by Bellucci et al. is visible from ground-based observatories and is included in our data. We detected no changes in Io’s spectrum at any of the five observed events during the approximately 60-90 min during which spectra were obtained following Io’s emergence from Jupiter’s shadow. The areas of the three strongest SO₂ bands in the region 3.5-4.15 μm were measured for each spectrum; the variation of the band areas with time does not exceed that which can be explained by the Io’s few degrees of axial rotation during the intervals of observation, and in no case does the change in band strength approach that seen in the Cassini VIMS data. Our data are of sufficient quality and resolution to show the weak 2.198 μm (4549.6 cm⁻¹) 4ν₁ band of SO₂ frost on Io for what we believe is the first time. At one of the events (June 22, 2004), we began the acquisition of spectra ∼6 min before Io reappeared from Jupiter’s shadow, during which time it was detected through its own thermal emission. No SO₂ bands were superimposed on the purely thermal spectrum on this occasion, suggesting that the upper limit to condensed SO₂ in the vertical column above Io’s surface was ∼4 × 10⁻⁵ g cm⁻².     

Hydroxyl airglow on Venus in comparison with Earth

Hydroxyl nightglow is intensively studied in the Earth atmosphere, due to its coupling to the ozone cycle. Recently, it was detected for the first time also in the Venus atmosphere, thanks to the VIRTIS-Venus Express observations. The main Δν=1, 2 emissions in the infrared spectral range, centred, respectively, at 2.81 and 1.46 μm (which correspond to the (1-0) and (2-0) transitions, respectively), were observed in limb geometry (Piccioni et al., 2008) with a mean emission rate of 880±90 and 100±40 kR (1R=10⁶ photon cm⁻² s⁻¹ (4πster)⁻¹), respectively, integrated along the line of sight. In this investigation, the Bates-Nicolet chemical reaction is reported to be the most probable mechanism for OH production on Venus, as in the case of Earth, but HO₂ and O may still be not negligible as mechanism of production for OH, differently than Earth. The nightglow emission from OH provides a method to quantify O₃, HO₂, H and O, and to infer the mechanism of transport of the key species involved in the production. Very recently, an ozone layer was detected in the upper atmosphere of Venus by the SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus) instrument onboard Venus Express (Montmessin et al., 2009); this discovery enhances the importance of ozone to the OH production in the upper atmosphere of Venus through the Bates-Nicolet mechanism. On Venus, OH airglow is observed only in the night side and no evidence has been found whether a similar emission exists also in the day side. On Mars it is expected to exist both on the day and night sides of the planet, because of the presence of ozone, though OH airglow has not yet been detected.In this paper, we review and compare the OH nightglow on Venus and Earth. The case of Mars is also briefly discussed for the sake of completeness. Similarities from a chemical and a dynamical point of view are listed, though visible OH emissions on Earth and IR OH emissions on Venus are compared.     

Properties of the clouds of Venus,as inferred from airborne observations of its near-infrared reflectivity spectrum

We have measured the shape and absolute value of Venus' reflectivity spectrum in the 1.2-to 4.0-μm spectral region with a circular variable filter wheel spectrometer having a spectral resolution of 1.5%. The instrument package was mounted on the 91-cm telescope of NASA Ames Kuiper Airborne Observatory, and the measurements were obtained at an altitude of about 41,000 feet, when Venus had a phase angle of 86°. Comparing these spectra with synthetic spectra generated with a multiple-scattering computer code, we infer a number of properties of the Venus clouds. We obtain strong confirmatory evidence that the clouds are made of a water solution of sulfuric acid in their top unit optical depth and find that the clouds are made of this material down to an optical depth of at least 25. In addition, we determine that the acid concentration is 84 ± 2% H_2SO4 by weight in the top unit optical depth, that the total optical depth of the clouds is 37.5 ± 12.5, and that the cross-sectional weighted mean particle radius lies between 0.5 and 1.4 μm in the top unit optical depth of the clouds. These results have been combined with a recent determination of the location of the clouds' bottom boundary [Marov et al., Cosmic Res.14, 637–642 (1976)] to infer additional properties about Venus' atmosphere. We find that the average volume mixing ratio of H₂SO₄ and H₂O contained in the cloud material both equal approximately 2× 10^?6. Employing vapor pressure arguments, we show that the acid concentration equals 84 ± 6% at the cloud bottom and that the water vapor mixing ratio beneath the clouds lies between 6 × 10^?4 and 10^?2.     

Europa’s disk-resolved ultraviolet spectra: Relationships with plasma flux and surface terrains

The full set of high-resolution observations from the Galileo Ultraviolet Spectrometer (UVS) is analyzed to look for spectral trends across the surface of Europa. We provide the first disk-resolved map of the 280 nm SO₂ absorption feature and investigate its relationship with sulfur and electron flux distributions as well as with surface features and relative surface ages. Our results have implications for exogenic and endogenic sources. The large-scale pattern in SO₂ absorption band depth is again shown to be similar to the pattern of sulfur ion implantation, but with strong variations in band depth based on terrain. In particular, the young chaos units show stronger SO₂ absorption bands than expected from the average pattern of sulfur ion flux, suggesting a local source of SO₂ in those regions, or diapiric heating that leads to a sulfur-rich lag deposit.While the SO₂ absorption feature is confined to the trailing hemisphere, the near UV albedo (300-310 nm) has a global pattern with a minimum at the center of the trailing hemisphere and a maximum at the center of the leading hemisphere. The global nature of the albedo pattern is suggestive of an exogenic source, and several possibilities are discussed. Like the SO₂ absorption, the near UV albedo also has local variations that depend on terrain type and age.     

