Photochemical processes on Titan A Revision from Cassini Observations Cheng Li1, X. Zhang1, J. A. Kammer1, M. C. Liang2, Y. L. Yung1 1California Institute of Technology 2Research Center for Environmental Changes, Academia Sinica, Taiwan

Photochemistry on Titan C2H2, C2H4,C2H6 C2H,C2H3,… N2+,N,N+ C3Hx, C4Hx,… CH,1CH2,3CH2 CH3 e- CH4 N2 surface

The simplest 1D photochemical model Chemical production Chemical loss Vertical transport 𝑃 − 𝐿 = − 𝜕 𝜕𝑧 (𝐾𝑛 𝜕𝑥 𝜕𝑧 ) Mixing ratio Number density Eddy diffusivity

CIRS limb view and FUV stellar occultation C2H2 C2H4 C2H6 Altitude (km) CH4 CH3CCH C3H8 C4H2 C6H6 Mixing ratio Vinatier et al., 2009; Kammer et al., 2011

Retrieval algorithm Levenberg–Marquardt Ar C2H2 1000 altitude Mixing ratio Levenberg–Marquardt

Molecular diffusivity Initial guess Retrieved eddy diffusivity

Altitude (km) Mixing ratio Vinatier et al., 2009; Kammer et al., 2011; Yelle et al., 2008 model 1 model 2 Altitude (km) CH4 C2H2 C2H4 C2H6 CH3CCH C3H8 C6H6 C4H2 Mixing ratio

Haze formation process revealed by the eddy diffusivity Liang et al., 2007 Stabilization due to haze heating Aerosol growth heating Detached haze layer

Conclusion Retrieved eddy diffusivity

How to estimate the sensitivity to chemical rate constants? 𝑆= 𝑖,𝑗 Δ 𝑥 𝑖 𝑥 𝑖 2 𝑖:𝑎𝑙𝑡𝑖𝑡𝑢𝑑𝑒 𝑗:𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑥 𝑖 :𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒 Δ 𝑥 𝑖 :𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒 𝑤ℎ𝑒𝑛 𝑟𝑎𝑡𝑒𝑠 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑗 𝑎𝑟𝑒 𝑑𝑜𝑢𝑏𝑙𝑒

Mixing tracer S

Reactions Rate constants Moses et al 2000 This work H + C2H4 + M  C2H5 + M 𝑘 0 =1.3× 10 −29 𝑒 − 380 𝑇 𝑘 ∞ =6.6× 10 −15 𝑇 1.3 𝑒 − 650 𝑇 𝑘 0 =5.4× 10 −25 𝑇 −1.46 𝑒 − 1300 𝑇 𝑘 ∞ =1.8× 10 −13 𝑇 0.7 𝑒 − 600 𝑇 CH3 + C2H5 + M  C3H8 + M 𝑘 0 =7.5× 10 −17 𝑇 −3 𝑒 − 300 𝑇 𝑘 ∞ =6.64× 10 −11 Increase by 5 times CH3 + C2H3 + M  C3H6 + M 𝑘 0 =5× 10 −27 𝑘 ∞ =1.1× 10 −11 H + C3H7 + M  C3H8 + M 𝑘 0 =4× 10 −19 𝑇 −3 𝑒 − 600 𝑇 𝑘 ∞ =2.49× 10 −10

How to estimate the eddy diffusivity? Molecular diffusivity adiabats 𝐾∝ 1 𝑛 Fulchignoni et al., 2005

Eddy diffusivity 𝑃−𝐿=− 𝜕 𝜕𝑧 (𝐾𝑛 𝜕𝑥 𝜕𝑧 ) Altitude(km) adiabats Altitude(km) Eddy diffusivity (cm2/s)

Previous work -- mixing ratio v.s. altitude(km) Lavvas et al, 07 Krasnopolsky, 09

Fractional change of C2H4 by doubling rate constants CH3  1CH2+H 2CH3 + M  C2H6 + M Altitude (km) H + C2H2 + M  C2H3 + M H + C2H4 + M  C2H5 +M

New reaction rate constants (solid lines) H + C2H4 + M  C2H5 + M 604K / 511K / 400K / 285K / 170K (Titan) Moses et al.,2000 This work Lightfoot et al., 1987