| Summary: | The atmosphere of Saturn's moon Titan features hydrocarbon clouds, for example those composed of methane and ethane, the properties of which are not completely understood. To improve the understanding of Titan's clouds, studies of the phase behavior of pure and mixed hydrocarbon aerosols were carried out.
The aerosol particles were generated in a bath gas cooling cell under conditions representative of Titan's lower atmosphere (78 K, 500-1000 mbar). The freezing and crystallization behavior of the aerosol particles were studied using rapid-scan infrared spectroscopy and the observed phases identified by comparison with thin film measurements.
Under the studied conditions ethane forms long-lived supercooled liquid droplets that are stabilized against freezing by the incorporation of methane and nitrogen gas, which are ubiquitous in Titan's atmosphere. The extent of the stabilization increases with increasing methane partial pressure. The studies demonstrate that supercooled ethane droplets could have an appreciable lifetime and might play an important role in the condensation of methane in Titan's atmosphere.
The influence of the presence of aerosols of other atmospheric components (acetylene, carbon dioxide, and water) on the freezing of supercooled hydrocarbon droplets was studied in heterogeneous nucleation experiments. The aerosols were found to accelerate the crystallization of hydrocarbon aerosols to differing degrees depending on their composition.
Numerous hydrocarbons exist as minor atmospheric components and might be of importance for Titan's clouds and lakes. Propane was found to form long-lived supercooled droplets at 78 K, which did not crystallize even in the presence of heterogeneous nuclei. n-Butane and n-pentane initially form amorphous-annealed particles. n-Butane particles were observed to undergo two phase transitions and n-pentane particles one. The surface and volume nucleation rate constants, JV and JS, were determined for all three of these transitions and are likely to be important for models of Titan's clouds.
Taken together, these studies provide a detailed picture of the phase behavior of hydrocarbon aerosols under conditions relevant to Titan. This is important for understanding cloud formation and the radiation budget on Titan. Furthermore, the recorded spectroscopic signatures provide a reference for astrophysical studies concerned with the detection and characterization of aerosols in Titan's atmosphere.
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