Planetary analogue studies of charge effects on cloud droplet behaviour

Ionisation in planetary atmospheres resulting from cosmic rays fragments atmospheric molecules resulting in the formation of free ions. The rate at which ions are produced varies with altitude and is determined by a combination of the cosmic ray flux and atmospheric density. The altitude at which this ion production rate peaks is known as the Pfotzer-Regener maximum which, on Earth, occurs at around 15-20 km. On Venus this maximum occurs at ~63 km, coinciding with the main cloud deck. This study investigates the effects enhanced ionisation may have on cloud droplets and their behaviour. Interactions between the ions produced and cloud droplets may have many consequences, including activation at lower saturation ratios, enhanced droplet coalescence and, for large charges, droplet breakup by Rayleigh instability.

This work explores the effects of ionisation on water droplets in the laboratory and also simulates some of the conditions occurring in the clouds of Venus. The main element of the experimental apparatus is an acoustic levitator that can allow individual droplets to be electrically isolated and observed. Measurements are taken by a CCD camera and processed using LabView image acquisition software. The droplets can be subjected to enhanced ionisation from a corona source and perturbed by using a 10 kV/m electric field placed across the droplet causing it to be deflected relative to its charge. The principal findings on water droplets were that higher charge led to a slower evaporation rate; however, higher charge also led to increased incidence of Rayleigh explosions which were observed during several of the experiments. Overall, the effect of charge slowing evaporation did not lead to a longer droplet lifetime due to mass loss occurring from the periodic Rayleigh instabilities. In order to simulate conditions more like the clouds of Venus, sulphuric acid droplets were also examined. It was found that even very dilute sulphuric acid was extremely resistant to evaporation, suggesting that the clouds of Venus may have very long-lived droplet lifetimes. This has wide-reaching implications as cloud droplets on Venus have been suggested to act as a substrate for possible microbial life in the clouds.