HiRISE observations of gas sublimation-driven activity in Mars’ southern polar regions: IV. Fluid dynamics models of CO2 jets

The High Resolution Imaging Science Experiment (HiRISE) onboard Mars Reconnaissance Orbiter (MRO) has been used to monitor the seasonal evolution of several regions at high southern latitudes on Mars and, in particular, the jet-like activity which may result from the process described by Kieffer (Kieffer, H.H. 2007]. J. Geophys. Res. (Planets) 112, E08005. doi:10.1029/2006JE002816) involving translucent CO₂ ice. In this work, we concentrate on attempting to model the dusty CO₂ gas jets using a computational fluid dynamics code. Models that included surface slopes of up to 20° (as an analogy to the jet activity seen in “Inca City”, 81°S, 296°E), wind (from 0 to 6 m s⁻¹), variable vent cross-section and length, particles (including a particle size distribution) and mass loading (with dust to gas ratios exceeding 1) were investigated. The structure of the resulting gas jets, the particle distribution within the jets, the deposition patterns (including their dependence on particle size), and the appearance of jets when viewed from different orientations (including from a nadir-pointing camera) have been investigated for a range of input parameters. The results provide predictions for the size-dependency of altitudes of particles within a plume and the distribution of particle sizes in the deposition fans. Where slopes are a dominant influence, larger particles are expected to be seen furthest from the vent. Where wind is dominant, smaller particles should travel to larger distances. Models producing deposition patterns consistent in length (∼80 m) and form with fans observed by HiRISE on MRO have been demonstrated. The models also suggest that downward flow of gas produced by drag effects from particles falling from the jet under gravity could provide a mechanism for the production of bright haloes which are observed to surround dark fan deposits in MOC, HiRISE and CRISM.