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Posted by: JRehling May 10 2012, 05:26 PM

The nature of the bright surface amounting to most of Titan's surface is entirely uncertain. Some of the main things we know are:

- It has a thickness between very thin (micrometers) and about 100 meters.

- If you name just about any substance expected to condense out of Titan's atmosphere, it's not it. (Spectral signal doesn't match.)

- On the VIMS dark "blue" plains where Huygens landed, the spectral signal of the bright terrain is a component that is most intense near the shoreline.

Titan has subdued topography, but if this area resembles the highlands near the Huygens landing site, there might not be vast avalanches, but crumbling on a small scale which can be significant when the layers are so small. Equatorial areas on Titan almost certainly get rainfall quite rarely, then catastrophic deluges. So models that seem plausible might include:

1) Thin sheets of dark dune material that accumulated for 10s or 100s of years as a minor albedo component being swept away by the rain, like washing a white car that hadn't had a carwash in 50 years.
2) Craggy features on a scale of centimeters or decimeters crumbling under the rainfall, reducing the shadows.

If we had a lander touch down on this sort of surface, show us the small scale morphology and analyze the chemistry of that bright coating (and what's below it), it would be a tremendous advance. Right now, we don't have a lot of information, and I'm not sure that the rest of Cassini's mission will fill in many of the blanks.

Posted by: ngunn May 10 2012, 10:52 PM

I think the bright stuff must be the actual 'bedrock' of Titan, not an overlay. It just isn't ordinary ice or anything else immediately recognisable. The main difference between Titan and it's Jovian relatives is the presence of an atmosphere. My guess is that materials exuded onto the surfaces of moons by tectonic processes are similar at Saturn and Jupiter but that they present themselves differently to the spectrometer because of the pressure difference. In a vacuum you get dust. Under pressure you get foams. Here, possibly, clathrate foams. Could their spectra conceal the fact that they're largely composed of water?

Posted by: Juramike May 10 2012, 11:42 PM

After a the bright material is subjected to rainfall, some areas (not all, that is really important) get brighter. But the "really bright material" goes back to "normal bright material" in a matter of months. (Barnes et al., LPSC, 2012; in full disclosure, I'm part of the "et al."). That time period is too quick to be due to covering by atmospheric fallout. The best explanation is that the "really bright material" is a transient coating that goes away (sublimes, blows away, etc.)

From RADAR dielectric constant data, it is very clear that the bright material, (or pretty much else all over Titan) is NOT water ice. RADAR probes deeper than VIMS. VIMS spectra indicates that the Equatorial Bright material is "less water-like" than the Dark Blue Unit, but that could also be a grain size effect.

The one exception is Sinlap ejecta. There, VIMS says that there is an expression of more water ice rich stuff on the surface AND effective dielectric constant data is atypically high - also consistent with water ice.

My favorite hypothesis is that Equatorial Bright terrain is insoluble "gunk" that came from the atmosphere, and that the dark blue unit is "soluble transported goop" that came off the Equatorial Bright terrain. It is also possible that treating the primary atmospheric deposit with hydrocarbon rains also modifies stuff so that primary atmospheric deposit = precursor Equatorial bright + precursor Dark Blue unit.

The organics you could get from Titan chemistry are pretty complex, it will be really difficult to tease specific surface organic molecules from orbit using only limited IR spectroscopy. (Heck, look at all the effort being used to determine tholin structures! They are using LC-MS-MS Orbitrap instruments!)

(Spectroscopically, methane clathrate would look just like water ice + methane, you'd need an uber uber high-resolution IR instrument to detect the subtle shift due to the methane C-H bond vibration bumping into it's water cage wall)

Posted by: ngunn May 12 2012, 09:57 PM

I'm looking for a substance that might form a rind on exposed ice outcrops on Titan and I need a chemistry lesson, so if you're there Mike (or anyone): how about tetrahydrofuran? How does it form and how does its spectrum fit?

Posted by: Juramike May 13 2012, 05:31 AM

Oxygen (and water) are both pretty rare starting materials n Titan's atmosphere,. So ether, alcohol, or any other oxygen-containing molecules should also be pretty rare. Tetrahydrofuran (THF)!, is a cyclic ether, so should also be very rare. There just isn't an easy way to get oxygen incorporated in Titan's atmospheric chemistry.

Posted by: ngunn May 13 2012, 09:37 AM

Thanks Mike. I was supposing the oxygen might be released from the ice as a result of attack by cosmic rays or betas from C14 in the air/rain/dust. If that were indeed happening what do you think would be the most likely compounds to form? (I picked THF more or less at random.)

Posted by: Juramike May 13 2012, 12:56 PM

IIRC the cosmic ray flux is lower due to the deep atmosphere. And there's not much ice exposed at the surface. So I would speculate any cosmic ray induced chemistry would affect the surface organics. There was a paper out a few years ago suggesting acetylene trimerization to benzene based on cosmic-ray induced chemistry.

Posted by: ngunn May 13 2012, 10:17 PM

I'm looking for a way for ice to disguise itself from the spectrometer by forming a surface coating without OH bonds. John Rehling pointed out that no likely atmospheric precipitate fitted the IR data for Titan's bright areas, so I thought: OK, maybe the stuff forms by reacting with ice. What has the ice got that the atmosphere hasn't? Oxygen. So I went looking for organic molecules containing oxygen but without OH bonds.

Undisguised ice on a wide scale is observed at one location only, and nobody's suggesting that that impact happened very recently, so I'm envisaging a coating process that would be effective on a long time scale. I think there is evidence also of a minority spectral contribution from uncoated ice in areas where materials have recently been violently redistributed by flash floods.

