Microphysical modeling of water ice aerosols in the ice giant atmospheres

The temperature regimes and observed species abundances indicate that water should condense in the stratospheres of each of the giant planets. Water reaches its condensation temperature at higher altitudes than hydrocarbon photochemical products, and water ice particles could then act as condensation nuclei for hydrocarbons deeper in the lower stratosphere. This is especially true for Uranus, where sluggish atmospheric mixing confines hydrocarbons to relatively low altitudes. Additionally, water ice particles could explain the high-altitude hazes seen in high-phase angle Voyager 2 images of Neptune. Using PlanetCARMA  - an aerosol microphysics model which simulates nucleation, condensation, evaporation, coagulation, and vertical transport in a column of atmosphere – we will describe particle number density profiles and size distributions of water ice particles in the atmospheres of Uranus and Neptune. Sensitivity tests include (1) nucleation – homogeneous vs. heterogeneous (including varying size, abundance, and contact parameter for the cloud condensation nuclei); (2) Vapor pressure equation; (3) water abundance and flux; and (4) degree to which physical processes are important, such as coagulation, condensation, and evaporation. Understanding the role of water ice in the ice giant atmospheres is important to further our understanding of the observed stratospheric hazes as well as the more optically thick methane clouds seen in the troposphere.