Nucleation and growth of carbon dioxide ice crystals in the Martian atmosphere

Carbon dioxide (CO2) ice clouds and snow play very important roles in controlling the present and past climate of Mars, primarily through their radiative effects. The radiative effects of clouds are determined by their physical characteristics, and efforts to model and understand these effects have been limited by the lack of knowledge regarding the sizes, shapes, and abundance of CO2 snow crystals on Mars. Direct observation of these quantities is very difficult and until such data are obtained, we must rely on theoretical models. In this study I present the first analysis of homogeneous, heterogeneous, and ion-induced nucleation of CO2 ice crystals in the Martian atmosphere and calculate the nucleation rates for each mechanism as a function of supersaturation. I also present the first microphysical model for the growth (and evaporation) of CO2 ice crystals in the Martian atmosphere. This model is quite general and can also be used for other ices in other planetary atmospheres. It self-consistently treats the kinetics of crystal growth and the transfer of mass and heat between the crystal and surrounding gas using a formulation which is valid for any vapor mixing ratio and any Knudsen number, including near unity. These capabilities are required for modeling CO2 snow on Mars because the Martian atmosphere is 95% CO2 gas, so growth is not controlled by vapor diffusion, and because the air density is so low that the molecular mean free path is comparable to expected particle sizes. Using this model I have calculated growth (evaporation) rates of CO2 ice crystals as a function of super(sub)saturation for Mars' present and past atmospheric conditions.; I have also developed a one-dimensional, steady-state model of CO 2 condensation and precipitation in the present polar night atmosphere of Mars. This model calculates growth rates required to balance latent heat release with radiative cooling at all altitudes; and using the single-particle microphysical model, I calculate the required supersaturation profile. This profile depends strongly on the flux of snow and which crystal growth mechanism is assumed, and its implications for nucleation are discussed. I also use the results in a multiple-scattering, radiative transfer model to calculate infrared opacities which are compared to past and present spacecraft data to place constraints on the amount and characteristics of CO2 snow in polar night.