Visible And Infrared Thermal Imaging Spectrometer Research Articles

Idunn Mons: Evidence for Ongoing Volcano-tectonic Activity and Atmospheric Implications on Venus

Abstract In 2010 the ESA Venus Express Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument first observed 1 μm emissivity anomalies over the top and eastern flank of Idunn Mons (46° S; 146° W), a 200 km wide volcano located in Imdr Regio, a volcano-dominated large volcanic rise of Venus. The anomalies suggest the presence of chemically unweathered and fresh volcanic deposits, which provided the first hint that volcanism in this area may have been active during the past few million years. Subsequent studies have investigated the geologic and atmospheric evolution at Idunn Mons, but no study has comprehensively investigated the evolution and the implication for recent activity in Idunn Mons. Previous work, using both VIRTIS data and Magellan radar emissivity data, confirmed the occurrence of unaltered basaltic lava flows at Idunn Mons. Building on that previous work, experimental laboratory studies have revealed that chemical weathering on Venus may act much faster than previously expected, which suggests very young ages for these flows. This inference has been supported by investigations of the tectonic fracturing surrounding Idunn Mons. Finally, atmospheric data from VIRTIS also show regional anomalies in the speed of the winds in the lower atmosphere over Imdr Regio, which may be related to very recent or ongoing volcanism. In this paper, we take a comprehensive approach, using atmospheric to surface measurements, including recent laboratory experiments, to constrain the evolution of Idunn Mons. Our work suggests that Idunn Mons may be geologically both volcanically and tectonically active today.

Using VIRTIS on Venus Express to Constrain the Properties of the Giant Dark Cloud Observed in Images of Venus by IR2 on Akatsuki

Abstract A cloud opacity contrast feature that has been called a “long-lived sharp disruption” has been seen in the atmosphere of Venus in the near-infrared using Akatsuki’s IR2 camera, most clearly at equatorial latitudes. This feature was found to have a consistent planet-circling period of 4.9 days, and subsequent searches of past imagery revealed that it has probably existed for at least 30 years, the duration of near-infrared investigation of the deep atmosphere of Venus. Guided by the remarkably consistent morphological appearance of this feature, we have identified at least one previous instance of it in the Venus Express Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) data. We take advantage of the spectroscopic capabilities of VIRTIS to retrieve atmospheric parameters in the vicinity of this feature that cannot be retrieved using the limited filter selection on board Akatsuki. We find that the changes in measurable quantities, such as cloud particle acid mass fraction, water vapor, carbon monoxide, cloud base altitude, and particle size, suggest that the changes that take place in the vicinity of this feature are restricted to the lower clouds of Venus (below 50 km). We hypothesize that further evolution of this feature (over timescales of days to weeks) results in measurable variations in these parameters at altitudes in the middle clouds of Venus (50–57 km), lending credence to its identification as a baroclinic trough or Kelvin front.

Modeling of observations of the OH nightglow in the venusian mesosphere

Venus airglow emissions have been unambiguously detected in the wavelength ranges of 1.40–1.49 and 2.6–3.14 μm in limb observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) onboard the Venus Express (VEx) spacecraft and are attributed to the OH(2–0) and OH(1–0) Meinel band transitions. The integrated (limb slant path) emission rates for these bands were measured by Piccioni et al. (2008).Photochemical (Caltech/JPL KINETICS) and global circulation (Venus Thermospheric General Circulation Model - VTGCM) model calculations suggest the observed OH emission is produced primarily via the Bates-Nicolet mechanism, as on the Earth, although Venus' background atmosphere is different than that of the Earth, but the modeled contribution of the HO2 + O → OH(v) + O2 reaction increases in the lower portion of the OH airglow layer. An overall difference of ~2 km in the peak heights of the OH(1–0) and OH(2–0) layers is seen in both the KINETICS and VTGCM simulations as a result of this change in the relative importance of H + O3 → OH(v) + O2 versus HO2 + O → OH(v) + O2 reactions with altitude.First time 3-D simulations of the OH Δv = 1 nightglow limb slant emission calculate a peak intensity of ~0.6 ± 0.3 MegaRayleighs at ~102 km altitude, an intensity that is consistent with Venus Express VIRTIS observations (Gérard et al. 2010; Soret et al. 2010, 2012) and KINETICS results. Soret et al. (2010) reported the intensity of the peak OH airglow increased from 0.30 to 0.40 MR from dusk to dawn but noted the observations used are not uniformly distributed and the observed emission is extremely variable, so a more detailed assessment of the observations was not possible. Our simulations show a decrease in the average OH(1–0) emission is symmetric about the midnight meridian, but the simulations find an asymmetric decrease from the equator to the poles. Consideration of transport and chemical lifetimes suggests modeling of OH above ~96 km requires explicit description of transport and vibrational-state-dependent chemistry.

