DSMC Simulation of the Plasma Bombardment on Io’s Sublimated and Sputtered Atmosphere

Chris Moore0 and Andrew Walker1

N. Parsons2, D. B. Goldstein1, P. L. Varghese1, L. M. Trafton1, D.A. Levin2

0Sandia National Labs1University of Texas at Austin

2Penn State University

50th AIAA Aerospace Sciences Meeting1/10/2012

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company,for the United States Department of Energy’s National Nuclear Security Administration

under contract DE-AC04-94AL85000.

Supported by the NASA Planetary Atmospheres and Outer Planets Research Programs. Computations performed at the Texas Advanced Computing Center.

Outline

• Brief motivation and background information on Io• Overview of physical models in our planetary DSMC Code

• Description of new physical models– Particle description of the plasma

– Surface sputtering due to energetic ions

– Ion reaction chemistry

– Photo-chemistry

• Atmospheric Simulations• Conclusions

2

Motivation

Jupiter Io

Plasma Torus

Io FluxTube

• Jovian plasma torus sweeps past Io’s atmosphere causing:• Heating• Chemistry• Changes to the global

winds• Enhanced gas columns

due to sputtering• Observed auroral

glows • Matching obs. can

be used to probe the torus conditions

• Io supplies the Jovian plasma torus:• Surface and atmospheric sputtering• Ionization• Charge exchange

Illustration by Dr. John Spencer

3

Background Information on IoFrost patch of condensed SO2

Volcanic plume with ring deposition

Jupiter

Io

Io FluxTube

Illustration by Dr. John Spencer

• Io is the closest satellite to Jupiter• Radius ≈ 1820 km (slightly larger than our moon)• Atmosphere sustained by volcanism and sublimation from SO2 surface frosts

• Dominant dayside atmospheric species is SO2; lesser species - S, S2, SO, O, O2

• Io is the most volcanically active body in the solar system• Volcanism is due to an orbital resonance with Europa and Ganymede which

causes strong tidal forces in Io4

Brief Overview of DSMC• DSMC simulates gas

dynamics using a “large” number of representative particles– Position, velocity, internal

state, etc. stored

• Particle collisions and movement are decoupled in a given timestep

• Particles are moved by integrating F=ma

• Binary collisions allowed to occur between particles in the same “collision cell”

5

Overview of our DSMC code• Atmospheric models

– Rotational and vibrational energy states– Sub-stepped emission– Variable gravity– Simulate plasma with particles– Chemistry: neutral, photo, ion, & electron

• Surface models– Non-uniform SO2 surface frosts– Comprehensive surface thermal model– Volcanic hot spots.– Residence time on the non-frost surface– Surface sputtering by energetic ions

• Numerical models– Spatially and temporally varying

weighting functions.– Adaptive vertical grid that resolves mfp– Sample onto to uniform output grid– Separate plasma and neutral timesteps

Time scalesChemistry 10-12 seconds

Surface sputtering 10-10 seconds

Plasma Timestep 0.005 seconds

Ion-Neutral Collisions 0.01 seconds - hours

Vibrational Half-life millisecond-second

Cyclotron Gyration 0.5 seconds

Neutral Time step 0.5 seconds

Neutral Collisions 0.1 seconds - hours

Residence Time seconds - hours

Ballistic Time 2-3 minutes

Flow Evolution Several hours

Eclipse 2 hours

SO2 Photo Half-life 36 hours

Io Day 42 hours6

Overview of our DSMC code• Atmospheric models

– Rotational and vibrational energy states– Sub-stepped emission– Variable gravity– Simulate plasma with particles– Chemistry: neutral, photo, ion, & electron

• Surface models– Non-uniform SO2 surface frosts– Comprehensive surface thermal model– Volcanic hot spots.– Residence time on the non-frost surface– Surface sputtering by energetic ions

• Numerical models– Spatially and temporally varying

weighting functions.– Adaptive vertical grid that resolves mfp– Sample onto to uniform output grid– Separate plasma and neutral timesteps

