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Selfsim
2013-Jun-06, 09:37 AM
PAHs what's more ...

Cassini sees precursors to aerosol haze on Titan (http://phys.org/news/2013-06-cassini-precursors-aerosol-haze-titan.html)


Scientists working with data from NASA's Cassini mission have confirmed the presence of a population of complex hydrocarbons in the upper atmosphere of Saturn's largest moon, Titan, that later evolve into the components that give the moon a distinctive orange-brown haze.
...
"We can finally confirm that PAHs play a major role in the production of Titan's lower haze, and that the chemical reactions leading to the formation of the haze start high up in the atmosphere," said this paper's lead author Manuel López-Puertas from the Astrophysics Institute of Andalucia in Granada, Spain. "This finding is surprising: we had long suspected that PAHs and aerosols were linked in Titan's atmosphere, but didn't expect we could prove this with current instruments."
...
These hydrocarbons also are peculiarly capable of sending out profuse amounts of infrared radiation even in the rarefied environment of Titan's upper atmosphere, where the collisions between molecules are not very frequent. The molecules are themselves an intermediate product, generated when radiation from the sun ionizes smaller molecules in the upper atmosphere of Titan that then coagulate and sink.

Paul Wally
2013-Jun-06, 08:46 PM
PAHs what's more ...

Cassini sees precursors to aerosol haze on Titan (http://phys.org/news/2013-06-cassini-precursors-aerosol-haze-titan.html)

Notice that they were surprised that the instruments picked it up, not because the PAHs are there.

Selfsim
2013-Jun-06, 10:06 PM
Notice that they were surprised that the instruments picked it up, not because the PAHs are there.The article is not clear about what alerted them in the first place ...
The team of scientists had been studying the emission from various molecules in Titan's atmosphere when they stumbled upon a peculiar feature in the data. One of the characteristic lines in the spectrum—from methane emissions—had a slightly anomalous shape, and the scientists suspected it was hiding something.They had detected benzene previously, however.

What an 'anomalous shape' in a spectral line is supposed to mean, is anyone's guess. Mine would be that the PAHs might have been of a very small amplitude and very close to particular methane lines making it difficult to resolve them(??)

The orbital configuration of the spacecraft may have had a lot to do with why the detection wasn't particularly straight-forward ...

The additional signal was found only during daytime, so it clearly had something to do with solar irradiation.

starcanuck64
2013-Jun-06, 10:33 PM
It's also interesting that Saturn's magnetic field is involved in the complex chemistry going on in Titan's atmosphere.


When sunlight or highly energetic particles from Saturn's magnetic bubble hit the layers of Titan's atmosphere above about 600 miles (1,000 kilometers), the nitrogen and methane molecules there are broken up.

TooMany
2013-Jun-06, 11:02 PM
What an 'anomalous shape' in a spectral line is supposed to mean, is anyone's guess.

It means it was not what is expected from methane alone. Read this version (http://www.jpl.nasa.gov/news/news.php?release=2013-188) which provides some details about how they arrived at their conclusions.

Selfsim
2013-Jun-06, 11:44 PM
It's also interesting that Saturn's magnetic field is involved in the complex chemistry going on in Titan's atmosphere.Yes .. there's lots going on out there which may be partially attributable to Saturn's magnetosphere.
I read somewhere that Saturns' magnetosphere is the third largest structure in the Solar System. Its signatures are detectable millions of kilometres downstream from the near planet phenomena. There are explosions of hot ionised (plasma) gas, correlated with periodically detectable signals nearer to the planet. Enceladus' cold dense ejecta may provide the fuel for the effects further out, also. How much effect this may have on Titan, (which is ~five times further out than Enceladus), is under constant study ..
Interesting stuff ..

Selfsim
2013-Jun-06, 11:50 PM
It means it was not what is expected from methane alone. Read this version (http://www.jpl.nasa.gov/news/news.php?release=2013-188) which provides some details about how they arrived at their conclusions.Wish we could see the spectrum they're talking about ..
So far we've had reported that methane, benzene and PAHs have resulted in a methane emission line which had 'a slightly anomalous shape'.

Swift
2013-Jun-07, 01:21 PM
Wish we could see the spectrum they're talking about ..
So far we've had reported that methane, benzene and PAHs have resulted in a methane emission line which had 'a slightly anomalous shape'.
I think you'll have to wait for the paper or presentation for that.

Selfsim
2013-Jun-07, 09:43 PM
Interestingly from the Wiki article on abiogenesis (PAH World Hypothesis): (http://en.wikipedia.org/wiki/Abiogenesis#PAH_world_hypothesis)

In September 2012, NASA scientists reported that PAHs, subjected to interstellar medium (ISM) conditions, are transformed, throughhydrogenation, oxygenation and hydroxylation, to more complex organics - "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively". Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers ofprotoplanetary disks."
I wonder whether the same 'disappearance' applies for PAHs in Titan's upper atmosphere?

Somehow, methinks the proximity of Cassini might overcome that limitation(??), although this may be because the molecules become too massive and form solids, (which would require greater energies for specifically, emissions, I suppose ...)

