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Atmospheric Chemistry of Titan
Littlebit
post Jun 1 2010, 07:59 PM
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QUOTE (Juramike @ May 31 2010, 08:36 PM) *
Here's my memory trick:
Carbenium ions (CH3+) are fun.
Carbonium ions (CH5+) are boring.

So three's a threesome and five is a crowd?

Fun stuff Mike. Ok, fun to a few of us. It seems like, with all the radical chemistry we should find more nitrated compounds. Where are they and/or why not?
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Juramike
post Jun 1 2010, 08:34 PM
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Nitrogen does get incorporated. It is primarily in the form of nitriles (RCN). There should be surface deposits of cyanide (HCN), acetonitrile (CH3CN), and acrylonitrile (H2C=CHCN), cyanoacetylene (HCCCN), and cyanogen (NCCN).

Tholins are also thought to be rich in nitrile functionalities.

I'm still digging through some of the literature, but it seems that the literature rates for nitrile formation are a little less nailed down than for hydrocarbon formation processes. The key starting point seems to be molecular dinitrogen getting blown apart to form nitrogen radical carbene (.:N:).


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Juramike
post Jun 2 2010, 04:15 AM
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So what does CH5+ do? Basically(!), not much. It is just a transporter for a proton that it really doesn’t want. Once it finds a better proton acceptor (HCN used as an example in the graphic below), it gives the proton away and reverts back to a happy neutral CH4 molecule. All done.

Attached Image


CH3+ on the other hand, is an electron-deficient intermediate. It wants electrons - badly. It is willing to steal them from another molecule. For instance, in the graphic above, CH3+ steals electrons from a methane (CH4) molecule, kicking out a hydrogen molecule in the process. These are all 2-electron processes and are shown with a full double-headed arrow. AS the two electrons from a C-H bond goes to form a new C-C bond, another two electrons from another C-H bond jump to make a bond between the two H atoms, thus making a dihydrogen molecule.

The end result is an ethyl cation. This is more stable since the electron-deficient carbon’s empty orbital has partial overlap with one of the C-H orbitals. This is called hyperconjugation. Carbenium ion stability is tertiary>secondary>primar>>methyl.
The ethyl cation can be thought of as an electron-deficient ethane cation. But it can also be thought of as protonated ethylene. If a suitable base is around, it can pass off the proton (methane wants it even less, but HCN will accept it) and generate ethylene. Ethylene is unsaturated and has a lot of new modes of reactivity due to the double bond – we’ll be seeing it much more. One can build a lot of neat things up from ethylene, and thus CH3+ is the grandaddy of all the neat stuff.

The game of proton “hot potato” cascades down due to the energies of proton affinity. Protons really, really hate to be alone. Even protonated helium is better than a helium atom + a lonely proton. The following graphic shows a big proton cascade for many Titan-relevant molecules:

Attached Image


It starts up high then goes down to the left. This chart and energy numbers are only relevant for gas phase species. Energies in the liquid (or even solid) phase will be different due to better solvation of the proton and protonated species (which is also solvent dependent – protonated water in hydrocarbon solvent will be less happy than if it was protonated water in water). Still, it should be no surprise that ammonia or other organic amines should be the ultimate proton sink on Titan’s surface.


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Juramike
post Jun 3 2010, 03:32 AM
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Even on electron quenching of the CH5+ radical, it is pretty unimpressive. It pretty much goes to CH3 radical and 2 H radical. Although these are mildy exciting, it is nothing compared to what can happen to CH3+.

On e-quench of CH3+, the amount of energy released blows the system apart to generate an even more reactive carbene some of the time. The rest of the time, it goes to even more reactive intermediates, the radical carbene .:CH (whoa!) or even the total-fraggo uber-reactive option to a carbon atom.

Attached Image


So even if these intermediates manage to not react with other species and finally hook back up with an electron, CH3+ generates the more reactive and exciting intermediates.


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ngunn
post Jun 3 2010, 09:01 PM
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About Titan chemistry, not necessarily atmospheric:
http://saturn.jpl.nasa.gov/news/newsreleas...elease20100603/
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Juramike
post Jun 4 2010, 12:38 AM
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Great catch!!

From the press release: "We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results – for example, reactions involving mineral catalysts."

For a quick peek at what kind of reactions acetylene can undergo, see some of the discussion in this [lengthy] post: (Surface Chemistry of Titan thread, post 227)


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Bill Harris
post Jun 4 2010, 10:02 PM
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Mike,

This has been a good discussion of the chemistry of Titan's atmosphere. A lot of food for thought.

I'm not a chemist (except for a smattering of geo-chem), but this is my view of Titanian (??) atmospheric chemistry:

The atmosphere is at a higher pressure, and since the gravity of Titan is less, this density varies less with altitude.

The atmospheric temperature of -95*K or so also gives an effectively dense atmosphere with small distances between molecules.

At this low temperature chemcal reactions take a very long time to occur and even intermediate compounds that are unstable under Earthly conditions are stable for subsequent reactions on Titan. We've probably not even thought of the catalysts that can be created.

With no magnetic field of it's own, Titan's atmosphere bears the full brunt of the solar wind, as well as cosmic radiation which add enough energy with that ionizing radiation to initiate some reactions.

Titan's atmosphere is a reaction vessel of, uh titanic proportions, under unearthly conditions.

