Formation and distribution of benzene on Titan

Abstract

[1] We present a study of the formation and distribution of benzene (C 6 H 6 ) 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 (C 6 H 5 ) with H atoms on the walls of the instrument and that the measured signal is a combination of (1) C 6 H 6 from the atmosphere and (2) C 6 H 6 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 $10 7 cm À2 s À1 , of the same order of magnitude as the production rate by neutral reactions of $4 Â 10 6 cm À2 s À1 . We show that benzene is quickly photolyzed in the thermosphere, and that C 6 H 5 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 À15 g cm À2 s À1 , corresponding to an accumulated surface layer of $3 m.

Reaction of the C2H Radical with 1-Butyne (C4H6): Low-Temperature Kinetics and Isomer-Specific Product Detection

“…A recent study by Vuitton and coworkers 41 provides insight into the formation and distribution of benzene, a C 6 hydrocarbon species, on Titan. The three-body propargyl recombination reaction (see Eq.…”

Composition and chemistry of Titan's thermosphere and ionosphere

“…Figure 3a,b shows low altitudes INMS CSN spectra from the inbound and outbound legs of T16 (Vuitton et al 2008). The spectra show a clear signal in the m/zZ62-63 and m/zZ73-79 regions that is interpreted as evidence for benzene based on the INMS calibration (Cui et al submitted).…”

Composition and chemistry of Titan's stratosphere

“…However, laboratory measurements at low temperature and pressure of the relative yields of the different isomers and of the benzene photolysis are crucially needed. Note that data from the Cassini/INMS (Ion and Neutral Mass Spectrometer) indicate an additional source of benzene in the ionosphere, at approximately 900 km (Waite et al 2007;Vuitton et al 2008). A general discussion of the ionospheric chemistry is presented by Vuitton et al (2009).…”

“…for the most recent ones, Wilson & Atreya 2004;Lebonnois 2005;Lavvas et al 2008a,b;Vuitton et al 2008). These models incorporate chemical sinks and losses (photodissociation, 2-and 3-body reactions) and vertical transport parametrized by an altitude-varying eddy mixing coefficient.…”

“…As discussed by Waite et al (2007), in situ data from the INMS and the Cassini plasma spectrometer also give evidence for production of organic aerosols at high altitudes (approx. 1000 km), with the formation of polycyclic aromatic hydrocarbons (PAHs) by ion chemistry (see discussion by Vuitton et al 2008) and the presence of massive negatively charged molecules.…”