Optical Properties of Venusian Clouds

Theme: Environmental Aerosols

Start date: Cohort 2: 2020

Supervisors: Prof. John Plane, Prof. Dan Marsh, Prof. Ben Murray


Despite many years of study, the atmosphere of Venus is not well understood. When the spectrum of light from Venus’s atmosphere is examined, there is an as yet unexplained region of absorption at approximately 320 – 500nm (Pérez‐Hoyos et al., 2018). This corresponds to the blue to ultraviolet region of the spectrum, or “near-UV”, although the exact range is quoted differently by different authors, (see for example Pollack et al., 1980; Frandsen, Wennberg, & Kjaergaard, 2016; Ekonomov et al., 1984; Limaye et al., 2019). The absorption is believed to occur in the upper cloud layer of Venus, which consists of droplets of an H2SO4/H2O mixture. The absorption cannot be explained by the droplets alone, or by the inclusion of SO2 gas, and so an additional particulate or aerosol absorber mixed in with the droplets or gas has been proposed (Ekonomov et al., 1984; Pollack et al., 1980). Although several candidates have been suggested, no chemical species has been conclusively proven to fit the observed absorption pattern and atmospheric chemistry of Venus. This project will examine the optical properties of sulphuric acid droplets containing candidate absorbers. This will be done initially in bulk liquid or aerosol distributions, and then in single droplets using optical tweezers. The project will also consider the physical and more general optical properties of the absorber candidates in droplets, including real refractive index and ability to maintain a spherical shape. Glory patterns – a series of concentric coloured rings of light produced in spherical droplets when light is internally reflected (Laven, 2005) – have been observed on Venus. The observed glories suggest the presence of absorber particles in the droplets. The particles must be entirely within the droplets as attaching particles to the droplet surface would result in distortion of the glory, which is not observed. Some candidates are impossible to insert into droplets and so, regardless of their agreement with the optical properties and absorption detected, cannot be the absorber (Petrova, 2018). 

The LMD (Le Laboratoire de Météorologie Dynamique) Venus atmosphere model comprehensively models the clouds of Venus. The physical, chemical, and optical properties of promising candidates from the laboratory experiments can be included in the model to simulate their behaviour in the Venusian atmosphere and therefore to check their suitability as candidates. As well as helping narrow down the search or strengthening the evidence for the identification of the absorber, the inclusion of a confirmed absorber in the model will improve its ability to model realistic behaviour to solve other problems. Computer modelling will be particularly useful if no single candidate seems sufficient to provide all of the absorption detected. Modelling would then be used to combine candidate absorbers in variable amounts and pairings to see if any are promising. If promising candidate mixtures are predicted, experimental tests of the mixtures could begin, and the process could proceed iteratively to identify the absorber ratios.