Plasma-induced Sputtering & Heating of Titan’s Atmosphere R. E. Johnson & O.J. Tucker

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

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

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

Non-thermal EscapeINMS data for N2 and CH4 Density (DeLaHayeet al. 2007) Hot component Parameter of the fit: Texo DSMC Thermal component CH4 & N2 escape significant but highly variable

Enhanced thermal escape at Titan? Slow Hydrodynamic Escape Model Loss of CH4 & N2 Dominated by Thermal Conduction(Strobel 2008;2009; Cui et al 2008; Yelle et al 2009) Thermal EscapeCharacterized by the Jeans Parameter,  = Gravitational Energy/Thermal Energy

Thermal Escape Rate vs. λ for principal species (Volkov et al. 2011: ApJ & Phys Fluids) Hydro- like  = escape rate from top of domain o,o = evaporative flux from surface Jeans -like Thermal escape at Titan: N2 & CH4 ~ Jeans Rate (Tucker &Johnson 2009)

Plasma Heating of the Thermosphere? ~exobase in plasma sheet in lobe N2 Density in Thermosphere (Westlake et al. 2011)

DSMC Model of INMS Data Cross sections with internal energy exchange exobase in lobe in plasma sheet exobase N2 H2 CH4 H2 CH4 N2 INMS Data from J. Bell

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

Summary Thermal Escape: including plasma heating No large enhancements over the Jeans rate CH4 & N2 density profiles consistent with DSMC for lobe & plasma data H2 : agreement only for plasma sheet data? Non-thermal escape:expansion in corona implies non-thermal escape

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

Sputtering & Heating of Corona Slow ion-neutral collision cross sections are large Incident Ions Exiting, Pick-up Ions N2,CH4,H2 + N2,CH4,H2 + 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

Effect of Neutral-Neutral Cross Sections on H2 profile and escape To ~132 K

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

+ Analytic Model Struck neutrals have a spectrum of recoil energies, E ~ Edeposited / E2 Recoils ejected if direction is upand E > Eescape Number ejected per ion incident ~ E deposited / Eescape Tested by simulations

+ 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

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

Fits to Hot Corona (De La Haye et al. 2007) 0.2 (<5) x1010 amu/cm2/s (DeLaHaye et al. 2007) 4-5 x1010 amu/cm2/s (Yelle et al. 2007)* 5 x1010 amu/cm2/s (Strobel 2007) CH4 Escape  1/7 the photo destruction rate (Yelle et al 2007) Total atmospheric mass lose present atmosphere in ~4.5Gyr

Atmospheric Loss Rate • 0.2 - 5 x1010amu/cm2/s (DeLaHaye et al. 2007) • 5 x1010amu/cm2/s (Strobel 2007) • 4-5 x1010 amu/cm2/s (Yelle et al. 2007).

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

Energetic Neutrals Image Part of the Corona H+ (10’s keV) + H2 H + H2+ (MIMI Instrument: I. Dandouras et al,) Plasma is variable but not unlike Voyager (Hartle et al. 2006)

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

Incident Flux ~16 amu (< ~ 0.75 keV)--> O+ (CHx+,N+) ~28 amu ( < ~1.25 keV)--> N2+ (HCNH+,C2H5+) Energy Flux EUV ~ 2 x1010 eV/cm2/s Plasma ~1.5-0.5 x1010 eV/cm2/s Energetic Ions ~0.5 x1010 eV/cm2/s Sillanpaa et al 2007; Ledvina 2007; Michael et al 2005

Model Global Average Escape Rates Escape of N atoms as N or N2 is ~ 4x1025 N s-1 Flux = 2 x107 N/cm2/s ~ 10% CH4 ~ 2 x106 /cm2/s ~ 10% H2  ~ 2 x107 /cm2/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)

Model Global Average Escape Rates Flux ≈

Average Energy Deposition UV-EUV ~ 2 x1010 eV/cm2/s Plasma ions ~0.4 (1.5) x1010 eV/cm2/s Energetic Ions (>10keV) ~0.5 x1010 eV/cm2/s Sillanpaa et al 2007; Ledvina 2007; Michael et al 2005 exobase Plasma ions (14, 28) UV+EUV >10keV O+ Ledvina, Tucker >10keV H+ Ledvina, Tucker

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

Simulations Energy spectra of N2 in the Transition Region & Corona Thermal core + suprathermal tail Below exobase  Above exobase Hot N2 populates corona

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

Plasma-induced Sputtering & Heating of Titan’s Atmosphere R. E. Johnson & O.J. Tucker