And at the surface of Titan, with water/ice and methane, nitrogen and ammonia there are undoubtedly clathrates/hydrates under unearthly conditions.

--Bill



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Juramike
post Jun 4 2010, 10:49 PM
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The low temperatures at the surface and lower atmosphere (<100 km) will hinder most thermal reactions we would consider easy at terrestrial temperatures.

But up high, the high energy photons + the thicker atmosphere at high altitude with methane make for a lot of really high-energy exotic chemistry. The really fun stuff happens above 200 km altitude. There's a peak at about 300-400 km, and another peak about 800 km, as different processes have their preferred altitudes.

So the fun stuff happens up high, then the products condense out around 130 km, then stays inert down to the surface.

[Some of the fun intermediates could get trapped and embedded in haze particles that make it down to the surface. (I'm guessin' that is the crux of the Lunine et al. abstract). Even at low temperatures, a carbenium ion thrown in solution with some unsaturated compounds might do some reacting.]


Aside from all this, one really important point is that photochemistry can occur near 0 K. So while ground-state (thermal) chemistry might be difficult at low temperatures, once you've photoexcited something, even at very low temperatures, you can do exciting (!) things. [Earths ozone layer protects us from these photons by absorbing the light and cycling through oxygen species.] So even in the cold haze decks of Titan, as long as UV can penetrate, it will do stuff.


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Juramike
post Jun 4 2010, 10:52 PM
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For the moment, the Clark et al. paper can be found here by scrolling through articles in press: http://www.agu.org/contents/journals/ViewP...?journalCode=JE
Unfortunately, access is required for any detailed information. There is not yet an abstract.


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Juramike
post Jun 6 2010, 07:58 PM
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Article in press: Strobel, D.F. Icarus (2010) Article-in-press. "Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions." doi: 10.1016/j.icarus.2010.03.003

If I understood this correctly, the models for H2 flux don't match up to the observations. This paper attempts to "follow the hydrogen" and do some atomic accounting to see how well the hydrogen matches up to models.

H2 is made by a lot of the atmospheric chemistry processes. (All those H. radicals and H2 on the back side of the equations.)
Much of it escapes from Titan off the top of the atmosphere. But according to the author, an equivalent amount goes down and away onto Titan's surface.
(6.6E27 H2 s-1 escape vs. 2.9E27 H2 s-1 down to surface and away : I'm assuming the units are actual molecules - thus 10600 moles of H2 escapes Titan every second.)

One interesting possibility the authors raise is that the H2 is sucked up into surface (or subsurface) reactions with heavier organics that then regenerate ("crack") CH4.

Even with this, the overall atmospheric escape of H2 still ensures that methane will eventually be converted to heavier organics.

Key quote: "...either the observations are not consistent with each other or that our understanding of CH4 and H2 photochemistry is flawed and needs some revision."

Since a lot of the molecular production rates for other species are bouncing around by orders of magnitude (I'll post a table soon) from one model to the next, it is not surprising that the hydrogen production rate is also not-so-well constrained. Titan chemistry is complicated.


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Juramike
post Jun 8 2010, 12:43 AM
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H2 is the key reagent that shunts the pathway to the CH5+ intermediate. Atmospheres with large amounts of H2 will generate CH5+ and thus give low yields of saturated organics, while planetary atmospheres with N2 as the diluant will have a richer organic chemistry via the CH3+ intermediate.

Attached Image
Attached Image


CH5+ (H2 present, boring chemistry): Jupiter, Saturn, Uranus, Neptune
CH3+ (N2 diluant – little H2, exciting chemistry): Titan, Pluto, Triton

Working out the full atmospheric chemistry of Titan will also help understand and be a possible preview for some of the chemistry that could occur on Triton and Pluto. It also provides a more complicated example of the carbon-based chemistry that could be happening on Jupiter and the other gas giants. It will also help shed light on processes that can occur on moons and planets and moons outside our solar system that have methane, and atmospheres with either H2 or N2/Ar.


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Juramike
post Jun 9 2010, 02:16 AM
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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.

Attached Image


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:

Attached Image


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:

Attached Image


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.


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remcook
post Jun 9 2010, 07:04 AM
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Starting on Monday: http://fd147.univ-rennes1.fr/articles.php?lng=en&pg=10 , which is heavy on Titan chemistry.
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Juramike
post Jun 10 2010, 02:58 AM
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Very cool!

Here is an EXCEL conditional formatted graphic that shows how the predicted fluxes have varied between one published model and the next. These are all surface height ratios (m.w. and predicted density taken into account) normalized to values from the Krasnopolsky 2009 model. Yellow-orange means it is ballpark same order of magnitude. Dark red shows that the model underpredicts relative to Kransopolsky 2009 by over an order of magnitude, while dark blue shows that the past model overpredicts the Krasnopolsky 2009 by over an order of magnitude.

Attached Image


As yet another example, 1-butene (CH2=CHCH2CH3, m.w. 56, a liquid at Titan temperatures) has some of the greatest variation between all the models.


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Bill Harris
post Jun 10 2010, 02:31 PM
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This is mindboggling. The potential scope of the chemistry on Titan is almost beyond comprehension.

Life is simply organic chemistry in action, with C-H-O-N reacting in a consistent and useful way. I don't want to get too philosophical about this, we may be the result of carbon being pre-programmed to act as carbon does.

Wonderfully informative charts and diagrams-- thanks, Mike.

--Bill


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