Winds in the Lower Cloud Level on the Nightside of Venus from VIRTIS-M (Venus Express) 1.74 μm Images

The horizontal wind velocity vectors at the lower cloud layer were retrieved by tracking the displacement of cloud features using the 1.74 µm images of the full Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS-M) dataset. This layer was found to be in a superrotation mode with a westward mean speed of 60–63 m s−1 in the latitude range of 0–60° S, with a 1–5 m s−1 westward deceleration across the nightside. Meridional motion is significantly weaker, at 0–2 m s−1; it is equatorward at latitudes higher than 20° S, and changes its direction to poleward in the equatorial region with a simultaneous increase of wind speed. It was assumed that higher levels of the atmosphere are traced in the equatorial region and a fragment of the poleward branch of the direct lower cloud Hadley cell is observed. The fragment of the equatorward branch reveals itself in the middle latitudes. A diurnal variation of the meridional wind speed was found, as east of 21 h local time, the direction changes from equatorward to poleward in latitudes lower than 20° S. Significant correlation with surface topography was not found, except for a slight decrease of zonal wind speed, which was connected to the volcanic area of Imdr Regio.

Surface weathering on Venus: Constraints from kinetic, spectroscopic, and geochemical data

On Venus, understanding of surface-atmosphere interactions resulting from chemical weathering is both critically important for constraining atmospheric chemistry and relative ages of surface features and multifaceted, requiring integration of diverse perspectives and disciplines of study. This paper evaluates the issue of surface alteration on Venus using multiple lines of evidence. Surface chemistry from Venera and Vega landers is inconsistent with significant breakdown from atmospheric interactions, with <2.0 wt% S or less observed. Consideration of kinetics and breakdown of basalt under Venus conditions indicates diffusion of Ca > Fe > Mg toward the oxidizing Venus atmosphere, favoring creation of anhydrite and carbonate-rich surfaces on basalts with minor addition of hematite. When related to Venus-analog experiments, the kinetic calculations suggest a maximum coating of ~30 μm over 500,000 years. These changes would result in a slight overall volume increase in the outermost surface materials, which in turn decreases surface rock FeO contents. Those variations can be detected from orbit because emissivity is correlated with total FeO, and the predicted magnitudes are consistent with Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) observations. Models of anhydrite and hematite coatings on basalt mixtures suggest that changes in emissivity (ε) spectra due to chemical weathering can result in ca. <0.08 shifts in total emissivity. Such gradations are small compared to the first-order effect of bulk composition on emissivity, which can cause up to ~0.80 emissivity shifts. For all these reasons, there is at present no evidence to suggest that impenetrable coatings of either hematite (ε = 0.8) or anhydrite (ε = 0.1) are present on Venus. Orbital measurements of surface emissivity on a global scale could therefore produce not only a map of rock type and surface composition based on transition metal contents (largely FeO) (Helbert et al., 2020) but also provide local scale assessments of fresh vs. mature lava flows on the surface.

