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Unmanned Spaceflight.com > Outer Solar System > Saturn > Cassini Huygens > Titan
AlexBlackwell
Hunter Waite et al. have a paper ("The Process of Tholin Formation in Titan's Upper Atmosphere") to be published in the May 11, 2007, issue of Science; there is also an accompanying Perspectives piece ("Titan's Organic Factory") by Sushil Atreya.

The embargo will be lifted in a hour or two, so if do not have embargo access, the direct links to the articles probably will not be accessible until then. However, you can click here for a brief description.

Also, Opher et al. have a paper ("The Orientation of the Local Interstellar Magnetic Field") with an accompanying Perspectives piece ("A Local Wiggle in the Turbulent Interstellar Magnetic Field") by Jokipii. Again, unless you have embargo access, click here for a brief description.
AlexBlackwell
The embargo has been lifted and the papers are now accessible.
AlexBlackwell
Cassini spacecraft reveals evidence of tholin formation at high altitudes in Titan's atmosphere
Southwest Research Institute (SwRI) News
May 10, 2007
Littlebit
QUOTE
Using measurements from a combination of mass/charge and energy/charge spectrometers on the Cassini spacecraft, we have obtained evidence for tholin formation at high altitudes (1000 kilometers) in Titan's atmosphere. The observed chemical mix strongly implies a series of chemical reactions and physical processes that lead from simple molecules (CH4 and N2) to larger, more complex molecules (80 to 350 daltons) to negatively charged massive molecules (8000 daltons), which we identify as tholins. That the process involves massive negatively charged molecules and aerosols is completely unexpected.

If Tholins are formed primarily at these very high altitudes, the total tholin mole count over time is likely much lower than prior predictions, limiting the potential for tholin sludge buildup on the surface. This does not help explain what the surface is, but it does help explain why it does not appear to be black and oily.
Juramike
QUOTE (Littlebit @ May 11 2007, 10:42 AM) *
If Tholins are formed primarily at these very high altitudes, the total tholin mole count over time is likely much lower than prior predictions, limiting the potential for tholin sludge buildup on the surface.


The big surprise was that tholin precursors are formed at really high (and rarefied) altitudes. I assume that increased tholin buildup and construction continues (and speeds up) as the precursors fall down to the stratosphere (and then as they get even bigger, down to the surface).

This article shows that even more of the atmosphere can be involved in tholin formation than originally thought.

(As the authors mentioned, some of the upper atmosphere ionized precursors could get ejected from the atmosphere and then go on to soot up some of the other moons)

-Mike
AlexBlackwell
QUOTE (AlexBlackwell @ May 10 2007, 08:04 AM) *
Hunter Waite et al. have a paper ("The Process of Tholin Formation in Titan's Upper Atmosphere") to be published in the May 11, 2007, issue of Science; there is also an accompanying Perspectives piece ("Titan's Organic Factory") by Sushil Atreya.

For those without access to Science, Sushil Atreya's article above is available here.
TheChemist
I don't know if this has been already mentioned, but there is a new Titan atmospheric photochemistry model published
in two papers in Planetary and Space Science, articles in press page. Apologies, this is a long and badly-formatted post.

The model article is :
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Coupling photochemistry with haze formation in Titan's atmosphere. Part I: Model description
In Press, Accepted Manuscript, Available online 12 September 2007,
P.P. Lavvas, A. Coustenis and I.M. Vardavas
PDF (812 K)
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The validation with Cassini Huygens data :

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"Coupling photochemistry with haze formation in Titan's atmosphere. Part II: Results and validation with Cassini/Huygens data"
In Press, Accepted Manuscript, Available online 12 September 2007,
P.P. Lavvas, A. Coustenis and I.M. Vardavas
PDF (2433 K)
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I think I can bend the rules and provide the abstracts rolleyes.gif (after all, they will be available to non-subscribers shortly)

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Abstract Part I
We introduce a new 1D coupled Radiative / Convective - Photochemical - Microphysical model for a planetary atmosphere and apply it to Titan. The model incorporates detailed radiation transfer calculations for the description of the shortwave and longwave fluxes which provide the vertical structure of the radiation field and temperature profile. These are used for the generation of the photochemistry inside the atmosphere from the photolysis of Titan's main constituents, nitrogen (N2) and methane (CH4).
The resulting hydrocarbons and nitriles are used for the production of the haze precursors, whose evolution is described by the microphysical part of the model. The calculated aerosol and gas opacities are iteratively included in the radiation transfer calculations in order to investigate their effect on the resulting temperature profile and geometric albedo. The main purpose of this model is to help in the understanding of the missing link between the gas production and particle transformation in Titan's atmosphere. In this part, the basic physical mechanisms included in the model are described. The final results regarding the eddy mixing profile, the chemical composition and the role of the different haze precursors suggested in the literature are presented in Part II along with the sensitivity of the results to the molecular nitrogen photoinization scheme and the impact of galactic cosmic rays in the atmospheric chemistry.
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Abstract Part II
"The new one-dimensional radiative-convective/photochemical/microphysical model described in Part I is applied to the study of Titan's atmospheric processes that lead to haze formation. Our model generates the haze structure from the gaseous species photochemistry. Model results are presented for the species vertical concentration profiles, haze formation and its radiative properties, vertical temperature/density profiles and geometric albedo. These are validated against Cassini/Huygens observations and other ground-based and space-borne measurements. The model reproduces well most of the latest measurements from the Cassini/Huygens instruments for the chemical composition of Titan's atmosphere and the vertical profiles of the observed species.
For the haze production we have included pathways that are based on pure hydrocarbons, pure nitriles and hydrocarbon/nitrile copolymers. From these, the nitrile and copolymer pathways provide the stronger contribution, in agreement with the results from the ACP instrument, which support the incorporation of nitrogen in the pyrolised haze structures. Our haze model reveals a new second major peak in the vertical profile of haze production rate between 500 and 900 km. This peak is produced by the copolymer family used and has important ramifications for the vertical atmospheric temperature profile and geometric albedo. In particular, the existence of this second peak determines the vertical profile of haze extinction.
Our model results have been compared with the DISR retrieved haze extinction profiles and are found to be in very good agreement. We have also incorporated in our model heterogeneous chemistry on the haze particles that converts atomic hydrogen to molecular hydrogen. The resultant H2 profile is closer to the INMS measurements, while the vertical profile of the diacetylene formed is found to be closer to that of the CIRS profile when this heterogenous chemistry is included."
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ngunn
More on this topic:
http://www.ucl.ac.uk/media/library/OrganicBuildingBlocks

