Surface Chemistry of Titan |
Surface Chemistry of Titan |
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#101
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![]() Senior Member ![]() ![]() ![]() ![]() Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 ![]() |
The bulk of the liquid in the lakes on Titan is going to be a methane/ethane combination, but a lot of polymeric organic material is going to have been washed in from the terrain, as well as blown in from the dunes. (The lakes would be a dune trap). That means ice-silt and tholin-coated ice silt.
The higher-order organic polymers may act as a surfactant to coat the ice particles. I would hazard that the hydrophilic functionalities of the polymer subunits would side with the ice particle surface and the hydrophobic parts of the polymer would face the methane solvent. This would allow some really gnarly emulsions to set up. A cross-section of a lake on Titan would look like a classic "nightmare extraction" sitting in a sep funnel in an organic chemistry laboratory. There would be a foamy goo or scum component floating on the surface, the methane/ethane layer as solvent possibly with low density organic shmeggums floating about, then a denser loose gelatinous organic polymer/solvent component full of organic yukkies (please excuse the med chem. technical jargon), then a more dense portion of organic polymer/organic solvent/water emulsion, and finally a water-ice silt bottom. Water (ice) and organics are immiscible. But hexanes and acetonitrile (CH3CN) are also immiscible. By analogy, methane should also be immiscible with CH3CN (which would be a solid at Titan’s temperature, but a lower density component than ice). This should make for yet another fun emulsion possibility. [I’ve seen waaaaay too many ugly emulsions in sep funnels with “simple” organic components.] With the complex organic chemistry at Titan, there is a very real possiblitiy of multiple layers of emulsions combined with an organic scum layer at the surface. I’m not sure how any of this would affect specular reflections or even radar penetration. The surface would not look like the pretty foam of a bubble batch but more like the curd on overcooked pea soup.. Would certain layers reflect radar better than others? Can you get specular reflection when there are bubbles or “floaters” on the surface? What if the “floaters” are soft low density organics? How do organic emulsion blobs reflect radar? The patterns we are seeing in the lower parts of the lakes may be channels in the goopy lower emulsion layers (think of the orange crud at the bottom of scummy ponds). I could imagine a scenario when higher density organic goo flows into the lakes and carves a path through the less dense emulsion. The real “bottoms” of the lakes may lie under meters of organic emulsion. Lakes on Titan may resemble more of an open pit hazmat toxic waste dump (although I still like to think of it as “a pristine prebiotic environment”). -Mike -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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#102
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![]() Senior Member ![]() ![]() ![]() ![]() Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 ![]() |
Here are two interesting papers on tholins:
Bernard et al. Icarus 2006, 185 301-307. "Reflectance spectra and chemical structue of Titan's tholins: Application to the analysis of Cassini-Huygens observations" Abstract is here. and also (** free article **) Imanaka et al Icarus 2004, 168, 344-366. "Laboratory experiments o fTitan tholin formed in cold plasma at various pressures: implications for nitrogen-containing polycyclic aromatic compounds in Titan's haze" and it is free and available here. Both papers show that there are many flavors of tholins. (The Bernard et al paper analyzed two.) The flavor is highly dependant on pressure (altitude of formation) and chemical make up of the atmosphere where it formed. Quote (from Imanaka et al paper): "Recent laboratory simulations have revealed that the optical properties anad elemental compositions of tholins also depend on experimental conditions, such as intial gas mixture and temperature [multiple references cited in paper]. So tholins are like Forrest Gump's chocolates: "You never know what you are going to get". The bummer is that it will be difficult/impossible? to distinguish one type from the other. Quote (from Bernard abstract): "...it is very unlikely to derive quantitative chemical information (e.g., N content, sp2/sp3 ratio) from remote sensing reflectance data". Bummer. And it gets worse. All the key fundamental IR bands that would be diagnostic for identifying key functional groups are obscured by Titan's methane atmosphere. (VIMS, ISS, and also including DISR data). Here is a quick list (in uM for astro-dudes, and cm-1 for chem geeks): alcohol OH 2.72-2.75 uM, 3640-3610 cm-1 (whoo-hoo, methane window) carboxylic acids (COOH) 2.86-3.85 (whoo-hoo, methane window) amines N-H 2.86 uM, 3400 cm-1 (whoo-hoo, methane window) C-H bonds 3.23-3.57 uM, 3100-2800 cm-1(obscured by CH4) CN (nitrile) 4.42-4.47 uM, 2240-2215 cm-1 (obscured by CH4) ketenimines (RRC=C=N-R) 5.0 uM, 2000 (whoo-hoo, methane window) C=O diagnostic bonds: esters (RCOOR) 5.76 uM, 1735 cm-1 (obscured by CH4) aldehydes (RCHO) 5.80 uM, 1725 cm-1 (obscured by CH4) ketones (RRC=O) 5.83 uM, 1715 cm-1 (obscured by CH4) a,b unsat ketones (RRC=CRCOR) 5.93 cm-1, 1685 cm-1 (obscured by CH4) aromatic C-H 6.25 uM, 1600 cm-1 (obscured by CH4) "fingerprint region" ethers, amines, etc 8.77-16 uM, 1140-600 cm (obscured by CH4) Note that only carboxylic acid, amines, and water (falls in same region), and ketenimines could be detected by IR. Many of the other interesting functional groups present in molecules "the list" (see above) CANNOT BE DETECTED. Another downer from the Bernard et al paper: "The last window around 4.95 uM (2020 cm-1) is not useful to distinguish tholin types and unfortunately neither the fundamental CH2/CH3 bands nor the CN/NC bands (4.48-4.58 uM) are observable through Titan's atmosphere." Major bummer: Possibly nitriles aplenty might be down there, but we won't detect them. Remember that tholins are formed in the atmosphere and maybe at the surface. There could be other chemistry going on at the surface as well. (Think of Earth: The big reaction at the surface, CO2 sequestering in the oceans to build metal ion carbonate imestone is not easily detectable in the atmosphere. Titan atmospheric chemistry could be rare and not representaive of the organic surface (and solution) chemistry occuring at the surface.) So all the compounds on the list above cannot be ruled out easily. In fact, even Swiss cheese cannot be ruled out at this point. (It could be argued that the porosity could influence the dielectric constant) Landing an NMR instrument, IR spectrometer, Raman spectroscopy, and what the heck, a polarimeter on the surface might be the only way to figure out what is going on. All coupled with a prep HPLC, LCMS, several types of chromatography columns, elemental analysis instruments, and a team of dedicated and cold-hardy analytical chemists seems warranted. What an uber-gnarly problem. -Mike -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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