A Comprehensive Numerical Model of Io’s Sublimation-Driven Atmosphere Andrew Walker David Goldstein, Chris Moore, Philip Varghese, and Laurence Trafton University of Texas at Austin Department of Aerospace Engineering Santa Fe DSMC Workshop September 16 th, 2009 Supported by the NASA Planetary Atmosphere Program In collaboration with Deborah Levin and Sergey Gratiy at Pennsylvania State University

Outline Background information on Io Overview of our DSMC code Gas dynamic results Conclusions Validation – Comparison to Observations (Time permitting)

Io is the closest satellite of Jupiter –Io radius ~1820 km It is the most volcanically active body in the solar system The primary dayside species, SO 2, was detected by the Voyager IR spectrometer in 1979 –Pearl et al. (1979) Since then many observations have failed to determine whether Io’s atmosphere is pre-dominantly volcanically or sublimation-driven. IoPlasma Torus Jupiter Background Information on Io

Frost patch of condensed SO 2 Volcanic plume with ring deposition Surface Temperature ~ 90 K – 115 K Length of Ionian Day ~ 42 hours Mean free path near the surface: noon ~ 10 m midnight ~ 100 km

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 Time scales Vibrational Half-lifemillisecond-second Time step0.5 seconds Between Collisions0.1 seconds - hours Residence TimeSeconds - Hours Ballistic Time2-3 Minutes Flow Evolution1-2 Hours Simulation Time2 hours Eclipse2 hours Io Day42 Hours

DSMC in 3D/Parallel 3D –The domain is discretized by a spherical grid –Domain extends from Io surface to 200 km in altitude –Encompasses all latitudes and longitudes Parallel –MPI –Tested up to 360 processors Parameters ~180 million molecules in domain – 1 degree resolution in latitude and longitude –Exponential vertical grid that resolves mean free path y x θ φ

Boundary Conditions – Frost Fraction SO 2 surface frost fraction from Galileo NIMS data (Doute et al., 2001): –Area fraction of SO 2 frost of a 1 o by 1 o element –High latitudes and longitudes from 0 o to 60 o interpolated –Within a computational cell, the rock and frost are assumed segregated with the relative abundances determined by the frost fraction –The frost fraction provides the probability for a molecule to hit frost or rock and the fractional area of each cell that sublimates

Boundary Conditions – Residence Time SO 2 residence time on rock: –When a molecule hits the rock surface, it sticks for a period of time dependent on the rock surface temperature [s] (Eq. 1)  H S (  H S /k B = 3460±40 K) : Surface binding energy of SO 2 on a SO 2 frost,  T S : Rock surface temperature  o (2.4×10 12 s -1 ) : Lattice vibrational frequency of SO 2 within surface matrix site. –Model assumes rock is coated with a thin monolayer of SO 2 Two residence time models tested: –The “short” residence time model uses Eq. 1. –The “long” residence time model uses Eq. 1 x –The “long” residence time model may be appropriate for a highly porous rock. SO 2 Sublimation & Condensation on SO 2 frost –Sublimation Rate = [#/m 2 -s] –Unit Sticking Coefficient

T frost Boundary Conditions – Surface Temperature T rock Dual frost/rock surface temperature: –Independent thermal inertias and albedos –Lateral heat conduction assumed negligible –Same peak temperature (115 K) –Model based on Saur and Strobel (2004) Temperature Dist. validated by Rathbun et al. (2004) –Rathbun et al. measured brightness temperature with Galileo PPR –Matched cooling rate during night

Column density predominantly (exponentially) controlled by surface frost temperature –Due to exponential dependence of SO 2 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 Vertical Column Density

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 Mach Number at 30 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 Translational Temperature at 3 km Altitude

Translational temperature –In equilibrium with the surface frost temperature at very low altitudes on dayside only (temperatures elevated near surface on nightside due to plasma heating) –Temperature rapidly increases due to plasma heating Rotational temperature –In thermal equilibrium with translation at altitudes below ~10 km on the nightside –Thermal equilibrium is maintained to higher altitudes on the dayside because of the higher collision rate –Cold “pocket” of gas (~60 K) at 3 km altitude on the dayside Thermal Non-Equilibrium T rot T trans

Conclusions 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 Rotational temperatures are not in equilibrium with translational temperatures: –Above ~10 km on the nightside –Above ~50 km on the dayside

Types of Available Observations Plume ImagesAuroral GlowsIR Map of Hot Spots IR Map of Passive Background Disk-Averaged Spectra Lyman-  inferred column densities

Composite Atmosphere – Sublimation + Volcanic A nightside Pele-type plume computed with our 2D DSMC code (Zhang et al., 2004) –The axi-symmetric plume calculation is rotated in 1 degree increments to form a full three-dimensional plume –The plumes (large Pele-type and smaller Prometheus type) are superimposed on the sublimation atmosphere by mass-averaging all of the properties Composite atmosphere showing density ~100 m above surface with two near limb slices showing density with altitude –Streamlines in white show flow away from peak frost temperature as well as deflection around plumes –10 persistently active volcanic plumes (Geissler et al., 2004; Pele and 9 prometheus-type) were superimposed

Comparison of our atmospheric simulations with inferred column densities from Lyman-  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 Comparison to Observations

Comparison of band depth vs. central longitude for several atmospheric cases (Gratiy et al., 2009) –The upper curve is a cos 1/4 (  ) variation with a 90 K nightside temperature –The lower curves are the temperatures needed to create a column densities inferred by Lyman-  observations. The empirical fit is also a cos 1/4 (  ) variation but with a 0 K nightside temperature. Comparison to Observations