Simulation of Titan’s atmospheric photochemistry

Author： Couturier-Tamburelli Isabelle Piétri Nathalie Gudipati Murthy S.

E-ISSN： 1432-0746|578|issue|A111-A111

ISSN： 0004-6361

Source： Astronomy & Astrophysics, Vol.578, Iss.issue, 2015-06, pp. : A111-A111

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Abstract

We studied the photochemistry of frozen ice of a polar Titan’s atmospheric molecule cyanodiacetylene (HC₅N) to determine the possible contribution of this compound to the lower altitude photochemistry of haze layers found on Titan. We used infrared analysis to examine the residue produced by irradiation of solid HC₅N at λ > 300 nm. The resulting polymer is orange-brown in color. Based on theoretical analysis and the general tendency of HC₅N and C₄N₂ to undergo similar ice photochemistry at longer wavelengths accessible in Titan’s lower atmosphere, we conclude that Titan’s lower atmosphere is photochemically active in the regions of cloud, ice, and aerosol formation. C₄N₂is a symmetric molecule with no net dipole moment whereas, HC₅N has a large dipole moment of ~4 D. Consequently, though both these molecules have very similar molecular weight and size, their sublimation temperatures are different, HC₅N subliming around 170 K compared to 160 K for C₄N₂. Based on our studies we conclude that in Titan’s atmosphere the cyanoacetylene class of molecules (HCN, HC₃N, HC₅N, etc.) would condense first followed by the dicyanoacetylenes (C₂N₂, C₄N₂, C₆N₂, etc.), leading to fractionation of different class of molecules. From the fluxes used in the laboratory and depletion of the original HC₅N signals, we estimate Titan’s haze ice photochemistry involving polar nitriles to be significant and very similar to their non-polar counterparts.