The Formation of Cirrus‐Like Water and Carbon Dioxide Ice Clouds in the Venusian Upper Haze Region

The upper haze region of the Venusian atmosphere (∼70–100 km) experiences localized cooling, probably due to gravity waves, creating transient cold pockets. Water ice clouds may form in these cold pockets when sulfuric acid droplets undergo hygroscopic growth and subsequent freezing. However, the freezing of sulfuric acid droplets under conditions relevant for the Venusian upper haze region has not been previously investigated (∼<170 K). We experimentally explored homogeneous H2O ice nucleation in H2SO4 solution droplets using a cryomicroscope, where droplet emulsions (micron-scale droplets) were created using a microfluidic device. Crystallization was observed down to ∼160 K at conditions close to those predicted by the water activity criterion, but crystallization was increasingly restricted by slow crystal nucleation and growth rates as the solutions became ultra-viscous or glassy at lower temperatures. Combining our measurement with satellite retrievals of temperature and water suggests that H2O ice may form homogeneously in liquid aqueous H2SO4 droplets or heterogeneously in glassy aqueous H2SO4 droplets. Resulting H2O ice crystals are unlikely to grow larger than ∼1 μm and will therefore persist in the upper haze layer of Venus forming detached layers. In extreme temperature minima, CO2 ice will likely nucleate heterogeneously on H2O ice, followed by rapid growth, sedimentation, and sublimation, resulting in downward transport of H2SO4, H2O, and other materials. Overall, our findings highlight the potential for cirrus-like clouds to form on Venus, but the extent to which they form strongly depends on temperature retrievals that carry a high degree of uncertainty.