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 Chaotic Chemistry

Titan is a synthetic chemist's worst nightmare. There are high energy processes that span the spectrum (!) of energies and modes of reactivities, plus the products of one reaction become the reactants for another. One recent model takes into account over 415 simultaneous reactions in the atmosphere. Even with all this complexity (a polite word for chaos), some of the models are beginning to get close to the observed Cassini and Huygens results.

One of the tricky bits is that Titan has different levels of reactivity. High-energy photochemistry (ion-neutral) chemistry dominates the upper atmosphere, while lower down "lower energy" radical reactions come into play. Finally, even pi-system photochemistry kicks in with longer wavelength light (200 nm or so) that is able to push with double bonds and pi-systems into an excited state. Once in the excited state, all sorts of things can happen. Photochemistry with excited states can occur even near 0 K. Deep below the haze layers, all the fun photons are absorbed, and the chemistries rely on ground state thermal chemistry - the day to day stuff we are used to. Here the energy barriers need to get crossed by the kinetic energy of the molecules themselves. And on frigid Titan, the molecules are gonna struggle to get up over that hill and over into the next valley. Here is a graphic that tries to put that all into perspective, with an example transformation for each type of reactivity:




The next graphic compares an ion-neutral route and a radical route to ethylene starting from methane. Both occur in Titan's atmosphere:



Of the two types of chemistry, ion chemistry is the more energetic, during the initiation it rips an electron out of the molecular or even atomic orbital. The radical cation either reacts or self-fragments to generate a cation AND a radical species. (two reactive intermediates!). Further reactions proceed until the last step where an electron eventually collides with the system. It took a lot of energy to rip the electon out, and when the electron is popped back in a huge amount of energy is released. This huge amount can't easily be released just through wimpy vibrational or translation mode changes - instead the molecule may frag up. (Picture driving your car along the road, suddenly two solid rocket boosters you've been carrying along are ignited, and your cars structural frame can't absorb the extra energy....) So even at the end, ion neutral chemistry will generate reactive radical intermediates that enter a radical reaction pathway...

In the scheme, a methyl group loses an electron, then blows out a hydrogen radical. The resulting cation sucks in the electrons from a methane molecule (I drew it as a 2e- exchange, it could be several 1e- exchanges also). This leaves it as a C2H5+ cation and kicks out molecular hydrogen (again, it could kick out two H radicals). [C2H5+ is a real important intermediate in Titan chemistry, we'll see him again, soon.] At some point, an electron drops into the system, with a huge release in energy. The least exciting option is a fragmentation to ethylene and the release of yet another hydrogen radical.

For the radical route, a homolytic cleavage (1e- split) creates a methyl radical (CH3.) and a hydrogen radical (H.). Both the methyl radical and hydrogen radical can go on to react with other species (H abstraction or addition to a double bond, for example.) In this scheme, the Hydrogen radical reacts with acetylene to create an ethylene radical, which can then propagate to do further stuff. To cut this story short, the ethylene radical encounters a hydrogen radical, they combine and make a neutral molecule full of happy paired electrons, ethylene. (In reality, it is an encounter with another ethylene radical that causes a disproportionation to ethylene and acetylene, the more energetic ethylene radical likely loses a hydrogen radical to another ethylene and is converted to acetylene.). Once the electrons are paired up, that terminates the propagation.

The two types of chemistry are very important in planetary atmospheres. It was the realization and inclusion of ion-neutral chemistries at the proper level of importance that has generated the most recent round of models. And these were driven by the discovery of large amounts of benzene in Titan's atmosphere (see post #1).