Simulation of gas dynamics, radiation and particulates in volcanic plumes on Io

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Zhang, Ju

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Volcanic plumes on Jupiter’s moon Io are modeled using the direct simulation Monte Carlo (DSMC) method. The main goal of this work is to improve the understanding of Ionian atmosphere itself and the internal processes that are responsible for the volcanic plumes with rarefied gas dynamics modeling techniques developed for aerospace engineering applications. A DSMC model including spherical geometry, variable gravity, internal energy exchange (discrete vibration-translation and continuous rotation-translation energy exchange) in the gas, infrared and microwave emission from the gas, multi-domain sequential calculation to resolve the fast emission event, opacity and two phase gas/particle flow, has been developed. Increasing confidence in our model has been built up through the encouraging matches to and agreements with a variety of observations, such as plume shape, vertical gas column density in the plumes, plume images, plume shadows, ring depositions, etc.. A concept of virtual vent is proposed for both volcanic tube and lava lake plumes. A parametric study of the two most important parameters at the virtual vent - velocity and temperature - is performed. Constraints are put on the vent conditions via the observables such as the canopy shock heights, peak gas deposition ring radii, vertical and tangential gas column densities, and total gas mass and emission power. Also, the flow of refractory 1 nm – 1 µm particles entrained in the gas is modeled with “overlay” techniques which assume that the background gas flow is not altered by the particles. The column density along the tangential lines-of-sight and the shadow cast by the plume are calculated and compared with Voyager and Galileo images. Encouraging matches are found between simulations and observations. The model predicts the existence of a canopy-shaped shock inside the gas plume, a multiple bounce shock structure around a dayside plume, a frost depletion by the gas bounce, concentration of emission in the vibrational bands in the vent vicinity and re-emission at the shocks for certain band. An upper limit on the size of spherical particles that can track the gas flow in the outer portion of the plumes is ∼10 nm. Particles of size ∼1 nm can track the gas flow well throughout the entire plume. A subsolar frost temperature in the range of ∼ 110 − 118 K is suggested.