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]

Titan’s atmospheric chemistry is driven by sunlight, and the fact that CH4 is present with large amounts of N2 as the diluant. The different wavelengths of light will hit different types of bonds, and will also penetrate to different depths. Very short wavelength photons (EUV) will not penetrate very far before hitting a nitrogen molecule and effectively blowing it into high-energy pieces. Slightly longer wavelength photons will penetrate a little deeper, only because the molecular transitions that would absorb this energy, located on methane molecules, are few and far between, the atmosphere is still composed of nitrogen molecules (95%). The “longer” UV wavelengths, powerful enough to excite double and triple bonds, penetrate deeper still (they are too wussy to interact with N2 or CH4 – they just pass on by) until they can find ethylene, acetylene and similar compounds. Finally, the longer wavelength UV light, the kind that gets absorbed by extended conjugated or aromatic pi-systems, penetrates to the haze layer. It also gets scattered by the haze particles.



Once initiated, Titan has a complex reaction manifold that simultaneously creates and uses intermediates to make organic products. Here is an oversimplified diagram that shows some of the intermediates and routes:



Each intermediate has several simultaneous routes for formation as well as for reaction (destruction) to make other products. Each reaction pathway has its own rate. Recent models have over 400 simultaneous rates. It is sum of the the simultaneous inbound, and simultaneous outbound routes that determine the overall amount (flux rate) of a given intermediate that gets made. And yes, this varies by altitude (and temperature) as well. So the 400+ simultaneous rates at 100 km will be different than the 400+ simultaneous rates at 1000 km.
Here is a table that summarizes the overall surface flux rates for many of the predicted common organic compounds on Titan:



It would only take one little change in one of the rates to propagate through the system and change all the values. Thus, it is not surprising that each author’s total flux rates are different.

For example, note that some authors predict huge amounts of ethylene being formed, while others predict none at all. Where none is predicted, it is because there are other processes that consume ethylene as fast as it is produced. Obviously, a measurement of C2H4 (m.w. 26) from the burp the Huygens GCMS measured on the surface would help constrain these models and kick off a whole new round of model improvements.