2012 IR&D Annual Report

Capability Development for Modeling Small Icy Satellites in the Solar System,

Principal Investigators
Daniel Boice
Raymond Goldstein

Inclusive Dates:  06/20/12 – 10/20/12

Background — Data from the Cassini spacecraft have transformed our view of the Saturnian system, including its icy satellite, Enceladus. The surprise discovery was the observation of plumes of material being emitted from a region near the south pole of Enceladus, later identified as composed primarily of water with entrained grains, likely of ice and dust. Similarities to activity of comets abound. Almost two-dozen flybys of Enceladus have provided a host of measurements of neutral and ion composition and dynamics in its surrounding environment. Moreover, a detailed understanding of how gas in the plumes and plasma in the environment surrounding Enceladus interact is lacking. SwRI's experience with modeling chemistry in cometary atmospheres allows for a unique quantitative approach to address key issues regarding the plumes of Enceladus and its surroundings.

Approach — Researchers analyzed the available composition data obtained by the Cassini Plasma Spectrometer (CAPS) during the fifth Enceladus flyby (E5) with a chemical dynamics model originally developed for comets in order to understand the processes that produce the observed ion composition. The combination of this unique dataset together with a novel modeling approach originally developed for chemistry in comets has been used to address key outstanding issues regarding this icy satellite. The sophisticated chemical model presents a powerful new tool with which to understand the complexity of the ion-neutral, gas-plasma interactions and to contribute significantly to understanding Enceladus and its surrounding environment.

Accomplishments — Researchers made comparisons between their new model results and the CAPS data analysis. The model has been enhanced by the inclusion of energetic electrons that surround Enceladus in the E-ring of Saturn as described by Cravens et al. (2011). These hot electrons interact primarily with the plume species via electron impact reactions. Additional electron impact reactions were added to the chemical network of the code to properly account for them. They produce another source of water group ions and further dissociate the neutral water, resulting in the formation of more H2O+, OH+, and O+ at the expense of H3O+. This results in the improved agreement of the current model with the preliminary model without the hot E-ring elections (Boice and Goldstein 2010). Results are encouraging even though there is work to be done concerning OH+ and O+. This will make an intriguing issue to be addressed in a proposal submitted to NASA for future work.

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