The Formation of Cirrus-Like Ice Clouds in Venus’ Upper Haze Layer

The upper haze region of Venus’ atmosphere (~70-90 km) has been shown to experience cold pockets, likely induced by gravity waves. Within this region, temperatures may become sufficiently low (< 160 K) to induce homogeneous ice nucleation in sulphuric acid droplets which might lead to ice cloud formation. Here, we first explored the homogeneous nucleation of ice in sulphuric acid solutions using a liquid nitrogen-cooled cryo-microscope setup, where water-in-oil droplet emulsions (with droplets of around 10-15 µm) are created using a microfluidic device. With this setup, we were able to extend the results from previous studies to lower temperatures and higher sulphuric acid concentrations. We observed crystallisation to 170 K, but this crystallisation was increasingly restricted by very slow crystal nucleation and growth rates at lower temperatures. Crystallisation was not observed below 154 K, consistent with the formation of ultra-viscous or glassy solutions.

To further explore the possibility of ice cloud formation on Venus, we also examined the observations of temperature, water vapour mixing ratio and pressure from the Solar Occultation in the InfraRed (SOIR) instrument onboard the Venus Express orbiter. Using this data, we determined that cooling around 80 km altitude would lead to the atmosphere becoming supersaturated with respect to ice, likely causing hygroscopic growth of sulphuric acid particles. We identified two possible trajectories due to this cooling. Either the conditions result in the growth and dilution of sulphuric acid droplets until homogeneous crystallisation conditions can be met, or the trajectory will cross into the glassy region, which would stop the droplets from being able to reach equilibrium. In this scenario, the formation of glassy aerosols will either stop any nucleation occurring, or they might provide solid surfaces on which heterogeneous nucleation occurs. Either through homogeneous or heterogenous nucleation, assuming a number concentration of 0.5 cm^-3, we would expect an average size of 0.6 µm ice crystals to form in the upper haze layer of Venus.  

Around 36% of the SOIR profiles reveal that these altitudes occasionally experience temperature extremes which are suitably cold (< 140 K) for the deposition of crystalline CO₂. A 1D model was developed to investigate the influence of gravity waves. This shows that under these conditions, crystals will grow rapidly in the cold phase of a wave to sizes large enough for precipitation downwards to the underlying warm phase where the CO₂ evaporates, effectively increasing the rate of sedimentation of sulphuric acid particles. Therefore, we suggest that water ice clouds form in large parts of the upper haze layer on Venus, with CO₂ ice clouds sometimes forming but rapidly precipitating and potentially redistributing sulphuric acid, water and other materials downwards.