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]

Into the Wierdness: Ion-neutral chemistry

(A major bummer about Titan chemistry literature is the lack of detailed electron-pushing mechanisms and especially frustrating is the lack of clear electron accounting. Many radical cations and even neutral radical in the figures do not show the unpaired electron on the structure. In my diagrams I'll try to detail the play-by-play.)

Much of Titan's chemistry is driven by upper atmosphere high-energy photochemistry. It is likely that plasma chemistry may play a small part, but the major driver is dayside solar radiation chemistry.

One of the biggest surprises of the Cassini mission is the amount of complex hydrocarbons (especially benzene!) found in the upper atmosphere. The thermo and ionosphere are pretty impressive chemical factories.

In these reaches ion-neutral chemistry plays a big role. The first step is the whacking of a molecule by a high energy photon (were talking Extreme ultraviolet, around 50-100 nm - this is mega-energy and waaay above your sunlamp). This is enough to blast an electron out of the molecular (or even atomic) orbital and create a wierd little species called a radical cation. This is a single electron process, so one of the electrons in the molecule is now unpaired, thus a radical. It is also charged positively, since an electron was ripped out of the system. Radical cations are pretty exotic here on Earth. They are usually only found in the vacuum ionization chamber of your local LCMS or GCMS. They do very weird things.

One of the things they like to do is bite into a covalent bond and make a charged species, and generate a neutral radical. (Think: Radical-cation + neutral --> Cation + uncharged radical)

Sometimes, structurally complex radical cations will frag up all by themselves in a similar way (radical-cation --> cation + uncharged radical). This happens in your local LCMS and GCMS, but that's a story for a different day.



Here is an example that shows a nitrogen molecule (N2) getting whacked by a high energy photon, making a radical cation, then homolytically (single electron style) biting into a neutral hydrogen atom. One hydrogen gets stolen and gloms onto the dinitrogen to make a cation, while the remant hydrogen atom (now a radical since the electron is unpaired in the atomic orbital) goes flying off. But the dinitrogen cation is not happy, it has a very weak Proton Affinity and wants to give away the proton. When it finds a neutral methane molecule it can donate the proton to the methane molecule. (Y'all can think of it as an electophilic attack by the proton on the methyl, or a nucleophilic attack by electrons in the C-H orbital to the proton - either way it is the same). Compared to most stuff we are used to, neither methane or the nitrogen really want to be protonated. But in this case the nitrogen want the proton much less than the methane. So the methane molecule gets stuck with it for the moment. (Kinda like regifting an ugly sweater).

So you end up with a CH5+ cation. Which is weird. Drawing a pentavalent carbon is the best way to get ridiculed by your colleagues, or lose 10 points on a test score, depending on your situation. In the rarefied upper atmosphere of Titan, it is sorta tolerated (remember, the methyl isn't super happy about that extra proton). Structurally, two of the hydrogen atoms of the carbonium ion (normal R3C+ is a "carbenium" ion) are in a 3-center 2-electron bond. You can think of it as a hydrogen molecule sidebound to a carbenium ion. So these two hydrogens are in a special relationship. (But they can exchange out with the others in the structure).

Think of the methyl carbonium ion as a super acid. It wants to get rid of that proton (but can't to dinitrogen).

It turns out that CH5+ plays a special role, it actually prevents exciting chemistry from happening.