The Solar Occultation in the InfraRed (SOIR) instrument onboard Venus Express sounded the Venus mesosphere and lower thermosphere using solar occultation geometry between April 2006 and December ...2014. The observations were all taken at the terminator. This paper reports on the water vapor vertical distribution above the clouds and geo-temporal variations, observed across the whole Venus Express mission. Water vapor profiles are sampled between 80 and 125 km from both the morning and evening side of the terminator. Calculations of the water vapor volume mixing ratio agree with those from previous studies, having an average atmospheric profile that varies between 0.56 ppmv and 2.45 ppmv. No significant variations were observed in observations taken between the morning and evening terminator. Short term variations of several Earth days dominate the data set, varying by up to a factor 18 over a 24 h period, from ~10 ppmv to ~0.55 ppmv at an altitude of ~111 km. Similar to previous trace gas observations by SOIR (HCl, HF and SO2), no significant spatial or long term trends are observed.
•First mesospheric water vapor vertical profiles at the Venus terminator, measured concurrently with carbon dioxide.•Average atmospheric profiles vary from 0.56 to 2.45 ppmv between 80 and 125 km.•The mean vertical profile is nearly constant with altitude.•No dependence with latitude or local solar time are observed.•A very large variability with time and location is observed, up to a factor 18, with no clear tendency pattern.
•Venus’ upper haze was observed at 70–95km by SPICAV IR.•Detached haze layers were detected at altitudes from 70 to 90km.•The aerosol scale height equals to 3.3±0.7km.•Particle size and number ...density profiles for bimodal and unimodal cases are retrieved.•Unimodal radius equals 0.54±0.25μm and is smaller in the polar region than in nonpolar regions.
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 0.65–1.7μm. We report results from 222 solar occultations observed from May 2006 to November 2014. The vertical resolution of measurements varies from 1 to 25km 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 95km. 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±0.7km. Detached haze layers were detected at altitudes from 70 to 90km. Particle size and number density profiles are retrieved from extinction coefficients using Mie scattering theory adopting H2SO4 refractive indices. Bimodal distribution of particles is consistent with data for some orbits with mean radius for mode 1 reff1=0.12±0.03μm and reff2=0.84±0.16μm 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 Reff=0.54±0.25μm, 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 ∼500cm−3 at 75km to ∼50cm−3 at 90km for mode 1, and from ∼1cm−3 at 75km to ∼0.1cm−3 at 90km for mode 2.
•Large SO2 and SO variability in the Venus atmosphere is observed.•SO2 abundance in the Venus atmosphere shows an inversion layer located around 70–75km.•GCM modelling indicates that dynamics may ...play a role in generating the inversion.
Recent observations of sulfur containing species (SO2, SO, OCS, and H2SO4) in Venus’ mesosphere have generated controversy and great interest in the scientific community. These observations revealed unexpected spatial patterns and spatial/temporal variability that have not been satisfactorily explained by models. Sulfur oxide chemistry on Venus is closely linked to the global-scale cloud and haze layers, which are composed primarily of concentrated sulfuric acid. Sulfur oxide observations provide therefore important insight into the on-going chemical evolution of Venus’ atmosphere, atmospheric dynamics, and possible volcanism.
This paper is the first of a series of two investigating the SO2 and SO variability in the Venus atmosphere. This first part of the study will focus on the vertical distribution of SO2, considering mostly observations performed by instruments and techniques providing accurate vertical information. This comprises instruments in space (SPICAV/SOIR suite on board Venus Express) and Earth-based instruments (JCMT). The most noticeable feature of the vertical profile of the SO2 abundance in the Venus atmosphere is the presence of an inversion layer located at about 70–75km, with VMRs increasing above. The observations presented in this compilation indicate that at least one other significant sulfur reservoir (in addition to SO2 and SO) must be present throughout the 70–100km altitude region to explain the inversion in the SO2 vertical profile. No photochemical model has an explanation for this behaviour. GCM modelling indicates that dynamics may play an important role in generating an inflection point at 75km altitude but does not provide a definitive explanation of the source of the inflection at all local times or latitudes
The current study has been carried out within the frame of the International Space Science Institute (ISSI) International Team entitled ‘SO2 variability in the Venus atmosphere’.