Length scalesAtomic interactions~10-9 m

Sputtering radius~10-7 m

Debye Length<1 m

Electron Larmor radius 3 m

Dayside neutral m.f.p.~10 m

Volcanic plume vents0.1–10 km

Ion-neutral m.f.p.500 m

Electron-ion m.f.p.~1 km

Ion Larmor radius 3 km

Atmospheric scale height10–100 km

Nightside neutral m.f.p.~100 km

Volcanic plumes100–500 km

Io’s radius1820 km

Jovian plasma torus~105 km

7

3D / ParallelBlown-up Viewof Low AltitudeAtmosphere withMeshFull Planet

360 processors

Processor Boundary

Single ProcessorDomain

• 3D• Spherical grid – northern

hemisphere• 3°×3° latitude/longitude cells• Non-uniform radial grid

• Parallel• MPI, 900 CPU’s

• Parameters• 360 million molecules

instantaneously• Simulated 10 hours to quasi-SS• ~25,000 computational hours

8

•Ion energies in the collision cascade regime

•Little sputtering contribution from electronic excitation

•Sputtering yield proportional to incident ion energy

Surface Sputtering

x

+

x+

Incident Ion Energy, Ei (eV)S

pu

tte

rin

gY

ield

,YA

,B(#

of

Bp

er

ion

)100 200 300 400 500 600

50

100

150

200

250

300

350

YA,B/(A,B) = 0.53Ei + 29

YO , SO , O , SO = 0.256

YS , SO , S , SO = 0.223

YS , SO

YO , SO

x

+

Black Symbols - Johnson , 1984Red Symbols - Chrisey , 1987

+

+2

2

2

2+

+

et al.et al.

+

+2

2

9

•Ion energies in the collision cascade regime

•Little sputtering contribution from electronic excitation

•Sputtering yield proportional to incident ion energy

•Sputtering yield exponential with surface frost temperature

•

Surface Sputtering

SO2 sputtering yield, S, versus SO2 frost temperature. Lanzerotti et al. (1982)

1.7

50

115

s

s

TYield

TYield

10

Surface Sputtering

Sputtered SO2 energy distribution. Boring et al. (1984)

•Ion energies in the collision cascade regime

•Little sputtering contribution from electronic excitation

•Sputtering yield proportional to incident ion energy

•Sputtering yield exponential with surface frost temperature

•

•Sputtered particles leave with Thompson energy distribution

1.7

50

115

s

s

TYield

TYield

11

Charged Particle Motion•Acceleration during move:

•Use predictor-corrector integrator

•Pre-computed (MHD) fields used•Electrons are assumed to move with the ions

•Debye length << m.f.p.

BvEm

eZe

r

Rga r

Io

ˆ

2

B-FieldE-Field(Out of the page)

Simulate simple ion motion and impact onto surface:

12

Heavy Interactions: MD/QCT1

• SO2 + O collisions simulated using Molecular Dynamics/Quasi-Classical Trajectories (MD/QCT)

•RK-4 integration of Hamiltonian equations

•Particles interact via their potentials

•Cases run for range of collider velocities and initial SO2 internal energies•Each case consists of 10,000 separate trajectories: Microcanonical sample unique impact parameters and initial SO2 component coordinates

• Potential Energy Surface• Total potential of SO2 + O system is the summation of the collisional interaction potential and molecular potential of the SO2 molecule

• Collisional interaction: Lennard-Jones 6-12 potential

• SO2 molecular potential: Murrell 3-body potential

• Allows for accurate dissociation of SO2 molecule to SO + O, O2 + S, or S + 2O 131Parsons, N. and Levin, D., 50th AIAA Aerospace Sciences Meeting: 2012-0227

Heavy Interactions

•MD/QCT (fast neutrals/ions) or theoretical cross section data vs. translational and internal energy•Linearly interpolate between nearest cross section data points

•If no MD/QCT data, use Arrhenius coefficients & TCE

Relative Velocity, Vrel (km/s)

Cro

ss

Section

(cm

2 )0 20 40 60 80 100

10-17

10-16

10-15

10-14

Total SO2 + O using MD/QCTVHS fit to MD/QCT data below 16 km/sMD/QCT SO2 + O non-reactiveSO2 + O SO + 2O, Eint=4.99 eVSO2 + O S + 3O, Eint=4.99eVO2 + O 3O

•Always use the total cross section to determine the reaction rate (number of selections and fraction accepted)

•VHS cross section « Total cross section above ~20 km/s

14

Photo-chemistry

• •Rate constants, kreact,s,i, assume quite sun

•Assume gas is optically thin•Optical depth over photo-dissociation wavelengths less than 0.1

•Give dissociation products an average excess kinetic energy

•Accurate below the exobase where products are collisionally equilibrated

Time (s)

No

rma

lize

dp

art

icle

nu

mb

er

0 100000 200000 300000 400000 500000 60000010-2

10-1

100

SO2

SOO2

SO

0-D box initialized with only SO2 particles. Lines are analytic, diamonds from DSMC.

