DSMC Simulation of the Plasma Bombardment on Io’s Sublimated and Sputtered Atmosphere Chris Moore 0 and Andrew Walker 1 N. Parsons 2, D. B. Goldstein 1, P. L. Varghese 1, L. M. Trafton 1, D.A. Levin 2 0 Sandia National Labs 1 University of Texas at Austin 2 Penn State University 50 th AIAA Aerospace Sciences Meeting 1/10/2012 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-AC04-94AL 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 Flux Tube 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 Io Frost patch of condensed SO 2 Volcanic plume with ring deposition Jupiter Io Io Flux Tube 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 SO 2 surface frosts Dominant dayside atmospheric species is SO 2 ; lesser species - S, S 2, SO, O, O 2 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

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 SO 2 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 scales Chemistry seconds Surface sputtering seconds Plasma Timestep0.005 seconds Ion-Neutral Collisions0.01 seconds - hours Vibrational Half-lifemillisecond-second Cyclotron Gyration0.5 seconds Neutral Time step0.5 seconds Neutral Collisions0.1 seconds - hours Residence Timeseconds - hours Ballistic Time2-3 minutes Flow EvolutionSeveral hours Eclipse 2 hours SO 2 Photo Half-life36 hours Io Day 42 hours 6

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 SO 2 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 scales Atomic interactions ~10 -9 m Sputtering radius ~10 -7 m Debye Length <1 m Electron Larmor radius3 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 radius3 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 ~10 5 km 7

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

Ion energies in the collision cascade regime Little sputtering contribution from electronic excitation Sputtering yield proportional to incident ion energy Surface Sputtering 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 SO 2 sputtering yield, S, versus SO 2 frost temperature. Lanzerotti et al. (1982) 10

Surface Sputtering Sputtered SO 2 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 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. B-Field E-Field (Out of the page) Simulate simple ion motion and impact onto surface: 12

Heavy Interactions: MD/QCT 1 SO 2 + 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 SO 2 internal energies Each case consists of 10,000 separate trajectories: Microcanonical sample unique impact parameters and initial SO 2 component coordinates Potential Energy Surface Total potential of SO 2 + O system is the summation of the collisional interaction potential and molecular potential of the SO 2 molecule Collisional interaction: Lennard-Jones 6-12 potential SO 2 molecular potential: Murrell 3-body potential Allows for accurate dissociation of SO 2 molecule to SO + O, O 2 + S, or S + 2O 13 1 Parsons, N. and Levin, D., 50 th AIAA Aerospace Sciences Meeting:

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 « T otal cross section above ~20 km/s 14

Photo-chemistry Rate constants, k react,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 0-D box initialized with only SO 2 particles. Lines are analytic, diamonds from DSMC. 1 Io Day Sunlight time 15

Simulation Conditions Io just before ingress → Plasma incident onto dusk terminator Assume uniform SO 2 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 Dusk Terminator Dawn Terminator Plasma Flow Sunlight 16

3D Results: SO 2 SO 2 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: O 2 O 2 produced via photo- dissociation on dayside Non-condensable O 2 gas dynamics very different, but day-to-night flow still present O 2 “finger” extends much further onto the nightside, ≈ to the dusk terminator Retrograde flow across nightside meets day-to-night flow at dusk terminator O 2 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 SO 2 Frost Sputtering primarily on the nightside and at high latitudes Dense atmospheric columns (> 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 SO 2 sputtering map suggests Na is the source of green aurora with sputtering blocked on dayside Dawn Terminator Nightside Na aurora? 20

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 Sunlight Dusk Terminator Dusk Terminator 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 O 2 flow feature generated at the dusk terminator Non-condensable O 2 pushed across the nightside to the dusk terminator where it meets the opposite day-to-night flow O 2 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 > 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

Electron Interactions Used measured (and theoretical) cross sections versus relative velocity (energy) For SO 2, SO, O 2, S 2, O, and S Because of the large number of reactions, precompute reaction probabilities vs. finite set of energies Account for anisotropic scattering SO 2 cross sections Trace molecular cross sections O cross sections

Physically, sputtered particles shouldn’t collide with each other Sputtering occurs over area of ~ m 2 → point source Collisions between sputtered particles tends to reduce the expansion velocity parallel to the surface Solution: Place sputtered particles at cell corner, weighted by the inverse distance from impact point to each corner Surface Sputtering L c, 0 L c,1 L c,2 L c, Surface Cell

Column Densities Due to flow, SO 2 column density no longer hydrostatic near dawn terminator on nightside Nightside SO 2 column density increases slightly towards pole SO 2 condensable, SO partially condensable, O 2 non- condensable –O 2 dominates nightside –SO extends further than SO 2 onto nightside O 2 shows large buildup in column density near dusk terminator 19