Titan's topography, strange.... Options

[ngunn]

After sleeping on it I'll now try to be more explicit about why I have difficulty with Jason's quasi-static scenario above.

I have no knowledge of how far sand particles must travel to build a dune sea or the streamlined teardrop shapes around islands, but we can see how far it must have travelled to clear Xanadu. To do that, any sand that formed in the middle of it must have drifted through at least one Xanadu radius. If the global eastward drift were much less than that we would see positive accumulations of sand on [i]both[/ sides of Xanadu, not just the western side. Instead we see a depletion of sand on the eastern side. I think that implies that the global eastward drift at least equals, and may exceed, the flow rate associated with the albedo cleaning process. In other words each sand particle must have drifted, over the history of the sand seas, a distance at least comparable with the dimensions of the sand seas themselves, and possibly much further. That in turn suggests that if Xanadu did not occasionally flush there should by now be a very much bigger pile of sand at its western margin.

[Jason W Barnes]

Well, if it's raining 1um grains, and the dunes are 300um, perhaps the 1um stuff blows around on short timescales and corns up into bigger clumps through freeze-thaw cycles. Or something. Snowdrifts definitely don't have the same flake size after just a few days.

No -- they tend to agglomerate into ice sheets. At least they do on my driveway if I don't shovel it in time!

Bob Brown at U of Arizona (VIMS Team Leader) suggested that maybe the grains are sintering together like hot metals, and I talk about that briefly in my sand dunes paper. Bottom line is that it's possible, but it would require the right compositional properties and resistance to both erosion and to sintering past the saltation size.

And they can't turn into a solid block under the weight of 100 meters of the stuff

- VIMS Jason

[stevesliva]

Bob Brown at U of Arizona (VIMS Team Leader) suggested that maybe the grains are sintering together like hot metals,

Both the ice and metal analogies are crystalline. I imagine that this goop wouldn't be. Always have to throw in a "or sumthin'" qualifier because I have no idea what Titan goop really does. Perhaps LN-cold wax is a better analogy. Give it a little heat over a long time period and it'll start sticking together, right?

[DFortes]

"No disrespect," to quote Jon Stewart, but this is why I don't like the application of goo, gloop, etc to Titan. Wax IS crystalline; you can check out some fascinating papers on the crystallography of candle wax by Dorset in Acta Cryst. (e.g., http://dx.doi.org/10.1107/S0108768195005465).
I have every expectation that the ices, hydrates, clathrates, AND the organic solids will be crystalline. I cover this in slightly more detail in my paper in press - http://dx.doi.org/10.1016/j.asr.2008.11.024

[Juramike]

Link for Dorset, Acta Crystallographica B51 (1995) 1021-1028. "The crystal structure of wax."
(Pay for article; no abstract): http://scripts.iucr.org/cgi-bin/paper?S0108768195005465

The authors showed that mixed paraffin waxes after evaporation from light peteroleum ether (mostly pentanes, probably the best room-temperature terrestrial analog for liquid methane) oriented themselves into laminar sheets.

When viewed from the side, the position of the carbon atoms is in a hexagonal arrangement (like a sheet of benzenes). The kinks of the chain are evidently 180 degrees out-of-phase with the chain below it. So the distance of 2 adjacent carbons is close, then far, then close as one moves along the chain.

The atoms at the center of the chain are pretty locked in, but at the edges the location is more random. (Probably due to voids or other defects since these are mixed component crystals)

(The structures in the article were not placed in the Cambridge Crystallographic Database).

However, the crystal structure of the more complex beeswax, was much less resolved. Since one would expect multiple functionalities on Titan surface deposit materials (tholin NMR analysis indicates multiple functional groups in the mix), I'd expect that beeswax might be a better analog.

[Although some of the functional groups might help orient the molecules in a crystal lattice]


Another article mentions that large long chain alkanes with varying lengths compounds can orient themselves in different ways. One form which can cause large voids in the structure.

http://scripts.iucr.org/cgi-bin/paper?S0108768195005465

But again, these are long straight-chain boring hydrocarbons.

AFAIK, the tholin formation/deposition experiment results have been goopy smears.


****

Titan's surface organics are presumably caused either by atmospheric fallout or by chemical reactions from stuff released at the surface (declathrate/reaction) or processing of stuff in hydrocarbon or aqueous solution in the subsurface.

Stuff falling out of the atmosphere should be in the amorphous phase (and fluffy).
It would need to be reprocessed by either solution (aqueous/organic), pressure or melting to dissolve/reorder/crystallize or reorder/recrystallize the materials.

I'm not quite sure I understand the mechanism by which non-volatile dry atmospheric fallout could recrystallize/reorder at 1.5 atm and 95 K.

(However, under pressure, in partial-solution, or at higher temperature the grains could agglomerate and recrystallize)

[Juramike]

Stiles et al. Icarus (2009) ASAP. "Determining Titan surface topography from Cassini SAR Data" doi: doi:10.1016/j.icarus.2009.03.032

Pretty pictures of "seams" along RADAR tracks give detailed elevation data inside the SAR Swaths.