1 Io Day

reactionsN

iisreactsreact tkP ,,, exp1

Sunlighttime

15

Longitude (degrees)

La

titu

de

(de

ge

es

)

03060901201501802102402703003303600

30

60

9090 92 94 96 98 100 102 104 106 108 110 112 114

Surface (frost)Temperature (K)

Subsolar Point

Dawn TerminatorDusk Terminator

Leading HemisphereTrailingHemisphere

XY

Z

Y, Subplasma Point (270 )

Constant NightsideTemperature (90 K)

Subsolar Point(351.1 )

Upstream PlasmaCorotational Direction

Trailing Hemisphere

X (0 )

Leading Hemisphere

Simulation Conditions

•Io just before ingress → Plasma incident onto dusk terminator•Assume uniform SO2 frost → No rock surface or residence time

•Assume simple radiative equilibrium surface temperature model•Do not account for Io’s rotation, thermal inertia

Y

Jupiter

Eclipse

Sunlight

Plasma Flow

Io

8.9°Io

Sub-Jovian spot; 0° longitude

Io’s orbit

X

DuskTerminator

DawnTerminatorPlasma

Flow

Sunlight

16

3D Results: SO2

•SO2 number density peaks near the subsolar point

•Day-to-night near surface flow develops from subsolar point

•Retrograde wind forms and high density “finger” extends past the dawn terminator due to plasma pressure

•Slight increase in the polar atmosphere due to preferential polar sputtering

Direction of Io’s rotation

17

3D Results: O2

•O2 produced via photo-dissociation on dayside

•Non-condensable O2 gas dynamics very different, but day-to-night flow still present

•O2 “finger” extends much further onto the nightside, ≈ to the dusk terminator

•Retrograde flow across nightside meets day-to-night flow at dusk terminator

•O2 diffuses towards the poles where it is stripped away or destroyed by the plasma

Dawn Terminator

18

3D Results: O+

•O+ density contours 4 km above Io’s surface

•High altitude ions stream along field lines to surface

•On the nightside, ions stream to the surface

•Upstream torus O+

density 2400 cm-3

•Dense dayside atmosphere prevents plasma penetration

•Enhancement on the dayside from plasma flow

19

Surface Sputtering of SO2 Frost

•Sputtering primarily on the nightside and at high latitudes•Dense atmospheric columns (> 1015 cm-2) block energetic ions from reaching the surface

•Obs. show green auroral glow only on Io’s nightside•Sodium is believed to be sputtered off Io’s surface

•Simulated SO2 sputtering map suggests Na is the source of green aurora with sputtering blocked on dayside

Longitude (degrees)

La

titu

de

(de

ge

es

)

03060901201501802102402703003303600

30

60

901.0E+25 1.9E+25 3.7E+25 7.2E+25 1.4E+26 2.7E+26 5.2E+26 1.0E+27

SputteringRate (s-1 km-2)

Subsolar Point

Dawn Terminator

Dusk Terminator

Subplasma Point

Nightside

Dawn Terminator

Nightside Na aurora?

20

Longitude (degrees)

La

titu

de

(de

ge

es

)

03060901201501802102402703003303600

30

60

900.0010 0.0040 0.0158 0.0631 0.2512 1.0000

NormalizedSputtering Rate

Subsolar Point

Dawn Terminator

Dusk Terminator

Subplasma Point

Nightside

Discussion•Direction of plasma flow relative to subsolar point important

•Subsolar point changes during Io’s orbit → Atmospheric dynamics will change as Io orbits Jupiter

•Sputtering only occurring near night time temperatures implies preferential scouring of surface by plasma from 270°–360°

•Eclipse inhibits formation of dayside atmosphere

•Plasma directly impacts this quadrent

•Io’s surface frost poor in this region

Prior to ingress

Y

Jupiter

Eclipse

Sunlight

Plasma Flow

Io

8.9°Io

Io’s orbit

X

Sunlight

Eastern Elongation

Plasma Flow

Plasma Flow

Sunlight

Dusk Terminator

Dusk Terminator

DuskTerminator

Io

Current simulation

21

Conclusions•The interaction of the Jovian plasma torus with Io’s atmosphere was simulated using the DSMC method.

•A sub-stepping method was used to time-resolve the movement and collisions of energetic ions and electrons from the Jovian plasma torus

•MD/QCT simulations were used to compute the cross-sections for heavy reactions

•Sputtering from Io’s surface by energetic ions and fast neutrals was included

•Formation of high density “finger” onto the nightside near the dawn terminator due to plasma pressure

•Interesting O2 flow feature generated at the dusk terminator

•Non-condensable O2 pushed across the nightside to the dusk terminator where it meets the opposite day-to-night flow

•O2 stagnates and forced to diffuse slowly towards the pole until it is stripped away and/or dissociated

•Sensitivity of sputtering on surface temperature can lead to sharp gradients in sputtering column density — Sputtering blocked by large columns > 1015 cm-2

•Concentrated at high latitudes and on low density nightside

•Possible cause of observed (Voyager, Galileo) frost-poor region of Io’s surface22