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

1. 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 Austin2Penn State University 50th AIAA Aerospace Sciences Meeting 1/10/2012 Supported by the NASA Planetary Atmospheres and Outer Planets Research Programs. Computations performed at the Texas Advanced Computing Center. 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.

2. 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

3. Motivation • 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 Jupiter Io Plasma Torus Io Flux Tube Illustration by Dr. John Spencer • Io supplies the Jovian plasma torus: • Surface and atmospheric sputtering • Ionization • Charge exchange 3

4. Background Information on Io Frost patch of condensed SO2 Jupiter Io Flux Tube Io Volcanic plume with ring deposition 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 Io 4

5. 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

6. Overview of our DSMC code Time scales Chemistry 10-12 seconds Surface sputtering 10-10 seconds Plasma Timestep0.005 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 hours • 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 6

7. Overview of our DSMC code Length scales Atomic 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 vents 0.1–10 km Ion-neutral m.f.p. 500 m Electron-ion m.f.p. ~1 km Ion Larmor radius 3 km Atmospheric scale height 10–100 km Nightside neutral m.f.p. ~100 km Volcanic plumes 100–500 km Io’s radius 1820 km Jovian plasma torus ~105 km • 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 7

8. 3D / Parallel • 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

9. Surface Sputtering • Ion energies in the collision cascade regime • Little sputtering contribution from electronic excitation • Sputtering yield proportional to incident ion energy 9

10. Surface Sputtering • 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 SO2 sputtering yield, S, versus SO2 frost temperature. Lanzerotti et al. (1982) 10

11. Surface Sputtering • 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 Sputtered SO2 energy distribution. Boring et al. (1984) 11

12. 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. Simulate simple ion motion and impact onto surface: B-Field E-Field (Out of the page) 12

13. 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 13 1Parsons, N. and Levin, D., 50th AIAA Aerospace Sciences Meeting: 2012-0227

14. 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 • 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

15. Photo-chemistry Sunlight • 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 1 Io Day 0-D box initialized with only SO2 particles. Lines are analytic, diamonds from DSMC. 15

16. Simulation Conditions Eclipse Io Dusk Terminator Dawn Terminator Plasma Flow Y Plasma Flow X 8.9° Io Jupiter Sunlight Sub-Jovian spot; 0° longitude Sunlight Io’s orbit • 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 16

17. 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

18. 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

19. 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

20. 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 Nightside Na aurora? Dawn Terminator 20

21. Eclipse Prior to ingress Dusk Terminator Io Plasma Flow Y Plasma Flow X 8.9° Io Eastern Elongation Jupiter Sunlight Dusk Terminator Io’s orbit Sunlight Plasma Flow Io Dusk Terminator Sunlight Discussion Current simulation • 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 21

22. 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 surface 22