Validation of the IPSL Venus GCM Thermal Structure with Venus Express Data

General circulation models (GCMs) are valuable instruments to understand the most peculiar features in the atmospheres of planets and the mechanisms behind their dynamics. Venus makes no exception and it has been extensively studied thanks to GCMs. Here we validate the current version of the Institut Pierre Simon Laplace (IPSL) Venus GCM, by means of a comparison between the modelled temperature field and that obtained from data by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) and the Venus Express Radio Science Experiment (VeRa) onboard Venus Express. The modelled thermal structure displays an overall good agreement with data, and the cold collar is successfully reproduced at latitudes higher than +/−55°, with an extent and a behavior close to the observed ones. Thermal tides developing in the model appear to be consistent in phase and amplitude with data: diurnal tide dominates at altitudes above 102 Pa pressure level and at high-latitudes, while semidiurnal tide dominates between 102 and 104 Pa, from low to mid-latitudes. The main difference revealed by our analysis is located poleward of 50°, where the model is affected by a second temperature inversion arising at 103 Pa. This second inversion, possibly related to the adopted aerosols distribution, is not observed in data.

The surface distributions of the production of the major volatile species, H2O, CO2, CO and O2, from the nucleus of comet 67P/Churyumov-Gerasimenko throughout the Rosetta Mission as measured by the ROSINA double focusing mass spectrometer

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) suite of instruments operated throughout the over two years of the Rosetta mission operations in the vicinity of comet 67P/Churyumov-Gerasimenko. It measured gas densities and composition throughout the comet's atmosphere, or coma. Here we present two-years' worth of measurements of the relative densities of the four major volatile species in the coma of the comet, H2O, CO2, CO and O2, by one of the ROSINA sub-systems called the Double Focusing Mass Spectrometer (DFMS). The absolute total gas densities were provided by the Comet Pressure Sensor (COPS), another ROSINA sub-system. DFMS is a very high mass resolution and high sensitivity mass spectrometer able to resolve at a tiny fraction of an atomic mass unit. We have analyzed the combined DFMS and COPS measurements using an inversion scheme based on spherical harmonics that solves for the distribution of potential surface activity of each species as the comet rotates, changing solar illumination, over short time intervals and as the comet changes distance from the sun and orientation of its spin axis over long time intervals. We also use the surface boundary conditions derived from the inversion scheme to simulate the whole coma with our fully kinetic Direct Simulation Monte Carlo model and calculate the production rates of the four major species throughout the mission. We compare the derived production rates with revised remote sensing observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) as well as with published observations from the Microwave Instrument for the Rosetta Orbiter (MIRO). Finally we use the variation of the surface production of the major species to calculate the total mass loss over the mission and, for different estimates of the dust/gas ratio, calculate the variation of surface loss all over the nucleus.

Stationary waves and slowly moving features in the night upper clouds of Venus

At the cloud top level of Venus (65-70 km altitude) the atmosphere rotates 60 times faster than the underlying surface, a phenomenon known as superrotation. Whereas on Venus's dayside the cloud top motions are well determined and Venus general circulation models predict a mean zonal flow at the upper clouds similar on both day and nightside, the nightside circulation remains poorly studied except for the polar region. Here we report global measurements of the nightside circulation at the upper cloud level. We tracked individual features in thermal emission images at 3.8 and 5.0 $\mathrm{\mu m}$ obtained between 2006 and 2008 by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-M) onboard Venus Express and in 2015 by ground-based measurements with the Medium-Resolution 0.8-5.5 Micron Spectrograph and Imager (SpeX) at the National Aeronautics and Space Administration Infrared Telescope Facility (NASA/IRTF). The zonal motions range from -110 to -60 m s$^{-1}$, consistent with those found for the dayside but with larger dispersion. Slow motions (-50 to -20 m s$^{-1}$) were also found and remain unexplained. In addition, abundant stationary wave patterns with zonal speeds from -10 to +10 m s$^{-1}$ dominate the night upper clouds and concentrate over the regions of higher surface elevation.

