Organic Synthesis in the Atmosphere of Titan: Modeling

and Recent Observations

Yuk Yung (Caltech), M. C. Liang (Academia Sinica), X. Zhang

(Caltech), J. Kammer (Caltech), D. Shemansky (SET)

NAI Titan Team Meeting 11-12 May 2011, Pasadena, CA

Outline of Today’s Talk

Titan: gas phase chemistry

Aerosol formation

Surface chemistry

Synergism with lab data

Solar Scattering

Stellar Occultation

J. Ajello

Mixing Ratios of Selected Species from Occultations

UVIS spectrum

Liang et al. 2007

tholin

CH4

Impact: 514 km

Optical Depth Images

c

Lavvas et al. 2008

EUV

FUV

auto

Auto-catalytic process

Hydrocarbon Abundances from TB Encounter

Tholin scale heights above 540 km are larger than any other species indicating formation at high altitudes and downward diffusion.

Photochemical results

Liang, Yung, Shemansky ApJ 2007

CH4; hydrostatic

CH4; non-hydrostaticHC3N

HCN

C6N2

C6H6 C6N2; condensation line

Gu et al. 2009

Model without Haze

C6Hx

Model with Haze Formation

C6Hx

[Vuitton, et al., 2006]

[Vuitton, et al., 2006]

[Vuitton, et al., 2006]

Ion observation

Outline of Today’s Talk

Titan: gas phase chemistry

Aerosol formation

Surface chemistry

Synergism with lab data

Solar Scattering

Stellar Occultation

J. Ajello

Liang et al. 2007

tholinCH4

Impact: 514 km

Stellar Occultation

Qe =−4xI (m2 −1m2 + 2

)

Qs =83

x4 m2 −1m2 + 2

2

x =2πrλ Single Scattering

Albedo (SSA):

SSA = Qs/Qe

Important Parameters

Goody and Yung 1989

Obs: 0.118

16 nm

Refractive Index from Khare and Sagan (1984)

SSA at 1875 Å

Shemansky et al. 2010

. 2Trainer, et al 2006

Tomasko et al. 2008: ~100 km

50 nm radius 3000 monmers

Comparisons

• Tholin Radius at 1040 km: 16 nm

Liang et al. (2007) “guessed” 12.5 nm from Stellar Occultation only

• Comparable to 25 nm (in radius) from Trainer et al. (2006) ; 40 nm from Bar-Nun et al. (2008)

• Lavvas et al. (2008) at 520 km (ISS):

~40 nm

Comparison of radius of tholins

T Tomasko et al. 2008

Outline of Today’s Talk

Titan: gas phase chemistry

Aerosol formation

Surface chemistry

Synergism with lab data

What happens to the Unsaturated

Hydrocarbons at the Surface?

COSMIC-RAY-MEDIATED FORMATION OF BENZENE ON THE SURFACE OF

SATURN’S MOON TITAN

Zhou et al. 2010

Benzene (PAH) Production on

SurfaceCosmic-ray flux on Titan’s surface (φCR =1e9 eV cm−2 s−1)

Yield of benzene from solid acetylene (from lab: Y = 5.6e-3 eV−1)Fraction of the surface of Titan covered by organics (Fo=0.2)

Fraction of organics that is acetylene (Fa=0.2)Time for turnover of the surface by geological processes (τ=2e6 yrs, lowest

estimate )

We get: M = 1.4e19 molecules cm−2

3.4 e−17 g cm−2 s−1

Outline of Today’s Talk

Titan: gas phase chemistry

Aerosol formation

Surface chemistry

Synergism with lab data

Inverse Model

Parameter Estimate

PredictionsAdjoint Forcing

Gradients(sensitivities)

Optimization

Forward Model Adjoint Model

Observations

Improved Estimate

-

t0 tf tf t0

Forward and adjoint models

<-- time evolution profiles

Lab: Adamkovics et al. (2003)

Liang et al. (submitted)

Jupiter (Moses 2005)

Titan (Moses 2005)

References

• Yung, Y. L., M. Allen, and J. P. Pinto. (1984). "Photochemistry of the Atmosphere of Titan: Comparison between Model and Observations." Astrophysical Journal Supplement Series 55(3): 465-506.

• Goody, R. M., and Y.L. Yung, Atmospheric Radiation: Theoretical Basis, 1989, Oxford University Press.

• Yung, Y. L., and W. D. DeMore, Photochemistry of Planetary Atmospheres, 1999, Oxford University Press.

Acknowledgements

We appreciate discussions with kinetics groups of Ralf Kaiser and Stan Sander, Mark Allen, Bob West, and support from NASA Cassini, OPR, NAI and PATM.