organic synthesis in the atmosphere of titan: modeling and recent observations yuk yung (caltech),...
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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.