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  1. Modeling the Sublimation-driven Atmosphere of Io with DSMCAndrew WalkerDavid Goldstein, Chris Moore, Philip Varghese, and Laurence TraftonUniversity of Texas at AustinDepartment of Aerospace Engineering41st LPSC ConferenceMarch 3rd, 2010Supported by the NASA Planetary Atmosphere ProgramIn collaboration with Deborah Levin and Sergey Gratiy at Pennsylvania State University

  2. Outline Background information on Io Overview of DSMC The basic method Our modifications Gas dynamic results Circumplanetary flow Global column densities Translational temperature Validation – comparison to observations Conclusions

  3. Frost patch of condensed SO2 Background Information on Io Io is the closest satellite of Jupiter Io radius ~1820 km It is the most volcanically active body in the solar system Many observations have failed to determine whether Io’s atmosphere is pre-dominantly volcanically or sublimation-driven. Volcanic plume with ring deposition • Surface Temperature ~ 90 K – 115 K • Length of Ionian Day ~ 42 hours

  4. The Basics of DSMC A spatial domain is decomposed into cells Representative molecules move and collide in these cells. Variables (temperature, density, etc.) are sampled from molecular properties in a given cell Cells can have a variety of boundary conditions: vacuum, specular/diffuse reflection, unit sticking, or periodic.

  5. Time scales Vibrational Half-life millisecond-second Time step 0.5 seconds Between Collisions 0.1 seconds - hours Residence Time Seconds - Hours Ballistic Time 2-3 Minutes Flow Evolution 1-2 Hours Simulation Time 2 hours Eclipse 2 hours Io Day 42 Hours Overview of our DSMC code Three-dimensional Parallel Important physical models Dual rock/frost surface model Temperature-dependent residence time Rotating temperature distribution Variable weighting functions Quantized vibrational & continuous rotational energy states Photo-emission Plasma heating

  6. Trock Boundary Conditions – Surface temperature & frost fraction Tfrost • Dual frost/rock surface temperature: • Independent thermal inertias and albedos • Same peak temperature (115 K) • Temperature Dist. validated by Rathbun et al. (2004) Galileo PPR data • Surface frost fraction from Doute et al. (2001)

  7. Vertical Column Density • Column density predominantly (exponentially) controlled by surface frost temperature • Due to exponential dependence of SO2 vapor pressure on surface frost temperature • Frost fraction has small (proportional) effect on column • Leads to slightly irregular column densities on dayside • Large irregularities on the nightside where the surface temperature is nearly constant • Winds have negligible effect on the column

  8. Mach Number at 30 km Altitude • Streamlines in white; Sonic line in dashed white; Surface temperature contours in thick black (104 K and 108 K) • Dusk vs. dawn asymmetry ( Horseshoe-shaped Shock) • Due to extended dawn atmospheric enhancement which blocks west-moving flow • Along the equator, Mach numbers peak at: • M=1.40 for eastward flow; M=0.84 for westward flow

  9. Translational Temperature at 3 km Altitude • Coldest (~100 K) near peak surface temperature • Plasma energy coming down column of gas is completely absorbed above this altitude • Very warm (~360 K) near the M=1.4 shock at the dusk terminator • Compressive shock heating

  10. Types of Available Observations Plume Images Auroral Glows IR Map of Hot Spots IR Map of Passive Background Lyman-ainferred column densities Disk-Averaged Spectra

  11. Comparison to Observations • Comparison of band depth vs. central longitude for several atmospheric cases (Gratiy et al., 2009) • The upper curve is a cos1/4(q) variation with a 90 K nightside temperature • The lower curves are the temperatures needed to create a column densities inferred by Lyman-a observations. The empirical fit is also a cos1/4(q) variation but with a 0 K nightside temperature. • Comparison of our atmospheric simulations with inferred column densities from Lyman-a observations • 115 K cases both show reasonable agreement with the peak of Feaga’s data (Feaga et al., 2009); however, the peak in Feaga’s data may be from additional volcanic column. • There are morphological differences at mid- to high latitudes between the simulations and observations

  12. Column density is predominantly controlled by the frost surface temperature • Small effects from the surface frost fraction and negligible effects from flow • The pressure-driven supersonic flow diverges from near the region of peak surface frost temperature toward the nightside • The extended dawn enhancement • blocks the westward flow • Supersonic to east, north, • and south of peak pressure • Horseshoe-shaped shock Conclusions

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