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Plasma-induced Sputtering & Heating of Titan’s Atmosphere R. E. Johnson & O.J. Tucker Goal Understand role of the plasma in the evolution of Titan’s atmosphere Pre-Cassini Understanding: Hydrogen Escape Thermal Carbon & Nitrogen Loss Non-thermal

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Thermal & pick-up plasma >10keV H + exobase UV EUV >10keV O + Smith et al. 2009; Shah et al. 2009; Sillanpaa et al 2007; Ledvina 2007; Luna et al. 2005; Michael et al 2005 Hot recoil production Thermal conduction Average Energy Deposition Highly Variable Titan in Plasma Sheet Modeling of the interaction: Sillanpaa, Snowdon, Ledvina, etc.

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Thermal Plasma & Pick-up Ions Exobase Non-thermal Escape Plasma-Induced Escape Corona Collisions Unlikely Thermosphere Collisions Likely Energetic Ions Thermal Conduction Thermal Escape Michael et al. 2005 DeLaHaye et al. 2007 Westlake et al. 2011 Bell et al. 2011 Use Direct Simulation Monte Carlo Method (DSMC) To Describe Response of Atmosphere

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Non-thermal Escape INMS data for N 2 and CH 4 Density (DeLaHaye et al. 2007) Parameter of the fit: T exo Hot component Thermal component CH 4 & N 2 escape significant but highly variable DSMC

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Thermal Escape Characterized by the Jeans Parameter, = Gravitational Energy/Thermal Energy Enhanced thermal escape at Titan? Slow Hydrodynamic Escape Model Loss of CH 4 & N 2 Dominated by Thermal Conduction (Strobel 2008;2009; Cui et al 2008; Yelle et al 2009)

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Hydro- like Jeans -like = escape rate from top of domain o,o = evaporative flux from surface Thermal escape at Titan: N 2 & CH 4 ~ Jeans Rate (Tucker &Johnson 2009) Thermal Escape Rate vs. λ for principal species (Volkov et al. 2011: ApJ & Phys Fluids)

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~exobase Plasma Heating of the Thermosphere? N 2 Density in Thermosphere (Westlake et al. 2011) in plasma sheet in lobe

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DSMC Model of INMS Data Cross sections with internal energy exchange exobase N2N2 CH 4 H2H2 escape rate (s -1 )N2N2 CH 4 H2H2 Lobe (DSMC) [Jeans rate] < 10 23 [2.2 x10 3 ] < 10 23 [3.7 x10 14 ] 1.0 x10 28 [8.5 x10 27 ] Plasma sheet (DSMC) [Jeans rate] < 10 23 [4.1 x10 10 ] < 10 23 [5.4 x10 18 ] 1.4 x10 28 [1.1 x10 28 ] in lobe in plasma sheet exobase INMS Data from J. Bell H2H2 CH 4 N2N2

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DSMC exobase in plasma sheet Temperatures Separate Well Below Exobase DSMC is useful

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Summary Thermal Escape: including plasma heating No large enhancements over the Jeans rate CH 4 & N 2 density profiles consistent with DSMC for lobe & plasma data H 2 : agreement only for plasma sheet data? Non-thermal escape: expansion in corona implies non-thermal escape

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Projection of the Electric Field on the Equatorial Plane (S. Ledvina) Ion flow across exobase is non-uniform Need 2&3D Simulations

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Sputtering & Heating of Corona Slow ion-neutral collision cross sections are large + Exiting, Pick-up Ions N 2,CH 4,H 2 + Incident Ions Non-thermal escape: is non-uniform & variable 1. Need morphology of the local plasma flux for a number of passes 2. Need to re-analyze the INMS data in the corona

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Effect of Neutral-Neutral Cross Sections on H 2 profile and escape collision modelRate x 10 28 H 2 s -1 hard sphere (HS).98 HS with internal energy1.1 variable (HS) with internal energy1.04 T o ~132 K

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Cassini Plasma Data: Ta M~16 M~28 M~16 M=2 M=1 M=2 Energy flux ratio (egress/ingress) near exobase ~ 1.3 1679km egress

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Analytic Model Struck neutrals have a spectrum of recoil energies, E ~ E deposited / E 2 Recoils ejected if direction is up and E > E escape Number ejected per ion incident ~ E deposited / E escape Tested by simulations +

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Monte Carlo Simulations (e.g. Bird; Shematovich et al. 2003) Track representative particles under gravity Monte Carlo choice of collision outcome Simulate an atmosphere Inject ions Change in atmospheric structure Count ejected molecules Equivalent to solving the Boltzmann equation for a gas Limiting factors: cross sections and range of densities +

