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Venus clouds: Sulfuric acid reactions and supercooling in Venus liquid cloud droplets

Presentation #502.10 in the session Venus I (Oral Presentation)

Published onOct 23, 2023
Venus clouds: Sulfuric acid reactions and supercooling in Venus liquid cloud droplets

The Venus clouds consist mostly of liquid droplets of sulfuric acid solutions (H2SO4/H2O) where chemical reactions occur from uptake (absorption) of ambient gas molecules into the droplets which leads to solution chemistry. We calculate the Henry’s Law constant, H*, for the Venus cloud droplets which is the ratio of the vapor pressure of gases next to the droplet to the concentration of the dissolved gas inside the droplet solution. For HCl uptake into the droplets, H* varies by ~ 6 orders of magnitude over the altitude range of the cloud deck (47-70 km) with H* increasing towards the top indicating that more HCl is absorbed into the droplets at higher altitudes because droplets are more dilute (contain more water). Chemical reactions in solutions yield products which are not produced in gas-phase-only reactions such as sulfonic acids from HCl + H2SO4 -> ClSO3H. In water solutions, H2SO4 creates acid hydrates of formula H2SO4.nH2O where n=1-8. Laboratory experiments in the literature show that these acid hydrate solutions have much lower freezing points than the pure solid hydrates. Thus, over the entire altitude range of the cloud deck from 47-70 km, Venus cloud droplets will be supercooled and never freeze. Cloud droplets can remain as glassy, viscous supercooled liquids that are unfrozen. Because of the concentrated acid strength in the droplets, the sulfuric acid in these water solutions will be mostly in the form of sulfuric acid monohydrate H2SO4.H2O with some dihydrate H2SO4.2H2O and trihydrate H2SO4.3H2O. The freezing points (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. These cloud materials could be deposited on any Venus aerial vehicles and cause damage and corrosion to the aircraft, affecting their flight capabilities and instrument observations. Thus, it is important to determine the chemistry and viscosity in the clouds and accommodate these characteristics in the design of any proposed aerial missions flying in the skies of Venus.

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