Molecular mass 10,000 !!
Webscientist
A living mist?
ngunn
QUOTE (Webscientist @ Nov 30 2007, 09:12 AM) *
A living mist?


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. For anything remotely resembling life to get going you need a cyclic process where the outcome of one cycle alters the conditions in which the next cycle occurs, thereby affecting its outcome, and so on. It's not impossible to imagine such things happening on Titan, though even if there are I'm sure there would still be semantic disputes about the term 'life'. However IMHO haze particle formation is not such a process.

If you were speaking 'poetically' please forgive me for being a terrible pedant.
Juramike
QUOTE (Webscientist @ Nov 30 2007, 04:12 AM) *
A living mist?


More like an anthracene snowflake.
TheChemist
Well, there is a lot of heterogeneous chemistry (still largely unknown) taking place on organic aerosol particulate matter (including PAHs) in our own atmosphere, but I have never seen it characterized as "living". I don't see why this should be different for Titan smile.gif
marsbug
My apologies for resurrecting this thread but re-reading the organic building blocks article posted by ngunn there's something I hope someone can clear up:

QUOTE
Prof Andrew Coates, researcher at University College Londonís Mullard Space Science Laboratory and lead author of the paper, says: ďCassiniís electron spectrometer has enabled us to detect negative ions which have 10 000 times the mass of hydrogen. Additional rings of carbon can build up on these ions, forming molecules called polycyclic aromatic hydrocarbons, which may act as a basis for the earliest forms of life.


He makes it sound as if the 10,000 dalton ion may act as the basis for a PAH, but unless I've seriously misremebered my atomic weights 10,000 daltons ought to be a huge sheet or stack of aromatic rings anyway huh.gif . I get the feeling I'm seriously misintepreting him but I've had no luck finding clarification, can anyone help? unsure.gif
Juramike
(I think I'm gonna have to reread these full papers as well...)

But shooting right off the cuff (and without rereading the papers), I'd interpret the brief statements in the abstract as saying that the large, complex, 8,000+ Dalton, negative ions [which may or may not be molecular, they could be a bunch of things all associated together, gotta check] could serve as a catalytic surface that would allow the absorption and construction of polyaromatic molecules (PAH's).

Reading some of the recent articles in press in Icarus regarding tholin characterization (no link to abstracts as yet), I'm still not sure if tholins are a discrete mega-molecule or a big mess of many types of molecules that holds onto discrete large molecules really well (incorporating them into pockets of the "lattice"). Either way, they are pretty insoluble, pretty intractable, and difficult to characterize. (Tholin types are very sensitive to the original gas mixture, the pressure, and the formation method - there is not yet a standard "Tholin" you can buy from Aldrich for chemists to play with. Many of the literature articles are generating and examining different sets of tholins - which makes things more difficult to cross-reference.)

Aqueous hydrolysis (acidic and basic) experiments with tholin materials seem to be able to release discrete and identifiable molecules from the mess, but then you've changed the original material.

I'm still unsure of the relationship between PAH's and tholins. I always thought of PAH's as a bunch of higher order benzenes stuck together with very few heteroatoms. Tholins appear to have things (and aromatics) derived from nitrile (CN) polymerization: things like triazine rings. I thought I read (somewhere?) that tholins do not have any aromatic C-H resonances observed by NMR experiments (DEPT? 1H-13C HETCORR? HSQC?).

Personally, I'm not going to be happy until someone can draw a structure on a white board and say "This is a tholin". I think at this point people can start to draw partial pieces and some pretty good guesses of possible pieces, but the whole elephant still is a long way from being described.

-Mike


marsbug
Thanks Mike! I've been doing some more research and I found this interesting paper, i hope you can access it. It gives me the impression of tholins being shards of wire mesh (PAH's) tangled into bundles of string (chains)!:

QUOTE
Tholins formed at low pressures contain the clusters of nitrogen-containing polycyclic aromatic compounds (N-PACs) in a matrix of carbon and nitrogen branched chain networks, which are connected tightly to each other with hydrogen bonding of N---H bonds. Tholin formed at high pressure (2300 Pa) consists of a polymer-like branched chain structure terminated with ---CH3, ---NH2, and ---CN with fewer aromatic compounds


It also mentions near the end that tholin production at lower pressures (higher altitudes) more accurately matches titans smog.
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 smile.gif )


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... laugh.gif rolleyes.gif
Juramike
QUOTE (marsbug @ Sep 22 2008, 01:19 PM) *
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
QUOTE (ngunn @ Nov 30 2007, 11:44 AM) *
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
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