Formation and distribution of benzene on Titan

Abstract

We present a study of the formation and distribution of benzene (C6H6) on Titan. Analysis of the Cassini Mass Spectrometer (INMS) measurements of benzene densities on 12 Titan passes shows that the benzene signal exhibits an unusual time dependence, peaking ∼20 s after closest approach, rather than at closest approach. We show that this behavior can be explained by recombination of phenyl radicals (C6H5) with H atoms on the walls of the instrument and that the measured signal is a combination of (1) C6H6 from the atmosphere and (2) C6H6 formed within the instrument. In parallel, we investigate Titan benzene chemistry with a set of photochemical models. A model for the ionosphere predicts that the globally averaged production rate of benzene by ion-molecule reactions is ∼107 cm-7 s-1, of the same order of magnitude as the production rate by neutral reactions of ∼4 × 106 cm-2 s-1. We show that benzene is quickly photolyzed in the thermosphere, and that C6H5 radicals, the main photodissociation products, are ∼3 times as abundant as benzene. This result is consistent with the phenyl/benzene ratio required to match the INMS observations. Loss of benzene occurs primarily through reaction of phenyl with other radicals, leading to the formation of complex aromatic species. These species, along with benzene, diffuse downward, eventually condensing near the tropopause. We find a total production rate of solid aromatics of∼10-15 g cm-2 s-1, corresponding to an accumulated surface layer of ∼3 m. Copyright 2008 by the American Geophysical Union.