SOIR is a high‐resolution spectrometer flying on board the ESA Venus Express mission. It performs solar occultations of the Venus high atmosphere, and so defines unique vertical profiles of many of ...the Venus key species. In this paper, we focus on the Venus main constituent, carbon dioxide. We explain how the temperature, the total density, and the total pressure are derived from the observed CO2 density vertical profiles. A striking permanent temperature minimum at 125 km is observed. The data set is processed in order to obtain a Venus Atmosphere from SOIR measurements at the Terminator (VAST) compilation for different latitude regions and extending from 70 up to 170 km in altitude. The results are compared to many literature results obtained from ground‐based observations, previous missions, and the Venus Express mission. The homopause altitude is also determined.
Key Points
Venus terminator mesosphere and thermosphere
Carbon dioxide measurement
Venus terminator model
Rapid variations of pressure, temperature and illumination at the day–night terminator have the potential to cause asymmetries in the abundance distribution of the atmosphere constituents along the ...line of sight (LOS) of a solar occultation experiment. Ozone, in particular, displays steep density gradients across the terminator of Mars due to photolysis. Nowadays, most of the retrieval algorithms for solar and stellar occultations rely on the assumption of a spherically symmetrical atmosphere. However, photochemically induced variations near sunrise/sunset conditions need to be taken into account in the retrieval technique in order to prevent inaccuracies.
We investigated the impact of gradients along the LOS of the solar occultation experiment SPICAM/Mars Express for the retrieval of ozone under sunrise/sunset conditions. In order to test the impact of different gradients, we selected four occultations at sunrise and at sunset each. Sunset occultations are located near the equator, while sunrise observations are situated at high latitudes in the South, because of the geometry of the orbit.
We used the diurnal variations in the ozone concentration obtained from a three-dimensional General Circulation Model (GEM-Mars) together with an adapted radiative transfer code (ASIMUT). The General Circulation Model (GCM) suggests that ozone variations strongly depend on latitude, altitude, and season. As shown by the model, near the equator and below 25 km, ozone changes only slightly with local time. Around 45 km, the density changes by several orders of magnitude across the terminator. At high latitudes in the South, during northern winter time, ozone variations at the terminator are negligible.
The impact of gradients on ozone retrievals is strongly related to the local atmospheric structure as predicted by the GCM. Sunset ozone retrievals are smaller than retrievals obtained assuming a spherically symmetrical atmosphere, with a maximum change of about 20%. At sunrise, the impact of gradients on the retrievals is negligible. This behavior can be explained by the specific geometry of sunrise observations, all situated at high latitudes in the South.
•We analyzed the impact of gradients along the LOS of a solar occultation experiment.•We focused on the retrieval of ozone using observations acquired by SPICAM/MEx.•We implemented a radiative transfer code (ASIMUT) to take into account gradients.•We used the diurnal variations in ozone concentration obtained from a GCM (GEM-Mars).•The impact of gradients on retrievals is strongly related to the GCM model results.
•Large spatial and temporal variability in the Venus SO2 is observed.•High variability both on short-term and short-scale are observed.•The long-term trend observed confirms the role of transport of ...air from lower altitudes.•The short-scale and short-term variations are likely connected to variations in vertical transport of SO2.
The vertical distribution of sulfur species in the Venus atmosphere has been investigated and discussed in Part I of this series of papers dealing with the variability of SO2 on Venus. In this second part, we focus our attention on the spatial (horizontal) and temporal variability exhibited by SO2. Appropriate data sets – SPICAV/UV nadir observations from Venus Express, ground-based ALMA and TEXES, as well as UV observation on the Hubble Space Telescope – have been considered for this analysis. High variability both on short-term and short-scale are observed. The long-term trend observed by these instruments shows a succession of rapid increases followed by slow decreases in the SO2 abundance at the cloud top level, implying that the transport of air from lower altitudes plays an important role. The origins of the larger amplitude short-scale, short-term variability observed at the cloud tops are not yet known but are likely also connected to variations in vertical transport of SO2 and possibly to variations in the abundance and production and loss of H2O, H2SO4, and Sx.
