Clouds and Hazes in Planetary Atmospheres

Citation

Gao, Peter (2017) Clouds and Hazes in Planetary Atmospheres. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z99W0CGS. https://resolver.caltech.edu/CaltechTHESIS:09302016-155917400

Abstract

Clouds and hazes are found in every significant planetary atmosphere in the Solar System, from the sulfuric acid clouds of Venus and the water clouds of Earth and Mars, to the photochemical hazes that pervade the giant planets, ice giants, Titan, and even Pluto. Beyond the Solar System, transmission spectroscopy of exoplanets have found that many are also bound in clouds and hazes, though their higher temperatures give rise to clouds of salts, rocks, and metals, and hazes of soots and sulfurs. Understanding the behavior and role of clouds and hazes in planetary atmospheres is instrumental in understanding atmospheres as a whole, as they are strongly coupled to other atmospheric processes. For example, highly reflective clouds can reduce the effective temperature of a planet, while UV absorbing hazes can increase local atmospheric temperatures. Clouds and hazes also act as reservoirs for important trace species and can be crucial to atmospheric chemical cycles.

In this Ph.D thesis, I use modeling and comparisons to observations to understand cloud and haze processes on multiple worlds within and beyond the Solar System. I use the Community Aerosol and Radiation Model for Atmospheres (CARMA) to simulate the sulfuric acid clouds of Venus in an attempt to find the cause of the spatial and temporal variability in the Venus upper haze, as observed by Venus Express. I show that the variability is likely caused by sustained updrafts lofting large cloud particles into the haze. I then modify CARMA to include fractal aggregate particles to investigate the properties of the photochemical haze on Pluto as observed by New Horizons, and find that the haze particles must be porous, and that they may act as nucleation cites for simple hydrocarbons. Finally, I add exotic condensates to CARMA to model clouds on exoplanets, where their existence has led to difficulties in finding the atmospheric compositions of these worlds. I show that not all species that can condense will, due to their material properties, and that the cloud optical depth is largely controlled by the rate of particle production via homogeneous nucleation. In addition, I investigate the effect a sulfur haze would have on the reflected light spectrum of giant exoplanets to prepare for upcoming direct imaging missions, and find that sulfur hazes significantly brighten these planets at long wavelengths, while darkening them at short wavelengths due to UV absorption. Finally, I retrieve the properties of water ice particles from Cassini observations of the plumes of Enceladus assuming that they are aggregates rather than spheres, and thereby unifying forward scattering observations with in situ measurements.

Thesis Files