Development of a chemistry model for DSMC simulation of the atmosphere of Io

A significant process in the formation of the unique atmosphere of Io, a Jovian moon, is collision-induced dissociation of sulfur dioxide. The rarefied nature of the Ionian atmo- sphere makes its simulation by DSMC a practical scientific exercise. However, there is a lack of reliable data on collisions of Ionian species at the conditions seen in its atmosphere. As such, collisions between SO₂ and O are studied through two separate methods: Molecular Dynamics/Quasi-Classical Trajectories (MD/QCT) and ReaxFF, a molecular dynamics method utitlizing a force field for reactive systems. Four possible reaction paths are considered: atomization of the SO₂ molecule, dissociation to SO, dissociation to O₂, and the formation of SO₃. Relative velocities and initial SO₂ internal energies relevant to Io's atmospheric conditions are used. The results from each chemistry model are analyzed and compared to each other, in particular the reaction cross sections and equivalent variable hard sphere cross sections. In general, higher collision energies are found to cause atomization of the SO₂. In addition, dissociation to SO is a significant process for many of the studied cases, but dissociation to O₂ is mostly a negligible process. Finally, formation of SO₃ occurs only at low impact velocities. The chemistry model for the MD/QCT method was then implemented in DSMC 0-D, time-dependant and 2-D axisymmetric simulations under conditions relevent to the Ionian atmosphere. The results of these analyses are examined, and compared to results obtained using the baseline Total Collisional Energy (TCE) model and the previous results of Deng et al.1 It is found that the new MD/QCT based model predicts less SO ₂ dissociation than previous models, but there is an uncertainty in fitting the collision cross section to a VHS form.