Retrieval of Venus' cloud parameters from VIRTIS nightside spectra in the latitude band 25°-55°N

Two years of data from the M-channel of the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS), on board the European Space Agency mission Venus Express operating around the planet Venus, are analysed. Nocturnal data from a nadir viewpoint in the latitude band 25°N-55°N are selected for their configuration advantages and maximisation of the scene homogeneity. A reference model, and radiance spectrum, is defined based on average accepted values of the Venus main atmospheric and cloud parameters found in the literature. Extensive radiative transfer simulations are performed to provide a synthetic database of more than 10 000 VIRTIS radiances representing the natural variability of the system parameters (atmospheric temperature profile, cloud H2OH2SO4 solution concentration and vertical distribution, particle size distribution density and modal radius). A simulated-observed fitting algorithm of spectral radiances in window channels, based on a weighting procedure accounting for the latitudinal observed radiance variations, is used to derive the best atmosphere-cloud configuration for each observation.Results show that the reference Venus model does not adequately reproduce the observed VIRTIS spectra. In particular, the model accounting for a constant sulphuric acid concentration along the vertical extent of the clouds is never selected as a best fit. The 75%/96% and 84%/96% concentrations (the first values refer to the upper cloud layers and the second values to the lower ones) are the most commonly retrieved models representing more than 85% of the retrieved cases for any latitudinal band considered. It is shown that the assumption of stratified concentration of aqueous sulphuric acid allows to adequately fit the observed radiance, in particular the peak at 1.74 μm and around 4 μm.The analysis of the results concerning the microphysics suggests larger radii for the upper cloud layers in conjunction with a large reduction of their number density with respect to the reference standard. Considerable variation of the particle concentration in the Venus' atmosphere is retrieved for altitudes between 60 and 70 km. The retrieved models also suggest that lower cloud layers have smaller particle radii and larger number density than expected from the reference model. Latitudinal variations of microphysical and chemical parameters are also analysed.

Idunn Mons on Venus: Location and extent of recently active lava flows

From 2006 until 2014 the ESA Venus Express probe observed the atmosphere and surface of the Earth's twin planet. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) has provided data that indicate the occurrence of recent volcanic activity on Venus. We selected the eastern flank of Idunn Mons - Imdr Regio's single large volcano – as the study area, since it was identified in VIRTIS data as one of the regions with relatively high values of thermal emissivity at 1µm wavelength. Using the capabilities of specific techniques developed in the Planetary Emissivity Laboratory group at DLR in Berlin, our study intends to identify location and extent of the sources of such anomalies, thus the lava flows responsible for the relatively high emissivity observed by VIRTIS over the eastern flank of Idunn Mons. We map the lava flow units on the top and eastern flank of Idunn Mons, varying the values of simulated 1µm emissivity assigned to the mapped units. For each configuration we calculate the total RMS error in comparison with the VIRTIS observations. In the best-fit configuration, the flank lava flows are characterized by high values of 1µm simulated emissivity. Hence, the lava flow units on the eastern flank on Idunn Mons are likely responsible for the relatively high 1µm emissivity anomalies observed by VIRTIS. This result is supported by the reconstructed post-eruption stratigraphy, displaying the relative dating of the mapped lava flows, that is independent of the 1µm emissivity modeling. Values of average microwave emissivity extracted from the lava flow units range around the global mean, which is consistent with dry basalts.

Themis Regio, Venus: Evidence for recent (?) volcanism from VIRTIS data

Themis Regio is interpreted to be a hotspot rise underlain by one or more mantle plumes. Many volcanic features in the Themis region have high emissivity anomalies in Venus Express Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) data. Other high emissivity anomalies have been found to correspond to volcanic flows interpreted to be relatively recent (Smrekar et al. [2010a] Science 328, 605–608). Similarly, at Themis the majority of the high emissivity anomalies closely correspond to flows associated with coronae and volcanic edifices, consistent with the interpretation that they represent recent volcanism. These volcanic features also have negative Bouguer gravity anomalies indicating low density at depth, consistent with hot mantle upwellings leading to pressure release melting. In addition, several volcanic features at Themis have low emissivity anomalies, suggestive of more evolved compositions. Combining geologic mapping data with gravity and emissivity data provide support that Themis is an active hotspot swell, with associated long-lived, volcanoes and coronae that are likely associated with small-scale upwellings.

