Atmospheric Chemistry of Titan Options

[Juramike]

Here is a "Benzene-O-Vision" graphic showing the amount of benzene and phenyl radicals at high altitudes on Titan. This is based on detections of benzene and phenyl radical (which recombined in the sample chamber to make benzene) using the INMS instrument during closest approach. The numbers are normalized to constant pressure altitude, roughly 1000 km.



The data was taken from Table 1 in: Vuitton et al, Journal of Geophysical Research 113 (2008) E05007. "Formation and distribution of benzene on Titan". doi: 10.1029/2007JE002997 [EDIT 5/24/10: Article freely available here] and overlaid on a map of Titan.

The authors mentioned that the errors in these measurements are 20%.

These detections are well above the detached haze layer. Most are at the same sun azimuth angle. (T23 observation had the lowest angle.) Assuming that the temporal difference is minimal (each dot is from a different flyby), there doesn't appear to be an obvious correlation with latitude.

This graphic does show that benzene is present even waaaay up in the thermosphere and ionosphere.

[Juramike]

Ethane (C2H6) [CH3CH3]



Almost all ethane is formed from the combination of two methyl radicals. The methyl radicals are initially generated in a whole bunch of ways previously described, a few are shown in the graphic above. The actual radical combination mechanism is not-so-straightforward, it is actually a three body reaction, with “M” in the reaction scheme designating an atom (or molecule) of a carrier gas.

What does this “M” do? I’m not exactly sure….clearly it comes out of the reaction as it enters it. It thus acts as a catalytic group or surface. My wild speculation is that it may help absorb (or impart) some of the kinetic energy required to make the reaction happen. The Krasnopolsky, 2009 Titan atmospheric model got this reaction rate and sequence from the Lavvas et al., 2008 Titan atmospheric model, who in turn modified it from Cody et al., 2003 (?) which was for Saturn/Neptune atmospheric modeling and had “M” = H2/He.

[In this case “M” is an inert gas. Confusingly, earlier ground state solution-phase chemistry literature (1961) showed that certain metal species (also labeled “M” in the literature reaction schemes) can help stabilize free radicals presumably by making an interaction between the metal and the radical. The additional stability helped the recombination reaction pathway, the yields were higher in the methyl radical combination to form ethane.]

Since this is a three-body gas-phase reaction, it is going to work much better in a more crowded environment lower in the atmosphere. It is very hard to get three molecules together in a rarified environment. This is one reason why many of the radical recombination reactions (most require a diluant gas “M” atom/molecule) will at lower altitudes.

Oddly enough, photodissociation of ethane to make ethyl radical is NOT the main way to form ethyl radical (C2H5.). Most ethyl radical will actually come from acetylene radical (HCC.) attacking ethane (C2H6). But we’ve got more mechanisms to describe before we get there..

Next up, the key intermediate ethylene (C2H4)...