DSMC Simulations of Io’s atmosphere

Abstract

<p><span lang="EN-US">Io is the innermost galilean satellite of Jupiter and object to extreme tidal forces. As a result of these forces Io is the most volcanically active body in our solar system. Its large volcanic plumes can rise up to several hundred kilometres above the surface and are one known source of Io's SO<sub>2</sub><span class="apple-converted-space"> </span>atmosphere. Additionally, the surface of the moon is covered with surface frost which sublimates in sunlight and condenses during the night or when Io enters eclipse behind Jupiter. Therefore, Io’s atmosphere is a result of the combination of volcanism and sublimation, but it is unknown exactly how these processes work together to create the observed atmosphere. That is why we want to present an approach on modelling Io’s atmosphere and provide a better understanding of the ongoing dynamic processes.</span></p> <p><span lang="EN-US">Both, the gas flow of the plume and the sublimation atmosphere, are modelled using the Direct Simulation Monte Carlo (DSMC) method first utilised by G. A. Bird [1]. The DSMC method is the most suitable for this case because the gas dynamics can be modelled over a great range of gas densities which is especially important for rarefied gas flows at high altitudes and on the night side of Io. It is a particle-based method which returns a 3D gas flow field as a result.<span class="apple-converted-space"> </span></span><span lang="EN-GB">While we currently focus on single species simulations, our DSMC code is designed to support multiple species enabling us to study the gas emission of other volcanic features as for example lava lakes in the future.</span><span lang="EN-US"> This allows us to investigate the influence and contributions of different processes to the atmosphere. The idea of our work is based on simulations done by McDoniel et al. [2].</span></p> <p><span lang="EN-US">In a first case, we are investigating the flow of SO<sub>2</sub><span class="apple-converted-space"> </span>gas from the source of a plume, into the umbrella-shaped canopy and eventually back onto the surface locally. Additionally, we also study the interaction of the plume with an ambient sublimation atmosphere. In a second case, data obtained by the Galileo SSI experiment is used to create a surface albedo map of Io which enables us to calculate a more precise global thermal model for the sublimation atmosphere. We can then place plumes on the surface and study the interaction of volcanic and sublimation effects globally. Finally, we are also able to implement dust particles in the plume and analyse the effect for different dust sizes. From these results we can calculate images of the column density and the reflectance which in a next step could be used to compare to observational data.</span></p> <p><span lang="EN-US">Overall, our goal is to gain a better understanding of the plume structure, the interaction with the ambient atmosphere and the overall contribution of different processes to Io's atmosphere in preparation for future missions such as JUICE, Europa Clipper and a possible future Io Volcano Observer.</span></p> <p><span lang="EN-US"> </span></p> <p><span lang="EN-US">[1] Bird, G. A. (1994).<span class="apple-converted-space"> </span><em>Molecular Gas Dynamics and the Direct Simulation of Gas Flows</em>.</span></p> <p>[2] McDoniel, W. et al. (2019).<span class="apple-converted-space"> </span><em>Simulation of Io’s plumes and Jupiter’s plasma torus</em>. Phys. Fluids 31, 077103. DOI: 10.1063/1.5097961.</p>