(These are also visible in the Science article.)

[ngunn]

A stray thought on the dust-into-sand coagulation process. Could the presence of beta radiation have anything to do with it? Newly fallen dust should be the most radioactive material on the surface of Titan. Any effect would be naturally time-limited, and spatially limited to perhaps a few millimetres.

[DFortes]

On the crystalline versus amorphous debate

There are some recent laboratory studies of hydrocarbon ices, investigating the amorphous to crystalline transition temperature. These are found to be 70 K in diaceteylene (Zhou et al. in press Planet. Space. Sci. doi:10.1016/j.pss.2009.02.003) and 93 K in vinylacetylene (Kim & Kaiser, 2009, Astrophys. J. Suppl. Ser. 181, 543, doi:10.1088/0067-0049/181/2/543). Since much of Titan's stratosphere is warmer, then one might expect such condensates to be crystalline.

There is also observation evidence (IR lattice modes) for crystalline organic ices in Titan's stratosphere - Khanna, 2005a,b (doi:10.1016/j.icarus.2005.02.014 & 10.1016/j.icarus.2005.03.011). I think there's a paper just out by Mike Flasar which reviews this kind of thing.

There is also the possibility that amorphous ices might be transformed to a crystalline state by UV radiation, as demonstrated by Chakarov & Kasemo (1998) in Phys. Rev. Lett. 81, 5181. These authors discuss relevance to comets and outer solar system atmosphere.

And last of all, we must recall that whilst diffusion rates may be incredibly slow at 95 K, the materials may be kicking about for several billions years. It is likely that any glassy material will devitrify on such timescales, bearing in mind that 95 K is not *that* cold, and water ice is expected to undergo orientational ordering on geological timescales below 70 K.

[Juramike]

Good points.

F.M Flasar and R.K Achterberg, Phil. Trans. R. Soc. A February 28, 2009 367:649-664 "The structure and dynamics of Titan's middle atmosphere." doi:10.1098/rsta.2008.0242
Abstract via PubMed here. (I can't access the full article)

Link to abstract for Kaiser article mentioned above here

Quote from kaiser abstract:

New laboratory spectra of crystalline and amorphous diacetylene ice have been recorded in the range of 7000–500 cm−1 (1.4–20 μm) to aid in the identification of solid diacetylene on Saturn's moon Titan. We have established that amorphous diacetylene ice is stable only at temperatures less than 70±1 K.

So, if it is possible to get atmospheric fallout grains to crystallize/sinter/structurally reorganize into larger grains, does that also imply that over enough time that a stable dune field could also "lock up" and fossilize out?

[Jason W Barnes]

So, if it is possible to get atmospheric fallout grains to crystallize/sinter/structurally reorganize into larger grains, does that also imply that over enough time that a stable dune field could also "lock up" and fossilize out?

Possibly so given crystallization and reorganization, but not strictly from sintering alone. The rate of growth for a particle, i.e. dr/dt, is proportional to the radius to the 4th power. So as the sintering grains grow, they do so more and more slowly as a function of time. This can naturally limit their final size to something in the right range, if you tweak the initial parameters properly.

- Jason

[stevesliva]

Possibly so given crystallization and reorganization, but not strictly from sintering alone. The rate of growth for a particle, i.e. dr/dt, is proportional to the radius to the 4th power. So as the sintering grains grow, they do so more and more slowly as a function of time. This can naturally limit their final size to something in the right range, if you tweak the initial parameters properly.

I was wondering if a little kinetic energy from wind transport helped growth, and that once a particle was too big to be blown around, that it also limited further growth.

If those particles are 300um in size, though, that would seem small enough to blow around unless the wind was pretty damn weak. And if they can blow around, that would mean a less fossilized dune field. That must have been discussed here already. Time to use teh google.

[ngunn]

The paper is in the current issue of 'Science' out now:
http://www.sciencemag.org/cgi/content/short/324/5929/921

[titanicrivers]

Wow such an incredible discussion ! It is time for some pretty images however.
The altimetry data paper of Bryan Stiles et al http://dx.doi.org/10.1016/j.icarus.2009.03.032 published earlier this year did provide sensible correlations for many of the radar swath images although that was not stressed in their paper. Altitudinal data for the T7 swath for example provides a good concordance of downslope gradient and surface flow-fluvial features.

[ngunn]

Brilliant, TR! I think that needs doing for all the SAR swathes.

[titanicrivers]

Brilliant, TR! I think that needs doing for all the SAR swathes.

Now you're really trying to keep me busy!!!
I hope you'll settle for just one more, today anyway. The topo map below again takes SAR altimetry data from Stiles paper http://dx.doi.org/10.1016/j.icarus.2009.03.032 and blends it with SAR swath images. The map shows the S poleward portion of T 39 (T39 part 1 in Perry's compilation) from -40 S to -80 S roughly along 210 deg W longitude. Highly dissected terrain with an unusual radar bright basin(?) highlight this swath.