The Spatial and Temporal Distribution of Nighttime Ozone and Sulfur Dioxide in the Venus Mesosphere as Deduced From SPICAV UV Stellar Occultations

The nighttime ozone and sulfur dioxide distributions were analyzed using the entire Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus (SPICAV) UV/Venus Express stellar occultation data set. After the discovery of an ozone layer at 100 km in the mesosphere reported by Montmessin, Bertaux, et al. (2011, https://doi.org/10.1016/j.icarus.2011.08.010), 132 other detections were made during the entire 8 years long observing period of the SPICAV UV instrument. In the rare detections, the peak abundances of O3accumulating in the mesosphere are observed with densities from 107to 108molecules⋅cm−3at 85–110 km. The ozone layer is estimated to vary from 1 to 30 ppbv at 85–95 km while at 95–105 km the volume mixing ratio is expected within an interval from 6 to 120 ppbv. Below 93 km, a puzzling decrease of mixing ratio is observed towards midnight at 30°N. Our work also provides an improved sequel to the analysis of the sulfur dioxide survey previously made in the upper mesosphere by Belyaev, Evdokimova, et al. (2017, https://doi.org/10.1016/j.icarus.2017.05.002). On average, the SO2content is found to remain constant throughout the vertical profile at a value of around 135 ± 21 ppbv between 85 and 100 km. Rapid and large variations prevent to conclude firmly on any time or space pattern of SO2. Observations of light attenuation while bright ultraviolet stars were rising or setting behind Venus permitted us to characterize the quantity of carbon dioxide (the main atmospheric component), sulfur oxide, and ozone between 85 and 110 km of altitude at night. They were performed by the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus (SPICAV) UV instrument on board the ESA Venus Express mission in 2006–2014. For the first time, this data set has been entirely analyzed with the benefit of recent calibration's improvements enabled more faithful restitution of gases’ abundances. For ozone, discovered on Venus in 2011, thanks to SPICAV, we have been able to detect maximum concentrations and to estimate its nightside characterization. At midlatitudes, a ratio between ozone and carbon dioxide densities, or O3mixing ratio, decreases below 93 km at midnight. This puzzling result is tentatively explained by chemical compounds transported by winds from the dayside hemisphere to the nightside and which are known to quickly react with ozone and destroy it. Sulfur dioxide has been found to vary considerably on a short time scale (days) but, on average, the SO2mixing ratio is constant within 85–100 km. Ozone density is within 107to 108cm−3in the 85–110 km altitude range from 2006 to 2015The altitude profile of the sulfur dioxide volume mixing ratio is vertically uniform around 135 ppbv between 85 and 100 kmTemporal variations of ozone and sulfur dioxide are analyzed Ozone density is within 107to 108cm−3in the 85–110 km altitude range from 2006 to 2015 The altitude profile of the sulfur dioxide volume mixing ratio is vertically uniform around 135 ppbv between 85 and 100 km Temporal variations of ozone and sulfur dioxide are analyzed