Mechanisms for the formation of benzene in the atmosphere of Titan

Polycyclic aromatic hydrocarbons (PAHs) are important interstellar species, and their precursor benzene (C 6 H 6 ) has been detected in our solar system. In this study the possibility of benzene formation in the atmosphere of Titan is investigated. Benzene abundance in Titan's atmosphere is found to be rather highly dependent on the assumed mechanism for benzene production. Assuming reactions involved in this mechanism to proceed at the rate corresponding to 300 K, a value of 5.4 × 10 −7 at 2 × 10 −5 mbar is found for the mole fraction of benzene. The primary mechanism responsible for this benzene abundance involves the recombination of propargyl (C 3 H 3 ) radicals. A source of benzene molecules through ion chemistry in the upper atmosphere is also investigated. The inclusion of heavy cyclic ions results in little change in the C 6 H 6 abundance at the peak in mole fraction, where [C 6 H 6 ] = 3.6 × 10 5 cm −3 , but does produce about a factor of 2 increase in the benzene mole fraction at the 10 −6 mbar level [C 6 H 6 ] = 1 × 10 4 cm −3 . This produces a negligible change in C 6 H 6 column abundance below this microbar level. In the stratosphere, Infrared Space Observatory (ISO) measurements of the benzene abundance have been fit by our nominal profile multiplied by a factor of 3.0 ± 0.5. Taking the lower value of this factor, the ISO fit corresponds to an altitude‐dependent benzene profile with a value of 9.8 × 10 −11 at 1 mbar. Benzene profiles determined in this study suggest an important path for the formation of higher‐order hydrocarbons, which may play a significant role in the formation of hazes in Titan's atmosphere.