Atmospheric composition, chemistry, and clouds

Venus’ atmosphere has a rich chemistry involving interactions among sulfur, chlorine, nitrogen, hydrogen, and oxygen radicals. The chemical regimes in the atmosphere range from ion-neutral reactions in the ionosphere to photochemistry in the middle atmosphere to thermal equilibrium chemistry and surface-atmosphere reactions in the lower atmosphere. This variety makes Venus an important planet to understand within the context of terrestrial-like planets, both in our own solar system and outside it. The primary chemical cycles are believed known but surprisingly few details about these cycles have been fully verified by concurrence among observations, experiments, and modeling. Good models have been developed that account for many properties of the cloud layers, but the size distribution, shape, and composition of the majority of the aerosol mass are still open issues. This chapter reviews the state of knowledge prior to the Venus Express mission for the composition, chemistry, and clouds of the neutral atmosphere on Venus. Observations by instruments on Venus Express, in combination with ground-based observations, laboratory experiments, and numerical modeling, should answer some of the major open questions regarding the composition, chemistry, and clouds of Venus’ atmosphere.