VIRTIS emissivity of Alpha Regio, Venus, with implications for tessera composition

The composition of Venus tessera terrain is unknown. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) aboard Venus Express (VEx) collects data that yields the surface emissivity at ∼1μm, which contains information convolving a number of surface properties, including composition. We examine the variation of emissivity in the vicinity of Alpha Regio, which is the largest exposure of tessera terrain imaged by VIRTIS. We find that the emissivity of Alpha Regio tessera is lower than adjacent plains materials and the deposits and flows of Eve corona, both of which have previously been interpreted to be basaltic. The emissivity of the bulk of Alpha is also lower than its western boundary, which is interpreted to comprise plains structurally deformed to the same degree as tessera terrain. This suggests that the lower emissivity of Alpha is independent of structural elements, macroscale roughness, or local sedimentation processes, and is due to material properties like composition or grain size. The deviation of the emissivity of Alpha from that of the plains for which a bulk basaltic composition is well supported corresponds to a significant difference in rock type or surface mineral assemblage. The 1μm emissivity of Alpha is consistent with rocks with low ferrous iron content. This includes felsic igneous rocks like granitoids that form under either water-rich or water-poor conditions. A water-rich origin would require both a hydrosphere and a plate recycling mechanism and thus be limited to the lifetime of surface water on Venus. Alternatively, granitoids could form via the differentiation of basaltic melts. The production of all tessera terrain by this mechanism would require the accumulation and preservation of felsic melts from a volume of mafic magma that exceeds what is preserved in the currently observed plains. Both mechanisms of granitoid formation would require that tessera terrain be formed prior to the emplacement of the plains, consistent with their stratigraphic position. Anorthosites also satisfy the emissivity signature and can form from copious amounts of partial melting of a mafic source. Low emissivity values are also consistent with carbonates, sulfates, phyllosilicates and their dehydration products, which may have formed via weathering of basalts under conditions of higher atmospheric PH2O. All of these hypotheses suggest the mineralogy of Alpha tessera records an extinct era of Venus history and is a key target for future exploration.

Water vapor near Venus cloud tops from VIRTIS-H/Venus express observations 2006–2011

This work aims to give a summary of the water vapor at the cloud top of Venus atmosphere using the complete set of observations made using high spectral resolution channel (-H) of Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), on board the ESA Venus Express orbiter, to measure the cloud top altitude and the water vapor abundance near this level. An initial analysis of these measurements by Cottini et al. (2012) was limited to data in 140 orbits in the period 2007–2008. These observations were limited to the Northern hemisphere due to observational geometry in this early part of the mission. In the present paper, the analysis is extended to a larger dataset covering the years 2006–2011, significantly improving the latitudinal coverage. Altitude of the cloud tops, corresponding to unit optical depth at a wavelength of 2.5µm, is equal to 69±1km at low latitudes, and decreases toward the pole to 62–64km. The water vapor abundance is equal to 3±1ppm in low latitudes and it increases reaching a maximum of 5±2ppm at 70–80° of latitude in both hemispheres, with a sharp drop in the polar regions. This can be explained by the specific dynamics of the atmosphere of Venus affecting the distribution of water vapor such as the transfer of water vapor in the Hadley cell and the dynamic in the polar vortex. The average height of the cloud tops and the H2O near this level are symmetric with respect to the equator. As a function of local solar time, the water vapor shows no particular dependence, and the cloud tops exhibit just a weak maximum around noon. Over 5 years of observations the average values of the cloud top altitude and the water vapor were quite stable in low and middle latitudes, while in high latitudes both quantities in 2009–2011 years are systematically higher than in 2006–2008. Short period variations increasing with latitude are observed, from approximately less than ±1km for cloud tops and ±1ppm for water vapor in low latitudes to, respectively, ±2km and ±2ppm in high latitudes. As a rule there is no correlation between variations of the cloud top altitude, the water vapor content, and the UV brightness. However, numerous examples can be found when UV dark features, with a characteristic size of a few degrees of latitude (several hundred kilometers), coincide with regions of higher cloud tops.

