Variations in the canopy shock structures of massive extraterrestrial plumes: Parametric DSMC simulation of 2007 Tvashtar observations

Abstract

The New Horizons flyby of the Jovian system yielded multiple images of the erupting Pele-class volcanic plume, Tvashtar. These images include a unique five-frame sequence or “movie” taken over 8 min that revealed short-term dynamical activity in the form of a traveling wave and evolving plume structure. The high-latitude Tvashtar plume was observed from various aspects during the flyby to exhibit a canopy shock and a depositional ring with thermal vent emission. The movie frames and other high-resolution images of Io taken during the New Horizons flyby by the Long Range Reconnaissance Imager (LORRI) were deconvolved to aid analysis of the plume dynamics.While our previous work (Hoey et al., 2016) modeled this plume's gas field in its steady state with rarefied gas dynamic methods, Tvashtar's virtual vent conditions were underconstrained by observation. Therefore, to constrain thermodynamic and geometric parameters at the virtual vent, we extend our models of the Tvashtar plume to perform a parametric series of 3-D simulations at much improved resolution. These simulations vary virtual vent aspect ratio and stagnation temperature, and implement new models for gas opacity in SO2 flows and for the condensation and sublimation of particles entrained in the plume.We arrive at a set of virtual vent conditions that incorporate or satisfy all available observational constraints and several novel ones, including the gas canopy height and aspect ratio and the depositional ring extent and orientation. The 2007 Tvashtar gas plume is shown to be consistent with a virtual vent of aspect ratio less than or equal to 3; stagnation temperature 1287 K; area 30 km2; and mass flow rate 2.0 × 105 kg/s SO2. Alternate canopy shock and depositional ring configurations are evaluated across a range of vent aspect ratios from 1 to 15 with results that will help to constrain the sources of other observed gas plumes.Finally, we develop an alternate hypothesis for the New Horizons observations of canopy unsteadiness in the Tvashtar plume; namely, that transient perturbations in the structure of the gas canopy shock itself produce a traveling notch-like structure that may even grow unstable, inducing temporary canopy collapse. This is as opposed to a high-amplitude density pulse at the source, or to source-entrained dust in a condensate model coupled to steady gas-field solutions.