Cloud chemistry on Venus: Sulfuric acid reactions and supercooling in Venus liquid cloud droplets

Cloud particles on Venus consist of liquid droplets of aqueous sulfuric acid solutions (H2SO4/H2O) that are the site of chemical reactions arising from uptake (absorption) of surrounding gas molecules into the droplets and subsequent solution chemistry. The Henry's Law constant, H*, for HCl in sulfuric acid in water solutions is derived which gives the ratio of the concentration of the HCl gas dissolved inside the liquid droplet to HCl gas outside in the surrounding atmosphere. Uptake of HCl gas into sulfuric acid liquid droplets in the Venus clouds is calculated as a function of both temperature and sulfuric acid concentration. For HCl solubility into H2SO4/H2O solutions, the Henry's Law constant, H*, varies by at least 7 orders of magnitude, from H* = 2.5 × 10−6 to 6 × 101 M/atm from the bottom to top of the cloud deck over the altitude range of 47–70 km. This indicates that uptake of HCl increases dramatically as Venus altitude increases because droplets are more dilute (containing more water) at higher altitudes and will allow more HCl into solution. If chemistry effects occur at higher sulfuric acid concentrations from the reaction of dissolved HCl with H2SO4, the Henry's Law constant is greater and also ranges over 7 orders of magnitude: H* = 3.3 × 10−4 - 8.1 × 103 M/atm from the bottom to top of the clouds. Sulfuric acid reactions with dissolved gases such as HCl, HF or HBr inside the droplets should produce sulfonic acids (HSO3Cl, HSO3F, HSO3Br) and related products. Phosphine, PH3, is absorbed rapidly by sulfuric acid solutions and will react to form other oxidized phosphorus species. Chemical reactions in droplet solutions may yield products which are not produced in gas-phase-only reactions. H2SO4 creates acid hydrates in water solutions of formula H2SO4.nH2O where n = 1–8. These solutions have much lower freezing points than the pure solid hydrates and so Venus cloud droplets will be supercooled and never freeze. Cloud droplets can remain liquid and unfrozen over the entire altitude range of the cloud deck from 47 to 70 km despite the much colder temperatures at the top. The freezing points (from supercooling) of these solutions could be as much as 60 degrees K below that of the pure hydrates, around 190 K, much lower than any observed Venus cloud temperatures. Viscosity, η, (runniness) of the Venus cloud droplets varies from low viscosity at 50 km (the consistency of motor oil - very runny, not viscous) to higher viscosity in the cold temperatures at 70 km (similar to honey - very viscous, slow-flowing). Accumulation of cloud droplet materials on a hypothetical Venus aircraft flying at typical airspeeds may be on the order of 10s of centimeters onto wings and moving parts. These acidic, corrosive materials could damage an aircraft and affect its flight capabilities and instrument observations. Chemistry and viscosity effects in the clouds should be taken into account in the design of any future Venus aerial exploration missions.