Absorption and reflectance spectra might be detectable though. I know they've detected napthalene in the gaseous state (http://en.wikipedia.org/wiki/Napthalene#Gaseous_naphthalene) as part of the mysterious 'Unidentified Interstellar Bands' (UIB) spectra:

The gaseous naphthalene found in space is different from crystalline form typically used in mothballs in that it has an additional hydrogen atom, with the empirical formula: C10H+9. The UIBs have been observed by astronomers, and until recently, there has been no solid evidence identifying the compounds responsible for them. The research has recently been publicized as "Mothballs in Space."

At the end of the day, the real issue with Titan is: "What replenishes the methane (on Titan)?" ...

TooMany
2013-Jun-07, 10:00 PM
At the end of the day, the real issue with Titan is: "What replenishes the methane (on Titan)?" ...

Perhaps hydrocarbon lakes?

Colin Robinson
2013-Jun-08, 02:04 AM
At the end of the day, the real issue with Titan is: "What replenishes the methane (on Titan)?" ...

True.

The reactions that turn methane into more complex organics, including these PAHs, do raise big questions about the "methane economy" of the atmosphere.

Questions like: How long has the methane in Titan's atmosphere been there? If it has been there since the formation of the solar system, and steadily converting into other stuff all that time, why then is there any methane left in the atmosphere at all? Or, if the methane got into the atmosphere more recently, then how did that happen?

One reason these questions are hard to answer, is that hydrogen is released when methane gets converted into complex organics. And hydrogen molecules in the upper atmosphere have the velocity to diffuse into space.

Some of the hydrogen in the atmosphere may recombine with the organics at or near the surface and decompose them into methane again, with the help of some catalytic agent. This is an exciting possibility, because it implies a life-like chemistry — catalysts for decomposing stuff are key constituents in microbial life.

Nonetheless, loss of hydrogen into space means that such recycling of methane could only be partially effective, so the overall methane level would still gradually reduce.


Perhaps hydrocarbon lakes?

Yes, the lakes would be a factor, but whether they are a big enough factor is another question.

Most theories about Titan's atmosphere involve current or recent release of methane into the atmosphere from the subsurface. Volcanic activity is one possible mechanism. Another theory is that Titan has its present atmosphere thanks to a fairly recent impact event.

Selfsim
2013-Jun-08, 02:44 AM
...One reason these questions are hard to answer, is that hydrogen is released when methane gets converted into complex organics. And hydrogen molecules in the upper atmosphere have the velocity to diffuse into space. .. ah .. but do they? (See my post #6 (http://cosmoquest.org/forum/showthread.php?144595-Complex-Hydrocarbons-Detected-in-Titan-s-Atmosphere&p=2136497#post2136497)).

Gotta wonder what Saturn's magnetosphere (which rotates) does to attempted escapees?

Colin Robinson
2013-Jun-08, 03:18 AM
.. ah .. but do they? (See my post #6 (http://cosmoquest.org/forum/showthread.php?144595-Complex-Hydrocarbons-Detected-in-Titan-s-Atmosphere&p=2136497#post2136497)).

Gotta wonder what Saturn's magnetosphere (which rotates) does to attempted escapees?

If I remember correctly, it has in fact been argued that hydrogen molecules leaving Titan still remain in orbit around Saturn, and some find their way back to Titan again.

The molecules aren't actually attempting to escape, they're just moving about randomly.

That's why Strobel found it anomalous that the concentration of hydrogen near the surface is as low as it is — the laws of physics imply that molecules moving randomly will diffuse in all directions.

Selfsim
2013-Jun-08, 03:48 AM
If I remember correctly, it has in fact been argued that hydrogen molecules leaving Titan still remain in orbit around Saturn, and some find their way back to Titan again.

The molecules aren't actually attempting to escape, they're just moving about randomly.

That's why Strobel found it anomalous that the concentration of hydrogen near the surface is as low as it is — the laws of physics imply that molecules moving randomly will diffuse in all directions.I've just reading up on this .. man, its complicated. Lots has been learned from Cassini encounters .. but its very heavy reading, and difficult to picture the shape of it all. It seems Titan (and Hyperion) orbit at distances close to Saturn's minimum dimensions and they occasionally cross this boundary and travel outside it. Found this easier to read fact sheet .. (http://www.britannica.com/EBchecked/topic/525169/Saturn/54281/The-magnetic-field-and-magnetosphere)
Energetic charged particles trapped in Saturn’s outer magnetosphere collide with neutral atoms in Titan’s upper atmosphere and energize them, causing erosion of Titan's upper atmosphere … but I wonder whether these (from Saturn) might also contribute to it? … (I wonder??) ...

Unlike the cases of Earth and Jupiter, Saturn’s charged-particle population is substantially depleted by absorption of the particles onto the surfaces of solid bodies that orbit within the field lines. Voyager data showed that “holes” exist in the particle populations on field lines that intersect the rings and the orbits of moons within the magnetosphere.
Interesting ...

publiusr
2013-Jun-08, 06:12 PM
Perhaps hydrocarbon lakes?

High test for me please.

Colin Robinson
2013-Jun-10, 02:58 AM
I wonder whether study of the behavior of PAHs on Titan will help to confirm or refute the "PAH world" model of abiogenesis (http://www.astrobio.net/interview/1992/the-aromatic-world)?