Mapping the mesospheric CO2 clouds on Mars: MEx/OMEGA and MEx/HRSC observations and challenges for atmospheric models

This study presents the latest results on the mesospheric CO₂ clouds in the martian atmosphere based on observations by OMEGA and HRSC onboard Mars Express. We have mapped the mesospheric CO₂ clouds during nearly three martian years of OMEGA data yielding a cloud dataset of ∼60 occurrences. The global mapping shows that the equatorial clouds are mainly observed in a distinct longitudinal corridor, at seasons L_s = 0-60° and again at and after L_s = 90°. A recent observation shows that the equatorial CO₂ cloud season may start as early as at L_s = 330°. Three cases of mesospheric midlatitude autumn clouds have been observed. Two cloud shadow observations enabled the mapping of the cloud optical depth (τ = 0.01-0.6 with median values of 0.13-0.2 at λ = 1 μm) and the effective radii (mainly 1-3 μm with median values of 2.0-2.3 μm) of the cloud crystals. The HRSC dataset of 28 high-altitude cloud observations shows that the observed clouds reside mainly in the altitude range ∼60-85 km and their east-west speeds range from 15 to 107 m/s. Two clouds at southern midlatitudes were observed at an altitude range of 53-62 km. The speed of one of these southern midlatitude clouds was measured, and it exhibited west-east oriented speeds between 5 and 42 m/s. The seasonal and geographical distribution as well as the observed altitudes are mostly in line with previous work. The LMD Mars Global Climate Model shows that at the cloud altitude range (65-85 km) the temperatures exhibit significant daily variability (caused by the thermal tides) with the coldest temperatures towards the end of the afternoon. The GCM predicts the coldest temperatures of this altitude range and the season L_s = 0-30° in the longitudinal corridor where most of the cloud observations have been made. However, the model does not predict supersaturation, but the GCM-predicted winds are in fair agreement with the HRSC-measured cloud speeds. The clouds exhibit variable morphologies, but mainly cirrus-type, filamented clouds are observed (nearly all HRSC observations and most of OMEGA observations). In ∼15% of OMEGA observations, clumpy, round cloud structures are observed, but very few clouds in the HRSC dataset show similar morphology. These observations of clumpy, cumuliform-type clouds raise questions on the possibility of mesospheric convection on Mars, and we discuss this hypothesis based on Convective Available Potential Energy calculations.     

SPICAV on Venus Express: Three spectrometers to study the global structure and composition of the Venus atmosphere

Spectroscopy for the investigation of the characteristics of the atmosphere of Venus (SPICAV) is a suite of three spectrometers in the UV and IR range with a total mass of 13.9 kg flying on the Venus Express (VEX) orbiter, dedicated to the study of the atmosphere of Venus from ground level to the outermost hydrogen corona at more than 40,000 km. It is derived from the SPICAM instrument already flying on board Mars Express (MEX) with great success, with the addition of a new IR high-resolution spectrometer, solar occultation IR (SOIR), working in the solar occultation mode. The instrument consists of three spectrometers and a simple data processing unit providing the interface of these channels with the spacecraft.A UV spectrometer (118–320 nm, resolution 1.5 nm) is identical to the MEX version. It is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation. In nadir orientation, SPICAV UV will analyse the albedo spectrum (solar light scattered back from the clouds) to retrieve SO₂, and the distribution of the UV-blue absorber (of still unknown origin) on the dayside with implications for cloud structure and atmospheric dynamics. On the nightside, γ and δ bands of NO will be studied, as well as emissions produced by electron precipitations. In the stellar occultation mode the UV sensor will measure the vertical profiles of CO₂, temperature, SO₂, SO, clouds and aerosols. The density/temperature profiles obtained with SPICAV will constrain and aid in the development of dynamical atmospheric models, from cloud top (∼60 km) to 160 km in the atmosphere. This is essential for future missions that would rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO⁺, and CO₂⁺, and its direct interaction with the solar wind. It will study the H corona, with its two different scale heights, and it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere.The SPICAV VIS-IR sensor (0.7–1.7 μm, resolution 0.5–1.2 nm) employs a pioneering technology: an acousto-optical tunable filter (AOTF). On the nightside, it will study the thermal emission peeping through the clouds, complementing the observations of both VIRTIS and Planetary Fourier Spectrometer (PFS) on VEX. In solar occultation mode this channel will study the vertical structure of H₂O, CO₂, and aerosols.The SOIR spectrometer is a new solar occultation IR spectrometer in the range λ=2.2–4.3 μm, with a spectral resolution λ/Δλ>15,000, the highest on board VEX. This new concept includes a combination of an echelle grating and an AOTF crystal to sort out one order at a time. The main objective is to measure HDO and H₂O in solar occultation, in order to characterize the escape of D atoms from the upper atmosphere and give more insight about the evolution of water on Venus. It will also study isotopes of CO₂ and minor species, and provides a sensitive search for new species in the upper atmosphere of Venus. It will attempt to measure also the nightside emission, which would allow a sensitive measurement of HDO in the lower atmosphere, to be compared to the ratio in the upper atmosphere, and possibly discover new minor atmospheric constituents.