DSMC Simulations of Irregular Source Geometries for Io’s Pele Plume William McDoniel David Goldstein, Philip Varghese, Laurence Trafton The University of Texas at Austin 42 nd DPS Meeting October 6 th, 2010 Supported by the NASA Planetary Atmospheres Program

Pele Canopy rises to over 300km Deposition ring is ~1200km across Temperatures in excess of 1000K observed via IR Ring changes over time, but remains ovoid 10km 120km

Source Geometries Previous plume simulations used only round vents. Irregularities in Pele’s structure likely caused by unsteadiness or source geometry. Steady irregularities due to source geometry, and this must be simulated Observational clues to the actual source: -consistently ovoid ring -black “butterfly wings” -Galileo IR image of part of Pele’s caldera -Galileo/Voyager images of the caldera

Basic DSMC Overview Simulates gas dynamics using a “large” number of representative particles Particle collisions and movement are de- coupled in a given timestep Binary collisions between particles in the same cell Particles move using F=ma

Plume Simulations Earlier axisymmetric simulations of Pele using DSMC. The plume expands, collapses back on itself, and forms a large canopy. The gas can bounce off of the surface, and form secondary rings, depending on surface temperature.

Cold Line Source Vent number density: 5 × m -3 (under-resolved) 240km 1200km 10km

Hot Line Source Focusing at hot conditions – 650K and 900m/s at the vent. Near-field number density contours at ground level and along the dashed line. Focusing is more pronounced than with cold cases. Four orders of magnitude difference between red jets and blue expansion regions. 13km 20km A A’ A

Cold Lava Lake Vent number density: ~2 × m -3. Orientation is almost exact. 120km 240km 1200km

Cold Lava Lake A’ A A BB’ B Number density contours through the plume along two planes. 240km

Other Sources?

Conclusions DSMC can provide insight into the source geometry of Ionian plumes. A curved line source, as seen in the Galileo IR image, can produce the features seen in observations of Pele’s deposition pattern. But the observed hot line cannot be the only source of plume material because it produces a ring with a different orientation. Gas must be produced elsewhere in the caldera, perhaps in a line across the top of the lava lake.