Plasma-induced Sputtering & Heating of Titan’s Atmosphere

R. E. Johnson & O.J. Tucker

GoalUnderstand role of the plasma in the evolution of Titan’s atmosphere

Pre-Cassini Understanding: Hydrogen Escape Thermal

Carbon & Nitrogen Loss Non-thermal

Thermal &

pick-up plasma

>10keV H+

exobaseUV 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

Thermalconduction

Average Energy DepositionHighly Variable

Titan in Plasma Sheet

Modeling of the interaction: Sillanpaa, Snowdon, Ledvina, etc.

Thermal Plasma& Pick-up Ions

Exobase

Non-thermalEscape

Plasma-Induced Escape

CoronaCollisions UnlikelyThermosphereCollisions Likely

Energetic Ions

ThermalConduction

ThermalEscape

Michael et al. 2005DeLaHaye et al. 2007

Westlake et al. 2011Bell et al. 2011

Use Direct Simulation Monte Carlo Method (DSMC)To Describe Response of Atmosphere

Non-thermal EscapeINMS data for N2 and CH4 Density

(DeLaHaye et al. 2007)

Parameter of the fit:

Texo

Hot component

Thermal component

CH4 & N2 escape significant but highly variable

DSMC

Thermal EscapeCharacterized by the Jeans Parameter,

= Gravitational Energy/Thermal Energy

Enhanced thermal escape at Titan?

Slow Hydrodynamic Escape ModelLoss of CH4 & N2 Dominated by Thermal Conduction

(Strobel 2008;2009; Cui et al 2008; Yelle et al 2009)

Hydro-like

Jeans-like

= escape rate from top of domaino,o = evaporative flux from surface

Thermal escape at Titan: N2 & CH4 ~ Jeans Rate (Tucker &Johnson 2009)

Thermal Escape Rate vs. λ for principal species

(Volkov et al. 2011: ApJ & Phys Fluids)

~exobase

Plasma Heating of the Thermosphere?

N2 Density in Thermosphere(Westlake et al. 2011)

in plasma sheetin lobe

DSMC Model of INMS DataCross sections with internal energy exchange

exobase

N2CH4H2

escape rate (s-1) N2 CH4 H2

Lobe (DSMC)[Jeans rate]

< 1023

[2.2 x103] < 1023

[3.7 x1014]1.0 x1028

[8.5 x1027]

Plasma sheet (DSMC)[Jeans rate]

< 1023

[4.1 x1010] < 1023

[5.4 x1018]1.4 x1028

[1.1 x1028]

in lobe in plasmasheet

exobase

INMS Data from J. Bell

H2 CH4 N2

DSMC

exobase

in plasma sheet

Temperatures Separate Well Below ExobaseDSMC is useful

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

Projection of the Electric Field on the Equatorial Plane (S. Ledvina)

Ion flow across exobase is non-uniform

Need 2&3D Simulations

Sputtering & Heating of CoronaSlow ion-neutral collision cross sections are large

+

Exiting, Pick-up Ions

N2,CH4,H2N2,CH4,H2

+

Incident Ions

Non-thermal escape: is non-uniform & variable 1. Need morphology of the local plasma flux fora 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

collision model Rate x 1028 H2 s-1

hard sphere (HS) .98

HS with internal energy 1.1

variable (HS) with internal energy 1.04

To ~132 K

Cassini Plasma Data: Ta

M~16

M~28

M~28M~16

M=2

M=1

M=1M=2

Energy flux ratio (egress/ingress) near exobase ~ 1.3

1679kmegress

Analytic Model

Struck neutrals have a spectrum of recoil energies, E ~ Edeposited / E2

Recoils ejected if direction is up and 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) Tx Energy Deposition Escape Flux (K) (eV/cm3/s) (109 amu/cm2/s)

TA ingress 150 100 1.5 ( <18)

egress 157 78 1.1 (<14)

TB egress 149 290 4.0 (<48)

T5 ingress 162 ~0 ~0

egress 154 60 0 .9 (<12)

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 x1010 amu/cm2/s (DeLaHaye et al. 2007)• 5 x1010 amu/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 CoronaH+ (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/sEnergetic Ions ~0.5 x1010 eV/cm2/s

Sillanpaa et al 2007; Ledvina 2007; Michael et al 2005

Incident Ions

N (x1025 s-1) N2 (x1025 s-1)Net N as N and N2

(x1025 s-1)

O+ , N2+ 2.5 0.7 3.9

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 4GyrFor comparison

If Io had a Titan like atmosphereLose ~ 100% in 0.14 Gyr

(Johnson, 2004)

Incident Ions

Energy FluxNet Ejecta

(14 + 28 amu)

O+, N2+ ~5x109eV/cm2/s

~6 x1026 amu/s3 x108 amu/cm2/s

Model Global Average Escape Rates

Flux ≈

Plasma ions (14, 28)

>10keV H+

Ledvina, Tucker

exobase

Average Energy Deposition

UV+EUV

>10keV O+

Ledvina, Tucker

UV-EUV ~ 2 x1010 eV/cm2/s Plasma ions ~0.4 (1.5) x1010 eV/cm2/sEnergetic Ions (>10keV) ~0.5 x1010 eV/cm2/s Sillanpaa et al 2007; Ledvina 2007; Michael et al 2005

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

Smith et al 2009

Luna et al 2005

Shah et al 2009

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