Ethane ice haze in the upper atmosphere of Uranus.

Abstract

A one-dimensional model is developed to study the effect of a tropospheric vapor sink on the formation and evolution of ethane ice haze in the stratosphere of Uranus. The microphysical processes modelled are nucleation of ethane vapor to neutral ice spheres, growth by vapor deposition and Brownian coagulation, and loss by sedimentation and sublimation. The vapor processes are mixing by eddy diffusion and destruction by a sink, which is a highly parameterized representation of cosmic ray induced ion chemistry. The sink is defined by the product of the mixing ratio and a Gaussian distribution with a center $z\sb0$, a maximum A and a width b. It is hypothesized that high energy cosmic rays, which penetrate deep into the troposphere, convert simple hydrocarbons into large ion clusters, and that further irradiation of these clusters leads to the formation of complex, involatile aerosol polymers. Numerous experiments were run without particles to determine the relationship between diffusion and destruction as the sink parameters were varied over a wide range of values. The results of these experiments were used to run 21 full model experiments, which included particles. The primary purpose of these experiments was to bracket the cosmic ray sink parameters that resulted in haze distributions most congruous with observations. The most plausible cases examined so far are those for which $z\sb0$ = $-$100 to $-$200 km relative to the tropopause (3.4 to 9.1 bars), b = 60 to 90 km, and A is large enough to deplete the ethane mixing ratio profile below the tropopause. These sinks predict stratospheric ice hazes containing 10$\sp{-15}$ g cm$\sp{-3}$ of frozen ethane in submicron particles numbering about 21 cm$\sp{-3}$. These hazes typically form at 40-43 km altitude (17 mb) and extend downward about 20-30 km. Mass and number column densities are on the order of 10$\sp{-10}$ g cm$\sp{-2}$ and 10$\sp7$ cm$\sp{-2}$, respectively. The most developed hazes have modal radii of 0.13 $\pm$ 0.02 $\mu$m at 39 km (20 mb), where the number densities are between 0.2 and 0.8 cm$\sp{-3}$. These results agree well with observations and suggest that a vapor sink of this nature is not only plausible but may play a key role in the stability and vertical development of the hazes in the upper atmosphere of Uranus.