Posted by: Juramike May 14 2012, 01:47 AM

RADAR runs deep. According to dielectric constant data, there's not much ice, at least to RADAR penetration depth.

Posted by: ngunn May 14 2012, 07:58 AM

So you need a different disguise to fool the radar. My suggestion for that is high porosity of the 'bedrock'.

(Thanks for the separate thread.)

Posted by: titanicrivers May 15 2012, 01:11 AM

For those like myself who are struggling to understand the basic makeup of Titan's surface I'd recommend the book "Titan from Cassini-Huygens" http://www.springer.com/astronomy/book/978-1-4020-9214-5?otherVersion=978-94-007-4452-3 Chapter 6 by Soderblom, Barnes, Brown, Clark, Janssen, McCord, Niemann and Tomasko entitled "Composition of Titan's Surface". Especially readible was the possible explanations of the low dielectric constant of Xanadu (p. 165) and the summary (6.6) pages 170-171. While the low-medium average dielectric constant is consistent with a surface of hydrocarbons as Mike points out, 'bedrock' of water ice or ammonia hydrate remain a possibility if they exist, as Nigel points out, as extremely porous or fractured surfaces.

Posted by: stevesliva May 15 2012, 02:10 AM

Makes me wonder how think a layer of goo and gunk sedimentary rocks lie over the ice. Are folks still thinking that the cobbles in the Huygens photos are ice? Not hunks of wax or petroleum jelly or whatever...?

Posted by: titanicrivers May 15 2012, 04:28 AM

Interestingly enough in the abstract of that chapter 6 mentioned above is the statement that the Descent Imager/Spectral Radiometer (DISR) of Huygens did not detect spectral features of the spectrally active hydrocarbon and nitrile compounds that had been anticipated (except for methane). The dark plains of the landing site had a visible and near-infrared spectral reflectance suggested a mixture of water ice, tholin-like materials and dark neutral material. The identification of water ice was only suggestive and not conclusive.

Posted by: Juramike May 15 2012, 09:01 PM

I have not seen any evidence that suggests that the cobbles at the Huygens Landing Site are water ice. I also haven't seen any evidence that they are organic compounds either. It's a really neat mystery. We have sorted, rounded cobbles, but we don't know their composition.

The DISR spectral data in Stefan et al. suggests spectral change as you go away from the bright terrain. Is that due to the sand or cobbles? Is the spectral change due to ice? Grain sizes? Or different organic components?

Posted by: ngunn May 15 2012, 10:13 PM

Glad you're there Mike because I need a chemist. Porosity can make the water ice invisible to radar (and posssibly, like pumice, bouyant on floods), but probably not invisible to infrared. Can you think of a plausible coating to disguise it in the IR?

Posted by: Juramike May 16 2012, 12:07 AM

Well, if the ice was really fractured and porous (IIRC, Janssen et al had it at organics + fractured, so if it is ice it would need to be REALLY porous), and it was coated with some organic gunk, then it might fit the data.

The coating could happen during erosional tumbling. Kinda like breaded chicken. So the coating could be thin, which gives the IR signature. The high porosity ices gives the low dielectric constant. That's one possibility.

As to actual compounds for such a coating, probably difficult to identify from just the limited IR. It could be a realy complex mixture.
If I had to guess functional groups, I'd wave my arms and guess aromatcs, alkenes, alkynes, nitriles, (guanidines? Amidines?), imines, amines, and hydrocarbons, and polymerized things. I'd exclude carboxylic acids, amides, ketones, aldehydes, ethers, and alcohols.

Posted by: titanicrivers Nov 29 2012, 01:04 AM

An interesting paper was published in Nature today by the CIRCS (composite infrared spectrometer) team members concerning suprisingly rapid (for Titan) seasonal changes in the South Polar upper atmosphere infrared spectra. Link to the paper found here http://www.nature.com/nature/journal/v491/n7426/full/nature11611.html
JPL summary article found here
http://www.jpl.nasa.gov/news/news.php?release=2012-374

Posted by: antipode Dec 2 2012, 10:21 PM

Many wont have access, but the Icarus abstract really says it all:

Does Ice Float in Titan’s Lakes and Seas?
Jason D. Hofgartner, Jonathan I. Lunine
Department of Astronomy and Center for Radiophysics and Space Research, Cornell
University, Ithaca, NY 14853, USA

Abstract
We model Titan’s lakes and seas as methane-ethane-nitrogen systems and
model the buoyancy of solids in these systems assuming thermodynamic
equilibrium. We find that ice will float in methane–rich lakes for all temperatures
below the freezing point of pure methane and that ice will also
float in ethane–rich seas provided the ice has an air porosity of greater than
5% by volume.
Keywords: Titan, Titan, Surface

P

Posted by: marsbug Mar 14 2014, 07:58 AM

My apologies for the thread necromancy, but http://www.rsc.org/chemistryworld/2014/03/saturn-moon-titan-prebiotic-soup didn't seem worth starting a thread on its own. I don't have the chemistry background to know if this is a process that would occur on Titan, but I think the melting point of isopentane is a little too high for Titan temperatures. However, if there is one pathway to get tholin to dissolve in liquid methane/ethane then perhaps there are others?

Posted by: ngunn Mar 14 2014, 07:28 PM

At the surface yes, but not in the upper atmosphere where the haze condenses, so I (as a physicist) think your query/observation could be very relevant. I hope you get insightful replies on the chemistry.