Modeling VIRTIS/VEX O2(a1∆g) nightglow profiles affected by the propagation of gravity waves in the Venus upper mesosphere

AbstractIn this work we describe a model of the perturbation of the O2(a1Δg) nightglow limb profiles by the action of gravity waves (GWs) propagating in the Venus' upper atmosphere. Data have been acquired by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) on board the European Space Agency mission Venus Express (VEX). The high variability observed in the shape of the O2(a1Δg) nightglow limb profiles between 80 and 120 km, often characterized by the presence of a double peak, suggests the occurrence of GWs at the considered altitudes. In order to model and derive the GWs properties, we apply to Venus a well‐known theory used to study terrestrial density fluctuations induced by the GWs propagation. The retrieved vertical wavelengths and amplitudes of the waves at the O2(a1Δg) layer altitude (~100 km) are of the order of 7–16 km and 3–14% respectively, complying with wave amplitude threshold for dynamical instability in the majority of the fitted cases. Temperature fluctuations would exceed 40% at higher altitudes (115–120 km) thus inducing either wave breaking or dissipation. Intrinsic horizontal phase velocities are expected to vary in the range 32 m/s and 85 m/s. GWs are detected in a wide range of latitudes from the midlatitudes up to the polar regions, and we cannot exclude existence of the sources of different nature. This study also confirms the high variability induced by the action of GW propagation in the airglow profiles of the terrestrial planets and points out the need for future missions to couple simultaneous complementary GW detection techniques in order to better constrain GW properties and understand their impact on the Venus general circulation.

Atmospheric thermal structure and cloud features in the southern hemisphere of Venus as retrieved from VIRTIS/VEX radiation measurements

Thermal structure and cloud features in the atmosphere of Venus are investigated using spectroscopic nightside measurements recorded by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) aboard ESA’s Venus Express mission in the moderate resolution infrared mapping channel (M-IR, 1–5μm). New methodical approaches and retrieval results for the northern hemisphere have been recently described by Haus et al. (Haus, R., Kappel, D., Arnold, G. [2013]. Planet. Space Sci. 89, 77–101. http://dx.doi.org/10.1016/j.pss.2013.09.020). Now, southern hemisphere maps of mesospheric temperature and cloud parameter fields are presented that cover variations with altitude, latitude, local time, and mission time. Measurements from the entire usable data archive are utilized comprising radiation spectra recorded during eight Venus solar days between April 2006 and October 2008.Zonal averages of retrieved temperature altitude profiles in both hemispheres are very similar and give evidence of global N–S axial symmetry of atmospheric temperature structure. Cold collar and warmer polar vortex regions exhibit the strongest temperature variability with standard deviations up to 8.5K at 75°S and 63km altitude compared with about 1.0K at low and mid latitudes above 75km. The mesospheric temperature field strongly depends on local time. At altitudes above about 75km, the atmosphere is warmer in the second half of night, while the dawn side at lower altitudes is usually colder than the dusk side by about 8K. Local minimum temperature of 220K occurs at 03:00h local time at 65km and 60°S. Temperature standard deviation at polar latitudes is particularly large near midnight. Temperature variability with solar longitude is forced by solar thermal tides with a dominating diurnal component. The influence of observed cloud parameter changes on retrieved mesospheric zonal average temperature structure is moderate and does not exceed 2–3K at altitudes between 60 and 75km. The mesospheric thermal structure was essentially stable with Julian date between 2006 and 2008.Global N–S axial symmetry is also observed in cloud structures. Cloud top altitude at 1μm slowly decreases from 71km at the equator to 70km at 45–50° and rapidly drops poleward of 50°. It reaches 61km over both poles. Average particle size in the vertical cloud column increases from mid latitudes toward the poles and also toward the equator resulting in minimum and maximum zonal average cloud opacities of about 32 and 42 and a planetary average of 36.5 at 1μm. Zonal averages of cloud features are similar at different solar days, but variations with local time are very complex and inseparably associated with the superrotation of the clouds.

