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]

HCN



The incorporation of nitrogen into Titan’s organics usually results in the introduction of a nitrile group (-CN), where a terminal carbon atom is bound through a triple bond to a nitrogen atom. The nitrogen has a lone pair of electrons on it, although these may or may not be drawn in. (i.e. –CN:) This lone pair would love to donate to a proton (or other atoms desperate for electrons such as Lewis acids.)

The whole cascade starts with the blowing apart of molecular nitrogen by strong UV light. There are lots of ways this can happen, one is shown above. In this case, “lower energy” ultraviolet light in the 80 - 100 nm range (which is still pretty dang powerful) excites the molecular nitrogen to the point that it goes total fraggo and liberates a “naked” nitrogen atom and an “excited naked” nitrogen atom*. (The “excited naked” nitrogen atom will be a big player in tholin formation mechanisms). Other ways to get there include ionization of molecular nitrogen with light below 80 nm, then electron recombination back to molecular nitrogen, which is a pretty violent process, and then a total fraggo reaction that again generates a naked nitrogen atom and an excited naked nitrogen atom.

The “naked” nitrogen atom can react with a methyl radical to form a transient CH3N nitrene complex** (my guess is it would be likely in a triplet or an unpaired excited singlet state) that then blows out hydrogen radical to give H2CN. radical. This can react with another hydrogen radical to then kick out molecular hydrogen (H2) and HCN.
[I’m not sure why this process goes stepwise, I would think it possible for the transient CH3N carbene to kick out two hydrogen radicals (if triplet) or molecular hydrogen (possible if unpaired excited singlet state?) all in one go].

According to Krasnopolsky et al., 2009, this sequence accounts for 72% of all HCN formed in Titan’s atmosphere. But a recent article shows that excited state nitrogen chemistry may be also very important and poorly modeled. The atmospheric nitrogen chemistry of Titan is still poorly constrained, but getting better with recent lab experiments and further modeling. We’ll use the Krasnopolsky results, but these will likely shift on publication of the next model.

*****


*an asterisk [*] is used to designate an excited state atom. This is an atom where one of the electrons has been boosted to a higher energy orbital. Normally the electrons are spin paired in the orbital in the ground state. In an excited state atom the electrons can still be spin paired, but one of the electrons is now in a boosted energy orbital. So it may be in the unpaired excited state singlet state.


**very careful electron counting is important here, there are two lone pairs on nitrogen with a bonus electron, so naked nitrogen is like a triradical with a lone pair [:N...]
For the whole reaction we get: radical + radical nitrene (nitryne) --> radicals pair - nitrene (2 lone pairs on nitrogen) --> nitrene --> radical + radical