Atmospheric Chemistry of Titan |
Atmospheric Chemistry of Titan |
May 2 2010, 03:38 AM
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Senior Member Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 |
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. -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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Jul 1 2010, 12:31 AM
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Senior Member Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 |
Benzene (C6H6) high altitude ion route
At higher altitudes, a different process occurs. This model is a little more speculative. The dominant proposed route starts with a large amount of diacetylene (C4H2) which gets photoionized to the radical cation. This then reacts with ethylene (by sucking in ethylene’s pi-electrons). The transient intermediate then kicks out atomic hydrogen (H.) and we are left with a C6 cation. This can cyclize via [3,3]-sigmatropic rearrangement, a concerted set of three two-electron processes. This gives us C6H5+. Reaction of this reactive intermediate with either molecular hydrogen (H2) or ethylene (C2H4) gives us protonated benzene (C6H7+) AKA the benzenium ion. If the reaction is with H2, H2 is sucked up into the system, if the reaction is with C2H4 (as drawn above) acetylene is spit out. The ethylene acts as a molecular hydrogen donor. On electronic recombination, benzene is liberated. Normally, electron recombination is a pretty harsh process and it can frag up most molecular cations when they recombine. But benzene has many vibrational modes associated with it and so can sometimes get through this OK. The speculative part is that the proposed Vuitton et al. 2009 literature model used a 10x higher amount of C4H2 than could be accounted for with the formation models for C4H2. Hopefully more measurements and theoretical work will help refine the formation models. But at least the model is consistent with Cassini observations of benzene abundance. -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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