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Recent observations of sulfur containing species (SO 2 , SO, OCS, and H 2 SO 4 ) 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 SO 2 and SO variability in the Venus atmosphere. This first part of the study will focus on the vertical distribution of SO 2 , 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 SO 2 abundance in the Venus atmosphere is the presence of an inversion layer located at about 70–75 km, with VMRs increasing above. The observations presented in this compilation indicate that at least one other significant sulfur reservoir (in addition to SO 2 and SO) must be present throughout the 70–100 km altitude region to explain the inversion in the SO 2 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 75 km 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 ‘SO 2 variability in the Venus atmosphere’. [ABSTRACT FROM AUTHOR]

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 SO 2 on Venus. In this second part, we focus our attention on the spatial (horizontal) and temporal variability exhibited by SO 2 . 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 SO 2 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 SO 2 and possibly to variations in the abundance and production and loss of H 2 O, H 2 SO 4 , and S x . [ABSTRACT FROM AUTHOR]

The thermal structure of Venus upper atmosphere (90–140 km) was investigated using stellar occultation measurements acquired by the SPICAV experiment on board Venus Express. The SPICAV ultraviolet channel provides CO 2 local density and temperature vertical profiles with a vertical resolution of < 7 km of both the southern and the northern hemispheres on the nightside (18:00–06:00 h local time). A permanent layer of warm air is observed at the mesopause in the altitude range 90–100 km. Temperature then decreases with increasing altitude reaching a minimum value around 125 km. Spatial and temporal changes in the thermal structure have been analyzed. Local time variations dominate the structure of Venus atmosphere at these altitudes: temperatures show an increase of ~ 20 K on the morning side compared to the evening side. The homopause altitude was also determined; it varies between 119 and 138 km of altitude, increasing from the evening side to the morning side. SPICAV temperature profiles were compared to several literature results from ground-based observations, previous spacecraft missions and the Venus Express mission. [ABSTRACT FROM AUTHOR]

The SOIR instrument on-board Venus Express is an infrared spectrometer covering the 2.2–4.3 μm spectral region. This instrument allows the detection of several key species of the Venus atmosphere, including CO 2 , CO, H 2 O/HDO, HCl, HF and SO 2 . From the CO 2 density measurements, temperature is inferred giving information on the thermal structure of the atmosphere. Here we described the kind of data (profiles, latitudinal average, etc.) that will be provided to the updated VIRA compilation. [ABSTRACT FROM AUTHOR]

SOIR on board Venus Express sounds the Venus upper atmosphere using the solar occultation technique. It detects the signature from many Venus atmosphere species, including those of SO 2 and CO 2 . SO 2 has a weak absorption structure at 4 µm, from which number density profiles are regularly inferred. SO 2 volume mixing ratios (VMR) are calculated from the total number density that are also derived from the SOIR measurements. This work is an update of the previous work by Belyaev et al. (2012) , considering the SO 2 profiles on a broader altitude range, from 65 to 85 km. Positive detection VMR profiles are presented. In 68% of the occultation spectral datasets, SO 2 is detected. The SO 2 VMR profiles show a large variability up to two orders of magnitude, on a short term time scales. We present mean VMR profiles for various bins of latitudes, and study the latitudinal variations; the mean latitude variations are much smaller than the short term temporal variations. A permanent minimum showing a weak latitudinal structure is observed. Long term temporal trends are also considered and discussed. The trend observed by Marcq et al. (2013) is not observed in this dataset. Our results are compared to literature data and generally show a good agreement. [ABSTRACT FROM AUTHOR]

Abstract: SPICAV IR, a part of SPICAV/SOIR suite on Venus Express, is a compact single pixel spectrometer for the spectral range of 0.65–1.7μm based on acousto-optical tunable filter (AOTF) technology. SPICAV IR is derived from SPICAM IR operating on Mars Express, the first AOTF spectrometer in the deep space, and adapted for Venus atmosphere measurements. The spectrometer sequentially measures spectra of reflected solar radiation from Venus on the dayside and the emitted Venus radiation in spectral “windows” on the nightside, and works also in solar occultation mode. The spectral range is 0.65–1.1μm with spectral resolution of 7.8cm−1, and 1–1.7μm with spectral resolution of 5.2cm−1. A description of this near-IR instrument, its calibration, in-flight performances, and the modes of operations on Venus’ orbit are presented. A brief overview of the science measurements is given: water vapor measurements in the mesosphere on the day-side and near surface on the nightside, mapping of the O2(a 1 Δ g ) emission at 1.27μm, aerosol studies via polarization and scattering solar radiation at the day-side, and measurements of aerosol properties at the tops of the clouds in solar occultations. [Copyright &y& Elsevier]