[X] My Assistant

[TheChemist]

The only NMR reference I can recall is in this Sagan paper from 1993, using solid state NMR.
Here is the abstract : (sorry for the format, but I have a class in 5 min )

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[marsbug]

Hmmm, to throw a thought out at random, perhaps there is another way of looking at the question of the tholins structure:
Since organic matter has such a wonderful affinity for long chains and ring structures it might make more sense to ask what structures, in a given area of titans atmosphere, are likely to survive the conditions in that region. Anything too heavy will be drawn down to the surface, anything vulnerable to UV wil get broken apart, and anything very eager to combine with other material will likely get pulled into another larger tholin structure.

There might be interesting exceptions, for example not all the ten thousand dalton ions in the upper atmosphere need be wide boys heading down.A few might have structures lending themselves to lower density, or other properties, that allows them to remain at that altitude.

To speculate wildly: If the tholin factory in the upper atmosphere has been running long enough it might even be dominated by a relative few structures that have the best survivability there, with others being pulled down to lower levels or smashed up by UV.
There might be the next few generations of lightweight plastic floating up there waiting to be found...

[Juramike]

To speculate wildly: If the tholin factory in the upper atmosphere has been running long enough it might even be dominated by a relative few structures that have the best survivability there, with others being pulled down to lower levels or smashed up by UV.

That's a pretty interesting concept...chemical evolution dictated by aerodynamics and UV stability. That could imply that some of the best catalysts might end up drifting downstairs or getting blasted apart. So the rates (if any) of PAH/tholin formation on the particulates might depend on the catalytic activity the longer lived fluffier particulates plus the catalytic activity of the shorter-lived (in that environment) particulates.

This sounds like it could also be an Earth atmospheric chemistry thing as well.
(And now that I've said it I'll bet someone will post a reference).

-Mike

[Juramike]

Found the recent NMR paper (and it's free)!

Derenne, S.; Quirco, E.; Szopa, C.; Cernogora, G.; Schmidt, B.; Less, V.; McMillan, P.F. LPSC 39 (2008) Abstract 1840. "New insights in tholin chemical structure using solid state (13)C and (15)N NMR spectroscopy."

They used solid state 13 C MAS (MAS = Magic Angle Spinning, you put a sample at a particular angle in the probe and you can acquire a decent signal.)

The authors write:
"no signal is observed between 125 and 150 ppm, pointing to a virtual lack of non-substituted [H-carrying] unsaturated [double or aromatic bonded] carbons." [For reference, benzene the carbons of benzene C-H's all resonate equivalently at 126 ppm. Substituting or fusing the benzene ring usually pulls the resonances downfield (bigger number)] "This is in agreement with the lack of protonated aromatic carbons suggested by infra-red spectroscopy."

So where the heck are the benzenes? (The authors pose this question in their conclusion.)

They tentatively identify aliphatic functions, amines, triazine rings, nitriles and possibly carbodiimides - basically things you can make by polymerizing HCN.
[Carbodiimides should hydrolyze to the corresponding ureas in the presence of water; they should also have a very intense IR absorption in the 2500 - 2000 cm (4-5 um) region.]

Things ruled out include: isonitriles (these were eliminated by examination of the 15N NMR spectrum - good, they really stink), , and C-H containing pyrroles, C-H containing indoles, and C-H containing benzenes.

Pyrimidines and pyridines could be present, but only if they are substituted by amino groups.


The compared the spectra of the tholins with the recently-in-the-news molecule melamine which is 2,4,6-triaminotriazine (amino-cyanide trimer). Some of the peaks matched up pretty well both in 13C NMR and 15N NMR. (So marsbug might've been bang on regarding plastics).

-Mike

[marsbug]

Let's take that one head on. The formation of each haze particle is presumably a quasi-random cumulative process leading in one direction only - its eventual deposition on the surface of Titan.

Actually I've looked for but not found much published on haze particles reaching the surface, which is a bit of a mystery. Has anything been mentioned at the DPS conference, I've not had time to follow it properly?

Edit: I realize that there are dunes covering some 20% of titans surface for which there is evidence of organic composition, but their relationship to the haze is unclear, and I've not heard anything on when these deposits were put down, or over how long. From what I've read it's assumed (reasonably, but still an assumption) that the organics in the atmosphere are precursors of the organic 'sand'.

I'm wondering if there are any papers or articles out there discussing haze particles being deposited on the surface today? It just seems like a big missing link if we can't point to a patch of deposits on titan and say: that came out of the haze.

[ngunn]

New CHARM presentation on this topic now:
http://saturn.jpl.nasa.gov/multimedia/products/CHARM.cfm