Numerical estimates for the chemical composition of Enceladus’ plume particles
1University of Oulu, Faculty of Science, Physics
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Oulu : O. Veteläinen,
The Saturnian moon Enceladus contains a liquid water ocean beneath its surface. Ocean water is released into space through cracks in the moon’s frozen crust, forming a plume of vapor and ice-grains. NASA’s Cassini mission detected complex organic molecules in these plume particles. The aim of this work is to numerically estimate the chemical composition of the ice-grains found in Enceladus’ plume. The ice-grains are assumed to form as bubble bursting aerosols at the ocean surface. Known scaling laws of film and jet drop sizes are combined with a monolayer model of the liquid surface layer, a method which could in principle be applied to any system with bubble bursting aerosols. The bulk ocean water is modeled as an aqueous solution of sodium chloride, sodium carbonate, sodium bicarbonate and slightly soluble organic compounds. The following organic compounds are considered as proxies for the organic compounds present on Enceladus: phenylalanine and its sodium salt as a proxy for an aromatic compound and an amino acid, benzoic acid and benzyl alcohol. The Cassini measurements also imply the existence of very large organic molecules on Enceladus, with molecular masses in excess of 200 u. As a proxy for such compounds we have chosen the humic-like substance Suwannee River Fulvic Acid. Both saturated and supersaturated cases for the organic concentrations are considered. The calculations describe highly enriched organic concentrations for smaller droplets and nearly bulk concentrations for larger ones, as one would expect from surface active compounds. The calculated droplets have lower concentrations of salts than organics, in contrast with Cassini measurements. The calculations could be improved by better estimations of solution surface tension, more sophisticated surface layer characterizations and a better understanding of the bulk ocean composition. Results are also hindered by a poor understanding of the bursting bubble distribution. Future models could account for aerosol dynamics as the droplets rise to the moon’ surface.
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