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Used sputter models Best Fit Energy Distributions These only give bounds for E > ~ 0.2eV Maxwellian + Analytic Kappa Function

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Fits to Hot Corona ( De La Haye et al. 2007) T x Energy Deposition Escape Flux (K) (eV/cm 3 /s) (10 9 amu/cm 2 /s) T A ingress 150 100 1.5 ( <18) egress 157 78 1.1 (<14) T B egress 149 290 4.0 (<48) T 5 ingress 162 ~0 ~0 egress 154 60 0.9 (<12) 0.2 (<5) x10 10 amu/cm 2 /s (DeLaHaye et al. 2007) 4-5 x10 10 amu/cm 2 /s (Yelle et al. 2007)* 5 x10 10 amu/cm 2 /s (Strobel 2007) CH 4 Escape 1/7 the photo destruction rate (Yelle et al 2007) Total atmospheric mass lose present atmosphere in ~4.5Gyr

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Atmospheric Loss Rate 0.2 - 5 x10 10 amu/cm 2 /s (DeLaHaye et al. 2007) 5 x10 10 amu/cm 2 /s (Strobel 2007) 4-5 x10 10 amu/cm 2 /s (Yelle et al. 2007).

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INMS EXOSPHERE DATA De La Haye et al. 2007 Therefore: Invert data Simulate the corona get best fit energy spectrum Power Law ~E -x Kappa Distributions Obtain heating rate

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Energetic Neutrals Image Part of the Corona H + (10’s keV) + H 2 H + H 2 + (MIMI Instrument: I. Dandouras et al,) Plasma is variable but not unlike Voyager (Hartle et al. 2006 )

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Area 2 x10^18 Sillanpaa O+ 4 x10^9eV/cm^2/s global Teng (Pick-up) 5.6 x10^7 Ledvina 5.6 x10^8/cm^2/s

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Incident Flux ~16 amu ( O + (CH x +,N + ) ~28 amu ( N 2 + (HCNH +,C 2 H 5 + ) Energy Flux EUV ~ 2 x10 10 eV/cm 2 /s Plasma ~1.5-0.5 x10 10 eV/cm 2 /s Energetic Ions ~0.5 x10 10 eV/cm 2 /s Sillanpaa et al 2007; Ledvina 2007; Michael et al 2005

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Incident Ions N (x10 25 s -1 )N 2 (x10 25 s -1 ) Net N as N and N 2 (x10 25 s -1 ) O +, N 2 + 2.50.73.9 Model Global Average Escape Rates Escape of N atoms as N or N 2 is ~ 4x10 25 N s -1 Flux = 2 x10 7 N/cm 2 /s ~ 10% CH 4 ~ 2 x10 6 /cm 2 /s ~ 10% H 2 ~ 2 x10 7 /cm 2 /s corresponds to < 1% of present atmosphere in 4Gyr For comparison If Io had a Titan like atmosphere Lose ~ 100% in 0.14 Gyr (Johnson, 2004)

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Incident Ions Energy Flux Net Ejecta (14 + 28 amu) O +, N 2 + ~5x10 9 eV/cm 2 /s ~6 x10 26 amu/s 3 x10 8 amu/cm 2 /s Model Global Average Escape Rates Flux ≈

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Plasma ions (14, 28) >10keV H + Ledvina, Tucker exobase Average Energy Deposition UV+EUV >10keV O + Ledvina, Tucker UV-EUV ~ 2 x10 10 eV/cm 2 /s Plasma ions ~0.4 (1.5) x10 10 eV/cm 2 /s Energetic Ions (>10keV)~0.5 x10 10 eV/cm 2 /s Sillanpaa et al 2007; Ledvina 2007; Michael et al 2005

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Effects Chemistry: dissociation, ionization & O + implantation Heating Atmospheric loss: thermal & nonthermal Source for Magnetosphere Evolution of atmosphere Goal: accurately describe escape processes

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Simulations Energy spectra of N 2 in the Transition Region & Corona Thermal core + suprathermal tail Below exobase Above exobase Hot N 2 populates corona

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Some Energy Deposition Rate Estimates Smith et al 2009 Luna et al 2005 Shah et al 2009 Michael et al 2005 Strobel 2009 In Plasma Sheet Thermal & Pick-up UV/EUV Solar med. Mimi O + H + max H + O + Mimi Median

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