VIRTIS/VEX observations of Venus: overview of selected scientific results

After six years in a polar Venus orbit, the visible and infrared thermal imaging spectrometer (VIRTIS) on ESA's Venus Express mission provided an enormous amount of new data, including a three-dimensional view of the atmosphere and information on global surface properties of the planet. An overview is given of selected scientific results achieved by use of VIRTIS data comprising atmospheric thermal structure, molecular and particulate composition, chemistry and dynamics, and surface features.

Spatial correlation of OH Meinel and O 2 infrared atmospheric nightglow emissions observed with VIRTIS-M on board Venus Express

We present the two-dimensional distribution of the O 2 a 1Δ–X 3Σ (0–0) band at 1.27 μm and the OH Δ v = 1 Meinel airglow measured simultaneously with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express. We show that the two emissions present very similar spatial structures. A cross-correlation analysis indicates that the highest level of correlation is reached with only very small relative shifts of the pairs of images. In spite of the strong spatial correlation between the morphology of the bright spots in the two emissions, we also show that their relative intensity is not constant, in agreement with earlier statistical studies of their limb profiles. We conclude that the two emissions have a common precursor that controls the production of both excited species. We argue that atomic oxygen, which produces O 2 ( 1Δ) molecules by three-body recombination and is the precursor of ozone formation, also governs to a large extent the OH airglow morphology through the H + O 3 → OH * + O 2 reaction.

Investigation of air temperature on the nightside of Venus derived from VIRTIS-H on board Venus-Express

We present the spatial distribution of air temperature on Venus’ night side, as observed by the high spectral resolution channel of VIRTIS (Visible and Infrared Thermal Imaging Spectrometer), or VIRTIS-H, on board the ESA mission Venus Express. The present work extends the investigation of the average thermal fields in the northern hemisphere of Venus, by including the VIRTIS-H data. We show results in the pressure range of 100–4 mbar, which corresponds to the altitude range of 65–80 km. With these new retrievals, we are able to compare the thermal structure of the Venus’ mesosphere in both hemispheres. The major thermal features reported in previous investigations, i.e. the cold collar at about 65–70°S latitude, 100 mbar pressure level, and the asymmetry between the evening and morning sides, are confirmed here. By comparing the temperatures retrieved by the VIRTIS spectrometer in the North and South we find that similarities exist between the two hemispheres. Solar thermal tides are clearly visible in the average temperature fields. To interpret the thermal tide signals (otherwise impossible without day site observations), we apply model simulations using the Venus global circulation model Venus GCM (Lebonnois, S., Hourdin, F., Forget, F., Eymet, V., Fournier, R. [2010b]. International Venus Conference, Aussois, 20–26 June 2010) of the Laboratoire de Météorologie Dynamique (LMD). We suggest that the signal detected at about 60–70° latitude and pressure of 100 mbar is a diurnal component, while those located at equatorial latitudes are semi-diurnal. Other tide-related features are clearly identified in the upper levels of the atmosphere.

Zonal winds at high latitudes on Venus: An improved application of cyclostrophic balance to Venus Express observations

Recent retrievals of zonal thermal winds obtained in a cyclostrophic regime on Venus are generally consistent with cloud tracking measurements at mid-latitudes, but become unphysical in polar regions where the values obtained above the clouds are often less than or close to zero. Using a global atmospheric model, we show that the main source of errors that appear in the polar regions when retrieving the zonal thermal winds is most likely due to uncertainties in the zonal wind intensity in the choice of the lower boundary condition. Here we suggest a new and robust method to better estimate the lower boundary condition for high latitudes, thereby improving the retrieved zonal thermal winds throughout the high latitudes middle atmosphere. This new method is applied to temperature fields derived from Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) data on board the Venus Express spacecraft. We obtain a zonal thermal wind field that is in better agreement with other, more direct methods based on either retrieving the zonal winds from cloud tracking or from direct measurements of the meridional slope of pressure surfaces.

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 O 2 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 O 2 ( 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.