DSMC Modeling of 3D Vent Geometries for Ionian Plumes

Introduction: Volcanic plumes potentially provide us with evidence of some of a planet's dominant geologic and atmospheric processes, and can be the primary agent controlling the environment and the evolution of a planetary surface over modest time scales. This is especially true for Io, where the volcanic plumes are an ongoing phenomenon of nearly global scale that resurfaces the moon at rates estimated at more than 0.02 cm/yr and possibly as high as 2 cm/yr. The morphology and composition of these plumes is our prime clue to the subsurface geologic processes and composition, which is critical to understanding Io's formation and evolution. Excellent progress has been made in modeling Io's plumes realistically by using Direct Simulation Monte Carlo (DSMC) techniques [1]. Until now, simulated plumes have been treated as evolving from disk-like circular vents, but many of Io's volcanoes do not conform to this simple geometry. For the various types of observed plumes, the simulation of plausible vent geo-metries requires three-dimensional representation that has not yet been included in models. Background: By remote observation, we can categorize Io's plumes, and certain features of the lava lakes and flows, by temperature (e.g., the Near-Infared Mapping Spectrometer (NIMS) on Galileo; the ISS data from Cassini), composition and geomorphology (e.g., the SSI instrument on Galileo), and spectral characteristics. As summarized by Geissler and Goldstein [2], the volcanoes have been classified into four types: " Pro-methean", associated with surface lava flows; " Pillani-an", showing short, hot, explosive eruptions of gas and particles; " Lokian", with sources such as lava flows and lakes confined within patera walls; and " stealth", which are undetectable by instruments but inferred to exist. As with terrestrial volcanoes, the modes can switch over time or be combined in a single location. Each of these types of plumes has distinctive source geometries, and several require more than axisymmet-ric modeling. Geissler and Goldstein suggest, however, that perhaps only two broad categorizations exist: plumes arising from lava impinging on pre-existing ice (Prometheus-type) and those in which the gas evolves directly from hot rock (Pele-type) [2]. Modeling: DSMC: Except for regions extremely close to the physical vents on the surface of the planet, Io's plumes are of density. We simulate rarefied atmospheric flow with the DSMC molecular method. This