On Understanding the Nature and Variation of the Venusian Middle Atmosphere Via Observations and Numerical Modeling of Key Tracer Species

Recent observations of enhanced amounts of SO2 at 100 km by Venus Express suggest that there is a hitherto unknown source of gaseous sulphur species in the upper atmosphere of Venus, in contrast to previous expectation. Some researchers argue that the photolysis of H2SO4 vapour derived from evaporation of H2SO4 aerosols provides a source of SO3, which upon photolysis yields SO2. Other chemical schemes not involving H2SO4 pathways (e.g. polysulphurs or those not including the influence of evaporation and condensation of sulphuric acid haze particles have been suggested, but still discovering the unknown source of gaseous sulphur species has remained elusive. Similarly, recent VEx and ground based observations show that obtaining self-consistent temperature and CO volume mixing ratio profiles is difficult. Moreover, numerical studies of CO show that reconciling differences between collected data and modeling is sensitive to the heating and cooling mechanisms in the atmosphere above 70 km and not well understood. Recent studies of the night OH airglow in the Venusian mesosphere provide insight into chemical and dynamical processes that control the composition and energy balance in the upper atmosphere. Analogous to O2 and NO airglow studies, SO2 , CO, and OH can be used as effective atmospheric tracers, which in lieu of wind data, give important clues to the dynamics of the Venus upper atmosphere. Numerical modeling of the chemistry and dynamics of the Venus middle atmosphere has been performed for effective data analysis and the subsequent interpretation of these key tracer species recently observed by VEx and legacy spacecraft data. Additionally, a microphysical cloud model has been employed to determine how chemistry and dynamics can affect the aerosol creation and distribution in Venus' upper atmosphere with the view to solving the longstanding problem of the source of the unknown UV absorber and cloud banding in Venus' upper atmosphere. We discuss the modeling work in context of the observations with the view to better understanding the nature and variability of the Venus middle atmosphere in short term as well as secular timescales.