A Comprehensive Numerical Model of Ios Sublimation-Driven Atmosphere

Andrew Walker

David Goldstein, Chris Moore, Philip Varghese, and Laurence Trafton

University of Texas at AustinDepartment of Aerospace Engineering

Santa Fe DSMC Workshop September 16th, 2009

Supported by the NASA Planetary Atmosphere Program

In collaboration with Deborah Levin and Sergey Gratiy at Pennsylvania State University

Outline

Background information on IoOverview of our DSMC codeGas dynamic resultsConclusionsValidation Comparison to Observations (Time permitting)

Io is the closest satellite of JupiterIo radius ~1820 kmIt is the most volcanically active body in the solar systemThe primary dayside species, SO2, was detected by the Voyager IR spectrometer in 1979Pearl et al. (1979)Since then many observations have failed to determine whether Ios atmosphere is pre-dominantly volcanically or sublimation-driven.

IoPlasma TorusJupiterBackground Information on Io

Background Information on Io

Frost patch of condensed SO2Volcanic plume with ring depositionSurface Temperature ~ 90 K 115 KLength of Ionian Day ~ 42 hoursMean free path near the surface:lnoon ~ 10 m lmidnight ~ 100 km

Overview of our DSMC codeThree-dimensionalParallelImportant physical modelsDual rock/frost surface modelTemperature-dependent residence timeRotating temperature distributionVariable weighting functionsQuantized vibrational & continuous rotational energy statesPhoto-emissionPlasma heating

Time scalesVibrational Half-lifemillisecond-secondTime step0.5 secondsBetween Collisions0.1 seconds - hoursResidence TimeSeconds - HoursBallistic Time2-3 MinutesFlow Evolution1-2 HoursSimulation Time2 hoursEclipse2 hoursIo Day42 Hours

DSMC in 3D/Parallel3DThe domain is discretized by a spherical gridDomain extends from Io surface to 200 km in altitudeEncompasses all latitudes and longitudesParallelMPITested up to 360 processorsParameters~180 million molecules in domain 1 degree resolution in latitude and longitudeExponential vertical grid that resolves mean free path

yx

Boundary Conditions Frost FractionSO2 surface frost fraction from Galileo NIMS data (Doute et al., 2001):Area fraction of SO2 frost of a 1o by 1o elementHigh latitudes and longitudes from 0o to 60o interpolatedWithin a computational cell, the rock and frost are assumed segregated with the relative abundances determined by the frost fractionThe 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 TimeSO2 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)

-DHS (DHS/kB = 346040 K) : Surface binding energy of SO2 on a SO2 frost, - TS : Rock surface temperature- no (2.41012 s-1) : Lattice vibrational frequency of SO2 within surface matrix site.Model assumes rock is coated with a thin monolayer of SO2Two residence time models tested:The short residence time model uses Eq. 1.The long residence time model uses Eq. 1 x 1000.The long residence time model may be appropriate for a highly porous rock.

SO2 Sublimation & Condensation on SO2 frostSublimation Rate = [#/m2-s]Unit Sticking Coefficient

TfrostBoundary Conditions Surface TemperatureTrockDual frost/rock surface temperature:Independent thermal inertias and albedosLateral heat conduction assumed negligibleSame 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 PPRMatched cooling rate during night

Column density predominantly (exponentially) controlled by surface frost temperatureDue to exponential dependence of SO2 vapor pressure on surface frost temperatureFrost fraction has small (proportional) effect on columnLeads to slightly irregular column densities on daysideLarge irregularities on the nightside where the surface temperature is nearly constantWinds have negligible effect on the columnVertical 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 flowAlong the equator, Mach numbers peak at:M=1.40 for eastward flow; M=0.84 for westward flowMach Number at 30 km Altitude

Coldest (~100 K) near peak surface temperaturePlasma energy coming down column of gas is completely absorbed above this altitudeVery warm (~360 K) near the M=1.4 shock at the dusk terminatorCompressive shock heatingTranslational 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 heatingRotational temperatureIn thermal equilibrium with translation at altitudes below ~10 km on the nightsideThermal equilibrium is maintained to higher altitudes on the dayside because of the higher collision rateCold pocket of gas (~60 K) at 3 km altitude on the daysideThermal Non-EquilibriumTrotTtrans

Conclusions

Column density is predominantly controlled by the frost surface temperatureSmall effects from the surface frost fraction and negligible effects from flowThe pressure-driven supersonic flow diverges from near the region of peak surface frost temperature toward the nightsideThe extended dawn enhancement blocks the westward flowSupersonic to east, north, and south of peak pressureHorseshoe-shaped shockRotational temperatures are not in equilibrium with translational temperatures:Above ~10 km on the nightsideAbove ~50 km on the dayside

Types of Available ObservationsPlume ImagesAuroral GlowsIR Map of Hot SpotsIR Map of Passive BackgroundDisk-Averaged SpectraLyman-a 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 plumeThe plumes (large Pele-type and smaller Prometheus type) are superimposed on the sublimation atmosphere by mass-averaging all of the propertiesComposite atmosphere showing density ~100 m above surface with two near limb slices showing density with altitudeStreamlines in white show flow away from peak frost temperature as well as deflection around plumes10 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-a observations 115 K cases both show reasonable agreement with the peak of Feagas data (Feaga et al., 2009); however, the peak in Feagas data may be from additional volcanic column.There are morphological differences at mid- to high latitudes between the simulations and observationsComparison 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 temperatureThe 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 to Observations