The SOIR instrument on board Venus Express regularly sounds the Venus atmosphere using the solar occultation technique. The density and temperature profiles are inferred from SOIR spectra recorded in ...the infrared. The method has been described in a previous publication (Mahieux et al., 2012. J. Geophys. Res. 117. doi:10.1029/2012JE004058.). This paper is devoted to the update of the VAST (Venus Atmosphere from SOIR measurements at the Terminator) compilation that was initiated in the above cited work, which gives the mean CO2 number density and temperature profiles for different latitude bins. The method has been improved and has been applied to more data. The new compilation which is given on the same latitudinal grid now distinguishes between the two sides of the terminator. The compilation also confirms the main thermal layering characteristics that were identified in the earlier version: the succession of a warm layer (230±30K, 1−σ standard deviation) at a pressure level of 3.2×10−7mbar (~140km), a very cold layer (125±32K) at 2.5×10−5mbar (~123km), a warm layer (204±17K) at 0.01mbar (~102km) and finally a colder layer at 0.4mbar (171±34K, ~87km). The layering of all the temperature profiles is explained by radiative rather than dynamical processes. The temporal temperature variation is larger than the mean latitudinal temperature variation. VAST is compared with temperature profiles obtained from other Venus Express instruments, VeRa and SPICAV–UV, and ground based measurements.
•Mesosphere and lower thermosphere Venus terminator temperature profiles.•Latitude variations studied.•Permanent very cold layer at 125km.
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.
Venus Express SOIR terminator profiles of CO2 densities and corresponding temperatures have been determined for 132 selected orbits obtained between 2006 and 2013. These recently recalibrated ...measurements provide temperature profiles at the Venusian terminator over approximately 70–160km, revealing a striking permanent temperature minimum (at about 125km) and a weaker temperature maximum (over 100–110km). In addition, topside temperatures (above 140km) reveal a warming trend consistent with a typical thermospheric structure. These features are reflected in the corresponding CO2 density profiles, and provide detailed constraints for global circulation models of the upper atmosphere.
New Venus Thermospheric General Circulation Model (VTGCM) simulations are presented for conditions appropriate to these SOIR measurements. In particular, solar minimum to moderate fluxes are specified and mean values of eddy diffusion and wave drag parameters are utilized. Recent upgrades to the VTGCM code now include more realistic lower boundary conditions at ~70km near cloud tops. Model temperature profiles are extracted from the terminators that correspond to five latitude bins presently used in the SOIR data analysis. Averaging of VTGCM temperature profiles in each of these bins (at each terminator) is conducted to match SOIR sampling. Comparisons of these SOIR and VTGCM temperature profiles are shown. Most notably, the observed temperature minimum near 125km and the weaker temperature maximum over 100–110km are generally reproduced by the VTGCM at the correct pressure/altitude levels. However, magnitudes of simulated and measured temperatures are somewhat different as a function of latitude. In addition, VTGCM evening terminator (ET) temperatures are simulated to be modestly warmer than corresponding morning terminator (MT) values, a result of stronger ET than MT zonal winds at/above about 130km. The SOIR terminator temperatures thus far do not reveal this consistent trend, suggesting the VTGCM climate based winds may not precisely represent the averaged conditions during SOIR sampling. Overall, these data-model comparisons reveal that both radiative and dynamical processes are responsible for maintaining averaged temperatures and driving significant variations in terminator temperature profiles.
•The observed SOIR temperature structure at the Venus terminators is generally reproduced by the VTGCM numerical model above ~110 km.•The magnitudes of VTGCM and SOIR temperatures are somewhat different as a function of latitude, and match best at low latitudes.•VTGCM evening terminator temperatures are simulated to be modestly warmer than morning values above 130 km, unlike SOIR measurements.•Both radiative and dynamical processes are responsible for maintaining SOIR bin averaged terminator temperatures.
The NOMAD spectrometer suite on the ExoMars Trace Gas Orbiter will map the composition and distribution of Mars׳ atmospheric trace species in unprecedented detail, fulfilling many of the scientific ...objectives of the joint ESA-Roscosmos ExoMars Trace Gas Orbiter mission. The instrument is a combination of three channels, covering a spectral range from the UV to the IR, and can perform solar occultation, nadir and limb observations. In this paper, we present the science objectives of the instrument and how these objectives have influenced the design of the channels. We also discuss the expected performance of the instrument in terms of coverage and detection sensitivity.
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•NOMAD UV and IR channels perform solar occultation, limb and nadir observations.•NOMAD will map the composition of the Martian atmosphere.•NOMAD studies the spatial and temporal variability of trace gases, dust and clouds.•NOMAD will help localize sources and sinks of trace gases.•NOMAD will refine processes in the water, carbon and dust cycles.
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