Nucleation properties of aerosols in the atmospheres of Mars and Titan

Mathematics – Logic

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Scientific paper

Mars, with a rich CO2 atmosphere and Titan, with an abundance of hydrocarbons, have both been proposed as models for the atmosphere of early Earth. In the current atmospheres of Mars and Titan, CO2 and hydrocarbons, respectively, may condense to form clouds. In this paper, we explore cloud formation processes in these two very different planetary atmospheres. Under early Martian conditions, infrared scattering by CO2 clouds could have warmed the planet's surface above freezing. However, the radiative effect of the clouds depends strongly on the nucleation and growth kinetics of the cloud particles. We experimentally examine the nucleation and growth of CO2 on water ice under Martian conditions. We find that a critical saturation of S=1.3 is required for nucleation, corresponding to a contact parameter of m=0.95. After nucleation, growth of CO2 proceeds rapidly without a surface kinetic barrier. Using a microphysical cloud model, our data suggest that CO2 clouds are best described as ``snow,'' having a small number of very large particles. Titan's atmosphere may have clouds of ethane and methane and may even support a cycle analogous to the hydrologic cycle on Earth. Alternatively, Titan's atmosphere may be highly supersaturated in organics if no suitable particles are available for nucleation. Titan's organic haze particles, dubbed tholins, may provide a suitable nucleation surface. We have prepared a laboratory sample of tholins and will be performing nucleation experiments of ethane on the tholins. .

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